drawRect not being called in my subclass of UIImageView - iphone

Question

I have subclassed UIImageView and tried to override drawRect so I could draw on top of the image using Quartz 2D. I know this is a dumb newbie question, but I'm not seeing what I did wrong. Here's the interface:
#import <UIKit/UIKit.h>
#interface UIImageViewCustom : UIImageView {
}
- (void)drawRect:(CGRect)rect;
#end
And the implementation:
#import "UIImageViewCustom.h"
#implementation UIImageViewCustom
- (id)initWithFrame:(CGRect)frame {
if (self = [super initWithFrame:frame]) {
}
return self;
}
- (void)drawRect:(CGRect)rect {
// do stuff
}
- (void)dealloc {
[super dealloc];
}
#end
I set a breakpoint on drawRect and it never hits, leading me to think it never gets called at all. Isn't it supposed to be called when the view first loads? Have I incorrectly overridden it?

Answer 1

It'll only get called if the view is visible, and dirty. Maybe the problem is in the code that creates the view, or in your Nib, if that's how you're creating it?
You'll also sometimes see breakpoints failing to get set properly if you're trying to debug a "Release" build.
I somehow missed the first time that you're subclassing UIImageView. From the docs:
Special Considerations
The UIImageView class is optimized to
draw its images to the display.
UIImageView will not call drawRect: in a
subclass. If your subclass needs
custom drawing code, it is recommended
you use UIView as the base class.
So there you have it. Given how easy it is to draw an image into your view using [UIImage drawInRect:], or by using CALayer, there's probably no good reason to subclass UIImageView anyway.

Answer 2

Try to add
Edit:
- (id)initWithFrame:(CGRect)frame{
if (self = [super initWithFrame:frame]) {
[self setClearsContextBeforeDrawing:YES];//add this line also
}
return self;
}
- (void)setNeedsDisplay{
[self setNeedsDisplayInRect:self.frame];
}
into your code.
hope this helps.
Thanks,
madhup

Answer 3

Not directly answering your question, but may solve your problem:
Why do this with a subclass of UIImageView? Subclassing can always be problematic, especially for classes that aren't designed to be subclassed--and I bet UIImageView isn't. If you just want to draw stuff on top of an image, then create a view with a transparent background, draw whatever you want, and place it directly over the image.

Answer 4

Depending on your specific case, it could make sense to use a UIButton with a background image instead of a UIImageView. You could even set userInteractionEnabled to NO and the result would be indistinguishable from a UIImageView.
In this case your UIButton subclass would simply include:
- (void)drawRect:(CGRect)rect
{
if (kSomethingSpecial) {
[self setBackgroundImage:[UIImage imageNamed:#"RedBackground.png"]
forState:UIControlStateNormal];
}
}
Disclaimer - I would only recommend this if you have plans to use the image as a button at least some of the time. I can't wholeheartedly endorse using a UIButton as a workaround for this drawRect behaviour in UIImageView, as I'm sure Apple has its reasons for writing their API this way, like Mark referenced.