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Re: How to properly flip a UIView's coordinate system
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Re: How to properly flip a UIView's coordinate system


  • Subject: Re: How to properly flip a UIView's coordinate system
  • From: Kyle Sluder <email@hidden>
  • Date: Sat, 21 Sep 2013 14:53:23 -0700

> On Sep 21, 2013, at 1:38 PM, Markus Spoettl <email@hidden> wrote:
>
> Hi,
>
>  I have a UIView on iOS that shares (a lot of) code with an NSView I have on OSX. That code relies on a LLO (lower left origin) coordinate system and it's not an option to change that.

You’re not gonna like where this conversation is going.

> So I transform the coordinate system of the view by setting
>
> [self setTransform:CGAffineTransformMakeScale(1, -1)];
>
> in the UIView variant's -initializer. That works well, it effects all areas of view code the right way, including drawing and touch-events. Just what I want. Or so it seems.
>
> Something must be wrong, because in some situations, when I animate the view's frame (using UIView's -animateWithDuration::: methods), the system seems to have trouble handling this view properly.

This is because the frame of a view is meaningless once you've applied a transform.

Unlike the Mac, on iOS the frame and bounds do not define two separate coordinate systems. A UIView's size in its superview’s coordinate space is defined by its bounds.width. So flippedness doesn't really make sense on iOS.

> So, it's the transform. Can I do something about that?

You can fix your code to be origin-agnostic.

--Kyle Sluder

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References: 
 >How to properly flip a UIView's coordinate system (From: Markus Spoettl <email@hidden>)

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