If you accept One-time password (OTP) via SMS/text in your app, there's a nice goodie in UITextInputTraits (which is inherited by UIKeyInput which is in turn inherited by UITextInput) which is adopted by UITextField and UITextView.

In short, you can do:

let textField = UITextField()
textField.textContentType = .oneTimeCode

or

let textView = UITextView()
textView.textContentType = .oneTimeCode

and when you receive the OTP via SMS/text, iOS will automatically show the OTP code in the QuickType bar for a few minutes letting you tap on it to automatically fill it into the textField/textView for you.

This is available since iOS 12. There's a few more types available in UITextContentType such as .password, .newPassword that is quite useful.

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Arrays are a common data structure in Swift:

let l = ["alice", "bob", "charlie"]
dump(l[2])

But array subscripting is unchecked, so this will crash at runtime:

dump(l[3])

Here's an extension to add a checked subscripting operator:

extension Array {
    subscript(checked index: UInt) -> Element? {
        if index < count {
            return self[Index(index)]
        } else {
            return nil
        }
    }
}

So you can do:

dump(l[checked: 2])
dump(l[checked: 3])

Since it returns an Optional, you can do:

if let element = l[checked: 2] {
    NSLog("Value: \(element)")
}

Note that the argument is UInt instead of Int or Index so the compiler can check when you pass in a negative index like this:

dump(l[checked: -1])

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Sometimes it's handy to put content into UITableView and want it to stretch the full height so that all the content is displayed without scrolling. Especially if you display the UITableView in a vertical UIStackView.

class ViewController: UIViewController {
    private let tableView = UITableView(frame: .zero, style: .plain)
    private var tableViewContentSizeObserver: NSKeyValueObservation?
    lazy private var tableViewHeightConstraint = tableView.heightAnchor.constraint(equalToConstant: 0)

    func init() {
        super.init(nibName: nil, bundle: nil)
        NSLayoutConstraint.activate([
            tableViewHeightConstraint
        ])
        tableView.isScrollEnabled = false
    }

    func foo() {
        tableView.reloadData()
        tableViewContentSizeObserver = tableView.observe(\UITableView.contentSize, options: [.new]) { [weak self] (_, change) in
            guard let strongSelf = self else { return }
            guard let newSize = change.newValue else { return }
            strongSelf.tableViewHeightConstraint.constant = newSize.height
        }
    }
}

That's it. The trick is to set isScrollEnabled to false, and observe the contentSize and use the height to update the constraint.

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Do you notice that if the call-in-progress or personal hotspot indicator bar is shown when you launch an app on a iPhone without a notch (i.e. iPhone 8, 8 Plus and earlier as of writing), the launch screen tends to be distorted? I've found that an easy fix is just to disable the status bar at launch with this in your Info.plist:

<key>UIStatusBarHidden</key>
<true/>

Then enable it immediately in your app delegate:

application.isStatusBarHidden = false

Let me know if you know of any other trick to do this.

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As I've done on a few occasions, I also like to drop little nuggets about external tools that aren't specifically for iOS app development, but which can be incredibly useful for.

Check out jq which in the author's own words:

jq is like sed for JSON data - you can use it to slice and filter and map and transform structured data with the same ease that sed, awk, grep and friends let you play with text.

It lets you filter and transform JSON.

An example from the tutorial:

$ curl 'https://api.github.com/repos/stedolan/jq/commits?per_page=5' | jq '.[] | {message: .commit.message, name: .commit.committer.name}'

The relevant part is:

jq '.[] | {message: .commit.message, name: .commit.committer.name}'

.[] returns each element of the array and pipe them to {message: .commit.message, name: .commit.committer.name}'.

You can count, for example with:

$ curl 'https://api.github.com/repos/stedolan/jq/commits?per_page=5' | jq '. | length'

Install it with:

brew install jq

Have fun!

Edit 20181007: Fix typo for getting length with the help of @dev_jac.

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Sometimes it's useful to keep a list of weak references to objects, especially to reduce the maintenance work needed to track when they are destroyed if you had kept a strong reference to them instead.

class C {}

This wouldn't work since they keep a strong reference:

class Watcher {
    var objects = [C]()
}

So you can define a WeakRef wrapper class:

class WeakRef<T: AnyObject> {
    weak var object: T?
    init(object: T) {
        self.object = object
    }
}

And this will work as expected:

class Watcher {
    var objects = [WeakRef<C>]()
}

Have fun with it!

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I have some guidelines I apply to myself to help reduce cyclic references caused by closures capturing self.

1. I never explicitly refer to self (as in self.someProperty or self.someFunc()) unless I absolutely have to.
2. I have a simple rule of thumb, if I see a self. in a closure, I double check to make sure it is necessary. This is because self. in a closure means the closure captures self and holds a strong reference to self. Most of the time, this can lead to cyclic references either now or later. I try my best to avoid them.

Because of (1) above, self. will only appear:

1. in init() and its variations
2. when I need to assign to a new local variable in a function and the new local variable has the same name as a function or a property
3. when closure captures self.

This means I can run grep -R "self\." . and do a quick scan when I suspect a memory leak.

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Suppose you have code like this:

func foo() {
    print("foo called")
}

func bar() {
    print("bar called")

    foo()
    //Some more code here.
}

bar()

The output is:

bar called
foo called

All good. Now, during debugging, you want to skip running foo() and all the other code in bar(), so you might add an early return statement like this:

func bar() {
    print("bar called")

    return

    foo()
    //Some more code here.
}

Do you see the problem?

It'll still execute foo() because it will consider return foo() to be a single statement and then skip the rest of the code in bar(). The way to prevent this is to use return; (i.e. end it with a semi-colon) or just comment out the rest of the code.

This is easy to miss and I was bitten by this recently.

PS: The compiler issues a warning for this, but it's easy to miss while debugging and making all sorts of code changes quickly to figure out the problem at hand.

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Swift has strong type checking. Generally, I've found that if you write your Swift apps without using force unwraps, and not deliberately stepping out of Swift's type checking, you end up with an app that hardly crashes. There is 1 exception though. Array random access via index is unchecked:

let a = [1, 2, 3]
print(a[100])

This will crash with:

Fatal error: Index out of range

In this case, Swift code that compiles may still crash. So, either check before accessing (explicitly, or by implementing and using your own checked subscript operator) unless you are sure.

