WWDC19

Table of Contents

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

• Data Flow Through SwiftUI - Thursday
  • Tools for Data Flow
    • @State private var
    • @Binding Property Wrapper
  • Working with External Data
    • Combine Publisher
    • BindableObject Protocol A.K.A RxSwift lol
    • @EnvironmentObject
  • Source of Truth
  • Building Reusable Components
  • Using State Effectively

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!