CODEWITHSUNDEEP
iphoneobjective-cmultithreading
aahrens
aahrens

Reputation: 5590

Perform Method With int Return Value in Background Thread

I'm trying to speed up my application performance by performing calculations in background threads but I'm having trouble doing this. Originally I had been using

[self performSelectorInBackground:@selector(calculateValue:) withObject:[words objectAtIndex:row]];

which was fine when my selector was a void method. However, I'm trying to do something similar to but obviously the below code isn't valid.

int value = [self performSelectorInBackground:@selector(calculateValue:) withObject:[words objectAtIndex:row]];

Any help is greatly appreciated.

Updated

Here is the route I'm currently going. I don't know how to call back to the main thread to send the updated value from computeWordValue to my cellForRowAtIndexPath

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    int value = [self performSelectorInBackground:@selector(calculateWordValue:) withObject:[wordsSection objectAtIndex:row]];
    NSString *pointValue = [[NSString alloc] initWithFormat:@"Point:%d",value];
    cell.pointLabel.text = pointValue;
 }

-(void)calculateWordValue:(NSString *)word {

  [self performSelectorOnMainThread:@selector(computeWordValue:) withObject:word waitUntilDone:YES];
}

-(int)computeWordValue:(NSString *)word {

  return totalValue; //This will be a randomly generated number
}

Upvotes: 0

Views: 2989

Answers (2)

Richard J. Ross III
Richard J. Ross III

Reputation: 55573

Heres a way I use to do it: 

 -(void) calculateValue:(id) obj
  {
       // calculate value
       [self performSelectorOnMainThread:@selector(didFinishCalculating:) withObject:[NSNumber numberWithInt:value]];
  }

 -(void) didFinishCalculating:(NSNumber *) val
 {
       // do what you need to do here
 }

This really doesn't solve your problem I don't think, but it should at least give you a starting point.

UPDATE:

Your new code shows me that you don't really need to perform this in the background, just cache the value using an NSDictionary or something. Here's an example:

 -(int) calculateValue:(id) obj
 {
        if ([valuesCache objectForKey:obj] == nil)
        {
            // calculate value
            [valuesCache setObject:[NSNumber numberWithInt:result] forKey:obj];
            return result;
        }
        else
        {
            return [[valuesCache objectForKey:obj] intValue];
        }
 }

Upvotes: 1

Marco Masser
Marco Masser

Reputation: 1086

There's no way -performSelectorInBackground: ... could return the value of the method you're calling because it actually returns before the selector was even executed. That selector will be executed on a background thread asap.

The solution is handling the result of your method asynchronously, as Richard pointed out (the method in his answer should be - (void)didFinishCalculating:(NSNumber*)val, because only objects can be passed in -performSelector: ... calls):

  • Perform your selector on a background thread
  • Call your result handler method on the main thread. You should do that on the main thread in just about any case because some things in Mac OS X and iOS are designed to just run on the main thread, like GUI updates.

Upvotes: 1

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