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Did you know that you can include underscores (_) and leading zeros (0) in numeric literals for clarity?

i.e.

print(001) //1
print(1_000_000) //1000000
print(1.234_567) //1.234567

Works with numbers written in base-2 and base-10 too:

print(0b1000_0001) //129
print(0xCC_CC_CC) //Grayish, If you encode RGB in 3 bytes

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I'm a big fan of using Swift's strong static typing to reduce bugs, and I recently stumbled upon a library that helps with that.

From the README, imagine you have this defined:

struct User {
  let id: Int
  let email: String
  let address: String
  let subscriptionId: Int?
}

struct Subscription {
  let id: Int
}

func fetchSubscription(byId id: Int) -> Subscription? {
  return subscriptions.first(where: { $0.id == id })
}

Notice that the id properties in both structs and subscriptionId properties are of type Int. This is pretty common. The problem is that Int is a very general type.

And if you call fetchSubscription(byId:) like this, you have introduced a bug:

let subscription = fetchSubscription(byId: user.id)

Strong static typing to the rescue! Tagged provides a simple way to solve it:

import Tagged

struct User {
  let id: Id
  let email: String
  let address: String
  let subscriptionId: Subscription.Id?

  typealias Id = Tagged<User, Int>
}

struct Subscription {
  let id: Id

  typealias Id = Tagged<Subscription, Int>
}

It basically lets you define unique types based off another type (the latter being Int in the example). So you can say that this struct's id and another struct's id property are of different types, despite both of them fundamentally being Int.

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If you haven't tried CodeRunner, you really ought to. It's basically a macOS app that lets you run code snippets in different languages, include Swift. It doesn't have the fancy features present in Swift Playgrounds and the text editor is quite rudimentary, but despite that, it is really fast to build and run Swift code, so for most built-in non-UI stuff, you can explore Swift code very quickly, e.g. if you wanted to build a DateFormatter and want to experiment with various date formats.

It's really worth checking out. There's a free trial available.

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PS: This is not a paid ad

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Anytime you have some logic in a class which checks 2 or more of its properties to represent a state or condition of some sort, consider wrapping that into an enum.

For example:

class Payment {
    var isFree: Bool
    var price: Int?
    var currency: String?

    init() { //... }

    function send() {
        if isFree {
            //do something
        } else {
            if let price = price, let currency = currency {
                //do something with price + currency
            } else {
                //handle this somehow
            }
        }
    }
}

Change it to:

class Payment {
    enum Type {
        case free
        case paid(price: Int, currency: String)
    }
    var type: Type

    init() { //... }

    function send() {
        switch type {
        case .free:
            //do something
        case .paid(let price, let currency):
            //do something with price + currency
        }
    }
}

The code ends up clearer — being explicit that it's handling 2 states, free and paid — and it's less likely to introduce bugs because the code paths are easier to follow and if you need to do something that depends on the type, the compiler will force you to handle all the possible states (free and paid).

The resulting code can sometimes end up being longer, but it's a worthy trade-off.

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Most apps need some kind of backing datastore and sometimes plist(s) are a perfectly fine option. When you need to handle more, structured data, you can consider SQLite, perhaps with a wrapper like FMDB. Alternatively, there is Realm which provides a local database and an optional cloud-sync service.

It's easy to get started, especially if you just want to use it for local storage.

Add it to your Podfile:

pod 'RealmSwift'

Install:

$ pod install --repo-update

Import the framework:

import RealmSwift

Define a class that is backed by Realm:

class Item: Object {
    @objc dynamic var itemId: String = UUID().uuidString
    @objc dynamic var body: String = ""
    @objc dynamic var timestamp: Date = Date()

    override static func primaryKey() -> String? {
        return "itemId"
    }
}

The properties that you want stored in Realm has to be declared with @objc and dynamic.

Create the database instance:

let realm = try! Realm()

Fetch objects:

items = realm.objects(Item.self).sorted(byKeyPath: "timestamp", ascending: false)

Create and save a new object:

try! realm.write {
    realm.add(item)
}

Update an object:

try! realm.write {
    item.body = "Something something"
}

Delete an object:

try! realm.write {
    realm.delete(item)
}

You can define parent-child relationships:

class Parent: Object {
    @objc dynamic var parentId: String = UUID().uuidString
    var items = List<Item>()

    override static func primaryKey() -> String? {
        return "parentId"
    }
}

You can perform schema migrations with a migration block:

config.migrationBlock = { migration, oldSchemaVersion in
    if oldSchemaVersion < 10 {
        migration.enumerateObjects(ofType: Item.className()) { oldObject, newObject in
            guard let oldObject = oldObject else { return }
            guard let newObject = newObject else { return }
            newObject["someChangedVarName"] = oldObject["oldChangedVarName"]
        }
    }

    if oldSchemaVersion < 20 {
        //...
    }
}

It's extremely quick to start and low effort to use for local storage.

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I have written about printing out the document directory in #172, but here's a obvious tweak on hindsight, along with updates for current Swift language changes:

#if DEBUG
    let docPath = FileManager.default.urls(for: FileManager.SearchPathDirectory.documentDirectory, in: FileManager.SearchPathDomainMask.userDomainMask).last?.path
    print("docPath: \(docPath)")
#endif

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I'm going to talk about something that Swift doesn't have.

I'm quite a fan of the pipe operator in Elixir where you can do:

"Elixir rocks"
    |> String.upcase()
    |> String.split()

> ["ELIXIR", "ROCKS"]

Swift doesn't provide the pipe operator, but there are multiple implementations floating around that implements it using operator overloading. One of the libraries gives as an example:

let isEven: (Int) -> Bool = { x in x % 2 == 0 }
[1,2,3,4,5]
    .filter(isEven)
    .map({$0 * 3})
    .reduce(0, +)

which can be rewritten with the library's pipe operator as:

let isEven: (Int) -> Bool = { x in x % 2 == 0 }
[1,2,3,4,5]
    |> (filter, isEven)
    |> (map, {$0 * 3})
    |> (reduce, 0, +)

I just want to point out that. The pipe operator in Elixir (and F#) works in a different way. Let's look at an example. With the pipe operator in Elixir, you should be able to rewrite this code:

bar(foo([1, 2, 3, 4].map({ $0 == 2 })))

into:

[1, 2, 3, 4].map({ $0 == 2 })
    |> foo
    |> bar

In other words, you kind of flip the code with nested function calls from right to left to left-to-right, so it's easier to understand without introducing local variables.

Just wanted to clear this up.

PS: Sorry, this issue is very late.

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According to Wikipedia:

currying is the technique of translating the evaluation of a function that takes multiple arguments (or a tuple of arguments) into evaluating a sequence of functions, each with a single argument.

