冬季促销!
Swift 命令模式讲解和代码示例
命令是一种行为设计模式， 它可将请求或简单操作转换为一个对象。
此类转换让你能够延迟进行或远程执行请求， 还可将其放入队列中。
复杂度：
流行度：
使用示例： 命令模式在 Swift 代码中很常见。 大部分情况下， 它被用于代替包含行为的参数化 UI 元素的回调函数， 此外还被用于对任务进行排序和记录操作历史记录等。
识别方法： 命令模式可以通过抽象或接口类型 （发送者） 中的行为方法来识别， 该类型调用另一个不同的抽象或接口类型 （接收者） 实现中的方法， 该实现则是在创建时由命令模式的实现封装。 命令类通常仅限于一些特殊行为。
以下示例可在
Swift Playgrounds
上使用。
感谢
Alejandro Mohamad 创建了Playground版本。
概念示例
本例说明了命令设计模式的结构并重点回答了下面的问题：
- 它由哪些类组成？
- 这些类扮演了哪些角色？
- 模式中的各个元素会以何种方式相互关联？
了解该模式的结构后， 你可以更容易地理解下面基于真实世界的 Swift 应用案例。
Example.swift: 概念示例
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: 执行结果
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)
真实世界示例
Example.swift: 真实世界示例
import Foundation
import XCTest
class DelayedOperation: Operation {
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 {
override func main() {
print("\(self): Windows are closed via HomeKit.")
}
override var description: String { return "WindowOperation" }
}
class DoorOperation: DelayedOperation {
override func main() {
print("\(self): Doors are closed via HomeKit.")
}
override var description: String { return "DoorOperation" }
}
class TaxiOperation: DelayedOperation {
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: 执行结果
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
