Correct way to handle CGImageRef in containers under ARC - ios

Question

I am new to incorporating arc in my projects. I am trying to understand __bridge and its little friends so that I can properly cast my CGImageRef's when adding and removing them from containers.
I am getting a "Potential leak of an object stored…" on one of my lines. Here is the basic cycle of my code:
CGImageRef renderedRef = CGBitmapContextCreateImage(UIGraphicsGetCurrentContext());
[_array addObject: (__bridge_transfer id)renderedRef];//_array is an iVar
then somewhere down the road I do this:
CGImageRef iRef = (__bridge_retained CGImageRef)array[0];
//then I do something fancy with iRef
//at the end of the method, I get "Potential leak of an object stored…"
//I have no idea what to do
//I've tried CGImageRelease(iRef); but it still doesn't change the warning.
Can someone shed some light on this? Also, I have tried just using __bridge but that doesn't make a difference.
Edit 1:
I expanded the analyzer results and followed what was happening. It was because I was using iRef in a method like so: [self doSomethingFancy:iRef]; and in that method, iRef was being retained but not released. So that fixes the warning, but I'm still a bit puzzled.
I am not quite clear on when to use the various __bridge casts. Under ARC, does the following increase the reference count?
CGImageRef iRef = (__bridge CGImageRef)array[0];
Also, at some point if I tell my _array iVar to removeAllObjects, will that decrement their reference counts properly?

Answer 1

// This WILL NOT increment the image's retain count.
CGImageRef iRef = (__bridge CGImageRef)array[0];
// This WILL increment the image's retain count.
CGImageRef iRef = (__bridge_retained CGImageRef)array[0];
Since __bridge_retained increments the retain count, you need to decrement the retain count at some point later. Because __bridge_retained acts like CFRetain, Apple created a __bridge_retained wrapper called CFBridgingRetain that reminds you of this:
// This WILL increment the image's retain count.
CGImageRef iRef = CFBridgingRetain(array[0]);
Whenever you see CFRetain or CFBridgingRetain, you know you need to release the object at some point later.
Similarly, you can use CFBridgingRelease instead of __bridge_transfer to decrement the retain count of a CF object. Example:
[_array addObject:CFBridgingRelease(renderedRef)];
You can use CFBridgingRelease to balance a CFRetain or a CFBridgingRetain. It returns an id that ARC manages.
A normal NSMutableArray retains each of its elements. You can tell it to become empty:
[_array removeAllObjects];
When you do this, it will release each of its elements, balancing the retain it performed on each element.
Thus there is no memory leak in this code:
CGImageRef image = CGImageCreate(...);
[_array addObject:CFBridgingRelease(image)];
[_array removeAllObjects];
or in this code:
CGImageRef image = CGImageCreate(...);
[_array addObject:CFBridgingRelease(image)];
CGImageRef image2 = CFBridgingRetain([_array lastObject]);
[_array removeLastObject];
CGContextDrawImage(gc, rect, image2);
CGImageRelease(image2);