Hi, I'm Hwee-Boon Yar

I write, ship and sell software products. iOS/OS X app developer based in Singapore, working remotely. This is my blog.


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Ruby lends itself to creating a nice DSL for Auto Layout, so why not? I have been using this for both iOS and OS X apps since mid-2014.


Use it with visual format strings:

['celf', inner_view, 'l', post_count_label].constraints do |celf, l, _|
  celf.h '|[l]|'
  celf.v '|-m-[l]-m-|', {'m' => 7}

You can also create constraint objects yourself. This is equivalent to the previous example:

['celf', inner_view, 'l', post_count_label].constraints do |celf, l, _|
  celf.left = l.left
  celf.right = l.right
  l.top = celf.top + 7
  l.bottom = celf.bottom - 7

Send #constraints with a block to an array with an even number of elements. They should be pairs – the name (a String) to be used in the format strings in the block, followed by the view it represents.

If there are n views involved, there should be n*2 elements in the array. The block takes n+1 arguments, and the last object is the dictionary mapping generated from the array (usually unused) and the rest of the elements are constraint proxy objects representing each view, in the order specified in the array.

To avoid confusion, it's best to keep the variable names in the block the same as the string names in the array.

With the exception of the first view listed in the array, translatesAutoresizingMaskIntoConstraints is automatically set to false for every view, so you'll want to specify the outermost view as the first pair.

A multipler can be supplied:

  l.width = celf.width * 0.5

The attributes are available as :left, :right, :top, :bottom, :leading, :trailing, :width, :height, :center_x, :center_y and :baseline.

You can mix the 2 creation methods:

['celf', inner_view, 'l', post_count_label].constraints do |celf, l, _|
  celf.left = l.left
  celf.right = l.right
  celf.v '|-m-[l]-m-|', {'m' => 7}

When specifying options argument describing the attribute and direction of layout, you can also use symbols such as :align_center_y (for NSLayoutFormatAlignAllCenterY). They are interchangeable. The options are available as :align_left, :align_right, :align_top, :align_bottom, :align_leading, :align_trailing, :align_center_x, :align_center_y, :align_baseline.

ie. these are equivalent:

cv.h '|[v1][v2][v3]|', nil, NSLayoutFormatAlignAllCenterY
cv.h '|[v1][v2][v3]|', nil, :align_center_y

You can write equality “statements” in a single line:

b0.center_y = b1.center_y = b2.center_y = b3.center_y = b4.center_y

There are 2 attributes in the constraint proxies that you would occasionally find useful:

  • last_constraint returns the last NSLayoutConstraint created. It's useful if you want to hold on to a NSLayoutConstraint and modify it in an animation.
  • next_priority sets the priority for the next NSLayoutConstraint you create.

Quirks & Gotchas

1. In a better world, the first code example would have been:

[inner_view, post_count_label].constraints do |celf, l, _|
  celf.h '|[l]|'
  celf.v '|-m-[l]-m-|', {'m' => 7}

But since Proc#parameters isn't available, the syntax to invoke constraints requires duplication of names. This syntax can be improved when Proc#parameters is available.

2. In RubyMotion, local variables aren't shadowed correctly by dynamic variables. So try not to use the same names for the block args as the variables holding your views. Referring to the 1st example, if we use post_count_label instead of l:

['celf', inner_view, 'post_count_label', post_count_label].constraints do
    |celf, post_count_label, _|
  celf.h '|[post_count_label]|'
  celf.v '|-m-[post_count_label]-m-|', {'m' => 7}
#At this point, after the block, post_count_label is the constraint proxy,
#and not the label as one might expect.


  1. Add this to your Gemfile: gem 'purplish-layout'
  2. Run bundle install


purplish-layout on GitHub





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