为什么CALayer的“ transform.scale”值与它的转换比例不匹配？

Question

想像您要缩放图层，然后稍后再读出该缩放比例（例如：在更改缩放比例的动画中）。

let desiredScale = 0.5
layer.transform = CATransform3DMakeScale(desiredScale, desiredScale, 1)
let readScale = layer.value(forKeyPath: "transform.scale") as? NSNumber)?.floatValue
print("scale: \(readScale)") // "scale: 0.6666666"

使用value(forKeyPath:)读出的刻度值将接近，但不太正确！

Answer 1

这可能是因为读出的值与整个3D比例有关（考虑x，y和z）。专门读取x或y比例尺将返回正确的值：

let desiredScale = 0.5
layer.transform = CATransform3DMakeScale(desiredScale, desiredScale, 1)
let wholeScale = layer.value(forKeyPath: "transform.scale") as? NSNumber)?.floatValue
let xScale = layer.value(forKeyPath: "transform.scale.x") as? NSNumber)?.floatValue
print("scale: \(wholeScale)") // "scale: 0.6666666"
print("scale: \(xScale)") // "scale: 0.5"