实现事件同步 - 信号量的坑

有问题代码：

  
   dispatch_async(self.queue_handleAppear, ^{
        dispatch_semaphore_wait(self.semphore_t, DISPATCH_TIME_FOREVER);
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"hhh任务:%@ 开始执行",message);
            [currentView makeToast:message duration:interval position:position withFinishBlock:^( BOOL completeRes) {
                if (completeRes) {
                    NSLog(@"hhh任务:%@ 结束执行",message);
                    dispatch_semaphore_signal(self.semphore_t);
                }
            }];
        });
    });
    
    注意： queue_handleAppear是串行队列

正确解决方式：

     dispatch_async(self.queue_handleAppear, ^{
        [self.condition lock];
        dispatch_async(dispatch_get_main_queue(), ^{
            [currentView makeToast:message duration:interval position:position withFinishBlock:^( BOOL completeRes) {
                if (completeRes) {
                    [self.condition unlock];
                }
            }];
        });
    })