如果以后会检测到触摸，应否发布添加到视图的UIImageView？

Question

I've learnt that if you create an object, you own it and need to release it when you're done with it. In this case I create an UIImageView and add it to my view like this:

myImageView = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"image.png"]];
[myImageView setFrame:CGRectMake(10,10,100,100)];
[self.view addSubview:myImageView]; 
[myImageView release];

If I later on want to detect touch on myImageView like this in my touchEnded-method:

if([touch view] == myImageView){
    NSLog(@"TOUCHED!");
}

This works, but is this correct, since I now used myImageView after releasing it? And how do I release myImageView from the self.view that I added it to earlier?

Answer 1

The cardinal rule of Cocoa memory management is that you should retain those things you care about, and release those things you do not care about. There are a very small number of exceptions to prevent retain loops (delegates and datasources are never retained), but this is the rule you should follow.

In this case, if you are storing the image in an ivar, I would continue to retain it, regardless of the fact that its superview will always retain it, and so you don't "have" to. But if the view is removed from its superview, you will wind up with a dangling pointer, and then you will crash, so I code defensively against that by retaining. If you used an accessor here (which you should), then this would be automatic and much safer.

Apple has grown more consistent about this in iOS, switching their recommendation about IBOutlets. On Mac, you do not retain your IBOutlets, but in iOS, Apple explicitly instructs you to do so. This is similar to the situation you are discussing, and I agree with Apple's move towards a safer approach.

Answer 2

As long as your myUIImageView object has a retain count > 0, it will still exist and you can continue using it. When you first add it as a subview, it gets a retain message so it's retain count is likely 2. Then you send it release, so its retain count is reduced to 1. This means it still exists in memory. Now, if you sent it release again, or sent it removeFromSuperView then its retain count would be zero, and you'd lose it.

Answer 3

The behavior is erratic, sometimes you may see it works, sometimes you get a crash.

If you want to use that variable to point to your image view, retain the ivar (by using a retain property). This way you ensure that the image view is available for your controller class to use.

Answer 4

Assuming myUIImageView is an ivar of your custom UIView subclass, your code will work as long as your image view stays in his superview. The image view instance may be deallocated and you may end with an invalid pointer referencing a deallocated object. At best you crash.

According to memory management guidelines, you should consider your image view <-> custom uiview subclass relation as wether:

• strong reference. You own the image view (as you created it), and are responsible for retaining / releasing it
• weak reference. You don't own the object, thus keeping a reference to it may be dangerous.

in your case, it's probably a strong reference. your myUIImageView ivar should be a nonatomic retained property of your object.

Answer 5

If you need to access your UIImage at some point in the future, you need to retain it.

How you do this is at your discretion, but you should not rely on the UIView to retrain your objects for you. The situation you've created works for now, but it's fragile.