CODEWITHSUNDEEP
objective-cobjective-c-blocksnsoperation
Kevin Renskers
Kevin Renskers

Reputation: 5940

Cancel a completion handler block from running

I'm using a library that does some work in the background and then calls a completion handler. All really standard. 

[[LastFm sharedInstance] getInfoForArtist:@"Pink Floyd" successHandler:^(NSDictionary *result) {
    // Do stuff...
} failureHandler:nil];

I'm actually using this inside a tableview: in every cell (subclass) I get information about an artist and show it. This is also the problem: when the cell is moved off screen and reused for another artist, the successHandler for the previous artist can still be executed, resulting in labels and images that change multiple times in rapid succession.

My thought was to create a NSOperationQueue, add the getInfoForArtist call inside of it, and make sure it can be cancelled:

NSBlockOperation *operation = [[NSBlockOperation alloc] init];
__weak NSBlockOperation *weakOperation = operation;

[operation addExecutionBlock:^{
    [[LastFm sharedInstance] getInfoForArtist:mediaItem.artist successHandler:^(NSDictionary *result) {
        if (weakOperation.isCancelled) {
            return;
        }
        // Do stuff...
    } failureHandler:nil];
}];

[self.queue addOperation:operation];

The problem is that weakOperation is always null inside the successHandler. If I change it to be __block instead of __weak, weakOperation is the correct instance, but's its isCancelled state is always NO.

I am calling [self.queue cancelAllOperations]; at the correct time, when the cell is moved off screen.

So my question is, how can I prevent the successHandler from running after the cell was reused for another artist?

Upvotes: 1

Views: 505

Answers (1)

Christopher Pickslay
Christopher Pickslay

Reputation: 17772

The problem is that you're calling an asynchronous API. The lifetime of your operation is the call to getInfoForArtist:successHandler:, which probably returns immediately. By the time the asynchronous callback is executed, the operation has been disposed of, which is why the reference is nil inside the callback block.

By the time the successHandler is executed, there's no point in canceling the operation--you're not going to save network resources, and you may as well save the results of the lookup locally. The UI problem is that you shouldn't reference the cell (or its subviews, if you reuse them) directly. You might consider storing the results in a local NSDictionary keyed on either the NSIndexPath of the row, or the artist ID. Then, in your cellForRowAtIndexPath:, first check that dictionary before making the LastFM call. In the successHandler callback, you could iterate through the tableView's visibleCells and try to load the data from your dictionary.

Upvotes: 2

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