SwiftUI: Global Overlay That Can Be Triggered From Any View

Question

I'm quite new to the SwiftUI framework and I haven't wrapped my head around all of it yet so please bear with me.

Is there a way to trigger an "overlay view" from inside "another view" when its binding changes? See illustration below:

I figure this "overlay view" would wrap all my views. I'm not sure how to do this yet - maybe using ZIndex. I also guess I'd need some sort of callback when the binding changes, but I'm also not sure how to do that either.

This is what I've got so far:

ContentView

struct ContentView : View {
    @State private var liked: Bool = false

    var body: some View {
        VStack {
            LikeButton(liked: $liked)
        }
    }
}

LikeButton

struct LikeButton : View {
    @Binding var liked: Bool

    var body: some View {
        Button(action: { self.toggleLiked() }) {
            Image(systemName: liked ? "heart" : "heart.fill")
        }
    }

    private func toggleLiked() {
        self.liked = !self.liked
        // NEED SOME SORT OF TOAST CALLBACK HERE
    }
}

I feel like I need some sort of callback inside my LikeButton, but I'm not sure how this all works in Swift.

Any help with this would be appreciated. Thanks in advance!

Answer 1

It's quite easy - and entertaining - to build a "toast" in SwiftUI!

Let's do it!

struct Toast<Presenting>: View where Presenting: View {

    /// The binding that decides the appropriate drawing in the body.
    @Binding var isShowing: Bool
    /// The view that will be "presenting" this toast
    let presenting: () -> Presenting
    /// The text to show
    let text: Text

    var body: some View {

        GeometryReader { geometry in

            ZStack(alignment: .center) {

                self.presenting()
                    .blur(radius: self.isShowing ? 1 : 0)

                VStack {
                    self.text
                }
                .frame(width: geometry.size.width / 2,
                       height: geometry.size.height / 5)
                .background(Color.secondary.colorInvert())
                .foregroundColor(Color.primary)
                .cornerRadius(20)
                .transition(.slide)
                .opacity(self.isShowing ? 1 : 0)

            }

        }

    }

}

Explanation of the body:

• GeometryReader gives us the preferred size of the superview , thus allowing the perfect sizing for our Toast.
• ZStack stacks views on top of each other.
• The logic is trivial: if the toast is not supposed to be seen (isShowing == false), then we render the presenting view. If the toast has to be presented (isShowing == true), then we render the presenting view with a little bit of blur - because we can - and we create our toast next.
• The toast is just a VStack with a Text, with custom frame sizing, some design bells and whistles (colors and corner radius), and a default slide transition.

I added this method on View to make the Toast creation easier:

extension View {

    func toast(isShowing: Binding<Bool>, text: Text) -> some View {
        Toast(isShowing: isShowing,
              presenting: { self },
              text: text)
    }

}

And a little demo on how to use it:

struct ContentView: View {

    @State var showToast: Bool = false

    var body: some View {
        NavigationView {
            List(0..<100) { item in
                Text("\(item)")
            }
            .navigationBarTitle(Text("A List"), displayMode: .large)
            .navigationBarItems(trailing: Button(action: {
                withAnimation {
                    self.showToast.toggle()
                }
            }){
                Text("Toggle toast")
            })
        }
        .toast(isShowing: $showToast, text: Text("Hello toast!"))
    }

}

I used a NavigationView to make sure the view fills the entire screen, so the Toast is sized and positioned correctly.

The withAnimation block ensures the Toast transition is applied.

---

How it looks:

It's easy to extend the Toast with the power of SwiftUI DSL.

The Text property can easily become a @ViewBuilder closure to accomodate the most extravagant of the layouts.

---

To add it to your content view:

struct ContentView : View {
    @State private var liked: Bool = false

    var body: some View {
        VStack {
            LikeButton(liked: $liked)
        }
        // make it bigger by using "frame" or wrapping it in "NavigationView"
        .toast(isShowing: $liked, text: Text("Hello toast!"))
    }
}

---

How to hide the toast afte 2 seconds (as requested):

Append this code after .transition(.slide) in the toast VStack.

.onAppear {
    DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
      withAnimation {
        self.isShowing = false
      }
    }
}

Tested on Xcode 11.1