View and subviews after transformation

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dlsxor21c OP

Created Jan ’21

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Hello I'm reading View Programming Guide for iOS, and I have a question.

I'm just wondering what this part means.

The coordinate system of each subview builds upon the coordinate systems of its ancestors. So when you modify a view’s transform property, that change affects the view and all of its subviews. However, these changes affect only the final rendering of the views on the screen. Because each view draws its content and lays out its subviews relative to its own bounds, it can ignore its superview’s transform during drawing and layout.

I understand that modifying transform affects the subviews, but I don't know why this changes affects only final rendering and even ignores superview's transform.

Could you explain why it happens with example or other article?

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Answer 1

Claude31 OP

Jan ’21

That means that during the transform action, the subviews are not modified.

But at the end of transform, when everything is redrawn, the subviews are redrawn in the new coordinate system.

AFAIU, It is just as if, during transform, there were snapshots of the subviews (aka freeze) and they were unfrozen after the end of transform.

In fact, it is the same with animations. And doc states (bold is mine):

You typically modify the transform property of a view when you want to implement animations. For example, you could use this property to create an animation of your view rotating around its center point. You would not use this property to make permanent changes to your view, such as modifying its position or size a view within its superview’s coordinate space. For that type of change, you should modify the frame rectangle of your view instead.

So that implies:

it can ignore its superview’s transform during drawing and layout.

that during transform (animation), if the subviews redraw, they may ignore the transform

The best to perfectly understand this subtleties, is to experiment a lot by yourself.

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