開發IOS時經常會使用到UIAlertView類，該類提供了一種標準視圖，可向用戶展示警告信息。當用戶按下按鈕關閉該視圖時，需要用委托協議（Delegate PRotocol）來處理此動作，但是要設置好這個委托協議，就得把創建警告視圖和處理按鈕動作的代碼分開。

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];

　}

}

假如想在一個類中處理多個UIAlertView，那么代碼會更加復雜，需要在delegate方法中加入對AlertView的tag的判斷。

如果能在創建UIAlertView時把處理每個按鈕的邏輯寫好，那就簡單多了，我們可以使用BLOCK來完成。

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

可以參考源碼：http://blog.projectrhinestone.org/preventhandler/