Add Async image loading

Examples/Time Travel/SwiftUITimeTravel/TimeTravelView/StateMachine.swift

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+/// Conforming types serve as the state of a time travelable application
+public protocol StateMachine {
+    
+    /// Events define things that can happen within your application that change its state.
+    ///
+    /// This might include things like text editing, button taps, or network responses.
+    associatedtype Event
+    
+    /// Applies an event to the current state.
+    mutating func update(with event: Event)
+}

Examples/Time Travel/SwiftUITimeTravel/TimeTravelView/Store.swift

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+import SwiftUI
+import Combine
+
+public final class Store<StateType>: BindableObject where StateType: StateMachine {
+    
+    private let initialState: StateType
+    private var subsequentStates: [StateType] = []
+
+    public let didChange = PassthroughSubject<Void, Never>()
+    
+    public init(state: StateType) {
+        initialState = state
+    }
+    
+    var allStates: [StateType] {
+        [[initialState], subsequentStates].flatMap({ $0 })
+    }
+    
+    var stateCount: Int {
+        1 + subsequentStates.count
+    }
+    
+    var currentStateIndex: Int = 0 {
+        didSet {
+            withAnimation {
+                didChange.send(())
+            }
+        }
+    }
+    
+    /// The current state of the store. This will update as time traveling occurs.
+    public var state: StateType {
+        allStates[currentStateIndex]
+    }
+    
+    /// Dispatches an event to be applied to the current state.
+    public func dispatch(event: StateType.Event) {
+        var newState = state
+        newState.update(with: event)
+        subsequentStates.append(newState)
+        currentStateIndex = stateCount - 1
+    }
+    
+}

Examples/Time Travel/SwiftUITimeTravel/TimeTravelView/TimeTravelBarView.swift

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+import SwiftUI
+
+struct TimeTravelBarView : View {
+    
+    @EnvironmentObject var store: Store<TodoState>
+    
+    var body: some View {
+        let indexBinding = Binding<Double>(
+            getValue: { Double(self.store.currentStateIndex) },
+            setValue: { self.store.currentStateIndex = Int($0) })
+
+        return Slider(value: indexBinding, from: 0, through: Double(store.stateCount-1))
+            .background(Color.white)
+            .frame(height: 44.0, alignment: .bottom)
+            .padding()
+    }
+}
+
+#if DEBUG
+struct TimeTravelBarView_Previews : PreviewProvider {
+    static var previews: some View {
+        TimeTravelBarView()
+    }
+}
+#endif

Examples/Time Travel/SwiftUITimeTravel/TimeTravelView/TimeTravelView.swift

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+import SwiftUI
+
+public struct TimeTravelView<StateType, Content>: View where StateType: StateMachine, Content: View {
+    
+    let initialState: StateType
+    
+    private let content: Content
+    
+    @State var store: Store<StateType>? = nil
+    
+    public init(initialState: StateType, content: () -> Content) {
+        self.initialState = initialState
+        self.content = content()
+    }
+    
+    public var body: some View {
+        
+        let store = self.store ?? Store(state: initialState)
+        if (self.store == nil) {
+            self.store = store
+        }
+
+        return VStack {
+            content
+            TimeTravelBarView()
+        }
+        .environmentObject(store)
+    }
+}
+
+#if DEBUG
+struct TimeTravelView_Previews : PreviewProvider {
+    static var previews: some View {
+        TimeTravelView(initialState: TodoState()) {
+            TodoListView()
+        }
+    }
+}
+#endif