Question

I’m programming an iPhone app using Objective-C.

Here’s the error Xcode gives me:

error: assignment of read-only variable 'prop.149'

The code:

// Create CATransition object
CATransition *transition = [CATransition animation];
// Animate over 3/4 of a second
transition.duration = 0.75;
// using the ease in/out timing function
transition.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut];
// Set transition type
transition.type = kCATransitionPush;
// Next line gives error: assignment of read-only variable 'prop.149'
transition.subtype = (flipsideView.hidden == YES ? kCATransitionFromRight : kCATransitionFromLeft);

What does the error mean, and how do I fix it?

Answer 1

Not sure exactly why, but the compiler is not able to deduce the evaluated type of the result of the ternary operator. Simply adding an explicit cast seems to work:

transition.subtype = (NSString *)(flipsideView.hidden == YES ? kCATransitionFromRight : kCATransitionFromLeft);

I’d file this as a compiler bug.