Understanding UIView’s transform property (final)

In previous part, we talked about 4 properties of UIView and relationships between them. This part we cover operations with transform and how to persist UIView information. Let’s go!

transform operations

transform is type of CGAffineTransform and it represents a matrix used for affine transformation. Speaking of the matrix and and affine transformation, it is beyond context of this article, but if you wonder and want to learn more, this wiki would be a good start.

In brief, transform includes 6 matter values a, b, c, d, x and y as the figure below.

a and d are for scale, x and y are for translation and a, b, c and d are for rotation. However, usually, we use only scale and rotation of transform as for translation, we can simply use center property to position a view in its superview.

When transform is used for only one transformation, it is straightforward to extract that information, e.g. factor of scale or angle of rotation. In most of cases, however, we use combination of scale and rotation, that makes transformation extraction more tangled. Thanks to mathematics, we already have equations for this and only need to convert to code.


Extract

Right below is snippet for scale extraction

func scaleOf(transform: CGAffineTransform) -> CGPoint {
let xScale = sqrt(transform.a * transform.a + transform.c * transform.c)
let yScale = sqrt(transform.b * transform.b + transform.d * transform.d)

return CGPoint(x: xScale, y: yScale)
}

and for rotation.

func rotationOf(transform: CGAffineTransform) -> CGFloat {
return CGFloat(atan2(transform.b, transform.a))
}

Restore

In reverse, CoreGraphics provides handy functions to create CGAffineTransform from transformation information.

view.transform = CGAffineTransformMakeRotation(angle_in_radian) // Create CGAffineTransform of rotation transformation

view.transform = CGAffineTransformMakeScale(xScale, yScale) // Create CGAffineTransform of scale transformation

Because order of transformation matters, in case of combination of scale and rotation, we should persist the whole transform, i.e., a, b, c, d, x and y values, and use it to restore itself.

view.transform = CGAffineTransform(a: a, b: b, c: c, d: d, tx: x, ty: y);

Persistence

When comes with UIView information persistence, especially with transform is applied, note following things:

  • frame is invalid so we shouldn’t use it to position or measure view.
  • Though frame is invalid, there is way to calculate actual values and use them for positioning or sizing of view.
  • center is recommended to position view.
  • bounds.size is recommended to measure view.

So what information we need to persist? They are position in superview, size and transformation. With transformation, we can persist transform as a whole or individual one.

Position

The best way is to persist center, as center is independent of transform, it is safe to position view regardless of transform.

let persistedCenter = view.center

However, in some scenarios, frame.origin may be useful, then we need calculation on restoration to convert frame.origin back to center. It will be there later.

let persistedPosition = frame.origin

Size

Both bounds.size and frame.size cannot be used for size persistence, because they do not reflect correct view’s size. bounds.size is original value before transformation. And frame.size is size of boundary view, not actual view.

So how? The answer is to use bounds.size and scale transformation and it is pretty simple.

let scale = scaleOf(view.transform)
let persistedSize = CGSize(width: view.bounds.size.width * scale.x, height: view.bounds.size.height * scale.y)

If we count actual view’s size in persistence, transform we persist should contain only rotation transformation. This is due of restore phrase later. Otherwise, restoration will get unnecessary calculation, now or later is of your choice. Then I choose now and it also makes more sense to persist real view’s size than its original one.

Transformation

How to persist position and size affects how to do with transformation, actually, only size does. As stated right above, if we persist actual view’s size, rotation is the only thing we need for transformation.

let persistedRotation = rotationOf(view.transform)

How about original view’s size? So we have to persist the whole transform by using its values.

let a = view.transform.a
let b = view.transform.b
let c = view.transform.c
let d = view.transform.d

With these values, it is easy to restore transform as talked above.

Restore

Make assumption that we are persisting actual view’s size, its rotation and center, now it is time to restore the view from that.

let view = UIView(frame: CGRect(x: 0, y: 0, width: persistedSize.width, height: persistedSize.height))
view.transform = CGAffineTransformMakeRotation(persistedRotation)
view.center = persistedCenter

How about frame.origin for position? Here we are.

let view = UIView(frame: CGRect(x: 0, y: 0, width: persistedSize.width, height: persistedSize.height))
view.transform = CGAffineTransformMakeRotation(persistedRotation)
view.center = CGPoint(x: persistedPosition.x + view.frame.size.width / 2, y: persistedPosition.y + view.frame.size.height / 2)

Conclusion

There are still many more to talk about UIView, however, with this article, transform property is not scary anymore, huh? From here, we can keep learning layer.anchorPoint which is also relevant to transform, animation with transform and other 3 properties, etc. Happy learning!

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