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Demonstrates setting a preference based on an action
import SwiftUI
struct ContentView: View {
var body: some View {
WrapperView {
VStack {
PrefView()
PrefView()
}
}
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}
struct BooleanKey: PreferenceKey {
static var defaultValue: Bool = false
static func reduce(value: inout Bool, nextValue: () -> Bool) {
let next = nextValue()
guard value == false else { return }
value = next
}
}
struct PrefView: View {
@State var pressed: Bool = false
var body: some View {
Button(action: { self.pressed.toggle() }) { Text(pressed ? "unpress me" : "press me") }
.preference(key: BooleanKey.self, value: pressed)
}
}
struct WrapperView<Content>: View where Content: View {
var content: () -> Content
@State var pressed: Bool = false
init(@ViewBuilder content: @escaping () -> Content) {
self.content = content
}
var body: some View {
VStack {
Text(pressed ? "pressed" : "unpressed")
content()
}
.backgroundPreferenceValue(BooleanKey.self) { value -> AnyView in
DispatchQueue.main.async { self.pressed = value }
return AnyView(Color.clear)
}
}
}
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