无法更新/修改SwiftUI View的@state var

问题描述

即使我看到正在调用updateMessage()，我也无法更新ExampleView的message变量.这是我的简化/复杂的SwiftUI Playground代码示例，无法正常工作.在updateMessage()中调用时，message变量不会更新.结果，UI Text()也未更新.

I'm unable to update the ExampleView's message var even though I can see updateMessage() is being called. Here is my simplified/convoluted SwiftUI example of Playground code that isn't working. The message var does not get updated when called in updateMessage(). As a result, the UI Text() is not updated either.

为什么@State变量message不更新?什么是正确的更新方式?

Why is the @State var message not updating? What is the correct way to update it?

import SwiftUI
import PlaygroundSupport

struct ContentView: View {
    let coloredLabel = ExampleView()

    var body: some View {
        VStack {
            coloredLabel
                .foregroundColor(Color.red)
                .padding()
            Button(action: {
                self.coloredLabel.updateMessage()
            }) {
                Text("Press me")
            }

        }
    }
}

struct ExampleView: View {
    @State private var message: String = "Hello"

    var body: some View {
        Text(self.message)
    }

    func updateMessage() {
        print("updateMessage ran") // this prints
        self.message = "Updated"
    }
}

PlaygroundPage.current.setLiveView(ContentView())

推荐答案

您应该只更改视图的State 内部，它是自己的主体块.如果需要从父视图更改它，则可能需要将值从父视图传递给它，并改为Binding.

You should only change State of a view Inside it's own body block. If you need to change it from a parent view, you may want to pass the value to it from parent and make it Binding instead.

struct ContentView: View {
    @State private var message = "Hello"

    var body: some View {
        VStack {
            ExampleView(message: $message)
                .foregroundColor(Color.red)
                .padding()
            Button("Press me") {
                self.message = "Updated"
            }
        }
    }
}

struct ExampleView: View {
    @Binding var message: String

    var body: some View {
        Text(message)
    }
}

---

如果需要将消息封装在ExampleView内，则可以使用Bool(或enum等)代替

---

If you need to encapsulate messages inside the ExampleView, you can use a Bool (or an enum or etc) instead:

struct ContentView: View {
    @State private var updated = false

    var body: some View {
        VStack {
            ExampleView(isUpdated: $updated)
                .foregroundColor(Color.red)
                .padding()
            Button("Press me") {
                self.updated = true
            }
        }
    }
}

struct ExampleView: View {
    @Binding var isUpdated: Bool
    private var message: String { isUpdated ? "Updated" : "Hello" }

    var body: some View {
        Text(message)
    }
}

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