无法更新/修改 SwiftUI 视图的 @state var

Question

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.

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())

Answer 1

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)
    }
}

---

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)
    }
}

Answer 2

actually, the variable does get updated, but your Content view doesn't get informed about it. This is what happens:

• ContentView gets called, it initializes coloredLabel with an ExampleView
• you press the button in ContentView
• self.coloredLabel.updateMessage() get's called
• the message is printed
• the variable self.coloredLabel.message is modified
• ContentView does not get redrawn, as it isn't notified about the change
• more specifically, coloredLabel inside your Stack doesn't get updated

now, you have different options: @State , @Binding and @PublishedObject, @ObservedObject . You need one of these Publishers, so your view actually notices that it needs to do something.

Either you draw a new ExampleView every time you press the button, in this case you can use a @State variable in ContentView :

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

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

        }
    }
}

struct ExampleView: View {
    var message: String

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

which is probably not what you want.

Next, you can use @Binding which was already suggested.

And last, you can use ObservableObject @ObservedObject, @Published

class ExampleState: ObservableObject {
    @Published var message: String = "Hello"
    func update() {
        message = "Updated"
    }
}

struct ContentView: View {
    @ObservedObject var state = ExampleState()

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

        }
    }
}

struct ExampleView: View {
    @ObservedObject var state: ExampleState

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

what this says is: class ExampleState: ObservableObject - this class has published variables that can be observed

to resume (that's how I understand it):

• "Hey, ContentView and ExampleView : if state.message (any value that state publishes) changes, you need to redraw your body"
• "And ExampleState : after updating your message variable, publish the new value!"

lastly - for completion - there is @EnvironmentObject , as well, that way you'd only have to pass the variable to the top-views and everything down the view hierarchy would inherit it.

Answer 3

One of the solutions that should work, but as guys say, you can work with @Binding

 struct ExampleView: View {

        var message: String = "Hello"

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

        mutating func updateMessage() {
            print("updateMessage ran")
            message = "Updated"
        }
    }