pg8wood · July 2, 2020 15:27
diff --git a/AnyProgressViewStyleTypeErasureExample.swift b/AnyProgressViewStyleTypeErasureExample.swift
 //
 //  ProgressViews.swift
 //  wwdc-2020
 //
 //  Created by Patrick Gatewood on 6/30/20.
 //
 // An attempt at creating my own type erasure similar to `AnyView`. Not recommended
 // unless we figure out a way to restore `AnyProgressviewStyle`'s wrapped view's type.
 // See:https://forums.swift.org/t/why-some-swiftui-views-have-body-swift-never/27372/8

 import SwiftUI
 import Combine

 // MARK: - Type Erasure

 protocol TypeErasing {
    var erasedValue: Any { get }
 }

 struct ProgressViewStyleTypeEraser<S: ProgressViewStyle>: TypeErasing {
    let style: S
    var erasedValue: Any {
        style
    }
 }

 // MARK: - Type-erased `ProgressViewStyle` attempt

 /// After some research, it seems this isn't possible yet without leveraging SwiftUI's internal implementations.
 /// See: https://forums.swift.org/t/why-some-swiftui-views-have-body-swift-never/27372/8
 struct AnyProgressViewStyle: ProgressViewStyle {
    private var eraser: TypeErasing
    
    init<S>(_ style: S) where S : ProgressViewStyle {
        eraser = ProgressViewStyleTypeEraser(style: style)
    }
    
    private var wrappedStyle: Any {
        eraser.erasedValue
    }
        
    /// In AnyView, this type is Never, telling SwiftUI to internally call its wrapped subview's `body` instead.
    typealias Body = AnyView
    
    func makeBody(configuration: Self.Configuration) -> Self.Body {
        // See the AnyProgressViewStyle documentation for why we have to do this.
        // Overall, this is not a pattern I'd recommend using moving forward.
        if let style = wrappedStyle as? DefaultProgressViewStyle {
            return AnyView(style.makeBody(configuration: configuration))
        } else if let style = wrappedStyle as? CircularProgressViewStyle {
            return AnyView(style.makeBody(configuration: configuration))
        } else if let style = wrappedStyle as? LinearProgressViewStyle {
            return AnyView(style.makeBody(configuration: configuration))
        }

        fatalError("Need to figure out to to restore the type of ProgressViewStyle!")
    }
 }

 // MARK: - The example view which made me want to attempt to make my own type erasure

 struct ProgressViewExampleView: View {
    @State private var cancellables = Set<AnyCancellable>()
    @State private var downloadAmount = 0.0
    
    // The offender. Since `ProgressViewStyle` has associated type constraints, we
    // can't store a state variable of type `ProgressViewStyle`
    @State private var currentProgressViewStyle: ProgressStyle = .default
    
    enum ProgressStyle: String, CaseIterable {
        case `default` = "Default"
        case linear = "Linear"
        case circular = "Circular"
        
        var body: some ProgressViewStyle {
            switch self {
            case .default:
                return AnyProgressViewStyle(DefaultProgressViewStyle())
            case .linear:
                return AnyProgressViewStyle(LinearProgressViewStyle())
            case .circular:
                return AnyProgressViewStyle(CircularProgressViewStyle())
            }
        }
    }

    var body: some View {
        VStack(alignment: .leading) {
            Text("Progress View Style") // label isn't rendered with SegmentedPickerStyle, so we need one here
                .font(.headline)
            Picker(selection: $currentProgressViewStyle, label: Text("Progress View Style")) {
                ForEach(ProgressStyle.allCases, id: \.self) {
                    Text($0.rawValue)
                }
            }
            .pickerStyle(SegmentedPickerStyle())
    
            Text("The default style will vary based on the platform.")
                .font(.caption2)
                .padding(.bottom, 20)

            ProgressView("Downloading…", value: downloadAmount, total: 100)
                .progressViewStyle(currentProgressViewStyle.body)
                .frame(minWidth: 0, maxWidth: .infinity)
            
