Question

This is written in my viewDidLoad method

 NSOperationQueue *queue = [NSOperationQueue new];
    NSInvocationOperation *op = [[NSInvocationOperation alloc] 
                                        initWithTarget:self
                                        selector:@selector(downloadImage) 
                                        object:nil];
    [queue addOperation:op]; 

// The the rest in other methods;

- (void)downloadImage{
    NSData* imageData = [[NSData alloc] initWithContentsOfURL:[NSURL URLWithString:person.picURL]];
    UIImage* image = [[UIImage alloc] initWithData:imageData] ;


    [self showImage: image];
}

I have tried adding the following codes above [self showImage: image]; and i ended up with an exception.

1.) [self performSelectorOnMainThread:@selector(showImage:) withObject:image waitUntilDone:NO];

2.) [self performSelectorInBackground:@selector(showImage:) withObject:image];

// This is the showImage code.

- (void)showImage:(UIImage *)img {

    if (img != nil) 
    {    
        myImageView = [[UIImageView alloc] initWithImage:img];

        myImageView.frame = CGRectMake(20, 20, 132, 124);

        [scrollView addSubview:myImageView];

        [pictureOfPerson setImage:img];        
    }
}

I am trying to Asynchronously download the image and cache it. The image gets downloaded Asynchronously but i am not sure if it gets cached.

1.) How to cache the image and use it in when the view loads again

2.) While the image is downloading, what if i click on another view. Then i need to stop the download. How can i write this code. I know that i have to write it in the viewDidDissapear method.

3.) Is my code correct. Have i missed anything or is there a better approach to do this? if so tutorial or some sample code please

Answer 1

I’d use GCD to download the image. It’s much simpler to use than NSOperation. Here’s an example:

UIImage *personPicture;
personPicture = [self.imageCache objectForKey:person.picURL];
if (!personPicture) {
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSData *imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:person.picURL]];
        personPicture = [UIImage imageWithData:imageData];
        [self.imageCache setObject:imageData forKey:person.picURL];
        dispatch_async(dispatch_get_main_queue(), ^{
            [self showImage:personPicture];
        });
    });
}
else {
    [self showImage:personPicture];
}

You could use a class property like an NSMutableDictionary to store the UIImage data and associate it with your URL. If it’s present, use it, if not, download the image.

-EDIT-
Adding the code to handle caching.