iOS check if delegate exists before call method

Question

I write iOS app and use imageStore library to lazy load images and cache them in memory. (https://github.com/psychs/imagestore)

On ViewController I create imagestore instance:

imageStore = [ImageStore new];
imageStore.delegate = self;

When image loaded successfuly, imagestore call delegate method

- (void)imageStoreDidGetNewImage:(ImageStore*)sender url:(NSString*)url

that doing reloadData on tableview to redraw cells. All works good. But there is the problem: if ViewController didUnload (go back in navigation controller) and image loaded, application finish with crash, because imagestore call method of unloaded ViewController.

I try to do following: 1) in ViewController I place this code in viewDidUnload section:

imageStore.delegate = nil;
imageStore = nil;

2) In imageStore I added checking for nil:

if(delegate != nil) {
  ...call delegate method
}

It works, but periodically app crash anyway.

Solution

Try putting this code on dealloc section.

imageStore.delegate = nil;
imageStore = nil;

In the same way the if clause is not necessary because any call to an nil object is ignored by the application, so if you have something like this:

id delegate = nil;    
[delegate callAnyMethod];

has no effect in your application behavior, in other hand if the call of the method delegate is optional you should asure that delegate responds to selector, something like this should do the trick:

if([delegate conformsToProtocol:@protocol(yourProtocolName)] && [delegate respondsToSelector:@selector(imageStoreDidGetNewImage:url:)]) {
       [delegate imageStoreDidGetNewImage:imageStore url:url];
}

Cheers!