Simple threading: performSelectorInBackground Use

Yep, you are right, my typo. Yet the code still does not work: it crashes on device and on simulator, it starts the activity indicator only after 'doLenghtyProcessing' is done.

'-Aside from that it looks like you're starting the activityIndicator where you'd want to stop it, ... '

Hmm, my understanding is that I start 'doLenghtyProcessing' in background thread and call the main thread to animate activity indicator, right?

- (IBAction)doStuff:(id)sender {
 [self performSelectorInBackground:@selector(doInBackground) withObject:nil];
}
 
- (void)doInBackground {
 NSAutoreleasePool * pool; 
 pool = [[NSAutoreleasePool alloc] init]; 
 [self doLengthyProcessing];
 
 [self performSelectorOnMainThread:@selector(animateIndicator) withObject:nil waitUntilDone:NO];
 [pool drain]; 
}
 
- (void)animateIndicator {
 [self.activityIndicator startAnimating];
}
 
- (void)doLengthyProcessing {
 // ...
}

Thanks for all your comments guys.
So I made changes, now it seems to work in simulator, but still crashes on device.

- (IBAction)doStuff:(id)sender {
 [self.indicator startAnimating]; // ok, start activity indicator before spawn 
 // a new thread.
 [self performSelectorInBackground:@selector(doInBackground) withObject:nil];
}
 
- (void)doInBackground {
 NSAutoreleasePool * pool; 
 pool = [[NSAutoreleasePool alloc] init]; 
 [self doLengthyProcessing]; //I am putting some image processing code here 
//just for the test. Since 'doInBackground' is running in background thread, any
//function calls inside 'doInBackground' should run in the background thread, 
//and should return before 'doInBackground' does, am I wrong?
 
 [self performSelectorOnMainThread:@selector(stopIndicator) withObject:nil waitUntilDone:NO];
 [pool drain]; 
}
 
- (void)stopIndicator {
 [self.activityIndicator stopAnimating];
}
 
- (void)doLengthyProcessing {
 // ...
}

I found a previous discussion thread about this issue:
https://devforums.apple.com/message/24220#24220

I know I have lots to learn about concurrency/multi-threading stuff. I can't find any iPhone sample code on this: simply putting an activity indicator so users can tell device is not frozen but something is going on.

Message was edited by: syy

It seems like my 3G iPhone doesn't like to do number crunching in a thread spawned by 'performSelectorInBackground'. The more I strip down my 'doLengthyProcess' function, the better the device behaves.

Strange though, I called the same function ('doLengthyProcess') before in an UIImagePickerControllerDelegate method:
- (void)imagePickerController:(UIImagePickerController *)picker
didFinishPickingImage:(UIImage *)image
editingInfo:(NSDictionary *)editingInfo

It shows an activity indicator while the function is processed, and things just work. I assume this delegate method is doing multi-threading, and my 'doLengthyProcess' function must be kosher:thread safe, etc.

Well, if anybody care about this problem. Here is what I did to fix the issue using Operation Object/Queue:

- (IBAction)doStuff:(id)sender {
 [self.indicator startAnimating]; 
 NSInvocationOperation *genOp = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(doInBackground) object:nil];
 
MyAppDelegate *appDelegate = (MyAppDelegate *)([UIApplication sharedApplication].delegate); 
 
[appDelegate.myQueue addOperation:genOp];
[genOp release];
}
 
- (void)doInBackground {
 NSAutoreleasePool * pool; 
 pool = [[NSAutoreleasePool alloc] init]; 
 [self doLengthyProcessing]; 
 
 [self performSelectorOnMainThread:@selector(stopIndicator) withObject:nil waitUntilDone:NO];
 [pool drain]; 
}
 
- (void)stopIndicator {
 [self.activityIndicator stopAnimating];
}
 
- (void)doLengthyProcessing {
 // ...
}

I don't know this is the best way but it seems to work.