转自 http://alldunne.org/2011/09/how-to-pause-or-end-a-uiview-animation-via-the-calayer/
-(void)pauseLayer:(CALayer*)layer
{
CFTimeInterval paused_time = [layer convertTime:CACurrentMediaTime() fromLayer:nil];
layer.speed = 0.0;
layer.timeOffset = paused_time;
}
-(void)resumeLayer:(CALayer*)layer
{
CFTimeInterval paused_time = [layer timeOffset];
layer.speed = 1.0f;
layer.timeOffset = 0.0f;
layer.beginTime = 0.0f;
CFTimeInterval time_since_pause = [layer convertTime:CACurrentMediaTime() fromLayer:nil] - paused_time;
layer.beginTime = time_since_pause;
}