element-ios/RiotSwiftUI/Modules/Common/ViewModel/StateStoreViewModel.swift

//
// 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


/// 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`.
@dynamicMemberLookup
class ViewModelContext<ViewState:BindableState, ViewAction>: ObservableObject {
    // MARK: - Properties

    // MARK: Private

    fileprivate let viewActions: PassthroughSubject<ViewAction, Never>

    // MARK: Public

    /// Get-able/Observable `Published` property for the `ViewState`
    @Published fileprivate(set) var viewState: ViewState

    /// Set-able/Bindable access to the bindable state.
    subscript<T>(dynamicMember keyPath: WritableKeyPath<ViewState.BindStateType, T>) -> T {
        get { viewState.bindings[keyPath: keyPath] }
        set { viewState.bindings[keyPath: keyPath] = newValue }
    }
    
    // MARK: Setup

    init(initialViewState: ViewState) {
        self.viewActions = PassthroughSubject()
        self.viewState = initialViewState
    }

    // 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.
class StateStoreViewModel<State: BindableState, StateAction, ViewAction> {

    typealias Context = ViewModelContext<State, ViewAction>

    // MARK: - Properties

    // 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
    
    var state: State {
        get { context.viewState }
        set { context.viewState = newValue }
    }
    
    // MARK: Setup

    init(initialViewState: State) {
        self.context = Context(initialViewState: initialViewState)
        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.
    @available(*, deprecated, message: "Mutate state directly instead")
    func dispatch(action: StateAction) {
        Self.reducer(state: &context.viewState, action: action)
    }

    /// 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
    @available(*, deprecated, message: "Mutate state directly instead")
    func dispatch(actionPublisher: AnyPublisher<StateAction, Never>) {
        actionPublisher.sink { [weak self] action in
            guard let self = self else { return }
            Self.reducer(state: &self.context.viewState, action: action)
        }
        .store(in: &cancellables)
    }

    /// 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
    }
}