按鈕文字倒計時效果

// 開啓倒計時效果（將此方法在按鈕點擊事件中執行）
- (void)openCountdown {
    //倒計時時間
    __block NSInteger time = 59;

    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);
    //每秒執行
    dispatch_source_set_timer(_timer,dispatch_walltime(NULL, 0), 1.0 * NSEC_PER_SEC, 0);

    dispatch_source_set_event_handler(_timer, ^{
        if(time <= 0){ //倒計時結束，關閉
            dispatch_source_cancel(_timer);
            dispatch_async(dispatch_get_main_queue(), ^{
                //設置按鈕的樣式
                [self.resendAuthCodeBtn setTitle:@"重新發送" forState:UIControlStateNormal];
                [self.resendAuthCodeBtn setTitleColor:[UIColor darkGrayColor] forState:UIControlStateNormal];
                self.resendAuthCodeBtn.userInteractionEnabled = YES;
            });
        }else{
            int seconds = time % 60;
            dispatch_async(dispatch_get_main_queue(), ^{
                //設置按鈕顯示讀秒效果
                [self.resendAuthCodeBtn setTitle:[NSString stringWithFormat:@"重新發送(%.2d)s", seconds] forState:UIControlStateNormal];
                [self.resendAuthCodeBtn setTitleColor:[UIColor darkGrayColor] forState:UIControlStateNormal];
                self.resendAuthCodeBtn.userInteractionEnabled = NO;
            });
            time--;
        }
    });
    dispatch_resume(_timer);
}