How does UIScrollView change the frame, but not the bounds? How does UIScrollView work?

Question

I understand the difference between frames and bounds I think, bounds are to the view's local coordinate system, while frames are to the superviews.

With scrollviews however, I'm a little confused.

When I have a UIImageView in a UIScrollView and pinch to zoom it larger, it seems the frame grows larger (width and height) increases the width and height of the corresponding UIImage as well.

However, it seems like the bounds of the UIImageView don't change at all.

Why is this? How do scroll views work in regards to all of this? I've read multiple questions and checked the documentation but this explanation is eluding me.

How does zooming in a UIScrollView affect its contents? Does it just change the frame of the zooming view but somehow not the bounds?

Answer 1

See the explanation in my book :

The scroll view zooms by applying a scaling transform to the scalable view; therefore the frame of the scalable view is scaled as well. Moreover, the scroll view is concerned to make scrolling continue to work correctly: the limits as the user scrolls should continue to match the limits of the content, and commands like scrollRectToVisible:animated: should continue to work the same way for the same values. Therefore, the scroll view automatically scales its own contentSize to match the current zoomScale. (You can actually detect this happening by overriding setContentSize in a UIScrollView subclass: you can see the scroll view adjusting its own content size as you zoom.)

Basically (though that is not quite clear from the above quotation) we have no business concerning ourselves with the frame of the scalable view - or any view - that has a non-identity transform applied to it, which is exactly the case here. That fact is made very clear by the Apple documentation on UIView. Thus you should not be looking at the frame - only the transform. The frame value that your are reading changes purely as a side-effect of the transform change.

As for the bounds of the scalable view - obviously the bounds do not change; that is the whole point of how a transform works. It maintains a constant center and bounds , so that subviews and drawing continue to operate correctly within the frame-of-reference coordinates despite the transform. My book talks you through an understanding of this as well.

On the other hand, the scroll view's own bounds can certainly change their origin, not least because that is exactly and identically what it means for a scroll view to scroll. Scrolling is a change of bounds origin, plain and simple. And this would not be surprising during zooming, because, as I just said, the content size has changed, so the content may be repositioned in order to keep displaying it coherently as you zoom.