Cocoa-Touch: performSelectorOnMainThread: weird behavior + crash

Question

I have a situation where I'm lazy loading images from the www.
It's a list of items, when one item is tapped, a detail view is pushed to a nav controller.

In that detail view the item has an image, which first is a default image, and I want to start loading it's image from a URL.

So what I do is create an object which once initialized detaches a new thread which in turn loads the content and afterwards notifies my view that the data is loaded:

// in MyLoader:
- (MyLoader *)initWithUrl:(NSURL *)url requester:(id)requester {
    self.url = url;
    self.requester = requester; // both are nonatomic, retain properties
    [self performSelectorInBackground:@selector(loadIt) withObject:nil];
}

- (void)loadIt {
    NSAutoreleasePool *arp = [[NSAutoreleasePool alloc] init];
    NSData *data = [NSData dataWithContentsOfURL:url];
    [requester performSelectorOnMainThread:@selector(dataReady) withObject:data waitUntilDone:YES;
    [arp release];
}

// in MyRequester:
- (void)somewhere {
    MyLoader *loader = [[[MyLoader] alloc] initWithUrl:someUrl requester:self] autorelease];
    // then I retain loader somewhere, it's more complicated but I have verified that it's properly retained.
}

A few notes:

1. First I thought there might be a problem with some of the variables. I put a breakpoint right before performSelectorOnMainThread and confirmed that data and requester were both OK.

2. Then I thought it was caused by passing the NSData across the threads, so I changed withObject:nil. It still crashes.

3. When I further investigated, the crash was very strange. I specified waitUntilDone:YES, I've placed a breakpoint in the requester's dataReady. But the performSelectorOnMainThread call returns (it reaches the breakpoint after it) while not reaching the breakpoint inside dataReady. BTW, - (void)dataReady:(NSData*)'s body for now only contains int x = 1; (to place a breakpoint on). Also, I've tried setting waitUntilDone:NO, it still crashes.

4. The selector isn't performed (the breakpoint is not reached), but the crash happens a short while after the call.

Does anyone have any idea what's wrong?

This is obvious, but just to be clear, if I just comment out the [requester performSelectorOnMainThread... part, it doesn't crash.

Also, here's a stack trace, but it's not helpful at all.

#0  0x00a71004 in ___TERMINATING_DUE_TO_UNCAUGHT_EXCEPTION___ ()
#1  0x93436e3b in objc_exception_throw ()
#2  0x0028aca6 in __NSThreadPerformPerform ()
#3  0x00a098e1 in CFRunLoopRunSpecific ()
#4  0x00a08c48 in CFRunLoopRunInMode ()
#5  0x0005a78d in GSEventRunModal ()
#6  0x0005a852 in GSEventRun ()
#7  0x0168a003 in UIApplicationMain ()
#8  0x000028d4 in main (argc=1, argv=0xbffff100) at /Users/myName/Document/appName/main.m:14

Answer 1

You have:

[requester performSelectorOnMainThread:@selector(dataReady) withObject:data waitUntilDone:YES;

should be:

[requester performSelectorOnMainThread:@selector(dataReady:) withObject:data waitUntilDone:YES;

notice: @selector(dataReady:) (with colon) Since you're passing an argument to the method, it's presumed data ready is defined something like:

- (void) dataReady:(NSData *)theData ...