ios - 在 SwiftUI 中更新其他变量时如何修改/重置@State

Question

我想重置一些@State每次@ObservedObject被“重新分配”。如何实现？

class Selection: ObservableObject {
    let id = UUID()
}

struct ContentView: View {
  @State var selectedIndex: Int = 0
  var selections: [Selection] = [Selection(), Selection(), Selection()]
  var body: some View {
        VStack {
            // Assume the user can select something so selectedIndex changes all the time
            // SwiftUI will just keep updating the same presentation layer with new data
            Button("Next") {
                self.selectedIndex += 1
                if self.selectedIndex >= self.selections.count {
                    self.selectedIndex = 0
                }
            }
            SelectionDisplayer(selection: self.selections[self.selectedIndex])
        }
  }
}

struct SelectionDisplayer: View {
    @ObservedObject var selection: Selection {
        didSet { // Wish there were something *like* this
            print("will never happen")
            self.tabIndex = 0
        }
    }

  @State var tapCount: Int = 0 // Reset this to 0 when `selection` changes from one object to another

  var body: some View {
        Text(self.selection.id.description)
            .onReceive(self.selection.objectWillChange) {
                print("will never happen")
        }
        Button("Tap Count: \(self.tapCount)") {
            self.tapCount += 1
        }
    }
}

我知道 onReceive但我不想修改状态以响应 objectWillChange而是当对象本身被关闭时。在具有引用类型的 UIKit 中，我将使用 didSet但这在这里行不通。

我确实尝试使用 PreferenceKey为此( gist )，但它似乎太过分了。

Answer 1

目前(Beta 5)，最好的方法可能是使用构造函数加上泛型 ObservableObject对于我想在数据更改时重置的项目。这允许一些 @State被保留，而一些被重置。

在下面的示例中，tapCount每次重置 selection更改，而 allTimeTaps不是。

class StateHolder<Value>: ObservableObject {
    @Published var value: Value
    init(value: Value) {
        self.value = value
    }
}

struct ContentView: View {

  @State var selectedIndex: Int = 0
  var selections: [Selection] = [Selection(), Selection(), Selection()]
  var body: some View {
        VStack {
            // Assume the user can select something so selectedIndex changes all the time
            // SwiftUI will just keep updating the same presentation though
            Button("Next") {
                self.selectedIndex += 1
                if self.selectedIndex >= self.selections.count {
                    self.selectedIndex = 0
                }
            }
            SelectionDisplayer(selection: self.selections[self.selectedIndex])
        }
  }
}

struct SelectionDisplayer: View {

    struct SelectionState {
        var tapCount: Int = 0
    }

    @ObservedObject var selection: Selection
    @ObservedObject var stateHolder: StateHolder<SelectionState>
    @State var allTimeTaps: Int = 0

    init(selection: Selection) {
        let state = SelectionState()
        self.stateHolder = StateHolder(value: state)
        self.selection = selection
    }

  var body: some View {
        VStack {
            Text(self.selection.id.description)
            Text("All Time Taps: \(self.allTimeTaps)")
            Text("Tap Count: \(self.stateHolder.value.tapCount)")
            Button("Tap") {
                self.stateHolder.value.tapCount += 1
                self.allTimeTaps += 1
            }
        }
    }
}

在寻找解决方案时，我很感兴趣地发现您无法初始化 @State init 中的变量.编译器会提示在访问 self 之前还没有设置所有属性。