command pattern

callbackを登録するようなもの
オペレーションの呼び出しと実行を分離する

example

commandはreceiverに移譲するだけのものから、それ自身が処理をするものまである。

protocol CommandProtocol {
    func exec() {}
}

class PrintCommand: CommandProtocol {
    private let receiver: receiver

    func exec() { receiver.print() }
}

class Receiver {
    func print() {
        print(nextLine)
    }
}


class Invoker {
    private let command: CommandProtocol

    init(command: CommandProtocol) {
        self.command = command
    }

    func acceptInput() {
        command.exec()
    }
}

swift の BackgroundTask は適当な時間で強制終了される。
commandパターンによりretryをサポートする。

final class BackgroundCommand {
    enum Status {
        case willBegin, doing, done, failed
    }

    private let receiver: Receiver
    private let input: ReceiverInput
    private var status: Status = .willBegin
    private var backgroundTaskID: UIBackgroundTaskIdentifier = .invalid

    init(receiver: Receiver, input: ReceiverInput) {
        self.receiver = receiver
        self.input = input
    }

    func exec() {
        status = .doing

        backgroundTaskID = UIApplication.shared.beginBackgroundTask(withName: "TASK_NAME", expirationHandler: { [weak self] in self.status = .failed })

        DispatchQueue.global(qos: .userInitiated).async { [weak self] in
            guard
                let r = receiver,
                let i = input,
                let taskID = backgroundTaskID else {
                self?.status = .failed
                return
            }

            r.action(i)

            UIApplication.shared.endBackgroundTask(taskID)
            self?.backgroundTaskID = .invalid
            self?.status = .done
        }
    }

    func retry() {
        guard status == .doing else { fatalError() } // 適当に処理

        receiver.restore() // receiverの内部状態が変わるとき
        status = .willBegin
        exec()
    }
}