How bindings work in SwiftUI views

SwiftUI-BindingExample.swift

//
//  SwiftUI-BindingExample.swift
//  SwiftUI-DataFlowExamples
//
//  Part of the series beginning at:
//  https://medium.com/@alexobenauer/demystifying-data-flow-through-swiftui-697017aba7e0
//

import SwiftUI

struct PlayButton : View {
    
    @Binding var isPlaying: Bool
    // This is a binding. It is a reference to a read-write value
    //  passed in from a parent view. Writing to the value causes
    //  SwiftUI to update all of the views which use it.
    
    var body: some View {
        Button(action: {
            self.isPlaying.toggle()
        }) {
            Text(isPlaying ? "Pause" : "Play")
        }
    }
}

struct ExampleParentView : View {
    @State private var isPlaying: Bool = false
    
    var body: some View {
        VStack {
            PlayButton(isPlaying: $isPlaying)
            // To pass a binding of state, you use the $ prefix,
            //  which gives you the binding rather than the value.
            
            if isPlaying {
                Text("Currently playing")
            }
            // This is automatically updated by SwiftUI when
            //  PlayButton changes the isPlaying value.
        }
    }
}