参考：

http://www.cocoachina.com/ios/20170122/18601.html

https://www.jianshu.com/p/d911cd16c100

https://www.jianshu.com/p/492be28d63c4

//情况一：不泄露

- (void)case1
{
    NSLog(@"case 1 Click");
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        self.name = @"case 1";
    });
}

//情况二：不泄露

- (void)case2
{
    NSLog(@"case 2 Click");
    __weak typeof(self) weakSelf = self;
    [self.teacher requestData:^(NSData *data) {
        typeof(weakSelf) strongSelf = weakSelf;
       strongSelf.name = @"case 2";
    }];
}

//情况三：泄漏

- (void)case3
{
    NSLog(@"case 3 Click");
    [self.teacher requestData:^(NSData *data) {
        self.name = @"case 3";
    }];
}

//情况四：不泄露

- (void)case4
{
    NSLog(@"case 4 Click");
    [self.teacher requestData:^(NSData *data) {
        self.name = @"case 4";
        self.teacher = nil;
    }];
}

//情况五：不泄露

- (void)case5
{
    NSLog(@"case 5 Click");
    Teacher *t = [[Teacher alloc] init];
    [t requestData:^(NSData *data) {
        self.name = @"case 5";
    }];
}

//情况六：不泄露

- (void)case6
{
    NSLog(@"case 6 Click");
    [self.teacher callCase6BlackEvent];
    self.teacher.case6Block = ^(NSData *data) {
        self.name = @"case 6";
        
        //下面两句代码任选其一即可防止内存泄漏，self.teacher 或者 case6Block 置为空都可以打破 retain cycle
        self.teacher = nil;
        //self.teacher.case6Block = nil;
    };
}

- (void)requestData:(void (^)(NSData *))block
{
    self.caseBlock = block;
    
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t) (0.3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        
        if(self.caseBlock != nil)
        {
            self.caseBlock(@"case");
        }
        
        //self.caseBlock = nil;//加上这句也可以防止内存泄漏
    });
}

- (void)callCase6BlackEvent
{
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        
        if(self.case6Block != nil)
        {
            self.case6Block(@"case 6");
        }
    });
}