We often used __weak to prevent the retain cycle problem in a block,
but it occurs another issue that weak variable would be nil if the
object pointed originally by weak variable released during we use it in block.
To fix this , we declare one more variable in block to
to retain the weak variable:
__weak typeof(self) weakSelf = self;
[self.APIService fetchRequest:request withCompletionBlock:
^(NSArray *infoList){
typeof(self) strongSelf = weakSelf;
if(strongSelf){
// do the callback work
} else {
// do nothing
// It is still possible that strongSelf
// get nil from weakSelf
}
}];
NOTE:
According to the comment in stackoverflow,
Keep in mind the strong reference cycle may not be this straightforward. For example, if your controller has a strong reference to an object, that object has a strong reference to a completion block, and then the completion block strongly references your controller, you'll still have a retain cycle, just with 3 objects instead of 2
We should need to check retain cycle with more care by this reminder.