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POWhy is it bad to re-use a CGRect for repositioning views?
 

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  1. POWhy is it bad to re-use a CGRect for repositioning views?
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    <p>I have a large number of views in my project where I do programmatic re-sizing and re-positioning of the subviews. I do this in <code>layoutSubviews</code>, using code like this:</p> <pre><code>CGRect rect = self.btnOne.frame; // change values for rect self.btnOne.frame = rect; // re-use same CGRect to reposition other views rect = self.btnTwo.frame; // change values self.btnTwo.frame = rect; // etc. ... </code></pre> <p>This type of code has always worked fine for me for a couple of years, now. I have recently added a bunch of code like this to a co-worker's project, and it's now starting to behave very strangely. For no reason I can figure out, the re-used <code>rect</code> object here now sometimes randomly changes its own values between when it's assigned the frame of a view and when it is re-assigned to the view's frame at the end of the block. The result is subviews that are in the incorrect position. It seems that the error usually takes the form of <code>rect.origin.x</code> somehow acquiring the value of <code>rect.origin.y</code> or vice versa.</p> <p>Even weirder, this problem only occurs on a real device; the exact same code running on the simulator does not have the problem.</p> <p>The fix is to use a uniquely-declared CGRect object for each subview, like so:</p> <pre><code>CGRect rect = self.btnOne.frame; // change values for rect self.btnOne.frame = rect; // declare new CGRect to reposition other views CGRect rect2 = self.btnTwo.frame; // change values self.btnTwo.frame = rect2; </code></pre> <p>When I make this change, everything goes back to working correctly. But <strong>I'm really interested in knowing why this happens, and especially knowing why it happens on a device but not on a simulator</strong>. It also seems to not happen until a co-worker makes changes to the view, although it's not clear to me how any change (not related to positioning of views) could do this.</p> <p>One other change is that the project the code was imported into is using ARC, whereas my original code was not.</p>
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    1. COThe line `self.btnOne.frame;` has no effect (and should give you a compiler warning). Is this your real code?
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      1. USrob mayoff
    2. COSince rect is on the stack, if you have an array elsewhere on the stack, it could be overwriting some part of rect if some code is writing past the end (or before the beginning) of the array (or other struct). You can put a watchpoint on the variable and see where it changes to figure out exactly what's causing it to change.
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      1. USuser1118321
    3. COCan you tell us if the views in your co-worker's project are affected by a transformation? Apple docs says this warning about the transform property: "If this property is not the identity transform, the value of the frame property is undefined and therefore should be ignored."
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      1. USTim Reddy
 

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