In Swift 3, currying was removed. But it is still a useful tool. If you still want to use currying, there's a library for that.

Given a function f:

func f(x: Int, y: Int, z: Int)

which you normally call with f(x: 1, y: 2, z: 3), you turn it into a series of functions that you can call in these order with currying:

let f0 = curry(f)
let f1 = f0(1)
let f2 = f1(2)
let result = f2(3)

This is useful, for e.g., because you can create the functions stored in f1, f2 as you process and gather the arguments.

While an alternative is just to store the values of x, y as we gather them in an object, as we write more functional code, currying is a very useful technique.

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Did you know that you can set conditions for your breakpoints?

1. Click at the gutter in Xcode editor on the line of code to set a breakpoint
2. Ctrl-click the breakpoint and choose Edit Breakpoint
3. Type in the condition. eg. i == 13

Conditional breakpoints will only break when it matches the condition.

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This is a common pattern:

let someModel = getCurrentModel()
functionThatIsAsynchronousEspeciallyAWebAPICall(model: someModel) { result in
    doSomething(with: result)
}

functionThatIsAsynchronousEspeciallyAWebAPICall() is a function that is asynchronous and sometimes takes a few seconds or more to complete and call its completion handler.

In an app that has user accounts or something similar, this becomes:

let account = getAccount()
functionThatIsAsynchronousEspeciallyAWebAPICall(account: account) { result in
    doSomething(with: result)
}

Depending on how you implement doSomething(with:), this might not always behalf correctly because by the time the completion handler is called, the user might have switched accounts. It's better to include a check that the current account is still the same.

let account = getAccount()
functionThatIsAsynchronousEspeciallyAWebAPICall(account: account) { result in
    let currentAccount = getAccount()
    guard currentAccount.username == account.username else { return }
    doSomething(with: result)
}

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Do you use UITableview without self-sizing cells?

I've found an odd behavior that happens when several conditions are met:

• not use self-sizing cells
• the number of rows display is such that it the total content height is slightly more than the table view's height
• you scroll to the bottom of the table

Sometimes when you call reloadData, the table may scroll so there is a gap at the bottom, below the last cell.

Anecdotally, this seems to be fixed by setting UITableview.estimatedRowHeight to 0.

If you experience this, try it out and let me know.

PS: Sorry, I'm late for this issue

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I discovered SwifterSwift recently. In its own words, it's a "A handy collection of more than 500 native Swift extensions to boost your productivity".

Some examples from https://github.com/SwifterSwift/SwifterSwift/blob/master/Examples/Examples.md:

// Remove duplicates from an array
[1, 2, 3, 1, 3].removeDuplicates() -> [1, 2, 3]

date.hour = 14

"hello world".split(by: " ") -> ["hello", "world"]

"123abc".reverse() -> "cba321"

"Hèllö Wórld!".latinize() -> "Hello World!"

√ 9 -> 3

view.addShadow(ofColor .black, radius: 3, opacity: 0.5)

image.kilobytesSize -> 114

imageView.download(from url, contentMode: .scaleAspectFit, placeHolder: UIImage?)

I'm still not sure if importing such libraries will essentially make you form your own dialect of Swift (maybe that's inevitable with all libraries), but there are definitely some gems in there and since the license is MIT, you can at least cherry pick what you want if you rather not import the entire library. So check it out!

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If you work with Core Data, you got to check out Core Data Editor. Remember Fred Brooks famous quote:

Show me your flowcharts and conceal your tables, and I shall continue to be mystified. Show me your tables, and I won’t usually need your flowcharts; they’ll be obvious.

It's true. I recently took over a project that uses Core Data and Core Data Editor lets me look at what are the types involved as well as data stored. It has a few handy features like letting you point to your simulator directory and DerivedData directory so that it can automatically list your apps.

As of writing, there's a downloadable release on GitHub.

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For most iOS apps, displaying the right data in the right way is very important. Traditionally, testing this is hard. It gets especially more complicated as your app feature set grows and you no longer know if a change affects another screen(s).

iOSSnapshotTestCase helps with that by letting us write snapshot tests, or what I like to call "screenshot-based asserts". You write tests cases with it, creating or navigating to the desired screen and then take a screenshot with it, comparing that to a reference screenshot which you made earlier.

It's easy to get started.

1. Add the iOSSnapshotTestCase pod with Cocoapods for your test target, run pod install

2. In Xcode, go to Product > Schemes > Edit Scheme, choose Test, add these to Environment Variables:

   FB_REFERENCE_IMAGE_DIR = $(SOURCE_ROOT)/$(PROJECT_NAME)Tests/Snapshots/ReferenceImages IMAGE_DIFF_DIR = $(SOURCE_ROOT)$/(PROJECT_NAME)Tests/Snapshots/FailureDiffs

   Make sure that $(PROJECT_NAME)Tests matches your test folder name in your project directory, otherwise tweak the path accordingly.

   The former will be used to store your reference images and the latter the results if they don't match.

3. Create a test class, but subclass FBSnapshotTestCase instead of XCTestCase.

4. In the test function, create your view controller/view and call FBSnapshotVerifyView() passing in the view to test
5. In setUp(), set recordMode to true and run the test case to create and store the reference screenshots in FB_REFERENCE_IMAGE_DIR. You'd want to make sure these screenshots represent how your app should behave.
6. From now on, run it with recordMode = false to compare against the reference screenshots — until you intentionally change your UI and you would need to update your reference snapshot(s). If a test fails, the result, including the difference will be stored in IMAGE_DIFF_DIR.
7. You want to add IMAGE_DIFF_DIR to your .gitignore file.

e.g of a test class:

class AnExampleSnapshotTest: FBSnapshotTestCase {
    override func setUp() {
        super.setUp()
        //Set to true only when creating reference snapshots
        //This line should not be checked into version control
        recordMode = true
    }

    func testExample() {
        //TODO create data, set up controller, etc
        let controller = MyViewController()
        //This will create a reference snapshot or compare against one, depending on the value of `recordMode`
        FBSnapshotVerifyView(controller.view)
    }
}

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We often need to go through elements in a collection, but only operating on elements that fulfill a certain condition. E.g. only if the number is odd:

for i in 1...10 {
    if i % 2 == 0 {
        print(i)
    }
}

Here's another way to do it, using filter():

for i in (1...10).filter({$0 % 2 == 0}) {
    print(i)
}

But this is probably the more idiomatic way to do it in Swift:

for i in 1...10 where i % 2 == 0 {
    print(i)
}

Note that you can use where clauses in a similar manner in other places, such as in switch-case statements.

