命令模式
简介
命令模式提供了一个对被调用方法的详情进行封装,以达到推迟调用或者可供不同元素调用的机制.使用命令模式的优点是1.可以执行撤销操作2.可以创造宏.
实现
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| protocol Command {
func execute();
}
class GenericCommand<T> : Command {
private var receiver: T;
private var instructions: T -> Void;
init(receiver:T, instructions: T -> Void) {
self.receiver = receiver; self.instructions = instructions;
}
func execute() {
instructions(receiver);
}
class func createCommand(receiver:T, instuctions: T -> Void) -> Command {
return GenericCommand(receiver: receiver, instructions: instuctions);
} }
class Calculator {
private(set) var total = 0; private var history = [Command]();
func add(amount:Int) {
addUndoCommand(Calculator.subtract, amount: amount);
total += amount;
}
func subtract(amount:Int) {
addUndoCommand(Calculator.add, amount: amount);
total -= amount;
}
func multiply(amount:Int) {
addUndoCommand(Calculator.divide, amount: amount);
total = total * amount;
}
func divide(amount:Int) {
addUndoCommand(Calculator.multiply, amount: amount);
total = total / amount;
}
private func addUndoCommand(method:Calculator -> Int -> Void, amount:Int) {
self.history.append(GenericCommand<Calculator>.createCommand(self,
instuctions: {calc in
method(calc)(amount);
})); }
func undo() {
if self.history.count > 0 {
self.history.removeLast().execute();
self.history.removeLast();
} }
}
|
这是在swift上的实现方法.
- 在代码中出现了一个这样的参数:
method:Calculator -> Int -> Void.注意,这是在swift中实例方法作为参数的写法.通常,swift类中调用实例方法的写法是:
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| ...
let printerObject = printer()
printerObject.printMessage("hello")
...
|
但是,一种替代的写法是:
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| ...
let printerObject = printer()
printer.printMessage(printerObject)("hello")
...
|
所以,method:Calculator -> Int -> Void实际上是Calculator的实例对象中加减乘除方法的类型.
- 在使用calcluator的每一个操作前,创建一个
command对象并且添加到history中.创建command对象时,需要两个参数,分别为receiver和instruction.receiver作为实例方法的第二种写法的第一个参数.instruction是一个block,使用了block的值捕获特性取得了method和amount.
