Effective Objective-C 2.0 编写高质量iOS与OS X代码 在既有类中使用关联对象存放自定义数据

static void *MyAlertViewKey = “MyAlertViewKey”;
- (void) askUserQuestion
{
     UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@“Question” message:@“What?” delegate:self cancelButtonTitle:@“cancel” otherButtonTitles:@“Continue”,nil];

     void (^block)(NSInteger) = ^(NSInteger buttonIndex){
          if(buttonIndex == 0){
               [self doCancel];
          } else {
               [self doContinue];
          }
     };
     
     objc_setAssociatedObject(alert, MyAlertViewKey, block, BJC_ASSOCIATION_COPY);
     [alert show];
}

- (void)alertView:(UIAlertView*)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{
     void (^block)(NSInteger) = objc_getAssociatedObject (alertView, MyAlertViewKey);
     block(buttonIndex);
}