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You have probably used Implicit Member Expressions without knowing what it's called:

var color: UIColor?
color = .white

i.e. the member of a type can be accessed from an inferred type.

You can access static functions:

foo(i: .createInstance())

Did you know that it works with .init() too?

class C {
    static func foo() -> C {
        return .init()
    }
}
func bar(c: C) {}

bar(c: .init())

Notice the 2 calls to init() above.

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Using Auto Layout with UIScrollView is surprisingly simple. You just need to add constraints to the content that will fill the entire scrollview by anchoring to the 4 edges of the scroll view. You don't need to set the content size of the scroll view explicitly. This is because the 4 edges actually refer to the scroll view's content view rather than the scroll view itself.

let scrollView = UIScrollView()
content.translatesAutoresizingMaskIntoConstraints = false

let content = UIView()
//TODO: Set up `content`, adding subviews, etc
content.translatesAutoresizingMaskIntoConstraints = false
scrollView.addSubview(content)

NSLayoutConstraint.activate([
    content.leadingAnchor.constraint(equalTo: scrollView.leadingAnchor),
    content.trailingAnchor.constraint(equalTo: scrollView.trailingAnchor),
    content.topAnchor.constraint(equalTo: scrollView.topAnchor),
    content.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor),

    //Of course you'll want to layout the scrollview itself. Likely occupying the entire parent
    scrollView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
    scrollView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
    scrollView.topAnchor.constraint(equalTo: view.topAnchor),
    scrollView.bottomAnchor.constraint(equalTo: footerBar.topAnchor),

    //TODO set up constraints for `content` as you would have if there was no `UIScrollView` involved
])

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UIWebView is (still) useful for showing web-based documents — such as help or FAQs — in your app. If the document background color and app's background color don't match and aren't white, you might sometimes see the the UIWebView's white background color appear for a split second the first time a document is loaded.

You can't fix this by setting UIWebView's background color, because it is ignored. Here's an easy way to fix this:

Set webview.isOpaque to false and make sure the UIViewController instance holding the UIWebView has the correct background color.

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Do you sometimes you want to change the title of a text-only UIButton while it is on the screen and the text flashes? The easiest way to fix that is to use the .custom button type instead of .system.

Here's some playground code to illustrate this:

import UIKit
import PlaygroundSupport

let view = UIView()
view.backgroundColor = .green
view.frame = CGRect(x: 0, y: 0, width: 200, height: 300)
let b = UIButton(type: .system) //Switch to .custom
b.frame = CGRect(x: 10, y: 10, width: 70, height: 24)
b.setTitle("foo", for: .normal)
b.setTitleColor(.red, for: .normal)
view.addSubview(b)

PlaygroundPage.current.liveView = view

DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
    b.setTitle("bar", for: .normal)
}

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Consider this code:

class S {
    var i: Int
    init(i: Int) {
        self.i = i
    }
}

class C {
    var prop1 = S(i: 0) {
        didSet {
            print("did set prop1, with i: \(prop1.i)")
        }
    }
}

var c = C()
print("i: \(c.prop1.i)")
var p = c.prop1
p.i = 1
print("i: \(c.prop1.i)")
c.prop1.i = 2

It prints:

i: 0
i: 1
i: 2

It prints 0, 1, 2 which is fairly obvious because S is a class — a reference type — so when p is actually a reference to the same instance of S, we are just operating on that same instance.

However, here's something that was surprising to me. What happens if we change S to be a struct? i.e.

Replace:

class {

with:

struct {

The output becomes:

i: 0
i: 0
did set prop1, with i: 2
i: 2

S is now a value type, so naturally, i remains at 0 because p is actually a copy of the struct held by the instance of C. Here's what surprised me though. c.prop1.i = 2 copies the struct, sets the new value of i (to 2) and assigns it back to the instance of C, triggering the didSet observer.

I recently wrote some code where this behavior bit me because I wasn't counting on didSet being triggered.

Beware!

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A common pattern for UI layout is to have a bar at the bottom of the screen like UITabBar. It has a fixed height in most cases, but with the iPhone X, you want it to stretch to the bottom of the screen, outside of the safe area while keeping it's contents (in UITabBar's case, the tab buttons) glued to the top of the bar.

Auto Layout anchors provide a very simple way to wire this up. In your view controller:

let buttonsHeight = 44
footerBar.topAnchor.constraint(equalTo: view.layoutGuide.bottomAnchor, constant: -buttonsHeight),
footerBar.bottomAnchor.constraint(equalTo: view.bottomAnchor),

The trick is to anchor the top and bottom of footerBar.

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This is super handy for implementing rounded corners when performance doesn't matter:

aView.clipsToBounds = true
aView.layer.cornerRadius = 20

But sometimes, maybe the design requires rounding only the top 2 corners. If it's an almost full-screen view, a simple way to do it is to let a view obscure the bottom 2 corners or just make sure it is offscreen (again, if performance is not an issue). But in iOS 11, there's a new property:

var maskedCorners: CACornerMask

This lets you set specific (corners) to mask. Here's some playground code:

import UIKit
import PlaygroundSupport

class VC: UIViewController {
    init() {
        super.init(nibName: nil, bundle: nil)
        view.backgroundColor = .black

        let square = UIView(frame: CGRect(x: 20, y: 20, width: 200, height: 200))
        square.backgroundColor = .white
        square.layer.cornerRadius = 20
        square.layer.maskedCorners = [.layerMinXMinYCorner, .layerMaxXMinYCorner]
        view.addSubview(square)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

PlaygroundPage.current.liveView = VC()

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Like most developers, I use git very often. As I'm a lazy developer, I like to shorten the commands that I use frequently, so I have set these up as aliases:

alias ga='git add'
alias gp='git add -p'
alias gs='clear && git status -s'
alias gd='git diff --color'
alias gdc='git diff --cached --color'
alias gdt='git difftool'
alias gc='git commit $1'
alias gca='git commit -a $1'
alias gb='git branch'
alias gl='git log --color'
alias glo='git log --color --pretty=oneline'
alias gt='git tag'
alias gco='git checkout $1'
alias gcp='git cherry-pick $1'
alias gdn='git diff --name-only'
alias gr='git remote'

I get heavy use out of gb, gco, and gdn.

Do you have aliases for git commands that you found particularly useful?

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If you build lots of forms — screens powered by table views that either read-only information or allow users to enter data such as names, dates/birthdays — Eureka is worth taking a look.

