Question about dealloc

[gfxcc]

why you implement dealloc like this?

• (void)dealloc
  {
  [self.scrollView removeObserver:self forKeyPath:@"contentOffset"];
  [self.scrollView removeObserver:self forKeyPath:@"contentInset"];
  self.scrollView = nil;
  }
• (void)willMoveToSuperview:(UIView *)newSuperview
  {
  [super willMoveToSuperview:newSuperview];
  if (!newSuperview) {
  [self.scrollView removeObserver:self forKeyPath:@"contentOffset"];
  [self.scrollView removeObserver:self forKeyPath:@"contentInset"];
  self.scrollView = nil;
  }
  }

why not just removeObserver in (void)willMoveToSuperview like this:

• (void)willMoveToSuperview:(UIView *)newSuperview
  {
  [super willMoveToSuperview:newSuperview];
  [self.scrollView removeObserver:self forKeyPath:@"contentOffset"];
  [self.scrollView removeObserver:self forKeyPath:@"contentInset"];
  self.scrollView = nil;
  }

Another question,
when the function dealloc will be called?

Thanks