效果图:

果冻效果.gif

这里自定义一个View(图中的蓝色视图)

• .h文件:

#import <UIKit/UIKit.h>

@interface BlockView : UIView

- (void)startAnimationFrom:(CGFloat)from to:(CGFloat)to;

- (void)completeAnimation;

@end

声明了两个方法,一个用于开始动画,另外一个用于通知视图已完成动画

• m 文件:

#import "BlockView.h"

@interface BlockView ()

@property (strong, nonatomic) CADisplayLink *displayLink;
@property (nonatomic) CGFloat from;
@property (nonatomic) CGFloat to;
@property (nonatomic) BOOL animating;
@end

@implementation BlockView

- (void)startAnimationFrom:(CGFloat)from to:(CGFloat)to {
    self.from = from;
    self.to = to;
    self.animating = YES;
    if (self.displayLink == nil) {
        self.displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(tick:)];
        
        [self.displayLink addToRunLoop:[NSRunLoop currentRunLoop]
                               forMode:NSDefaultRunLoopMode];
    }
}

- (void)completeAnimation {
    self.animating = NO;
    [self.displayLink invalidate];
    self.displayLink = nil;
}

- (void)tick:(CADisplayLink *)displayLink {
    [self setNeedsDisplay];
}

- (void)drawRect:(CGRect)rect {
    CALayer *layer = self.layer.presentationLayer;
    
    CGFloat progress = 1;
    if (!self.animating) {
        progress = 1;
    } else {
        progress = 1 - (layer.position.y - self.to) / (self.from - self.to);
    }
    
    CGFloat height = CGRectGetHeight(rect);
    CGFloat deltaHeight = height / 2 * (0.5 - fabs(progress - 0.5));
    NSLog(@"delta:%f", deltaHeight);
    
    CGPoint topLeft = CGPointMake(0, deltaHeight);
    CGPoint topRight = CGPointMake(CGRectGetWidth(rect), deltaHeight);
    CGPoint bottomLeft = CGPointMake(0, height);
    CGPoint bottomRight = CGPointMake(CGRectGetWidth(rect), height);
    
    UIBezierPath* path = [UIBezierPath bezierPath];
    [[UIColor blueColor] setFill];
    [path moveToPoint:topLeft];
    [path addQuadCurveToPoint:topRight controlPoint:CGPointMake(CGRectGetMidX(rect), 0)];
    [path addLineToPoint:bottomRight];
    [path addQuadCurveToPoint:bottomLeft controlPoint:CGPointMake(CGRectGetMidX(rect), height - deltaHeight)];
    [path closePath];
    [path fill];
}

@end

• 在动画开始的时候，初始化displayLink，指定tick方法;
• 动画结束的时候invalidate displayLink:
• 每一次调用tick时，我们需要根据当前的位置重绘边缘，所以只需调用setNeedsDisplay即可：
• 在drawRect中，我们计算当前动画的progress，然后进行绘制。

需要注意的是，我们需要通过self.layer.presentationLayer来获取动画过程中的位置信息。
另外,搭建视图时,我将自定义的View和控制器的View设置成了同样的背景色,而在绘制图层时填充了蓝色,这样才能看到果冻效果,如果忽略了背景色,可能会导致复制代码后运行起来看不出果冻效果,可以通过慢速动画发现,自定义View的上边缘会有黑色区域浮动.