Question

I’m working on some CoreAnimation stuff. A navigation controller with a few view controllers. And the view controllers have UISCrollViews for different “pages” of a “brochure.” On each page, there might be some animation that gets triggered when the user flips to that page.

I was trying something like this (for one-shot animations).

void (^animationBlock)() =
  ^{
    static bool alreadyTriggered = NO;
    if(alreadyTriggered)
      return;

    [CATransaction begin];
    [CATransaction setCompletionBlock:
     ^{
       alreadyTriggered = YES;
     }];

    // Do me some animations...

    [CATransaction commit];
  };

  NSMutableDictionary* pageBlocks = [[NSMutableDictionary alloc] init];
  [pageBlocks setObject:[animationBlock copy] forKey:<animation's name>];
  [self.animationBlocks setObject:pageBlocks forKey:<some page number>];

  [pageBlocks release];
  [animationBlock release];

“animation’s name” and “some page number” are placeholders for the sake of explanation (they are arbitrary NSString literals).

And the code that triggers these animations is:

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
  int pageNumber = floor(self.scrollView.contentOffset.x / self.scrollView.frame.size.width);
  NSMutableDictionary* pageBLocks = [self.animationBlocks objectForKey:[NSString stringWithFormat:@"page%i",pageNumber]];
  [CATransaction begin];
  for(id key in pageBLocks)
    ((void (^)())[pageBLocks objectForKey:key])();
  [CATransaction commit];
 }

So far so good, only that If I pop the brochure from the navigation controller (aka calls dealloc on the brochure) and then push it in again, the static bool is still set.

My thoughts:

– am I retaining the block?
I don’t know.. I’m calling release after adding it (with copy) to the dictionary and also the brochure’s dealloc method calls release on the dictionary.

– am I keeping another copy of the static bool somewhere?
My first bool is allocated when I declare the block as static within the scope of a method.. well depends on Objective-C’s activation record scheme which I haven’t looked into. But assuming so, that copy is gone when releasing the object on popViewcOntroller. And another copy of it from invoking copy on the block when adding it to the dictionary should be released when the dictionary is killed?

am I retaining the whole brochure object itself? I didn’t completely get it from the Apple docs, but they say if I access an instance variable by reference I retain self. I tried releasing self from inside the block and everything keeps running just fine…?

Answer 1

Make the block return a value and then decide whether or not to remove the block from the dictionary.

// ...
    BOOL shouldRemove = block();

    if (shouldRemove) {
        [pageBlocks removeObjectForKey:key];
    }
// ...

---

Let’s test the static variable

@interface TestClass : NSObject
@end

@implementation TestClass

- (void(^)(void))block;
{
    return [[^{

        static BOOL staticBOOL = NO;

        NSLog(@"%d", staticBOOL);

        staticBOOL = YES;

    } copy] autorelease];
}  

@end

int main(int argc, char *argv[]) {
    NSAutoreleasePool *p = [[NSAutoreleasePool alloc] init];

    TestClass *test1 = [[TestClass alloc] init];
    test1.block();
    test1.block();

    TestClass *test2 = [[TestClass alloc] init];
    test2.block();
    test2.block();

    [p release];
}

This outputs

#=> 2012-04-23 00:43:38.501 Untitled[8380:707] 0
#=> 2012-04-23 00:43:38.503 Untitled[8380:707] 1
#=> 2012-04-23 00:43:38.503 Untitled[8380:707] 1
#=> 2012-04-23 00:43:38.504 Untitled[8380:707] 1

How do we solve the problem? I would probably remove the object from the dictionary once it had been executed like this

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView;
{
    NSInteger pageNumber = floor(self.scrollView.contentOffset.x / self.scrollView.frame.size.width);
    NSMutableDictionary *pageBlocks = [self.animationBlocks objectForKey:[NSString stringWithFormat:@"page%i", pageNumber]];

    [CATransaction begin];

    for (id key in [pageBlocks copy]) {
        void (^block)(void) = [pageBlocks objectForKey:key];
        if (block) {
            block();
        }
        [pageBlocks removeObjectForKey:key];
    }

    [CATransaction commit];
}