メールの検証コードのカウントダウンの上の文字はどうしてきらめきます

buttonはシステムスタイルなのでbuttonをカスタムスタイルに変えれば問題解決

コード:

-(void)startTime{
    __block int timeout= 60; // 
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_source_t _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0,queue);
    dispatch_source_set_timer(_timer,dispatch_walltime(NULL, 0),1.0*NSEC_PER_SEC, 0); // 
    dispatch_source_set_event_handler(_timer, ^{
        if(timeout<=0){ // ， 
            dispatch_source_cancel(_timer);
            dispatch_async(dispatch_get_main_queue(), ^{
                [_getIdentifyBtn setTitle:@" " forState:UIControlStateNormal];
                _getIdentifyBtn.userInteractionEnabled = YES;
            });
        }else{
            dispatch_async(dispatch_get_main_queue(), ^{

                [UIView beginAnimations:nil context:nil];
                [UIView setAnimationDuration:1];
                [_getIdentifyBtn setTitle:[NSString stringWithFormat:@"%zd ",timeout] forState:UIControlStateNormal];
                [UIView commitAnimations];
                _getIdentifyBtn.userInteractionEnabled = NO;
            });
            timeout--;
        }
    });
    dispatch_resume(_timer);
    
}