Command を Swift で
Command は、 振る舞いに関するデザインパターンの一つで、 リクエストや簡単な操作をオブジェクトに変換します。
変換により、 コマンドの遅延実行や遠隔実行を可能にしたり、 コマンドの履歴の保存を可能にしたりできます。
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使用例： Command パターンは、 Swift コードではよく見かけます。 最もよく使われるのは、 UI 要素をアクションでパラメーター化する時のコールバックの代わりとしてです。 また、 タスクをキューに入れたり、 操作履歴の管理などでも使われます。
見つけ方： Command パターンは、 抽象またはインターフェース型 （送り手） 中の行動的メソッド （複数） が違う抽象またはインターフェース型 （受け手） 中のある一つのメソッドを起動することから識別できます。 受け手は、 生成時にコマンドの実装によりカプセル化されています。 コマンドのクラスは通常特定のアクションに限定されています。
以下の例は Swift Playgroundsで利用できます。
Playgroundバージョンを作成してくれた Alejandro Mohamadに感謝します。
概念的な例
この例は、 Command デザインパターンの構造を説明するためのものです。 以下の質問に答えることを目的としています：
- どういうクラスからできているか？
- それぞれのクラスの役割は？
- パターンの要素同士はどう関係しているのか？
ここでパターンの構造を学んだ後だと、 これに続く、 現実世界の Swift でのユースケースが理解しやすくなります。
Example.swift: 概念的な例
import XCTest /// The Command interface declares a method for executing a command. protocol Command { func execute() } /// Some commands can implement simple operations on their own. class SimpleCommand: Command { private var payload: String init(_ payload: String) { self.payload = payload } func execute() { print("SimpleCommand: See, I can do simple things like printing (" + payload + ")") } } /// However, some commands can delegate more complex operations to other /// objects, called "receivers." class ComplexCommand: Command { private var receiver: Receiver /// Context data, required for launching the receiver's methods. private var a: String private var b: String /// Complex commands can accept one or several receiver objects along with /// any context data via the constructor. init(_ receiver: Receiver, _ a: String, _ b: String) { self.receiver = receiver self.a = a self.b = b } /// Commands can delegate to any methods of a receiver. func execute() { print("ComplexCommand: Complex stuff should be done by a receiver object.\n") receiver.doSomething(a) receiver.doSomethingElse(b) } } /// The Receiver classes contain some important business logic. They know how to /// perform all kinds of operations, associated with carrying out a request. In /// fact, any class may serve as a Receiver. class Receiver { func doSomething(_ a: String) { print("Receiver: Working on (" + a + ")\n") } func doSomethingElse(_ b: String) { print("Receiver: Also working on (" + b + ")\n") } } /// The Invoker is associated with one or several commands. It sends a request /// to the command. class Invoker { private var onStart: Command? private var onFinish: Command? /// Initialize commands. func setOnStart(_ command: Command) { onStart = command } func setOnFinish(_ command: Command) { onFinish = command } /// The Invoker does not depend on concrete command or receiver classes. The /// Invoker passes a request to a receiver indirectly, by executing a /// command. func doSomethingImportant() { print("Invoker: Does anybody want something done before I begin?") onStart?.execute() print("Invoker: ...doing something really important...") print("Invoker: Does anybody want something done after I finish?") onFinish?.execute() } } /// Let's see how it all comes together. class CommandConceptual: XCTestCase { func test() { /// The client code can parameterize an invoker with any commands. let invoker = Invoker() invoker.setOnStart(SimpleCommand("Say Hi!")) let receiver = Receiver() invoker.setOnFinish(ComplexCommand(receiver, "Send email", "Save report")) invoker.doSomethingImportant() } } Output.txt: 実行結果
Invoker: Does anybody want something done before I begin? SimpleCommand: See, I can do simple things like printing (Say Hi!) Invoker: ...doing something really important... Invoker: Does anybody want something done after I finish? ComplexCommand: Complex stuff should be done by a receiver object. Receiver: Working on (Send email) Receiver: Also working on (Save report) 現実的な例
Example.swift: 現実的な例
