iOS-关键帧动画

//Animate using keyframe animation        [UIView animateKeyframesWithDuration:[self transitionDuration:transitionContext] delay:0.0 options:0 animations:^{            [UIView addKeyframeWithRelativeStartTime:0.0 relativeDuration:0.15 animations:^{                //90 degrees (clockwise)                snapshot.transform = CGAffineTransformMakeRotation(M_PI * -1.5);            }];            [UIView addKeyframeWithRelativeStartTime:0.15 relativeDuration:0.10 animations:^{                //180 degrees                snapshot.transform = CGAffineTransformMakeRotation(M_PI * 1.0);            }];            [UIView addKeyframeWithRelativeStartTime:0.25 relativeDuration:0.20 animations:^{                //Swing past, ~225 degrees                snapshot.transform = CGAffineTransformMakeRotation(M_PI * 1.3);            }];            [UIView addKeyframeWithRelativeStartTime:0.45 relativeDuration:0.20 animations:^{                //Swing back, ~140 degrees                snapshot.transform = CGAffineTransformMakeRotation(M_PI * 0.8);            }];            [UIView addKeyframeWithRelativeStartTime:0.65 relativeDuration:0.35 animations:^{                //Spin and fall off the corner                //Fade out the cover view since it is the last step                CGAffineTransform shift = CGAffineTransformMakeTranslation(180.0, 0.0);                CGAffineTransform rotate = CGAffineTransformMakeRotation(M_PI * 0.3);                snapshot.transform = CGAffineTransformConcat(shift, rotate);                _coverView.alpha = 0.0;            }];