View selector return value when debugging in Xcode

Question

I am running through an iPhone development tutorial and I have a weird bug that I am investigating using the debugger. I have the following code that checks to see if a object is in bounds. I am trying to see what the value of ball.center.x is at a certain point, but because center is a property accessor selector, I don't get the value when I hover over it in the Xcode debugger.

if (ball.center.x > self.view.bounds.size.width || ball.center.x < 0) {
    ballVelocity.x = -ballVelocity.x;
}

Is there a way to do this? I think I must just be missing something. I suppose I could update the code to assign the value to a variable that I could then watch in the debugger, but that seems like a sloppy work around to a common problem.

Thanks!

Answer 1

I'll assume center is an CGPoint.

In addition to the Xcode GUI debugger, there's also a gdb prompt available when you're debugging. Try this:

p* ball

or

p ball.center

Answer 2

and then there's also the venerable quick and dirty:

if (ball.center.x > self.view.bounds.size.width || ball.center.x < 0) {
    NSLog(@"ball center: %d",ball.center.x);
    ballVelocity.x = -ballVelocity.x;
}

Answer 3

Try this gdb command:

p (CGPoint)[ball center] 

I use this trick with UIView.frame, it works.

Answer 4

There is even easier way to do this than usingthe gdb console. From main menu choose:

Run -> Variables View -> View in expression window

And in the expression window's text field type:

(CGPoint)[ball center]

The main problem with expression windows showing the 'out of scope' error message is it almost never knows the return type of a function (or method), so it does not know how to present the returned variable. Apple gives us a clue what to do is such cases here under the 'Tips on using the Expressions window'.