SwiftUI : Why preferenceKey is not read/called

Question

Created Dec ’20

Replies 6

Boosts 0

Views 4.8k

Participants 4

From the code below, I can't figure out why Preference key is not read from the view put in background of HStack. Strangely, If I put that BGViewSetter to the Text Views inside ForEach loop, it works. Unless I have ommitted something too obvious, it looks like a bug. Anybody can confirm it? Thanks

xcode12.2
ios 14.2

Code Block struct BGViewSetter: View {
    var body: some View {
        GeometryReader { geometry in
            RoundedRectangle(cornerRadius: 25.0)
                .fill(Color.init(#colorLiteral(red: 0.9372549057, green: 0.3490196168, blue: 0.1921568662, alpha: 1)))
                .preference(key: TestRectPreferenceKey.self,
                            value: TestRectPreference(frame: geometry.frame(in: .global)))
        }
    }
}
    struct FinalView : View
    {
        @State var offsetX : CGFloat = .zero
        @State var info = ""
        @State var size : CGFloat = .zero
        var body: some View
        {
                VStack
                {
                    Text("FinalView : \(info)")
                        HStack
                        {
                            ForEach( 1 ..< 10)
                            { i in
                                Text("\(i)")
                                    .frame(width: 100)
                                    .opacity(0.8)
                            }
                        }
                        .background(BGViewSetter())
                        .animation(.easeOut)
                        .gesture(
                            DragGesture()
                                .onChanged
                                { gesture in
                                    self.offsetX = gesture.translation.width
                                }
                                .onEnded
                                { _ in
                                    self.offsetX = .zero
                                }
                        )
                        .offset(x: self.offsetX)
                    Text("Footer")
                    Divider()
                    Spacer()
                }
                .onPreferenceChange(TestRectPreferenceKey.self)
                                    {
                                            self.size = $0.frame.height
                                            self.info = "Pref Chng : \(self.size)"
                                    }
        }
    }
    struct PreferenceKeyTest_Previews: PreviewProvider {
        static var previews: some View {
            FinalView()
        }
    }

Boost

Answer 1

Claude31 OP

Dec ’20

I wanted to test, but I miss TestRectPreferenceKey definition.

0

Answer 2

jtmorris OP

Dec ’20

Accepted Answer

I've run into the same issue. Setting preference key in background() view fails. Same code works fine on ios 13.

1

Answer 3

1littledave OP

Dec ’20

My Mistake for not including TestRectPreference Implementation. Below is the code :

Code Block struct TestRectPreference : Equatable
{
    var frame : CGRect
}
struct TestRectPreferenceKey : PreferenceKey
{
    typealias Value =  TestRectPreference
    static var defaultValue:  TestRectPreference =  TestRectPreference(frame: CGRect.zero)
    static func reduce(value: inout  TestRectPreference, nextValue: () ->  TestRectPreference)
    {
        value.frame = nextValue().frame
    }
}

0

Answer 4

Claude31 OP

Dec ’20

There is no ContentView in the code.

What is the view you call in the SceneDelegate ? Looks like it is BGViewSetter. Correct ?

Do you mean you have to change to

Code Block                 ForEach( 1 ..< 10)
                { i in
                    Text("\(i)")
                        .frame(width: 100)
                        .opacity(0.8)
                        .background(BGViewSetter())
                }
            }

For it to work ?

I tried and did not notice any difference (when running in simulator, not in Preview)
In all cases, I get a rounded corner red view with nothing inside.
When testing,g in Preview, I get 3 buttons visible at top (4, 5, 6) in both cases.

What are you expecting as a result ?
What do you get ?
How do you know it is called / not called ?

0

Answer 5

1littledave OP

Dec ’20

@Claude31

Yes, Your code is what I meant as working (reading the frame of the background View of each Text views).

I am expecting Preferencekey to read the background View of HStack inside which contains

Code Block Text("\(i)")

I get "0.0" frame size. It's NOT supposed to be "0.0"
If you look at the info above red bar "Final View : Pref Change : <has to be some number>" not 0.0. If you put preferencekeyed background as like your code, you will see some number. I want to read HStack view not individual Text views.

Since "jtmorris" mentioned that it works on ios13, seems like updated compiler issue (inconsistencies) .

0

Answer 6

Dzero OP

Mar ’22

It's been a year. I hope you've solved your problem. Try removing value.frame = nextValue().frame should solve your problem

1