iOS开发——View的透明属性hidden、alpha、opaque

Hidden、Alpha、Opaque的区别

在iOS中，每一个View都有Hidden、Alpha、Opaque三个关于透明的属性，官方文档介绍以下：

1. @property(nonatomic) CGFloat alpha;
This value affects only the current view and does not affect any of its embedded subviews.Changes to this property can be animated.
2. @property(nonatomic, getter=isHidden) BOOL hidden;
The default value is NO.
A hidden view disappears from its window and does not receive input events. It remains in its superview’s list of subviews, however, and participates in autoresizing as usual.Hiding a view with subviews has the effect of hiding those subviews and any view descendants they might have. This effect is implicit and does not alter the hidden state of the receiver’s descendants.
3. @property(nonatomic, getter=isOpaque) BOOL opaque;
This property provides a hint to the drawing system as to how it should treat the view.If set to YES, the drawing system treats the view as fully opaque, which allows the drawing system to optimize some drawing operations and improve performance. If set to NO, the drawing system composites the view normally with other content. The default value of this property is YES.
You should always set the value of this property to NO if the view is fully or partially transparent.

大概意思以下：

1. alpha
   • 这个属性只能影响当前视图，不能连带影响子视图
   • 能够当作动画进行动态改变
   • alpha = 0 时仍旧接收事件，可是这个操做比Hidden开销大
2. hidden
   • 默认为NO，就是显示状态
   • hidden的视图再也不接收事件
   • hidden的视图仍然在父视图的子视图列表里，并且响应自适应autoresizing的事件
   • hidden的视图全部子视图也会被Hidden并且它们的Hidden属性不会被改变
3. opaque
   • 若是opaque设置为YES，那么视图会被当作全视图来对待，系统会重绘整个视图
   • 若是opaque设置为NO，那么系统会减小开销，以其中的内容来断定重绘的视图
   • 若是把视图的背景色设置为透明那个，那么opaque最好设置为NO，减小开销