import Foundation import XCTest class DelayedOperation: Operation, @unchecked Sendable { private var delay: TimeInterval init(_ delay: TimeInterval = 0) { self.delay = delay } override var isExecuting : Bool { get { return _executing } set { willChangeValue(forKey: "isExecuting") _executing = newValue didChangeValue(forKey: "isExecuting") } } private var _executing : Bool = false override var isFinished : Bool { get { return _finished } set { willChangeValue(forKey: "isFinished") _finished = newValue didChangeValue(forKey: "isFinished") } } private var _finished : Bool = false override func start() { guard delay > 0 else { _start() return } let deadline = DispatchTime.now() + delay DispatchQueue(label: "").asyncAfter(deadline: deadline) { self._start() } } private func _start() { guard !self.isCancelled else { print("\(self): operation is canceled") self.isFinished = true return } self.isExecuting = true self.main() self.isExecuting = false self.isFinished = true } } class WindowOperation: DelayedOperation, @unchecked Sendable { override func main() { print("\(self): Windows are closed via HomeKit.") } override var description: String { return "WindowOperation" } } class DoorOperation: DelayedOperation, @unchecked Sendable { override func main() { print("\(self): Doors are closed via HomeKit.") } override var description: String { return "DoorOperation" } } class TaxiOperation: DelayedOperation, @unchecked Sendable { override func main() { print("\(self): Taxi is ordered via Uber") } override var description: String { return "TaxiOperation" } } class CommandRealWorld: XCTestCase { func testCommandRealWorld() { prepareTestEnvironment { let siri = SiriShortcuts.shared print("User: Hey Siri, I am leaving my home") siri.perform(.leaveHome) print("User: Hey Siri, I am leaving my work in 3 minutes") siri.perform(.leaveWork, delay: 3) /// for simplicity, we use seconds print("User: Hey Siri, I am still working") siri.cancel(.leaveWork) } } } extension CommandRealWorld { struct ExecutionTime { static let max: TimeInterval = 5 static let waiting: TimeInterval = 4 } func prepareTestEnvironment(_ execution: () -> ()) { /// This method tells Xcode to wait for async operations. Otherwise the /// main test is done immediately. let expectation = self.expectation(description: "Expectation for async operations") let deadline = DispatchTime.now() + ExecutionTime.waiting DispatchQueue.main.asyncAfter(deadline: deadline) { expectation.fulfill() } execution() wait(for: [expectation], timeout: ExecutionTime.max) } } class SiriShortcuts { static let shared = SiriShortcuts() private lazy var queue = OperationQueue() private init() {} enum Action: String { case leaveHome case leaveWork } func perform(_ action: Action, delay: TimeInterval = 0) { print("Siri: performing \(action)-action\n") switch action { case .leaveHome: add(operation: WindowOperation(delay)) add(operation: DoorOperation(delay)) case .leaveWork: add(operation: TaxiOperation(delay)) } } func cancel(_ action: Action) { print("Siri: canceling \(action)-action\n") switch action { case .leaveHome: cancelOperation(with: WindowOperation.self) cancelOperation(with: DoorOperation.self) case .leaveWork: cancelOperation(with: TaxiOperation.self) } } private func cancelOperation(with operationType: Operation.Type) { queue.operations.filter { operation in return type(of: operation) == operationType }.forEach({ $0.cancel() }) } private func add(operation: Operation) { queue.addOperation(operation) } } Output.txt: 実行結果
User: Hey Siri, I am leaving my home Siri: performing leaveHome-action User: Hey Siri, I am leaving my work in 3 minutes Siri: performing leaveWork-action User: Hey Siri, I am still working Siri: canceling leaveWork-action DoorOperation: Doors are closed via HomeKit. WindowOperation: Windows are closed via HomeKit. TaxiOperation: operation is canceled 
