Cocoa/iPhone app, centering a label in a UIView

Question

What's the best way to center a label in a UIView? If you do something such as

UILabel *myLabel = [[UILabel alloc] initWithFrame:CGRectMake(view.frame.origin.x / 2, view.frame.origin.y / 2, 50.0, 50.0)];

Then you're setting the origin point of the label to the center of the view. The best bet would be to set the center of the view to that point using the center property. So I tried using the following code:

UIView *aView = [[[UIView alloc] initWithFrame:[[UIScreen mainScreen] applicationFrame]] autorelease];
aView.backgroundColor = [UIColor darkGrayColor];
CGRect frame = aView.frame;

UILabel *aLabel = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, 125.0f, 30.0f)];
[aLabel setCenter:CGPointMake(frame.origin.x / 2, frame.origin.y / 2)];

That yields a label which is pretty much outside the bounds of the view in the upper left hand corner.

Thanks.

Answer 1

To position any child horizontally centered in a parent you would calculate its position like so;

childX = (parentWidth - childWidth) / 2

(This also applies to height).

Answer 2

The main thing you are doing wrong is taking half the origin values, rather than half the sizes

However, you don't need to even calculate that in this case - just do something like the following:

UIView *aView = [[[UIView alloc] initWithFrame:[[UIScreen mainScreen] applicationFrame]] autorelease];
aView.backgroundColor = [UIColor darkGrayColor];

UILabel *aLabel = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, 125, 30)];
aLabel.center = aView.center;

(note, you don't need to force those coordinates to floats - in this case writing them as ints seems more readable).

Also, it's a matter of style - but since you're already using property syntax (aView.backgroundColor) you may as well use it for the center property too :-)