SwiftUI

Question

我刚刚更新到 XCode 11.4，我的一些代码已经停止工作。我在 ObservableObject 中有一些 @Published 结构变量。以前，当我更新结构上的属性时，didSet 方法会在已发布的属性上触发，但现在不再是这种情况了。此行为是否有可能在 Swift 的最新更新中被设计更改？

这是一个简单的例子：


import SwiftUI

struct PaddingRect {
  var left: CGFloat = 20
  var right: CGFloat = 20
}

final class SomeStore : ObservableObject {
  @Published var someOtherValue: String = "Waiting for didSet"

  @Published var paddingRect:PaddingRect = PaddingRect() {
    didSet {
      someOtherValue = "didSet fired"
    }
  }
}

struct ObserverIssue: View {
  @ObservedObject var store = SomeStore()

  var body: some View {
    VStack {
      Spacer()

      Rectangle()
        .fill(Color.yellow)
        .padding(.leading, store.paddingRect.left)
        .padding(.trailing, store.paddingRect.right)
        .frame(height: 100)

      Text(store.someOtherValue)

      HStack {
        Button(action: {
          // This doesn't call didSet
          self.store.paddingRect.left += 20

          // This does call didSet, ie. setting the whole thing
//          self.store.paddingRect = PaddingRect(
//            left: self.store.paddingRect.left + 20,
//            right: self.store.paddingRect.right
//          )

        }) {
          Text("Padding left +20")
        }

        Button(action: {
          self.store.paddingRect.right += 20
        }) {
          Text("Padding right +20")
        }
      }

      Spacer()
    }
  }
}

struct ObserverIssue_Previews: PreviewProvider {
    static var previews: some View {
        ObserverIssue()
    }
}

属性更新，但 didSet 没有触发。

是否可以获取结构的嵌套属性来触发发布者的 didSet 方法？

Answer 1

属性观察者观察属性。问题来自与属性包装器相关的新 Swift 语法。在您的情况下，您尝试观察 Published 的值（它是定义专用属性包装器的结构）是否发生了变化，而不是包装的属性.

的值

如果您需要监控 PaddingRect 中的左值或右值，直接观察该值即可。

import SwiftUI


struct PaddingRect {
    var left: CGFloat = 20 {
        didSet {
            print("left padding change from:", oldValue, "to:", left)
        }
    }
    var right: CGFloat = 20 {
        didSet {
            print("right padding change from:", oldValue, "to:", right)
        }
    }
}

final class SomeStore : ObservableObject {
    @Published var someOtherValue: String = "Waiting for didSet"
    @Published var paddingRect:PaddingRect = PaddingRect()
}

struct ContentView: View {
    @ObservedObject var store = SomeStore()

    var body: some View {
        VStack {
            Spacer()

            Rectangle()
                .fill(Color.yellow)
                .padding(.leading, store.paddingRect.left)
                .padding(.trailing, store.paddingRect.right)
                .frame(height: 100)

            Text(store.someOtherValue)

            HStack {
                Button(action: {
                    // This doesn't call didSet
                    self.store.paddingRect.left += 20

                    // This does call didSet, ie. setting the whole thing
                    self.store.paddingRect = PaddingRect(
                        left: self.store.paddingRect.left + 20,
                        right: self.store.paddingRect.right
                    )

                }) {
                    Text("Padding left +20")
                }

                Button(action: {
                    self.store.paddingRect.right += 20
                }) {
                    Text("Padding right +20")
                }
            }

            Spacer()
        }
    }
}

struct ContentView_Preview: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

或者利用 Published projected value is Publisher 并将 next modifier 应用于任何 View

.onReceive(store.$paddingRect) { (p) in
            print(p)
        }

Answer 2

您可以在 class 本身中订阅 @Published 价值流。

final class SomeStore: ObservableObject {
    @Published var someOtherValue: String = "Waiting for didSet"
    @Published var paddingRect: PaddingRect = PaddingRect()
    private var subscribers: Set<AnyCancellable> = []
    
    init() {
        $paddingRect.sink { paddingRect in
            print(paddingRect) // 
        }.store(in: &subscribers)
    }
}

Note that the sink closure will be called on willSet, though.

SwiftUI - 是否有可能在更改 @Published 结构时触发 didSet？

SwiftUI - is it possible to get didSet to fire when changing a @Published struct?

ios

swift

swift5

observableobject