Merge pull request qunten#55 from daviyang35/patch-2

消除三目运算符歧义，便于理解

Author: qunten

8-显式动画/显式动画.md

@@ -120,7 +120,7 @@ self.colorLayer.backgroundColor = color.CGColor;
 {
 
  //set the from value (using presentation layer if available)
- animation.fromValue = [layer.presentationLayer ?: layer valueForKeyPath:animation.keyPath];
+ animation.fromValue = [(layer.presentationLayer ? layer.presentationLayer : layer) valueForKeyPath:animation.keyPath];
  //update the property in advance
  //note: this approach will only work if toValue != nil 
  [CATransaction begin];