Layout protocol in SwiftUI

Protocol Requirements

• sizeThatFits(proposal:subviews:cache:) reports the size of the composite layout view.
  • i.e. decides where this view will appear
• placeSubviews(in:proposal:subviews:cache:) assigns positions to the container’s subviews.
  • i.e. tells the subviews where to appear
  • Note: this method proposes positions to the subviews. The subviews can choose to comply or ignore.

Known Limitations

• Layout protocol cannot be lazy
  • See: Quote from “The SwiftUI Layout Protocol - Part 1 - The SwiftUI Lab”

Docs

• Compose custom layouts with SwiftUI - WWDC22 - Videos - Apple Developer
• Layout | Apple Developer Documentation

Helpful Resources

• SwiftUI Lab Deep Dive
• The Layout Protocol - Swift Talk - objc.io
• SwiftUI Layout： The Mystery of Size | Fatbobman’s Blog

SwiftUI Lab Deep Dive

• The SwiftUI Layout Protocol - Part 1 - The SwiftUI Lab
  • What is the Layout Protocol?
  • Family Dynamics of the View Hierarchy
  • Our First Layout Implementation
  • Container Alignment
  • Custom Values: LayoutValueKey
  • Default Spacing
  • Layout Properties and Spacer()
  • Layout Cache
  • Great Pretenders
  • Switching Layouts with AnyLayout
• Part 1 Conclusion
• The SwiftUI Layout Protocol - Part 2 - The SwiftUI Lab
  • Custom Animations
  • Bi-directional Custom Values
  • Avoiding Layout Loops and Crashes
  • Recursive Layouts
  • Layout Composition
  • Another Composition Example: Interpolating Two Layouts
  • Using Binding Parameters
  • A Helpful Debugging Tool
  • Final Thoughts