            Spacer()
        }
        .padding()
        .navigationTitle("Progress Views")
        .onAppear {
            Timer.publish(every: 1, on: .main, in: .common)
                .autoconnect()
                .sink() { _ in
                    downloadAmount = (downloadAmount + 2).truncatingRemainder(dividingBy: 100)
                }
                .store(in: &cancellables)
        }
    }
 }

 struct ProgressViews_Previews: PreviewProvider {
    static var previews: some View {
        ProgressViewExampleView()
    }
 }
	//
	// ProgressViews.swift
	// wwdc-2020
	//
	// Created by Patrick Gatewood on 6/30/20.
	//
	// An attempt at creating my own type erasure similar to `AnyView`. Not recommended
	// unless we figure out a way to restore `AnyProgressviewStyle`'s wrapped view's type.
	// See:https://forums.swift.org/t/why-some-swiftui-views-have-body-swift-never/27372/8

	import SwiftUI
	import Combine

	// MARK: - Type Erasure

	protocol TypeErasing {
	var erasedValue: Any { get }
	}

	struct ProgressViewStyleTypeEraser<S: ProgressViewStyle>: TypeErasing {
	let style: S
	var erasedValue: Any {
	style
	}
	}

	// MARK: - Type-erased `ProgressViewStyle` attempt

	/// After some research, it seems this isn't possible yet without leveraging SwiftUI's internal implementations.
	/// See: https://forums.swift.org/t/why-some-swiftui-views-have-body-swift-never/27372/8
	struct AnyProgressViewStyle: ProgressViewStyle {
	private var eraser: TypeErasing

	init<S>(_ style: S) where S : ProgressViewStyle {
	eraser = ProgressViewStyleTypeEraser(style: style)
	}

	private var wrappedStyle: Any {
	eraser.erasedValue
	}

	/// In AnyView, this type is Never, telling SwiftUI to internally call its wrapped subview's `body` instead.
	typealias Body = AnyView

	func makeBody(configuration: Self.Configuration) -> Self.Body {
	// See the AnyProgressViewStyle documentation for why we have to do this.
	// Overall, this is not a pattern I'd recommend using moving forward.
	if let style = wrappedStyle as? DefaultProgressViewStyle {
	return AnyView(style.makeBody(configuration: configuration))
	} else if let style = wrappedStyle as? CircularProgressViewStyle {
	return AnyView(style.makeBody(configuration: configuration))
	} else if let style = wrappedStyle as? LinearProgressViewStyle {
	return AnyView(style.makeBody(configuration: configuration))
	}

	fatalError("Need to figure out to to restore the type of ProgressViewStyle!")
	}
	}

	// MARK: - The example view which made me want to attempt to make my own type erasure

	struct ProgressViewExampleView: View {
	@State private var cancellables = Set<AnyCancellable>()
	@State private var downloadAmount = 0.0

	// The offender. Since `ProgressViewStyle` has associated type constraints, we
	// can't store a state variable of type `ProgressViewStyle`
	@State private var currentProgressViewStyle: ProgressStyle = .default

	enum ProgressStyle: String, CaseIterable {
	case `default` = "Default"
	case linear = "Linear"
	case circular = "Circular"

	var body: some ProgressViewStyle {
	switch self {
	case .default:
	return AnyProgressViewStyle(DefaultProgressViewStyle())
	case .linear:
	return AnyProgressViewStyle(LinearProgressViewStyle())
	case .circular:
	return AnyProgressViewStyle(CircularProgressViewStyle())
	}
	}
	}

	var body: some View {
	VStack(alignment: .leading) {
	Text("Progress View Style") // label isn't rendered with SegmentedPickerStyle, so we need one here
	.font(.headline)
	Picker(selection: $currentProgressViewStyle, label: Text("Progress View Style")) {
	ForEach(ProgressStyle.allCases, id: \.self) {
	Text($0.rawValue)
	}
	}
	.pickerStyle(SegmentedPickerStyle())

	Text("The default style will vary based on the platform.")
	.font(.caption2)
	.padding(.bottom, 20)

	ProgressView("Downloading…", value: downloadAmount, total: 100)
	.progressViewStyle(currentProgressViewStyle.body)
	.frame(minWidth: 0, maxWidth: .infinity)

	Spacer()
	}
	.padding()
	.navigationTitle("Progress Views")
	.onAppear {
	Timer.publish(every: 1, on: .main, in: .common)
	.autoconnect()
	.sink() { _ in
	downloadAmount = (downloadAmount + 2).truncatingRemainder(dividingBy: 100)
	}
	.store(in: &cancellables)
	}
	}
	}

	struct ProgressViews_Previews: PreviewProvider {
	static var previews: some View {
	ProgressViewExampleView()
	}
	}