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我正在尝试学习swift中的计算属性..并且知道我需要setter来设置计算属性的值.我正试图但卡住了..请帮助我如何设置区域中的值与setter属性...和如果你能告诉我如何使用setter属性以及何时使用它会很棒

 class ViewController: UIViewController {
        var width : Int = 20
        var height : Int = 400
        var area: Int{
            get{
                return width * height
            }set(newarea){
                area = newarea*10
          //these line gives me an warning and area is not set
            }
        }

        override func viewDidLoad() {
            super.viewDidLoad()
            println("\(width)")
            println("\(height)")
            println("\(area)") 
         //  gives an error while setting value to computed properties...       area = 5000
         //  for that we need getter and setter properties in area...
            area = 490
            println("the new area …