How EnvironmentObjects work in SwiftUI views

SwiftUI-EnvironmentObjectExample.swift

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

import SwiftUI
import Combine

// This is a simple example of a class conforming to the
//  BindableObject protocol.
final class ViewPreferences : BindableObject {
    let didChange = PassthroughSubject<ViewPreferences, Never>()
    
    var showThePasscode: Bool = false {
        didSet {
            didChange.send(self)
        }
    }
}

struct PasscodeViewer : View {
    @EnvironmentObject var viewPreferences: ViewPreferences
    // This is an @EnvironmentObject. It is a reference to a binding which is
    //  provided by an ancestor view using .environmentObject, and originates
    //  from outside of the view hierarchy. You can write to the value, and
    //  whenever it is updated by any source, SwiftUI updates all of the views
    //  that use it accordingly.
    
    var body: some View {
        VStack {
            if viewPreferences.showThePasscode {
                Text("Hello World!")
            }
            else {
                Text("**********")
            }
            
            // We can pass a binding to our @EnvironmentObject with
            //  the $ prefix. Then, the Toggle component writes to
            //  the showThePasscode value, which causes all of the
            //  views that depend on the value to be re-rendered
            //  by SwiftUI.
            Toggle(isOn: $viewPreferences.showThePasscode) {
                Text("Show the passcode")
            }
            
            // We can also have custom subviews with access to the same
            //  @EnvironmentObject, without having to pass anything along.
            SomeSubView()
        }
    }
}

struct SomeSubView : View {
    @EnvironmentObject var viewPreferences: ViewPreferences
    
    var body: some View {
        Group {
            if viewPreferences.showThePasscode {
                Text("Showing passcodes")
            }
            else {
                Text("Obfuscating passcodes")
            }
        }
    }
}

#if DEBUG
struct PasscodeViewer_Previews : PreviewProvider {
    static var previews: some View {
        
        let viewPreferences = ViewPreferences()
        return PasscodeViewer()
            .environmentObject(viewPreferences)
        // With an @EnvironmentObject, we provide the object on one ancestor
        //  view, giving all of the views in its subview hierarchy access to it.
        
    }
}
#endif