問題
最近練習用Protocol-Oriented Programming實做可移動以及縮放的View，遇到一個很有趣的問題，假設有以下兩個protocol
protocol Draggable: class {
var view: UIView { get }
var initialLocation: CGPoint { get set }
}
extension Draggable where Self: UIView {
var view: UIView { get { return self } }
var parentView: UIView? { get { return self.view.superview } }
}
protocol Scalable: class {
var view: UIView { get }
}
extension Scalable where Self: UIView {
var view: UIView { get { return self } }
var parentView: UIView? { get { return self.view.superview } }
}然後實做一個View遵循這兩個protocol
public class VWView: UIView, Draggable, Scalable {
var initialLocation: CGPoint = CGPointZero
}這時候會發生compile error，告訴你
Type 'VWView' does not conform to protocol 'Scalable'
Type 'VWView' does not conform to protocol 'Draggable'
怎麼一回事呢？
原來是因為兩個protocol都有var view: UIView { get { return self } }的default implementations，編譯器不曉得該用哪一個，乾脆來一個編譯錯誤。
解法
其實解決方法很簡單，就是在class裡實做這個值，這樣編譯器就知道該用class裡面的這個了。
public class VWView: UIView, Draggable, Scalable {
var view: UIView { get { return self } }
var initialLocation: CGPoint = CGPointZero
}
