Lets say you are not only downloading tweets (text) from the web, but actual chunks of raw data, such as images. You may have a method that returns a UIImage, such as: -(UIImage*)cachedImageForURL:(NSURL*)url{ //Preferably look for a cached image id cachedObject = [cachedImages objectForKey:url]; if (cachedObject == nil) { //set loading placeholder in our cache dict [cachedImages setObject:LoadingPlacedholder forKey:url]; //Create and queue a new image loading op ImageLoadingOp *operation = [[ImageLoadingOp alloc] initWithImageURL:url target:self action:@selector(didFinishLoadingImageWithResult:)]; [operationQueue addOperation:operation]; [operation release]; } else if(![cachedObject isKindOfClass:[UIImage class]]) { //were already loading the image, dont kick off another request cachedObject = nil; } return cachedObject; } Except notice this time the operation is not… Read More

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