CODEWITHSUNDEEP
iosnsoperationqueuecancellation
siva krishna
siva krishna

Reputation: 1179

NsoperationQueue Cancel all operations is not cancelled until it finishes the operation

In my view i have image view, the data for image view comes form Url, the images are around 1-3 MB . If the user Swipes then i want to load the next image, Every thing works fine if swiped slowly, But when i swiped Fastly I want to Cancel the previous operation and start with new url.

For Ex. if user swipes 4 times if the operations for 2nd and 3rd images is in the middle, i want to cancel those and start downloading 4 th image

But now In the place of 4th image I’m getting first 2nd image follows 3rd and then 4 th image appears.

Here is my sample code 

- (void)handleSwipeLeft:(UISwipeGestureRecognizer *)aSwipeGestureRecognizer {


    [BackgroundOperation cancelAllOperations];  // To cancel previous one 


    [self performSelector:@selector(LoadImage) withObject:nil afterDelay:0.1];


}

-(void)LoadImage
{
    BackgroundOperation=[[NSOperationQueue alloc]init];  


    imgvww.image=[UIImage imageNamed:@"loader.png"]; // Place holder    till download finishes 


   [BackgroundOperation addOperationWithBlock:^
     {
         UIImage *img=[UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:[[self.ItemDetails objectAtIndex:0] objectForKey:@"ImageUrl"]]]];  // Getting data from URL 

         [[NSOperationQueue mainQueue] addOperationWithBlock:^{


             imgvww.image=img;  //Adding to image view after completion 


         }];

     }];
 }

Thank you.

Upvotes: 10

Views: 14415

Answers (3)

Yariv Nissim
Yariv Nissim

Reputation: 13343

Canceling the operation will only update its isCancelled property to YES.
To be able to cancel the operation, you should do the following: 

NSBlockOperation * op = [NSBlockOperation new];
__weak NSBlockOperation * weakOp = op; // Use a weak reference to avoid a retain cycle
[op addExecutionBlock:^{
    // Put this code between whenever you want to allow an operation to cancel
    // For example: Inside a loop, before a large calculation, before saving/updating data or UI, etc.
    if (weakOp.isCancelled) return;

    // Do something..
];

Upvotes: 3

user4151918
user4151918

Reputation:

Canceling an operation merely sets its isCancelled Flag to true.

You're responsible for checking to see if your operation has been cancelled, before it starts to run (or while it is running, if it is a long-running operation).

You can check if your operation is cancelled within an operation block but I'd recommend subclassing, instead of using a block.

Upvotes: 6

DBoyer
DBoyer

Reputation: 3122

Calling cancelAllOperations on an NSOperationQueue will simply call cancel on each of its operations. If the NSOperation does not override cancel then its never going to get cancelled.

There is no concept of cancelling an NSBlockOperation once it has started. The block simply executes and thats that.

If you want to specify special cancel behaviour (like cancelling your image download) you need to subclass NSOperation and override cancel.

There are many examples of this in AFNetworking or SDWebImage

To cancel an image download you need to wrap up an NSURLSesionDownloadTask in an NSOperation and then override cancel to call cancel on the NSURLSesionDownloadTask

Upvotes: 7

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