The README is very good, so I'll just show an example from there:

import Eureka

class MyFormViewController: FormViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        form +++ Section("Section1")
            <<< TextRow(){ row in
                row.title = "Text Row"
                row.placeholder = "Enter text here"
            }
            <<< PhoneRow(){
                $0.title = "Phone Row"
                $0.placeholder = "And numbers here"
            }
        +++ Section("Section2")
            <<< DateRow(){
                $0.title = "Date Row"
                $0.value = Date(timeIntervalSinceReferenceDate: 0)
            }
    }
}

I've found Eureka very handy if your app mostly uses the original look and feel of iOS. If you need to customize the forms UI — while Eureka supports that too, by letting you specify your own custom row and cell classes — I've found it might be a bit more messy than I hoped.

Eureka can be installed via Cocoapods or Carthage.

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Wanted to share the updated version of my typical pre-commit hook since 2 years ago. It's better at only picking up additions rather than diffs (additions + removals).

I've also included checks for prints and NSLogs.

Sometimes I work on projects in a team and want to make sure that I don't accidentally check in changes to .xcodeproj, so I have a check for that too.

My commit workflow then includes commenting and uncommenting out section of this pre-commit file while I hit warnings and comparing it with the changes I have in mind. Finally I'll just do a git commit -m "some message" -n (with the additional -n switch) to commit, bypassing the precommit hook.

#!/bin/sh

if git rev-parse --verify HEAD >/dev/null 2>&1; then  
  against=HEAD
else  
  # Initial commit: diff against an empty tree object
  against=4b825dc642cb6eb9a060e54bf8d69288fbee4904
fi

exec 1>&2

#Workaround for filenames with spaces in them (because for loop breaks use space by default)
SAVEIFS=$IFS
IFS=$(echo "\n\b")

for ENTRY in `git diff-index --cached --name-status $against -- | cut -c1,3-`; do  
  CHANGE=`echo "$ENTRY" | cut -c1`
  FILE=`echo "$ENTRY" | cut -c2-`
  if [ $CHANGE = "A" ] || [ $CHANGE = "M" ]; then
      if [ ${FILE: -26} == ".xcodeproj/project.pbxproj" ]; then
          echo "$FILE" 'contains .xcodeproj/project.pbxproj! Are you sure?'
          IFS=$SAVEIFS
          exit 1
      fi
      if [ ${FILE: -6} == ".swift" ] || [ ${FILE: -2} == ".m" ] || [ ${FILE: -2} == ".h" ]; then
          if git diff --cached --color=always "$FILE" | perl -wlne 'print $1 if /^\e\[32m\+\e\[m\e\[32m(.*)\e\[m$/' | grep -q '\t'; then
              echo "$FILE" 'contains tabs!'
              IFS=$SAVEIFS
              exit 1
          fi
      fi
      if git diff --cached --color=always "$FILE" | perl -wlne 'print $1 if /^\e\[32m\+\e\[m\e\[32m(.*)\e\[m$/' | grep -q 'kkk'; then
          echo "$FILE" 'contains kkk!'
          IFS=$SAVEIFS
          exit 1
      fi
      if git diff --cached --color=always "$FILE" | perl -wlne 'print $1 if /^\e\[32m\+\e\[m\e\[32m(.*)\e\[m$/' | grep -q ' NSLog'; then
          echo "$FILE" 'contains NSLog!'
          IFS=$SAVEIFS
          exit 1
      fi
      if git diff --cached --color=always "$FILE" | perl -wlne 'print $1 if /^\e\[32m\+\e\[m\e\[32m(.*)\e\[m$/' | grep -q ' print'; then
          echo "$FILE" 'contains print!'
          IFS=$SAVEIFS
          exit 1
      fi
      if git diff --cached --color=always "$FILE" | perl -wlne 'print $1 if /^\e\[32m\+\e\[m\e\[32m(.*)\e\[m$/' | grep -q ' dump'; then
          echo "$FILE" 'contains dump!'
          IFS=$SAVEIFS
          exit 1
      fi
      if git diff --cached --color=always "$FILE" | perl -wlne 'print $1 if /^\e\[32m\+\e\[m\e\[32m(.*)\e\[m$/' | grep -q '007'; then
          echo "$FILE" 'contains 007!'
          IFS=$SAVEIFS
          exit 1
      fi
  fi
done

IFS=$SAVEIFS
exit 0

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Swift is a strongly typed language, so it makes sense when using it to tweak your tools and code to take advantage of type-checking. This week's nugget goes along that line.

The library R.swift automatically generates static references to resources such as font files and image files so that you don't have to refer to them by string. Example from the README:

let icon = UIImage(named: "settings-icon")
let font = UIFont(name: "San Francisco", size: 42)
let color = UIColor(named: "indictator highlight")
let viewController = CustomViewController(nibName: "CustomView", bundle: nil)
let string = String(format: NSLocalizedString("welcome.withName", comment: ""), locale: NSLocale.current, "Arthur Dent")

becomes:

let icon = R.image.settingsIcon()
let font = R.font.sanFrancisco(size: 42)
let color = R.color.indicatorHighlight()
let viewController = CustomViewController(nib: R.nib.customView)
let string = R.string.localizable.welcomeWithName("Arthur Dent")

This avoids the chance of typing a string wrongly and not catching it before submitting a build to the app store.

Installation is simple:

1. Include the pod (R.swift)
2. Add a Run Script phase before Compile Sources: "$PODS_ROOT/R.swift/rswift" generate "$SRCROOT"
3. Add the R.generated.swift file to your project

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If you use custom fonts in your apps, it can be a hassle to track and maintain the UIAppFonts key in your Info.plist file, especially as you need to figure out each font name yourself. Here's an excellent timesaver: FontBlaster is a library that programmatically loads custom fonts for you. You just need to do this:

1. Include the font files in your project like you'd normally do
2. Include FontBlaster. (Supports Carthage and CocoaPods)
3. Run FontBlaster.blast()

You no longer need the UIAppFonts key in Info.plist.

I learnt about FontBlaster from Michael Tsai.

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Related to #162 Reclaim Disk Space from Xcode Usage, if you run:

$ xcrun simctl delete unavailable

simctl will remove old simulators which your current Xcode (xcode-select -p) cannot use.

If you work with multiple versions of Xcode, you have to be careful. This will probably delete simulators that another version of Xcode can use.

