The collection view will very likely have corresponding ivars for the animation
block and its completion handler. If that's the case, it should (shall) create
a copy of the anonymous animation (updates) and completion block when receiving
them in `performBatchUpdate:completion:`.
Now, `finish` is an "imported field" of the anonymous completion handler. The
copy operation of the anonymous completion handler will cause the block
`finish` to be copied as well. Since the outer block lives on the heap, the
imported fields of `finish` will live there, too. Effectively, imported fields
will be "copied" into the storage area of the outermost block.
It seems to me, that block `finish` should have a ref count of +1 within the
anonymous completion handler.
Vlad, could you please verify your observations and possibly create a simple
test class mimicking the behavior of the collection view (no animation, just a
dummy async method), e.g.
dispatch_block_t completion = ^{
NSLog(@"invoking completion");
};
dispatch_block_t updates = ^{
NSLog(@"invoking updates");
};
dispatch_block_t finish = ^{
NSLog(@"invoking finish");
if (completion) {
completion();
}
};
if (animated) {
self.testClass performDummyMethod:^{
updates();
} completion:^(BOOL finished) {
finish();
}];
When TestClass actually makes copies of the block when setting its ivars
`_updates` and `_completion`, it should work (IMO).
See also: "Block Implementation Specification"
<http://clang.llvm.org/docs/Block-ABI-Apple.html> (especially, "Imported const
copy of Block reference).
Andreas
On 09.08.2013, at 16:57, Vlad Alekseev <ippo...@me.com> wrote:
> Hey!
>
> I have a method where I update my collection view's layout parameter and want
> to have a completion block invoked when animation completes:
>
> - (void)transitionAnimated:(BOOL)animated
> completion:(dispatch_block_t)completion
> {
> dispatch_block_t updates = ^{
> self.layout.maximumScale = self.maximumScale;
> };
>
> dispatch_block_t finish = ^{
> if (completion) {
> completion();
> }
> };
>
> if (animated) {
> self.collectionView.userInteractionEnabled = NO;
> [self.collectionView performBatchUpdates:^{
> updates();
> } completion:^(BOOL finished) {
> self.collectionView.userInteractionEnabled = YES;
> finish();
> }];
> }
> else {
> updates();
> [self.layout invalidateLayout];
> finish();
> }
> }
>
> It works as expected if collection view contains some items. But it crashes
> if collection view is empty. And it crashes here:
>
> } completion:^(BOOL finished) {
> self.collectionView.userInteractionEnabled = YES;
> finish(); // ---- CRASH because finish == NULL
> }];
>
> Debugger says that finish is nil:
>
> (lldb) p finish
> (dispatch_block_t) $1 = <parent is NULL>
>
> What is going on with that block? Any ideas why it gets NULL-ified?
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