NSView淡入淡出

淡入动画：

[NSAnimationContext beginGrouping];

[[NSAnimationContext currentContext] setDuration:2.0];

[[theSuperview animator] addSubview:theView];

[NSAnimationContext endGrouping];

淡出动画：

[NSAnimationContext beginGrouping];

[[NSAnimationContext currentContext] setDuration:2.0];

[[theView animator] removeFromSuperview];

[NSAnimationContext endGrouping];