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Maybe it's an existing project, or for some other reason you are still using Objective-C, here's a nugget for you:

__auto_type i = @123;
NSLog(@"class: %@", NSStringFromClass([i class]));

const __auto_type j = @123;
j = @1;

That last line will compile with an error:

Cannot assign to variable 'j' with const-qualified type 'NSNumber *__strong const'

Typing _auto_type and const __auto_type looks a little unwieldy, so we can define a few macros:

#define let const __auto_type
#define var __auto_type

and use them instead:

var i = @123;
NSLog(@"class: %@", NSStringFromClass([i class]));

let j = @123;
j = @1; //Compilation error

(Probably more useful to store the macros in a new header file and #import them)

I picked up this tip from @_nb

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Did you know that LLDB includes an REPL?

Set a breakpoint in your Xcode project, say the method that gets run when you hit a button. In the debugger proompt, type repl. i.e.:

(lldb) repl

Then paste this in:

let v = UIView()
v.frame = CGRect(x: 20, y: 20, width: 200, height: 20)
v.backgroundColor = .red
let window = UIApplication.shared.windows[0]
window.rootViewController?.view.addSubview(v)
CATransaction.flush()

Hit enter.

You should see that the new red view appears immediately upon CATransaction.flush() while you are still in the debugger. It is important that you are not breaking in application(_:, didFinishLaunchingWithOptions:) for flush() to "load" your changes immediately.

Type : and hit enter to exit the REPL.

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As a printf debugger, I have a little trick I like to use when logging hierarchy function calls. For example:

struct Node {
    var name: String
    var children = [Node]()

    init(_ name: String) {
        self.name = name
    }
    mutating func addChild(child: Node) {
        children.append(child)
    }
    func printHierarchy() {
        print("\(name)")
        for each in children {
            each.printHierarchy()
        }
    }
}

var root = Node("Topmost")
var n1 = Node("n1")
var n2 = Node("n2")
var n3 = Node("n3")
var n4 = Node("n4")
n3.addChild(child: n4)
n2.addChild(child: n3)
root.addChild(child: n1)
root.addChild(child: n2)

root.printHierarchy()

This prints:

Topmost
n1
n2
n3
n4

which isn't that useful. But if we modify printHierarchy() to add an indent argument with a default value of 0:

func printHierarchy(indent: Int=0) {
    print("\(String(repeating: "  ", count: indent))\(name)")
    for each in children {
        each.printHierarchy(indent: indent + 1)
    }
}

Our output becomes:

Topmost
  n1
  n2
    n3
      n4

This is especially useful when we are logging more information in each node to debug method calls up and down the hierarchy.

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If you perform your own custom view draw by overriding UIView draw(_:) in your subclass, and you trigger it by calling setNeedsDisplay(_:), it's a performance boost if you restrict drawing to just the dirty rectangle passed to draw(_:):

override func draw(_ rect: CGRect) {
    //Only draw inside rect, and not the entire UIView specified by the `bounds` property
}

On macOS, the functions getRectsBeingDrawn(_:count:) and needsToDraw(_:) can be used to discover whether you want to draw, but they are not available on iOS.

I've found that a simple way the handle this is to set the dirty rect as the clipping region:

override func draw(_ rect: CGRect) {
    let context = UIGraphicsGetCurrentContext()
    context?.saveGState()
    let path = UIBezierPath(rect: rect)
    path.addClip()
    //Perform your drawing in `bounds` here
    context?.restoreGState()
}

I'm recently building an app that involves panning many views that implement custom drawing and just introducing clipping makes the performance improve from being dog slow to buttery smooth.

And if you want to go one step further, you can use the "context" technique I mentioned in #168 to clean up the code:

Implement this:

func withGraphicsContext(block: (CGContext?) -> ()) {
    let context = UIGraphicsGetCurrentContext()
    context?.saveGState()
    block(context)
    context?.restoreGState()
}

And you can rewrite draw(_:):

override func draw(_ rect: CGRect) {
    withGraphicsContext { context in
        let path = UIBezierPath(rect: rect)
        path.addClip()
        //Perform your drawing in `bounds` here
    }
}

There is a technote which mentions:

Each UIView is treated as a single element. When you request a redraw, in part or whole, by calling -setNeedsDisplayInRect: or -setNeedsDisplay:, the entire view will be marked for updates

I did some testing and it seems like the statement is either outdated or wrong. But even if it becomes true in the future, clipping wouldn't break your code. So happy drawing!

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In #232 we looked at how we can use Shared Web Credentials to share login information between your iOS app and Safari by reading/writing to the keychain.

There's a new feature in iOS 11 for apps called Password AutoFill. It lets iOS populate your username and password fields with information from the keychain chosen by the user with a single tap. This goes one step further to reduce the friction for users to move from using your website to running your app.

If you have seen this Password AutoFill QuickType bar in an app, it is AutoFill in action:

If you tap the button, it should attempt to authenticate using Touch ID and then show you a list of logins to choose from to fill in to the login form. If the app haven't done anything to enable AutoFill, iOS uses some heuristics to try to make it work.

You might even see some a username appearing in the AutoFill QuickType bar, letting you tap on it to choose it right away.

This is all done automatically by iOS

But if want to be sure AutoFill works correctly and show the correct username to the user in the AutoFill QuickType bar, you can set this up explicitly by:

1) Setting the content type of yoru username and password field:

usernameField.textContentType = .username
passwordField.textContentType = .password

2) Enable the associated domains entitlements in Xcode and set up a site associate file on your website. You would have done this if you have already set up Shared Web Credentials.

If you visit the login screen in your app again, you should notice the AutoFill QuickType bar pop up correctly and the username matching your app should appear.

Here's the WWDC video explaining Password AutoFill if you need more details.

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In UIView and its subclasses, there are many properties — such as frame and alpha — that can be animated when they change. i.e. you can do something this and changes will be animated:

UIView.animate(withDuration: 2) {
    self.label.frame = CGRect(x: 150, y: 200, width: 50, height: 20)
    self.button.tintColor = .white
}

Unfortunately, this doesn't work for UILabel's color property. Here's the good news: You can approximate this using transition(with:duration:options:animations:completion:) instead:

UIView.transition(with: label, duration: 2, options: .transitionCrossDissolve, animations: {
    self.label.textColor = .red
}, completion: nil)

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I first read about this idea of namespacing using enums in the article Easy Namespacing in Swift by @khanlou.

For example, for my library Zhi,

I have a file which only has these 2 lines to create a "namespace":

public enum Zhi {
}

Then in other files, I can do this to add to the namespace:

public extension Zhi {
    class StyleParser {
        //...
    }
}

As mentioned in the article, a major limitation is it doesn't work for protocols. So this doesn't work:

public extension Zhi {
    protocol P {}
}

But otherwise, using enums as a namespace is a great way to organize your code.

