How to apply multiple transforms in Swift

问题:

I would like to apply multiple transforms to a UIView (or subclass of UIView), such as translate, rotate, and scale. I know that two transforms can be applied with CGAffineTransformConcat, but how do I do it if I have three or more transforms?

I have seen these questions:

• Applying multiple transforms to a UIView / CALayer
• Using Multiple CGAffineTransforms On Text Matrix

but these questions are asking something different, and the given answers just talk about applying two transforms with CGAffineTransformConcat. Also, they use Objective-C rather than Swift.

回答1:

You can apply multiple transforms by stacking them on top of each other.

var t = CGAffineTransform.identity
t = t.translatedBy(x: 100, y: 300)
t = t.rotated(by: CGFloat.pi / 4)
t = t.scaledBy(x: -1, y: 2)
// ... add as many as you want, then apply it to to the view
imageView.transform = t

Or more compactly (but not necessarily as readable):

imageView.transform = CGAffineTransform.identity.translatedBy(x: 100, y: 300).rotated(by: CGFloat.pi / 4).scaledBy(x: -1, y: 2)

This series of transforms produces the image on the right:

Thanks to this answer for teaching me how to do it.

Notes

• The order in which you apply the transforms matters. For example, if the transforms were done in the opposite order it would produce the following result.

  t = t.scaledBy(x: -1, y: 2)
  t = t.rotated(by: CGFloat.pi / 4)
  t = t.translatedBy(x: 100, y: 300)
  

See also

• CGAffineTransform Reference (docs)
• Transforms (docs)
• Swift: Translating and Rotating a CGContext, A Visual Explanation (iOS/Xcode)
• Demystifying CGAffineTransform

This answer has been tested with Swift 4

回答2:

In Swift 3, these have been replaced by functions on CGAffineTransform itself, which can be chained.

extension CGAffineTransform {
    public func translatedBy(x tx: CGFloat, y ty: CGFloat) -> CGAffineTransform
    public func scaledBy(x sx: CGFloat, y sy: CGFloat) -> CGAffineTransform
    public func rotated(by angle: CGFloat) -> CGAffineTransform
}

so for example

let transform = CGAffineTransform(scaleX: 1.0, y: 3.0).translatedBy(x: 12, y: 9).rotated(by: 17.0)