CALayer Resize很慢

Question

我在编写一些代码时出现了一些性能问题，这些代码是我为使用touch调整CALayer大小而编写的。它工作正常，但动画远远不够活泼，落后于触摸位置。

CGPoint startPoint;
CALayer *select;

- (CGRect)rectPoint:(CGPoint)p1 toPoint:(CGPoint)p2 {
     CGFloat x, y, w, h;
     if (p1.x < p2.x) {
         x = p1.x;
         w = p2.x - p1.x;
     } else {
         x = p2.x;
         w = p1.x - p2.x;
     }
     if (p1.y < p2.y) {
         y = p1.y;
         h = p2.y - p1.y;
     } else {
         y = p2.y;
         h = p1.y - p2.y;
     }
     return CGRectMake(x, y, w, h);
 }

 - (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
     UITouch *t1 = [[[event allTouches]allObjects]objectAtIndex:0];
     CGPoint loc = [t1 locationInView:self];
     startPoint = loc;
     lastPoint = CGPointMake(loc.x + 5, loc.y + 5);

     select = [CALayer layer];
     select.backgroundColor = [[UIColor blackColor]CGColor];
     select.frame = CGRectMake(startPoint.x, startPoint.y, 5, 5);
     [self.layer addSublayer:select];
 }

 - (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
     UITouch *t1 = [[[event allTouches]allObjects]objectAtIndex:0];
     CGPoint loc = [t1 locationInView:self];
     select.bounds = [self rectPoint:startPoint toPoint:loc]; 
 }

有没有更好的方法来实现这一目标？

Answer 1

滞后是因为您正在更改图层的bounds属性，这是一个可动画的属性。

使用CALayers（CA代表核心动画......）对动画属性的任何更改都将默认为动画。这称为隐式动画。默认动画需要0.25秒，因此如果您经常更新它，比如在处理触摸期间，这将加起来并导致可见的延迟。

要防止这种情况，您必须声明动画事务，关闭隐式动画，然后更改属性：

[CATransaction begin];
[CATransaction setDisableActions:YES];
layer.bounds = whatever;
[CATransaction commit];

Answer 2

Swift 3/4中接受的答案：

CATransaction.begin()
CATransaction.setDisableActions(true)
layer.bounds = whatever
CATransaction.commit()

值得一提的是，这也适用于.frame属性，例如当您调整AVPlayerLayer的大小时：

override func layoutSubviews() {
    CATransaction.begin()
    CATransaction.setDisableActions(true)
    playerLayer.frame = self.bounds
    CATransaction.commit()
}