iOS：CAShapeLayer 路径转换

我用过CAShape Layer在对路径进行基本变换之前 - 从较小的圆圈到较大的圆圈。足够好了，但后来我尝试将三角形变成圆形；它有效，但转变很奇怪。换句话说，从一种形状到另一种形状，在形成最终形状之前，它会“翻转”、“扭曲”。对于相同的形状，没有问题——从一个圆到一个圆——但是对于不同的形状，转换很奇怪。

我想知道这是否是预期的方式？或者是否有其他技巧或解决方法，我们可以使用以下方法将一个形状平滑地按比例转换为另一种不同的形状CAShape Layer（或者是否有其他方法可以做到这一点；不一定是CAShape Layer）？提前致谢。

通过仔细选择控制点等，您可能可以绘制一条绘制为三角形的路径，但其线段和控制点的数量与您要绘制的圆形相同。像这样（所有数值均假设 iPhone 屏幕，但要点是通用的）：

@implementation ViewController
{
    CAShapeLayer* shapeLayer;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    shapeLayer = [[CAShapeLayer alloc] init];

    CGRect bounds = self.view.bounds;
    bounds.origin.x += 0.25 * bounds.size.width;
    bounds.size.width *= 0.5;
    bounds.origin.y += 0.25 * bounds.size.height;
    bounds.size.height *= 0.5;

    shapeLayer.frame = bounds;
    shapeLayer.backgroundColor = [[UIColor redColor] CGColor];

    [self.view.layer addSublayer: shapeLayer];
    [self toCircle: nil];
}

CGPoint AveragePoints(CGPoint a, CGPoint b)
{
    return CGPointMake((a.x + b.x) * 0.5f, (a.y + b.y) * 0.5f);
}

- (IBAction)toCircle:(id)sender
{
    UIBezierPath* p = [[UIBezierPath alloc] init];

    [p moveToPoint: CGPointMake(80, 56)];
    [p addCurveToPoint:CGPointMake(144, 120) controlPoint1:CGPointMake(115.34622, 56) controlPoint2:CGPointMake(144, 84.653778)];
    [p addCurveToPoint:CGPointMake(135.42563, 152) controlPoint1:CGPointMake(144, 131.23434) controlPoint2:CGPointMake(141.0428, 142.27077)];
    [p addCurveToPoint:CGPointMake(48, 175.42563) controlPoint1:CGPointMake(117.75252, 182.61073) controlPoint2:CGPointMake(78.610725, 193.09874)];
    [p addCurveToPoint:CGPointMake(24.574375, 152) controlPoint1:CGPointMake(38.270771, 169.80846) controlPoint2:CGPointMake(30.191547, 161.72923)];
    [p addCurveToPoint:CGPointMake(47.999996, 64.574379) controlPoint1:CGPointMake(6.9012618, 121.38927) controlPoint2:CGPointMake(17.389269, 82.24749)];
    [p addCurveToPoint:CGPointMake(80, 56) controlPoint1:CGPointMake(57.729225, 58.957207) controlPoint2:CGPointMake(68.765656, 56)];
    [p closePath];

    [CATransaction begin];
    CABasicAnimation *pathAnimation = [CABasicAnimation animationWithKeyPath:@"path"];
    pathAnimation.duration = 3.f;
    pathAnimation.fromValue = (id)shapeLayer.path;
    pathAnimation.toValue = (id)p.CGPath;
    [shapeLayer addAnimation:pathAnimation forKey:@"path"];
    [CATransaction setCompletionBlock:^{
        shapeLayer.path = p.CGPath;
    }];
    [CATransaction commit];

    double delayInSeconds = 4.0;
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
    dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
        [self toTriangle: nil];
    });

}

- (IBAction)toTriangle: (id)sender
{
    UIBezierPath* p = [[UIBezierPath alloc] init];

    // Triangle using the same number and kind of points...
    [p moveToPoint: CGPointMake(80, 56)];
    [p addCurveToPoint: AveragePoints(CGPointMake(80, 56), CGPointMake(135.42563, 152))  controlPoint1:CGPointMake(80, 56) controlPoint2:AveragePoints(CGPointMake(80, 56), CGPointMake(135.42563, 152))];
    [p addCurveToPoint:CGPointMake(135.42563, 152) controlPoint1:AveragePoints(CGPointMake(80, 56), CGPointMake(135.42563, 152)) controlPoint2:CGPointMake(135.42563, 152)];
    [p addCurveToPoint:AveragePoints(CGPointMake(135.42563, 152), CGPointMake(24.574375, 152)) controlPoint1:CGPointMake(135.42563, 152) controlPoint2:AveragePoints(CGPointMake(135.42563, 152), CGPointMake(24.574375, 152))];
    [p addCurveToPoint:CGPointMake(24.574375, 152) controlPoint1:AveragePoints(CGPointMake(135.42563, 152), CGPointMake(24.574375, 152)) controlPoint2:CGPointMake(24.574375, 152)];
    [p addCurveToPoint: AveragePoints(CGPointMake(24.574375, 152),CGPointMake(80, 56)) controlPoint1:CGPointMake(24.574375, 152) controlPoint2:AveragePoints(CGPointMake(24.574375, 152),CGPointMake(80, 56)) ];
    [p addCurveToPoint:CGPointMake(80, 56) controlPoint1:AveragePoints(CGPointMake(24.574375, 152),CGPointMake(80, 56)) controlPoint2:CGPointMake(80, 56)];
    [p closePath];

    [CATransaction begin];
    CABasicAnimation *pathAnimation = [CABasicAnimation animationWithKeyPath:@"path"];
    pathAnimation.duration = 3.f;
    pathAnimation.fromValue = (id)shapeLayer.path;
    pathAnimation.toValue = (id)p.CGPath;
    [shapeLayer addAnimation:pathAnimation forKey:@"path"];
    [CATransaction setCompletionBlock:^{
        shapeLayer.path = p.CGPath;
    }];
    [CATransaction commit];

    double delayInSeconds = 4.0;
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
    dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
        [self toCircle: nil];
    });

}

Even with段数和控制点的数量相同，动画still看起来有点奇怪，可能不是你想要的。原因是 CA 似乎将所有线段和控制点一对一匹配，然后对所有点在它们之间进行线性插值。由于控制点和生成的路径之间的关系不是线性的（在本例中为三次），因此线性插值控制点位置不会导致路径以线性方式移动。如果运行此代码，您可以看到，当它转换时，三角形的一侧有奇怪的驼峰，其中圆形路径在圆弧中具有其他点。

更一般地说，期望 CA 以某种特定的方式在两条任意路径之间神奇地变形（而事实证明这种方式在外观上是理想的）是不合理的。即使努力手工构建这些路径，以便它们可以祈祷变形，它们仍然不像我想象的那样。

通过使用扁平路径（即由许多小直线而不是弯曲路径元素组成的路径）来实现所需的效果可能更合理。即使这看起来也很重要，因为您再次需要两条路径具有相同数量的线段，并且您必须构造这些线段，以便公共点是整个路径上正确数量的线段小路。

总而言之：这是一个相当复杂的问题，CoreAnimation 提供的简单/免费的解决方案不太可能满足您的需求。