Merge branch 'langleyd/4781_swiftui_template_examples' of https://github.com/vector-im/element-ios into langleyd/4781_swiftui_template_example2

RiotSwiftUI/Modules/Common/ViewModel/BindableState.swift

@@ -0,0 +1,37 @@
+// 
+// Copyright 2021 New Vector Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+import Foundation
+
+/// Represents a specific portion of the ViewState that can be bound to with SwiftUI's [2-way binding](https://developer.apple.com/documentation/swiftui/binding).
+protocol BindableState {
+    /// The associated type of the Bindable State. Defaults to Void.
+    associatedtype BindStateType = Void
+    var bindings: BindStateType { get set }
+}
+
+extension BindableState where BindStateType == Void {
+    /// We provide a default implementation for the Void type so that we can have `ViewState` that
+    /// just doesn't include/take advantage of the bindings.
+    var bindings: Void {
+        get {
+            ()
+        }
+        set {
+            fatalError("Can't bind to the default Void binding.")
+        }
+    }
+}

RiotSwiftUI/Modules/Common/ViewModel/StateStoreViewModel.swift

@@ -20,79 +20,131 @@ import Combine
 import Foundation
 import Combine
 
-protocol BindableState {
-    associatedtype BindStateType = Void
-    var bindings: BindStateType { get set }
-}
-
-extension BindableState where BindStateType == Void {
-    var bindings: Void {
-        get {
-            ()
-        }
-        set {
-            fatalError("Can't bind to the default Void binding.")
-        }
-    }
-}
 
+/// A constrained and concise interface for interacting with the ViewModel.
+///
+/// This class is closely bound to`StateStoreViewModel`. It provides the exact interface the view should need to interact
+/// ViewModel (as modelled on our previous template architecture with the addition of two-way binding):
+/// - The ability read/observe view state
+/// - The ability to send view events
+/// - The ability to bind state to a specific portion of the view state safely.
+/// This class was brought about a little bit by necessity. The most idiomatic way of interacting with SwiftUI is via `@Published`
+/// properties which which are property wrappers and therefore can't be defined within protocols.
+/// A similar approach is taken in libraries like [CombineFeedback](https://github.com/sergdort/CombineFeedback).
+/// It provides a nice layer of consistency and also safety. As we are not passing the `ViewModel` to the view directly, shortcuts/hacks
+/// can't be made into the `ViewModel`.
 @available(iOS 14, *)
 class ViewModelContext<ViewState:BindableState, ViewAction>: ObservableObject {
-   
+    // MARK: - Properties
+
+    // MARK: Private
+
     private var cancellables = Set<AnyCancellable>()
     fileprivate let viewActions: PassthroughSubject<ViewAction, Never>
-    
+
+    // MARK: Public
+
+    /// Set-able/Bindable `Published` property for the bindable portion of the `ViewState`
     @Published var bindings: ViewState.BindStateType
+    /// Get-able/Observable `Published` property for the `ViewState`
     @Published fileprivate(set) var viewState: ViewState
 
+    // MARK: Setup
+
     init(initialViewState: ViewState) {
         self.bindings = initialViewState.bindings
         self.viewActions = PassthroughSubject()
         self.viewState = initialViewState
         if !(initialViewState.bindings is Void) {
+            // If we have bindable state defined, forward its updates on to the `ViewState`
             self.$bindings
                 .weakAssign(to: \.viewState.bindings, on: self)
                 .store(in: &cancellables)
         }
     }
-    
+
+    // MARK: Public
+
+    /// Send a `ViewAction` to the `ViewModel` for processing.
+    /// - Parameter viewAction: The `ViewAction` to send to the `ViewModel`.
     func send(viewAction: ViewAction) {
         viewActions.send(viewAction)
     }
 }
 
+/// A common ViewModel implementation for handling of `State`, `StateAction`s and `ViewAction`s
+///
+/// Generic type State is constrained to the BindableState protocol in that it may contain (but doesn't have to)
+/// a specific portion of state that can be safely bound to.
+/// If we decide to add more features to our state management (like doing state processing off the main thread)
+/// we can do it in this centralised place.
 @available(iOS 14, *)
-class StateStoreViewModel<State:BindableState, StateAction, ViewAction> {
-    
-    typealias Context = ViewModelContext<State, ViewAction>
-    
-    let state: CurrentValueSubject<State, Never>
+class StateStoreViewModel<State: BindableState, StateAction, ViewAction> {
 
+    typealias Context = ViewModelContext<State, ViewAction>
+
+    // MARK: - Properties
+
+    // MARK: Private
+
+    private let state: CurrentValueSubject<State, Never>
+
+    // MARK: Public
+
+    /// For storing subscription references.
+    ///
+    /// Left as public for `ViewModel` implementations convenience.
     var cancellables = Set<AnyCancellable>()
+
+    /// Constrained interface for passing to Views.
     var context: Context
-    
+
+    // MARK: Setup
+
     init(initialViewState: State) {
-        
         self.context = Context(initialViewState: initialViewState)
         self.state = CurrentValueSubject(initialViewState)
-        self.state.weakAssign(to: \.context.viewState, on: self)
+        // Connect the state to context viewState, that view uses for observing (but not modifying directly) the state.
+        self.state
+            .weakAssign(to: \.context.viewState, on: self)
             .store(in: &cancellables)
+        // Receive events from the view and pass on to the `ViewModel` for processing.
         self.context.viewActions.sink { [weak self] action in
             guard let self = self else { return }
             self.process(viewAction: action)
         }
         .store(in: &cancellables)
     }
-    
+
+    /// Send state actions to modify the state within the reducer.
+    /// - Parameter action: The state action to send to the reducer.
     func dispatch(action: StateAction) {
         Self.reducer(state: &state.value, action: action)
     }
 
-    class func reducer(state: inout State, action: StateAction) {
-
+    /// Send state actions from a publisher to modify the state within the reducer.
+    /// - Parameter actionPublisher: The publisher that produces actions to be sent to the reducer
+    func dispatch(actionPublisher: AnyPublisher<StateAction, Never>) {
+        actionPublisher.sink { [weak self] action in
+            guard let self = self else { return }
+            Self.reducer(state: &self.state.value, action: action)
+        }
+        .store(in: &cancellables)
     }
 
-    func process(viewAction: ViewAction) {
+    /// Override to handle mutations to the `State`
+    ///
+    /// A redux style reducer, all modifications to state happen here.
+    /// - Parameters:
+    ///   - state: The `inout` state to be modified,
+    ///   - action: The action that defines which state modification should take place.
+    class func reducer(state: inout State, action: StateAction) {
+        //Default implementation, -no-op
+    }
 
+    /// Override to handles incoming `ViewAction`s from the `ViewModel`.
+    /// - Parameter viewAction: The `ViewAction` to be processed in `ViewModel` implementation.
+    func process(viewAction: ViewAction) {
+        //Default implementation, -no-op
     }
 }