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I wrote a library for working with Auto Layout in Swift. It lets you tweak Auto Layout constraints and watch your app update immediately without a rebuild + run cycle. It’s called Zhi.

The library allows you to specify your auto layout constraints in separate .styles files. It supports:

• Live reloading of Auto Layout constraints as you save the .styles files, without a rebuild + run cycle
• Apple's Visual Format Language: H:[p(==v2)]|
• Support an equations-based syntax: v1.width = v2.width + 10

In addition, the .styles files also support:

• Metrics can be specified
• Colors — rgb(), rgba() or named colors in UIColor, e.g. red
• Image filenames
• Button title and label text
• Font name, size
• Dynamic type style names, e.g. callout or caption1
• and more

This means that you can tweak these properties without rebuilding your app during development.

Refer to the README for installation and usage.

Try it out and let me know what you think.

I created a Patreon page. I have been writing iOS Dev Nuggets for more than 5 years. If you want to support me, visit Patreon and consider pledging.

As usual, any tips or comments are welcomed via email.

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In Swift, the lazy keyword changes a property so that it is only calculated when it's accessed. This not only makes it useful for performance reasons, it's also incredibly useful for initializing your class when a non-optional property depends on another property for initialization.

Let's look at an example:

class C {
    var x: UIView
    var y: UIView = {
        var v = UIView()
        x.addSubview(v)
        return v
    }()

    init() {
        x = UIView()
    }
}

This class definition compiles with an error because the value of x is used when computing the value of y:

error: instance member 'x' cannot be used on type 'C'

This wouldn't work even if you change the definition of x to:

var x = UIView()

There's an easy fix — just change y to be lazy:

class C {
    var x: UIView
    lazy var y: UIView = {
        var v = UIView()
        x.addSubview(v)
        return v
    }()

    init() {
        x = UIView()
    }
}

Because lazy properties are only calculated when they are first used — and you don't use them in your initializer before x is initialized, this is a very simple fix.

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When creating iOS app preview videos, it's sometimes useful to where your fingers touches on the screen. This is particularly useful if you have multi-finger gestures involved.

There are several libraries that let you do this, one of them is EUMTouchPointView.

Integrating the library is simple. Instead of creating a UIWindow instance, create a EUMShowTouchWindow instance.

When you run your app, touch indicators will appear.

Remember to exclude the library for your app store submission.

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One of the most common way to reduce coupling when creating a new class is to create a delegate(s) for it. This is a common pattern used in Cocoa (Touch) classes, from simple examples like UIImagePickerController and UIImagePickerControllerDelegate to more complicated examples like UITableView and UITableViewDelegate, and UITableViewDataSource.

You can implement delegates using protocols. e.g.

protocol DelegateOfA {
    func onSomethingHappened()
}

class A {
    var delegate: DelegateOfA?

    init() {}
    func doSomething() {}
}

Use them like this:

class SomeDelegate: DelegateOfA {
    func onSomethingHappened() {
        print("I know something happened")
    }
}

let aDelegate = SomeDelegate()

let a = A()
a.delegate = aDelegate
a.doSomething()

//And this may/will be invoked somehow and fire your delegate methods:
//a.delegate?.onSomethingHappened()

One thing I have done more recently is to use blocks for these delegates:

class A {
    var onSomethingHappened: (()->())?

    init() {}
    func doSomething() {}
}

Use it like this:

let a = A()
a.onSomethingHappened = {
    print("I know something happened")
}
a.doSomething()

//And this may/will be invoked somehow and fire your delegate blocks:
a.onSomethingHappened?()

The overall code is slightly shorter, but more importantly, both the object creation of A and the delegate code are close together, making it easier to read, understand and maintain.

Try it out!

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Picked up this tip from @cabel related to sudo:

If you have a Macbook Pro with Touch ID (i.e. Touch Bar) support, you can enable Touch ID for sudo by adding this line as the 1st line after auth sufficient pam_tid.so in the file /etc/pam.d/sudo:

auth sufficient pam_tid.so

If Touch ID prompt doesn't appear until you keyed in the wrong password, chances are you added the line correctly, but not at the top.

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Sometimes we want a simple confirmation if a line of code has been reached, especially when we need to test on device, untethered. One trick I like to use is to play a system sound:

import AudioToolbox

AudioServicesPlaySystemSound(1109)

Of course, if you are tethered, you can use a breakpoint to play a sound.

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If you create a struct and have a property that is of the same type, e.g:

struct Node {
    var parent: Node?
    init() { }
}

You get this error:

value type 'Node' cannot have a stored property that recursively contains it

There are a few ways to work around this. One is to use a "box" class:

class Box<T> {
    var value: T
    init(value: T) {
        self.value = value
    }
}

And change your Node struct to:

struct Node {
    var parent: Box<Node>?
    init() { }
}

Alternatively, wrap it with an array:

struct Node {
    var parent = [Node]()
    init() { }
}

But when you see this error, it's also good to reflect on whether you are building your data structure correctly. in this example our struct is called Node, naturally nodes may refer to their parent. But since structs are value types, we are always copying the parent. If we build a tree of nodes, this copying behavior is not what we want. The simpler and probably more correct answer here is to just Node to be a class instead:

class Node {
    var parent: Node?
    init() { }
}

This will remove the error and more importantly, provide the correct semantics.

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Often when comparing implementations of the same function, I need to do some simple profiling to compare the total time taken. The simplest approach is to create a Date instance before and after the operation and calculate the difference. These are usually long operations and I want to track progress. This becomes a bit more troublesome when the operation is asynchronous. So I wrote a simple class to help:

import Foundation

class ProgressReporter {
    let startTime = Date()
    var name: String
    var total: Int
    var step: Int

    init(name: String, total: Int = -1, step: Int = 100000) {
        self.name = name
        self.total = total
        self.step = step
        print("\(name.firstUppercased)...")
    }

    func update(current: Int, completion: (()->())? = nil) {
        if current % step == 0 {
            print("Progress \(current)/\(total)")
        }
        if current == total {
            completed()
            completion?()
        }
    }

    func completed() {
        print("Completed")
        print("Time taken to \(name): \(Date().timeIntervalSince(startTime)) secs")
    }
}

extension String {
    var firstUppercased: String {
        guard let first = first else { return "" }
        return String(first).uppercased() + dropFirst()
    }
}

You can use it like this:

func getFileNames(someInput: [String]) {
    let progress = ProgressReporter(name: "get filenames", total: someInput.count)
    var results = [String]()
    for e in results {
        someAsyncOperation(e) {
            progress.update(current: results.count) {
                //do something with results
            }
        }
    }
}

It'll print something like this:

Get filenames
Progress 100000/2000000
Progress 200000/2000000
Progress 300000/2000000
...
Progress 2000000/2000000
Completed
Time taken to get filenames: 100 secs

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Do you sometimes have this need: edit a file or a few files in one application and then switch to another app and want to select those files? E.g. you edit iOS app screenshots in a graphics editor and then want to upload them in Safari.

