UIImages /UIButtons behave differently every other time program is compiled - ios

Question

I'm making a simple flashcard application. Here's a partial screenshot of one of my app storyboards:
It contains one UIImageView ("turnImage") on top of two buttons ("yesButton", "noButton"), and an additional third button which covers the whole view and overlaps with all the other elements (invisible in this screenshot). When this view is originally displayed, the images on the two buttons are set to nil and the UIImageView is set to display a "tap to change" graphic (with the third invisible button actually catching the tap):
[yesButton setImage:nil forState:UIControlStateDisabled];
[noButton setImage:nil forState:UIControlStateDisabled];
turnImage.image = [UIImage imageNamed:#"card-flip-01.png"];
yesButton.enabled = NO;
noButton.enabled = NO;
When the image is tapped (or the user touches anywhere else on the screen), the image changes to the "correct" and "incorrect" buttons, from which the user can pick whether or not they knew the word (not shown in these screenshots):
turnImage.image = nil;
yesButton.enabled = YES;
noButton.enabled = YES;
[yesButton setImage:[UIImage imageNamed:#"card-answer-correct-01.png"] forState:UIControlStateNormal];
[noButton setImage:[UIImage imageNamed:#"card-answer-wrong-01.png"] forState:UIControlStateNormal];
The above screenshots show the normal behaviour. However, I only get this behaviour on every other compilation - the rest of the time, the initial screen (which is supposed to display only the "tap to change" graphic) shows all three images, and the "tap to change" graphic is stretched differently from normal:
Aside for the graphical glitch, the program works fine: tapping anywhere gets the user to the next screen, where the two buttons are displayed correctly.
I get this both in the simulator and when run on a device. In the simulator, the behaviour changes every other time that the program is run; resetting the simulator content and settings does not affect this. On a device, the behaviour changes whenever I tell Xcode to run it on the device. If I unplug the device from a computer and start/shut down the application without changing its code, the behaviour does not change. (It is the combination of these factors that's making me assume the change in behaviour is linked to compilations.)
I tried closing down Xcode and starting it again, as well as rebooting the computer. These had no effect. I'm running the most recent version of Xcode.
The above code snippets aren't exactly the same as in my program: they've been simplified a bit for readability. There shouldn't be any functional changes, but just to be sure, here's the original code as well:
- (void)viewDidLoad{
// I've cut irrelevant stuff that happens before
[yesButton setImage:nil forState:UIControlStateDisabled];
[noButton setImage:nil forState:UIControlStateDisabled];
flipped = NO;
turnImage.image = [UIImage imageNamed:#"card-flip-01.png"];
[self setResponseNavigationButtons:flipped];
}
- (void) setResponseNavigationButtons:(BOOL)status{
if (status){
turnImage.image = nil;
[yesButton setImage:[UIImage imageNamed:#"card-answer-correct-01.png"] forState:UIControlStateNormal];
[noButton setImage:[UIImage imageNamed:#"card-answer-wrong-01.png"] forState:UIControlStateNormal];
} else {
turnImage.image = [UIImage imageNamed:#"card-flip-01.png"];
[yesButton setImage:nil forState:UIControlStateNormal];
[noButton setImage:nil forState:UIControlStateNormal];
}
yesButton.enabled = status;
noButton.enabled = status;
}
setResponseNavigationButtons is called with YES when changing to the other view.

Answer 1

D'oh, found the error. While rebuilding the project for the Nth time, I finally noticed the warning I had missed before:
Warning: Multiple build commands for output file ... card-flip-01.png
Apparently I had two different images with the same name in my resources, and the compiler would alternate between which one it would grab. Renaming the wrong one fixed the problem - it was transparent, and was letting the yes/no images show through.