Add StateStoreViewModel and publisher extensions for convenienec.

RiotSwiftUI/Modules/Common/ViewModel/StateStoreViewModel.swift

@@ -0,0 +1,93 @@
+// 
+// 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
+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.")
+        }
+    }
+}
+
+@available(iOS 14, *)
+class ViewModelContext<ViewState:BindableState, ViewAction>: ObservableObject {
+   
+    private var cancellables = Set<AnyCancellable>()
+    
+    let inputActions: PassthroughSubject<ViewAction, Never>
+    @Published var inputState: ViewState.BindStateType
+    @Published fileprivate(set) var viewState: ViewState
+
+    init(initialViewState: ViewState) {
+        self.inputState = initialViewState.bindings
+        self.inputActions = PassthroughSubject()
+        self.viewState = initialViewState
+        if !(initialViewState.bindings is Void) {
+            self.$inputState
+                .weakAssign(to: \.viewState.bindings, on: self)
+                .store(in: &cancellables)
+        }
+    }
+}
+
+@available(iOS 14, *)
+class StateStoreViewModel<State:BindableState, StateAction, ViewAction> {
+    
+    typealias Context = ViewModelContext<State, ViewAction>
+    
+    let state: CurrentValueSubject<State, Never>
+
+    var cancellables = Set<AnyCancellable>()
+    var context: Context
+    
+    init(initialViewState: State) {
+        
+        self.context = Context(initialViewState: initialViewState)
+        self.state = CurrentValueSubject(initialViewState)
+        self.state.weakAssign(to: \.context.viewState, on: self)
+            .store(in: &cancellables)
+        self.context.inputActions.sink { [weak self] action in
+            guard let self = self else { return }
+            self.process(viewAction: action)
+        }
+        .store(in: &cancellables)
+    }
+    func dispatch(action: StateAction) {
+        reducer(state: &state.value, action: action)
+    }
+
+    func reducer(state: inout State, action: StateAction) {
+
+    }
+
+    func process(viewAction: ViewAction) {
+
+    }
+}