ScreenState.swift

ScreenState.swift

public enum ScreenState<Value, Error: Swift.Error> {
    case loading
    case failed(Error)
    case empty
    case loaded(Value)

    public var isLoading: Bool {
        if case .loading = self { return true }
        return false
    }

    public var isEmpty: Bool {
        if case .empty = self { return true }
        return false
    }

    public var value: Value? {
        if case .loaded(let value) = self { return value }
        return nil
    }

    public var error: Error? {
        if case .failed(let error) = self { return error }
        return nil
    }

    /// 自身が `.loaded` であれば状態を変更しない。そうでなければ状態を `.loading` に変更する。
    public mutating func setLoadingIfNeeded() {
        switch self {
        case .failed, .empty:
            self = .loading
        case .loading, .loaded:
            break
        }
    }

    /// 自身が `.loaded` 以外の状態であれば状態を `.failed` に変更し、trueを返す。
    /// そうでなければ状態を変更せず、falseを返す。
    public mutating func handleError(_ error: Error) -> Bool {
        switch self {
        case .loaded:
            return false
        case .loading, .failed, .empty:
            self = .failed(error)
            return true
        }
    }
}

public struct ScreenStateProjector<Value, Failure: Error, Loading: View, Empty: View, Loaded: View, Failed: View>: View {
    public let state: ScreenState<Value, Failure>

    private let refreshAction: (() -> Void)?
    private var loading: () -> Loading
    private var empty: ((() -> Void)?) -> Empty
    private var loaded: (Value) -> Loaded
    private var failed: (Failure, (() -> Void)?) -> Failed

    public init(
        _ state: ScreenState<Value, Failure>,
        refreshAction: (() -> Void)?,
        @ViewBuilder onLoading: @escaping (() -> Loading) = {
            LoadingView(isLoading: true)
        },
        @ViewBuilder onFailed: @escaping (Failure, (() -> Void)?) -> Failed,
        @ViewBuilder onEmpty: @escaping ((() -> Void)?) -> Empty = {
            EmptyStateView(action: $0)
        },
        @ViewBuilder onLoaded: @escaping (Value) -> Loaded
    ) {
        self.state = state
        self.refreshAction = refreshAction
        self.loading = onLoading
        self.empty = onEmpty
        self.loaded = onLoaded
        self.failed = onFailed
    }

    public init(
        _ state: ScreenState<Value, Failure>,
        refreshAction: (() -> Void)?,
        @ViewBuilder onLoading: @escaping (() -> Loading) = {
            LoadingView(isLoading: true)
        },
        @ViewBuilder onEmpty: @escaping ((() -> Void)?) -> Empty = {
            EmptyStateView(action: $0)
        },
        @ViewBuilder onLoaded: @escaping (Value) -> Loaded
    ) where Failed == ErrorMessageView<Failure> {
        self.state = state
        self.refreshAction = refreshAction
        self.loading = onLoading
        self.empty = onEmpty
        self.loaded = onLoaded
        self.failed = {
            ErrorMessageView($0, action: $1)
        }
    }

    public var body: some View {
        switch state {
        case .loading:
            loading()
        case .failed(let error):
            failed(error, refreshAction)
        case .empty:
            empty(refreshAction)
        case .loaded(let value):
            loaded(value)
        }
    }
}