开发IOS时经常会使用到UIAlertView类，该类提供了一种标准视图，可向用户展示警告信息。当用户按下按钮关闭该视图时，需要用委托协议（Delegate PRotocol）来处理此动作，但是要设置好这个委托协议，就得把创建警告视图和处理按钮动作的代码分开。

UIAlertView *inputAlertView = [[UIAlertView alloc] initWithTitle:@"Add a new to-do item:" message:nil delegate:self cancelButtonTitle:@"Dismiss" otherButtonTitles:@"Add", nil];

//UIAlertViewDelegate protocol method

- (void)alertView:(UIAlertView *)alertView clickButtonAtButtonIndex:(NSInteger)buttonIndex {

  if (buttonIndex == 0) {

    [self doCancel];

  } else {

　　[self doContinue];

　}

}

假如想在一个类中处理多个UIAlertView，那么代码会更加复杂，需要在delegate方法中加入对AlertView的tag的判断。

如果能在创建UIAlertView时把处理每个按钮的逻辑写好，那就简单多了，我们可以使用BLOCK来完成。

一、使用Category +Associate Object +Block

//UIAlertView+FHBlock.h

typedef void (^FHAlertViewCompletionBlock)(UIAlertView *alertView, NSInteger buttonIndex);

@interface UIAlertView (FHBlock) <UIAlertViewDelegate>

@property(nonatomic,copy) FHAlertViewCompletionBlock completionBlock;

@end

//UIAlertView+FHBlock.m

#import <objc/runtime.h>

@implementation UIAlertView (FHBlock)<UIAlertViewDelegate>

- (void)setCompletionBlock:(FHAlertViewCompletionBlock)completionBlock {

  objc_setAssociatedObject(self, @selector(completionBlock), completionBlock, OBJC_ASSOCIATION_COPY_NONATOMIC);

  if (completionBlock == NULL) {

    self.delegate = nil;

  }

  else {

    self.delegate = self;

  }

}

- (FHAlertViewCompletionBlock)completionBlock {

  return objc_getAssociatedObject(self, @selector(completionBlock));

}

#pragma mark - UIAlertViewDelegate

- (void)alertView:(UIAlertView *)alertView clickButtonAtButtonIndex:(NSInteger)buttonIndex {

  if (self.completionBlock) {

    self.completionBlock(self, buttonIndex);

  }

}

@end

二、inheritance+Block

//FHAlertView.h

typedef void(^AlertBlock)(UIAlert* alert,NSInteger buttonIndex);

@interface FHAlertView:UIAlertView

@property(nonatomic,copy) AlertBlock completionBlock;

//FHAlertView.m

#pragma mark - UIAlertViewDelegate

- (void)alertView:(UIAlertView *)alertView clickButtonAtButtonIndex:(NSInteger)buttonIndex {

  if (self.completionBlock) {

    self.completionBlock(self, buttonIndex);

  }

}

@end

可以参考源码：http://blog.projectrhinestone.org/preventhandler/