LeadKit/Sources/Classes/DataLoading/GeneralDataLoading/GeneralDataLoadingViewModel.swift

//
//  Copyright (c) 2018 Touch Instinct
//
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//  of this software and associated documentation files (the Software), to deal
//  in the Software without restriction, including without limitation the rights
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//  copies of the Software, and to permit persons to whom the Software is
//  furnished to do so, subject to the following conditions:
//
//  The above copyright notice and this permission notice shall be included in
//  all copies or substantial portions of the Software.
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//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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//  THE SOFTWARE.
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import RxSwift
import RxCocoa

/// ViewModel that loads data from given data source with loading state tracking.
open class GeneralDataLoadingViewModel<ResultType>: BaseViewModel, GeneralDataLoadingHandler, DisposeBagHolder {

    public typealias LoadingModel = GeneralDataLoadingModel<ResultType>
    public typealias DataSourceType = Single<ResultType>
    public typealias LoadingState = GeneralDataLoadingState<DataSourceType>

    private let loadingModel: LoadingModel

    private let loadingStateRelay = BehaviorRelay<LoadingState>(value: .initial)

    // MARK: - DisposeBagHolder

    public let disposeBag = DisposeBag()

    /// Initializer with single result sequence and empty result checker closure.
    ///
    /// - Parameters:
    ///   - dataSource: A single element sequence.
    ///   - customErrorHandler: Custom error handler for state update. Pass nil for default error handling.
    ///   - emptyResultChecker: Closure for checking result on empty state.
    public init(dataSource: DataSourceType,
                customErrorHandler: LoadingModel.ErrorHandler? = nil,
                emptyResultChecker: @escaping LoadingModel.EmptyResultChecker = { _ in false }) {

        loadingModel = LoadingModel(dataSource: dataSource,
                                    customErrorHandler: customErrorHandler,
                                    emptyResultChecker: emptyResultChecker)

        loadingModel.stateDriver
            .drive(loadingStateRelay)
            .disposed(by: disposeBag)

        bindLoadingState(from: loadingStateDriver)

        loadingModel.reload()
    }

    /// Returns observable that emits current loading state.
    open var loadingStateObservable: Observable<LoadingState> {
        return loadingStateRelay.asObservable()
    }

    /// Returns driver that emits current loading state.
    open var loadingStateDriver: Driver<LoadingState> {
        return loadingStateRelay.asDriver()
    }

    /// By default returns true if loading state == .result.
    open var hasContent: Bool {
        return currentLoadingState.hasResult
    }

    /// Returns current result if it exists.
    public var currentResult: ResultType? {
        return currentLoadingState.result
    }

    /// Current state of loading process.
    private(set) public var currentLoadingState: LoadingState {
        get {
            return loadingStateRelay.value
        }
        set {
            loadingStateRelay.accept(newValue)
        }
    }

    /// Manually update loading state.
    /// Should be used only in specific situations on your own risk!
    ///
    /// - Parameter newState: New loading state.
    public func updateStateManually(to newState: LoadingState) {
        currentLoadingState = newState
    }

    /// Replaces current data source of loading model with new one.
    ///
    /// - Parameter dataSource: A single element sequence.
    public func replaceDataSource(with newDataSource: DataSourceType) {
        loadingModel.replaceDataSource(with: newDataSource)
    }

    /// Reload data.
    public func reload() {
        loadingModel.reload()
    }

    // MARK: - GeneralDataLoadingHandler

    open func onLoadingState() {
        // override in subclass
    }

    open func onResultsState(result: ResultType) {
        // override in subclass
    }

    open func onEmptyState() {
        // override in subclass
    }

    open func onErrorState(error: Error) {
        // override in subclass
    }
}