装饰器模式：Swift 实现

装饰器 Decorator

The components and decorators are departed, we can add new components and decorators freely. And we can make different compositions(with different decorations) to be different products.
组件模块和装饰器是分开的，我们可以自由地添加组件和装饰器。我们也可使用不同的组合形式（基于不同装饰方式），来得到不同的产品。

// Base interface between classes.
protocol Component {
    
    var text: String? {
     get set}
    func execute()
}

// Concrete Component
class Bike: Component {
    
    var text: String?
    func execute() {
    
        text = "bike"
    }
}

// Concrete Component
class Moto: Component {
    
    var text: String?

    func execute() {
    
        text = "moto"
    }
}

// Base Decorator
class BaseDecorator: Component {
    
    var text: String?
    
    private var c: Component
    func execute() {
    
        c.execute()
        
        if let t = c.text {
    
            text = "<" + t + ">"   // Here the text is the property of the current concrete decorator.
        } else {
    
            text = "<>"
        }
    }    
    
    func getText() -> String {
    
        if let s = text {
    
            return s
        } else {
    
            return ""
        }
    }
    func setText(_ str: String?) {
    
        text = str
    }
    func getC() -> Component {
    
        return c
    }
    // inject an instance
    init(_ c: Component) {
    
        self.c = c
    }
    
}

// Concrete Decorator
class PaintDecorator: BaseDecorator {
    
    override func execute() {
    
        super.execute()
        extra()
    }
    func extra() {
    
        let s = getText()
        setText("(Paint)\(s)(Paint)")
    }
}

// Concrete Decorator
class AttachTagDecorator: BaseDecorator {
    
    override func execute() {
    
        super.execute()
        extra()
    }
    func extra() {
    
        let s = getText()
        setText("(Tag)\(s)(Tag)")
    }
}

let a = Bike()
let tag = AttachTagDecorator(a)
let paint = PaintDecorator(tag)

paint.execute()                // (Paint)<(Tag)<bike>(Tag)>(Paint)
print(paint.getText())

let b = Moto()
let bpaint = PaintDecorator(b)
let bTag = AttachTagDecorator(bpaint)
bTag.execute()                // (Tag)<(Paint)<moto>(Paint)>(Tag)
print(bTag.getText())

上面代码中，组件有 Bike 和 Moto，装饰器有 AttachTagDecorator 和 PaintDecorator。对 Bike 先贴标签再绘图，而对 Moto 是先画图再贴标签，获得的结果中可以看出两个组件被包装的方式不一样。

// (Paint)<(Tag)<bike>(Tag)>(Paint)
// (Tag)<(Paint)<moto>(Paint)>(Tag)