A significant part of the time and effort spent while building iOS apps is spent on building and tweaking the user interface. The edit-build-run cycle can sometimes be time-consuming. Reducing the number of this build+run cycles is part of the attractiveness of tools such as React Native and the RubyMotion RedPotion framework. This is often referred to as code live-reloading. But using these tools usually mean switching completely away from Apple's toolchain.
However, Injection for Xcode lets you do it directly from Xcode. It supports both Objective-C and Swift. There's an excellent video in the README that illustrates how it works as well as a description of how to set it up, as well as its limitations. You can install it via Alcatraz. In the author's own words: Injection for Xcode is "nagware" that invites you to make a donation after two weeks.
This is a great time-saver when you build and tweak your user interface. I suggest you give it a try.