iOS arc和mrc下重写set方法

ARC下
@property (nonatomic, weak/assign) id delegate;
相当于

• (void)setDelegate:(id< MyDelegate >)delegate
  {
  if (_delegate != delegate) {
  _delegate = delegate;
  }
  }

MRC下
@property (nonatomic, strong/retain/copy) id< MyDelegate > delegate;
相当于

• (void)setDelegate:(id< MyDelegate >)delegate
  {
  if (_delegate != delegate) {
  [_delegate release];
  _delegate = [delegate retain/copy];
  }
  }