替代NSTimer的block计时器！！ !

既然NSTimer容易造成内存泄露，那就给初出茅庐的小程序员们带来一个惊喜吧！
话不多说上代码：
LIST

-(void)startTime{
    timeout_int=0;
    dispatch_queue_t queue_global =dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_source_t _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0,0,queue_global);
    dispatch_source_set_timer(_timer,dispatch_walltime(NULL, 0),1.0*NSEC_PER_SEC, 0);
    dispatch_source_set_event_handler(_timer, ^{
        __weak ViewController *weakself =self ;
        if(timeout_int>10){ //倒计时结束，关闭
            dispatch_source_cancel(_timer);
            dispatch_async(dispatch_get_main_queue(), ^{
               weakself.numberLabel.text = @"结束";
            });
        }else{
            // int minutes = timeout_int / 60;
            int seconds = timeout_int % 61;
            NSString *strTime = [NSString stringWithFormat:@"%.2d", seconds];
           dispatch_async(dispatch_get_main_queue(), ^{
                //设置界面的按钮显示
                weakself.numberLabel.text = strTime;
            });
            timeout_int++;
              }
    });
    dispatch_resume(_timer);
}