CoreAnimation旋转变换动画方向

Question

有没有一种简单的方法来指定图像在目标 C 中应该顺时针还是逆时针旋转？我有一个带有指针的速度计（一种反映汽车价格的设计） - 它工作正常，除非指针向下旋转并从屏幕转到另一侧更接近。我希望它始终逆时针旋转以转到较低的数字，顺时针旋转以转到较高的数字。这是我的代码段：

// figure out the minimum and maximum prices shown on the 
// speedometer
float min = [_a.text floatValue] * 1000;
float max = [_g.text floatValue] * 1000;

// set calibration between number labels to be small or large
int cal = SMALL_CALIBRATION;
if ([_stickerPriceSetting floatValue] >= TRIGGERPRICE)
    cal = LARGE_CALIBRATION;

// set an animation duration of 1.5 seconds for the needle rotation
[UIView beginAnimations:nil context:NULL];
[UIView setAnimationDuration:1.5];

// low prices (off speedometer)
if (price < (min - cal)) 
    _needle.transform = CGAffineTransformMakeRotation(- M_PI/12); 

// price too high (off speedometer)
else if (price  > (max + cal)) 
    _needle.transform = CGAffineTransformMakeRotation(M_PI * 13/12); 

// need to have needle point to the price
else {
    float delta = (max - min);
    float price_point = price - min;
    _needle.transform = CGAffineTransformMakeRotation(price_point/delta * M_PI);
}

 [UIView commitAnimations];

Answer 1

您可以通过动画图层属性 transform.rotation 来完成此操作：

// figure out the minimum and maximum prices shown on the 
// speedometer
float min = [_a.text floatValue] * 1000;
float max = [_g.text floatValue] * 1000;

// set calibration between number labels to be small or large
int cal = SMALL_CALIBRATION;
if ([_stickerPriceSetting floatValue] >= TRIGGERPRICE)
    cal = LARGE_CALIBRATION;

CGFloat angle;
if (price < min - cal)
    angle = -M_PI / 12;
else if (price > max + cal)
    angle = M_PI * 13 / 12;
else
    angle = M_PI * (price - min) / (max - min);

[UIView animateWithDuration:1.5 animations:^{
    [(id)_needle.layer setValue:[NSNumber numberWithFloat:angle]
        forKeyPath:@"transform.rotation"];
}];

如果导入 QuartzCore/QuartzCore.h，您将不需要 (id) 演员。

Answer 2

我的车速表也有同样的问题。在对变换属性进行动画处理时，您无法指定旋转方向。它总是会走最短的路。我通过做 2 个动画来解决这个问题，以防角度大于 180 度（第二个动画在第一个动画完成后开始）。您必须注意正确的动画时间，以便动画流畅。