SwiftUI Question passing a variable to another view

Question

I am wanting to pass a Float Variable from one view to another new view.

In the code below there is a Float value called mhzValue which is set by way of the Slider, the slider changes the value and Text is then displaying within the view.. When the user taps on the Navigation Button to display the new view, I would like to be able to take the mhzValue and display it in the new view in a text box, as well as set it as another variable.

Hope that makes sense.

Please see some sample code below..

Thank you.

Craig

import SwiftUI
struct ContentView : View {
    @State private var mhzValue : Float = 0
    var body: some View {
        // Navigation View
        NavigationView {

            VStack{
                Text("Test Value:")
                    .font(.headline)
                    .color(.blue)
                    .padding(.leading, -180.0)

                //Get Slider Value
                Slider(value: $mhzValue, from: 1, through: 55, by: 1)
                    .padding(.horizontal)
                //Display Slider Value
                Text("\(Int(mhzValue)) Value")
                    .font(.title)
                    .fontWeight(.semibold)
                    .color(.blue)
                // Naviagtion Button and send value of mhzValue to new View
                NavigationButton(destination: NextView()){
                    Image(systemName: "plus.square.fill")
                        .foregroundColor(.white)
                        .font(/*@START_MENU_TOKEN@*/.title/*@END_MENU_TOKEN@*/)
                        .frame(width: 150.0, height: 16.0)
                        .padding(15)
                        .background(Color.red)
                        .cornerRadius(10.0)
                }
            }
        }
    }
}

// New View to show Slider Value
struct NextView : View {
    var body: some View {
        Text("Display Slider Value Here:")

    }
}

#if DEBUG
struct ContentView_Previews : PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}
#endif

I see you've declared ContentView to be "the source of truth", to use Apple's term for mhzValue. Why is it set to private? That makes little sense. What happens if you remove that, and make sure NextView is binded to it? Without a bit more context, you might even just make it @EnvironmentObject. — user7014451
– user7014451, Commented Jun 30, 2019 at 2:01
Hi Thank you... I have been playing with Swift for a couple of years now and continue to have problems in remembering... Old age... Thanks for your comments they are much appreciated !! Craig — Craig Shine
– Craig Shine, Commented Jun 30, 2019 at 9:42

Christopher Monsour · Accepted Answer · 2019-06-30 02:53:14Z

57

This is easily done with Bindings. Because mhzValue is marked with the @State property wrapper, it has an associated Binding. You can therefore declare a @Binding variable in your second view, and initialize it with the Binding to the original variable.

struct NextView : View {
    @Binding var mhzValue: Float
    ...
}

When you specify NextView as the destination for your navigation button, pass it a Binding to mhzValue. (The dollar-sign syntax is a shorthand way to refer to the binding.)

struct ContentView : View {
    @State private var mhzValue : Float = 0
    ...
    NavigationButton(destination: NextView(mhzValue: self.$mhzValue)){...}
    ...
}

You can then use mhzValue inside NextView:

struct NextView : View {

    @Binding var mhzValue: Float

    var body: some View {
        VStack{
            Text("Display Slider Value Here:")
            Text("\(Int(mhzValue)) Value")
                .font(.title)
                .fontWeight(.semibold)
                .color(.blue)
        }
    }
}

Any changes you make to mhzValue within NextView will effectively be changes to ContentView.mhzValue.

edited Jun 30, 2019 at 2:53

answered Jun 30, 2019 at 2:40

Christopher Monsour

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3 Comments

user7014451 Over a year ago

Good answer. One question about it. As I noted in my comment, mhzValue is marked as private. Does this matter? (I'm actually surprised it builds without some warning or error.)

Craig Shine Over a year ago

This is wonderful ... thank you... As I mention above I am having problems remembering ... I don't think I will ever grasp it.. therefore I will rely on people like you for your wonderful and continued help.. Thanks you .. Craig.

New iOS Dev Over a year ago

Thanks nice answer, i have one question for same.. How can we pass "mhzValue" to thirdView? is this possible in swiftUI?

Claytog · Accepted Answer · 2022-10-19 00:49:54Z

5

The value can be passed through an init. i.e.

struct ViewToSendValue: View {
    var body: some View {
        ViewToReceiveValue(valueToPass: "some string")
    }
}

struct ViewToReceiveValue: View { 

    var valueToPass: String = ""

    init(valueToPass: String) {
        self.valueToPass = valueToPass
    }

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

answered Oct 19, 2022 at 0:49

Claytog

1,01814 silver badges10 bronze badges

6 Comments

Kamil Over a year ago

That's the easiest method so far.

trilogy Over a year ago

But that only happens once. So if you make a change on the first view to that value, it won't reflect in the second view.

Hayden Over a year ago

@trilogy that isn't true. You can try it out in a sample project.

trilogy Over a year ago

@Hayden an “init” is like a constructor, it only runs once on creation. Using Bindings is the correct way.

Hayden Over a year ago

"an “init” is like a constructor, it only runs once on creation" is correct, but SwiftUI will re-initialize the view if its dependencies change. In the above example, if you create a mechanism to change valueToPass at runtime (like pressing a button), and add a breakpoint in ViewToReceiveValue.init, you can see that it's called once on launch, and once after the button is pressed. This approach is preferred (and just as performant) over bindings if you want one-way data flow. Bindings allow the child to modify the property connected to the parent, which isn't always desirable.

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