Action based on a variable update in SwiftUI

Question

I have the following kind of code in a SwiftUI app.

And I want to perform a certain action (which is going to update my UI) when the state variable answerFlag has been touched (in the CustomButton view).

I would like to get some hints on the way to go for that.

My SwiftUI knowledge is still too limited or I do not yet master the use of what I know. The couple of things I tried did not work.

struct ContentView: View {
    @State var answerFlag:Bool = false
    ......

    var body: some View {
        VStack {
            Spacer()
            CustomButton(text: answerText,
                         validAnswer: correctAnswer,
                         replyFlag: $answerFlag)
            Spacer()
        }.background(theBGColor)
            .onTapGesture {
                ... do some useful thing ...
            }
        // I need to perform some action when answerFlag has changed !!??
    }
}


struct CustomButton: View {
    var text,validAnswer: String
    @Binding var replyFlag:Bool

    var body: some View {
        Text(text)
        .font(.largeTitle)
        .fontWeight(.bold)
        .foregroundColor(.primary)
        .padding(.horizontal, 6.0)
        .background(Color.brown)
        .cornerRadius(5.0)
        .overlay(RoundedRectangle(cornerRadius: 5.0)
        .stroke(lineWidth: 2.0)
        .foregroundColor(Color.gray))
        .onTapGesture {
            print("BUTTON-HIT:\(self.text)::\(self.validAnswer)")
            if self.text == self.validAnswer {print("OK")}
            else {print("NG")}
            self.replyFlag = true
        }
    }
}

Your question is a little too vague. What kind of action did you have in mind? The following have different answers: 1) I want to update my model with stats on right/wrong counts, 2) I want to display an alert saying “correct!”, 3) I want to display a Text saying “That is correct!” — vacawama
– vacawama, Commented Aug 29, 2020 at 11:59
I would also look into didSet. Look at this link and ctrl + f for "didSet" docs.swift.org/swift-book/LanguageGuide/Properties.html — Sanzio Angeli
– Sanzio Angeli, Commented Aug 29, 2020 at 17:41

Asperi · Accepted Answer · 2020-08-29 11:44:47Z

1

Here is possible way...

SwiftUI 2.0

var body: some View {
    VStack {
        Spacer()
        CustomButton(text: answerText,
                     validAnswer: correctAnswer,
                     replyFlag: $answerFlag)
        Spacer()
    }.background(theBGColor)
        .onTapGesture {
            ... do some useful thing ...
        }
    // I need to perform some action when answerFlag has changed !!??
        .onChange(of: answerFlag) { newValue in
            // ... do anything heeded here
        }
}

SwiftUI 1.0+

Similarly to above add to some view the following

import Combine

    ...
    .onReceive(Just(answerFlag)) { newValue in
        // ... do anything heeded here
    }

answered Aug 29, 2020 at 11:44

Asperi

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