SwiftUI onTapGesture不适用于Mac app中的ObservedObject [英] SwiftUI onTapGesture does not work with ObservedObject in Mac app

问题描述

我想基于另一个视图(LeftView)中轻击手势的操作来更新视图(RightView).下面是我的示例代码来完成此操作:

I would like to update a view (RightView) based on actions from a tap gesture in another view (LeftView). Below is my example code to accomplish this:

AppState.swift

import Foundation

class AppState: ObservableObject {
    @Published var tapCount: Int = 0

    func incrementTapCount() {
        self.tapCount += 1
        print("tap count \(self.tapCount)")
    }
}

LeftView.swift

import SwiftUI

struct LeftView: View {
    @ObservedObject var appState = AppState()

    var body: some View {
        VStack {
            Text("Left View")
        }
        .frame(width: 100, height: 200)
        .onTapGesture {
            print("tapped left view")
            self.appState.incrementTapCount()
        }
    }
}

RightView.swift

import SwiftUI

struct RightView: View {
    @ObservedObject var appState = AppState()

    var body: some View {
        VStack {
            Text("Right View")
            Text("Tap Count: \(self.appState.tapCount)")
        }.frame(width: 200, height: 200)
    }
}

ContentView.swift

import SwiftUI

struct ContentView: View {
    var body: some View {
        HStack {
            LeftView()
            RightView()
        }.frame(width: 300, height: 200)
    }
}

不幸的是，RightView文本不会随着点击数更新.但是，如果我在ObservableObject类中使用计时器来更新拍子计数，则RightView会正确更新.

Unfortunately, the RightView text does not update with the tap count. However, if I use a timer in the ObservableObject class to update the tap count, then the RightView updates correctly.

import Foundation

class AppState: ObservableObject {
    @Published var tapCount: Int = 0

    init() {
        Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { timer in
            self.tapCount += 1
        }
    }

    func incrementTapCount() {
        self.tapCount += 1
        print("tap count \(self.tapCount)")
    }
}

为什么LeftView中的轻击手势不会更新RightView文本?

Why does the tap gesture in LeftView not update the RightView text?

推荐答案

LeftView 和 RightView 都分别初始化了自己的单独的 AppState 对象，所以他们不会观察到相同的状态.

LeftView and RightView are each initializing their own separate AppState objects, so they aren't observing the same state.

在两个视图中，都将 @ObservedObject 保留为未初始化的属性:

In both of your views, leave the @ObservedObject as an uninitialized property:

LeftView.swift&RightView.swift

@ObservedObject var appState: AppState

在SwiftUI视图中具有未初始化的属性时，您需要在每次使用该视图时为其提供一个值.

When you have an uninitialized property like this in a SwiftUI view, you need to provide a value for it any time you want to use the view.

PreviewProvider 使用视图的实例，因此Xcode可以在画布上向您显示该视图，并且只需要您为提供一个 placeholder 值.appState .该值并不是很重要(除了Xcode画布，它没有在其他任何地方使用)，因此您只需要提供正确类型的值即可:

The PreviewProvider uses an instance of your view so Xcode can show it to you on the canvas, and it only needs you to supply a placeholder value for appState. The value isn't really important (it isn't used anywhere but the Xcode canvas) so you just need to provide something of the correct type:

#if DEBUG
struct LeftView_Previews: PreviewProvider {
    static var previews: some View {
        LeftView(appState: AppState())
    }
}
#endif

最后，在 ContentView 中，您需要将对单个 shared AppState 的引用传递到 LeftView 和 RightView ，这样它们就可以观察相同的对象:

Finally in ContentView you'll want to pass a reference to a single shared AppState down to LeftView and RightView so they can each observe the same object:

ContentView.swift

import SwiftUI

struct ContentView: View {
    var sharedAppState = AppState()

    var body: some View {
        HStack {
            LeftView(appState: sharedAppState)
            RightView(appState: sharedAppState)
        }.frame(width: 300, height: 200)
    }
}

现在两个视图都在观察 AppState 的相同实例，当它们的属性更改时，它们都将更新.

Now that both views are observing the same instance of AppState, they'll both update when its properties change.

您还可以使用 @EnvironmentObject 在应用程序中所有所有视图之间共享状态，但是由于这里我们只关注两个视图，因此 @ObservedObject是一个务实的选择.

You can also use @EnvironmentObject to share state between all the views in your app, but since we're only concerned with two views here @ObservedObject is a pragmatic choice.

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