SwiftUI-更改@Published结构时是否有可能触发didSet? [英] SwiftUI - is it possible to get didSet to fire when changing a @Published struct?

问题描述

我刚刚更新到XCode 11.4，并且我的一些代码已停止工作.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 方法将在已发布的属性上触发，但情况不再如此.这种行为是否有可能在Swift的最新更新中被设计改变了?

I have just updated to XCode 11.4 and some of my code has stopped working. I have some @Published struct variables in an ObservableObject. Previously, when I updated properties on the struct, the didSet method would fire on the published property, but that's not the case anymore. Is it possible that this behaviour has changed by design in the latest update to Swift?

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()
    }
}

该属性会更新，但 didSet 不会触发.

The property updates, but didSet does not fire.

是否可以获取结构的嵌套属性以触发发布者的 didSet 方法?

Is it possible to get nested properties of a struct to trigger the didSet method of the publisher?

推荐答案

属性观察者观察该属性.问题出在与属性包装器有关的新Swift语法上.在您的情况下，您尝试观察发布值(这是定义专用属性包装器的结构)的值是否发生了变化，而不是包装后的属性的值发生了变化.

The property observer observes the property. The trouble goes from new Swift syntax related to property wrappers. In your case you try to observe if value of Published (which is a struct defining the specialized property wrapper) did change, not the value of the wrapped property.

如果需要监视PaddingRect中的左或右值，只需直接观察此值即可.

If you need to monitor left or right values in PaddingRect, simply observe this values directly.

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()
    }
}

或利用已发布的预测值是Publisher 的优势，然后将next修饰符应用于任何视图

Or take the advantage that Published projected value is Publisher and aply next modifier to any View

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

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