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WWDC19

Table of Contents

=================

Data Flow Through SwiftUI - Thursday

Session materials: https://developer.apple.com/videos/play/wwdc2019/226/

Tools for Data Flow

  • Property
  • BindableObject
  • @State
  • @Binding
  • @Environment

@State private var

  • Indicates that this value can change over time and not leading to mutation of self like a normal property value
  • It is a property wrapper
  • It is a good practice to define them private to limit it to the specified view
  • SwiftUI can observe a change on a @State variable
  • Every @State is a source of truth for the view

@Binding Property Wrapper

  • Read and write without ownership
  • Derived from @State
  • Allow a child component to access a state via binding
  • It's a good practice to use them to keep the source of truth to one
struct ContentView: View {
    @State private var foo: SomeType
    var body: some View {
       Button($foo) // Passing the binding here to the child view
      }
    }
}

struct Button: View {
  @Binding private var foo: SomeType
    var body: some View {
      // use the binding here without ownership
      Button(action: {
          withAnimation { self.foo.toggle()}
        })
    }
}
  • Usage of @State and @Binding eliminates the need for a view controller to handle

Working with External Data

Combine Publisher

  • Single abstraction
  • Main thread: use .receive(on:)
someStack.onReceive(some.currentPublisher) { // Rx! }

BindableObject Protocol A.K.A RxSwift lol

  • Reference type
  • External
  • Pass directly with @ObjectBinding
  • Automatic dependency tracking
struct MyView: View {
  @ObjectBinding var model: MyModelObject
  ....
}

MyView(model: modelInstance)

class MyModel: BindableObject {}
  • Views in SwiftUI are value types but @ObjectBinding works with reference passing

@EnvironmentObject

  • Similar to @ObjectBinding with an addition of indirect passing
    • Acts like a singleton reference to an @ObjectBinding which is available to be used in various SwiftUI views
  • Data applicable to an entire hierarchy
  • Convenience for indirection
  • e.g accent color, theme, right to left etc

Source of Truth

  • @State
    • View-local
    • Value
    • Framework managed
  • BindableObject
    • External
    • Reference
    • Developer managed

Building Reusable Components

  • Read-only: Swift property, Environment
  • Read-write: @Binding
    • First class reference to data
    • Use $ to derive from source
  • Prefer immutable access

Using State Effectively

  • Limit use if possible

  • Use derived Binding or value

  • Prefer BindableObject for persistence

  • Example: Button highlighting

  • Always question state vs binding needs!