Using blocks to download image, freezing UI

Question

In my app I am downloading image using blocks but it is freezing my UI. I have one network class which contains method to download image,

-(void)downloadImageWithCompletionHandler:^(NSData *aData, NSError *error)aBlock;

I am calling above method in my view controller to download image. So once the image is downloaded I am using NSData to show in image view. The network class method uses NSURLConnection methods to download the image.

   [[NSURLConnection alloc] initWithRequest:theURLRequest delegate:self];

Once the data download is complete I am calling completion handler block of the view controller.

But I am not sure why my UI is freezing? Can anyone help me find where I am doing wrong?

Thanks in advance!

do it asynchronously -> move the request to be called in a background thread — Julian, Aug 05 '15 at 12:32
This method of NSURLConnection will not download data in background? Then whats the use of delegates methods? — Nuzhat Zari, Aug 05 '15 at 12:33
see this for great tutorial on sessions: http://www.raywenderlich.com/51127/nsurlsession-tutorial — Teja Nandamuri, Aug 05 '15 at 12:35
It will download data asynchronously https://developer.apple.com/library/prerelease/ios/documentation/Cocoa/Reference/Foundation/Classes/NSURLConnection_Class/index.html#//apple_ref/doc/uid/20001697-SW9. — Nuzhat Zari, Aug 05 '15 at 12:36
You should use asynchronous downloading of images like this http://stackoverflow.com/questions/15668160/asynchronous-downloading-of-images-for-uitableview-with-gcd — Joe Hallenbeck, Aug 05 '15 at 12:37
Please check the link I have posted above. NSURLConnection also allows asynchronous download. — Nuzhat Zari, Aug 05 '15 at 12:40

score 1 · Answer 1 · answered Aug 05 '15 at 13:44

- (void) setThumbnailUrlString:(NSString *)urlString
{
    NSString *url= [NSString stringWithFormat:@"%@",urlString];
    //Set up Request:
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc]init];
    [request setURL:[NSURL URLWithString:url]];

    NSOperationQueue *queue=[[NSOperationQueue alloc] init];
    if ( queue == nil ){
        queue = [[NSOperationQueue alloc] init];
    }
    [NSURLConnection sendAsynchronousRequest:request queue:queue completionHandler:^(NSURLResponse * resp, NSData     *data, NSError *error)
     {
         dispatch_async(dispatch_get_main_queue(),^
                        {
                            if ( error == nil && data )
                            {
                                UIImage *urlImage = [[UIImage alloc] initWithData:data];
                                _headImageView.image=urlImage;
                                _backgroundImageView.image=urlImage;
                            }
                        });
     }];
}

score 0 · Answer 2 · answered Aug 05 '15 at 14:01

You need to download the image in background thread to avoid freezing the UI thread.There is a simple demo to achieve this.

- (void)downloadImageWithCompletionHandler:(void(^)(NSData *aData, NSError *error))aBlock {

    NSURLRequest *theURLRequest = nil;  // assign your request here.
    NSOperationQueue *mainQueue = [NSOperationQueue mainQueue];
    [NSURLConnection sendAsynchronousRequest:theURLRequest queue:mainQueue completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {
        // UIThread.
        aBlock(data,connectionError);
    }];
}

how to call this method.

[self downloadImageWithCompletionHandler:^(NSData *aData, NSError *error) {
    // get UIImage.
    UIImage *image = [UIImage imageWithData:aData];
}];

score 0 · Accepted Answer · answered Oct 05 '15 at 09:56

0

I figured out the problem. Problem was not in the block or using NSUrlConnection method, it is working properly. Problem was, I was saving data in file once I download it. This operation was happening on main thread which was blocking the UI.

answered Oct 05 '15 at 09:56

Nuzhat Zari

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Using blocks to download image, freezing UI

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