Then this tip is for you:

1. File > New Smart Folder
2. Click This Mac
3. Click +
4. Click Kind and change it to Last modified date
5. Make sure within last is selected
6. Enter 1 (days)
7. View > as List
8. Click the Date Last Opened column header to sort descending
9. Click Save
10. Make sure Add to Sidebar is checked
11. Enter a name (e.g. Recent) and click Save

The Recent smart folder should appear in your Finder sidebar.

With our example, after editing you just need to click the Recent smart folder either in Finder (or in other cases the Open file dialog) to access the most recently modified files.

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This is a continuation of #260 where I talk about including the app version and device Name in feedback/support emails. After thinking about it, I created Apple Device Names. It provides a simple API that maps iPhone10,6 → iPhone X.

It's just a curl command away:

$ curl http://appledevicenames.com/device/iPhone10,6

iPhone X

So to generate something like this:

For info/diagnostic purposes:
  App v1.0.0 build 23 on iPhone X, iOS 11.0.3

You can do this in Swift:

//Stick this somewhere
private func deviceModel() -> String? {
    var keys = [CTL_HW, HW_MACHINE]
    var requiredSize = 0
    let fetchSizeResult = sysctl(&keys, UInt32(keys.count), nil, &requiredSize, nil, 0)
    guard fetchSizeResult == 0 else { return nil }
    var data = [CChar](repeating: 0, count: requiredSize)
    let fetchResult = sysctl(&keys, UInt32(keys.count), &data, &requiredSize, nil, 0)
    guard fetchResult == 0 else { return nil }
    return String(validatingUTF8: data)
}

//Stick this somewhere
func withDeviceName(block: @escaping (String) -> Void)  {
    let fallbackName = "Unknown"
    if let model = deviceModel(),
    let url = URL(string: "https://appledevicenames.com/device/\(model)") {
        URLSession.shared.dataTask(with: url) { data, _ , _ in
            DispatchQueue.main.async {
                if let data = data, let name = String(data: data, encoding: .utf8) {
                    block(name)
                } else {
                    block(fallbackName)
                }
            }
        }.resume()
    } else {
        block(fallbackName)
    }
}

//Calling code
withDeviceName { deviceName in
    let appVersion = Bundle.main.object(forInfoDictionaryKey: "CFBundleShortVersionString")
    let buildNumber = Bundle.main.object(forInfoDictionaryKey: "CFBundleVersion")
    let osVersion = UIDevice.current.systemVersion
    if let appVersion=appVersion, let buildNumber=buildNumber {
        let body = "For info/diagnostic purposes:\n  App v\(appVersion) build \(buildNumber) on \(deviceName), iOS \(osVersion)"
        //body = "For info/diagnostic purposes:\n  App v1.0.0 build 23 on iPhone X, iOS 11.0.3"
        //Use body
    }
}

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There's always been an app size limit for cellular downloads. It's 150 MB as of writing.

For those who are along the borderline, here's a great way to check how big your app is:

1. Go to iTunes Connect
2. My Apps
3. Click the app
4. Click Activity
5. Under All Builds
6. Click the version number to expand
7. Click on the build
8. Click App Store File Sizes

This reveals a list of device types and their corresponding download sizes.

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In my apps, I include a button for users to send me feedback emails. I will pre-fill the email with a line like this:

For info/diagnostic purposes:
    MY_APP_NAME v1.0 build 101 on iPad Pro 9.7 Wifi + Cellular, iOS 10.0

This provides some basic information that can help me understand their issue or feedback better.

I generate this with code like this:

let appVersion = Bundle.main.object(forInfoDictionaryKey: "CFBundleShortVersionString")
let buildNumber = Bundle.main.object(forInfoDictionaryKey: "CFBundleVersion")
let osVersion = UIDevice.current.systemVersion
if let appVersion=appVersion, let buildNumber=buildNumber {
    let body = "For info/diagnostic purposes:\nMY_APP_NAME v\(appVersion) build \(buildNumber), iOS \(osVersion)"
    //Use body
}

If you want to include the device name (iPhone X, etc) property, you can use https://github.com/InderKumarRathore/DeviceGuru.

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In #135 CAShapeLayer, Animated Views and XCPShowView(), we spoke about using CALayer and UIBezierPath to draw and animate outlines.

Here's another common thing you can do with it — Rounded corners:

let canvas = UIView()
canvas.frame = CGRect(x: 0, y: 0, width: 200, height: 300)
canvas.backgroundColor = .blue

let v2 = UIView()
v2.frame = CGRect(x: 20, y: 20, width: 50, height: 60)
v2.backgroundColor = .red
v2.layer.cornerRadius = 8
canvas.addSubview(v2)

And if you only want to round certain corners, use UIRectCorner:

let v3 = UIView()
v3.frame = CGRect(x: 20, y: 100, width: 50, height: 60)
v3.backgroundColor = .green
canvas.addSubview(v3)

let cornerRadius = 12
let layer = CAShapeLayer()
layer.path = UIBezierPath(roundedRect: v3.bounds, byRoundingCorners: [.topLeft, .bottomRight], cornerRadii: CGSize(width: cornerRadius, height: cornerRadius)).cgPath
v3.layer.mask = layer

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This guide from PaintCode is a useful graphical illustration of various iPhone sizes, now updated to include iPhone 8 (Plus) and iPhone X.

There's also additional notes about the iPhone X here.

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In Obj-C, you can use asserts like this:

NSAssert([self foo], @"Something something");

Similarly in Swift:

assert(foo())

In Obj-C, asserts are controlled by ENABLE_NS_ASSERTIONS (Enable Foundation Assertions in Xcode Build Settings) which is default to NO in release builds.

But in Swift, asserts are controlled by the Swift optimization level. With a release build — which defaults SWIFT_OPTIMIZATION_LEVEL (Swift Compiler - Code Generation > Optimization Level) to -O, the assert() doesn't run. It runs with -Onone.

It's not complicated once we realise these are 2 different flags, but it's still a good practice to only run code which has no side effects with an assert to present such errors.

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