Demystifying SwiftUI Previews

Understand how SwiftUI previews work, why they pause, and how to fix it.

Transcript

1. Demystifying
   SwiftUI previews
   Marina Gornostaeva | @hybridcattt
   

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2. Agenda
   ! Other use-cases
   " Preview lifecycle
   # The build process
   $ How live reloading works
   ⏸ Reduce previews pausing
   

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3. ! Other use-cases
   

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4. ! Other use-cases
   

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5. " Lifecycle of a SwiftUI preview
   • Build once
   • Live reload on changes
   • Pause if change is unsupported by live reloading
   • Repeat
   

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6. # Build process
   • For DEBUG
   • Detect by ENABLE_PREVIEWS build setting
   • Use ProcessInfo in runtime
   

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7. $ Dynamic replacement
   • Swift feature called @_dynamicReplacement
   • Pitched in 2018
   • Supports changes in implementation of a function, computed variable, initializer or
   subscript
   struct MyStruct {
   dynamic func x() {
   print("x")
   }
   }
   extension MyStruct {
   @_dynamicReplacement(for: x())
   func y() {
   print("y - replaced dynamically")
   }
   }
   

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8. $ Limitations of live reloading
   • It’s applied to everything in the current ile
   • Supported: changes to bodies of functions, properties, initializers
   • Not supported:
   • Editing comments
   • Signature changes
   • Adding/removing types, functions, properties
   • Initial value changes
   • Changes in other iles
   

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9. ⏸ Improving live reloading
   • Use computed vars
   • Check build scripts
   • Auto-resume previews
   

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10. Thanks &
    @hybridcattt
    https://hybridcattt.com/blog/ ixing-swiftui-previews
    

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