财产获取者和安置者 [英] Property getters and setters

问题描述

通过这个简单的类，我得到了编译器 warning

试图修改/访问 x 在它自己的设置器/获取器中

，当我这样使用时：

  var p：point = Point（）
 px = 12 
  

我得到一个EXC_BAD_ACCESS。

  class Point {
 
 var x：Int { 
 set {
x = newValue * 2 //错误
} 
 get {
 return x / 2 //错误
} 
} 
 // ... ... 
} 
  

解决方案

Setter和Getters应用于计算的属性；这样的属性在实例中没有存储-来自getter的值应从其他实例属性中计算出来。在您的情况下，没有要分配的 x 。支持ivars。您不能-您需要某事来备份计算出的属性。试试这个：

  class Point {
 private var _x：Int = 0 // _x-> backingX 
 var x：Int {
 set {_x = 2 * newValue} 
 get {return _x / 2} 
} 
} 
  

具体地说，在Swift REPL中：

  15> var pt = Point（）
 pt：Point = {
 _x = 0 
} 
 16> pt.x = 10 
 17> pt 
 $ R3：点= {
 _x = 20 
} 
 18> pt.x 
 $ R4：Int = 10 
  

With this simple class I am getting the compiler warning

Attempting to modify/access x within its own setter/getter

and when I use it like this:

var p: point = Point()
p.x = 12

I get an EXC_BAD_ACCESS. How can I do this without explicit backing ivars?

class Point {

    var x: Int {
        set {
            x = newValue * 2 //Error
        }
        get {
            return x / 2 //Error
        }
    }
    // ...
}

解决方案

Setters and Getters apply to computed properties; such properties do not have storage in the instance - the value from the getter is meant to be computed from other instance properties. In your case, there is no x to be assigned.

Explicitly: "How can I do this without explicit backing ivars". You can't - you'll need something to backup the computed property. Try this:

class Point {
  private var _x: Int = 0             // _x -> backingX
  var x: Int {
    set { _x = 2 * newValue }
    get { return _x / 2 }
  }
}

Specifically, in the Swift REPL:

 15> var pt = Point()
pt: Point = {
  _x = 0
}
 16> pt.x = 10
 17> pt
$R3: Point = {
  _x = 20
}
 18> pt.x
$R4: Int = 10

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