如何顯示倒數計時器

Question

我不知道該怎么做。 我有一個按鈕，一旦點擊它，我想在五秒鍾內開始一個過程，而在用戶等待時，我想顯示倒數-5，4，3，2，1

我無法使用nstimer嘗試此操作，但這不起作用。 有什么建議么？ 謝謝

Answer 1

您可以使用dispatch_after()並利用塊捕獲范圍的方式來跟蹤剩余的秒數。

- (IBAction)buttonHandler:(UIButton *)sender
{
    if (self.countingDown)
        return;

    [self startCountDown];
}

- (void)startCountDown
{
    self.countingDown = YES;
    self.button.enabled = NO;
    int seconds = 5;
    [self countDownFor:seconds];
}


- (void)countDownFor:(int)seconds
{
    self.countDownLabel.text = [NSString stringWithFormat:@"%d",seconds];
    if (seconds == 0) {
        self.countingDown = NO;
        self.button.enabled = YES;
        return; 
    }

    double delayInSeconds = 1.0;
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
    dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
        [self countDownFor:(seconds - 1)];
    });
}

Answer 2

使用GCD和塊的優雅解決方案：

    __block void (^runBlock)(int) = ^(int i) {
        countdownLabel.text = [NSString stringWithFormat:@"%d", i];
        if(i>0) {
            dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, 1*NSEC_PER_SEC);
            dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
                runBlock(i-1);
            });
        } else {
            runBlock = nil; // Breaking retain cycle
            // Time to start your great action!
            // ...
        }
    };

    runBlock(5);

Answer 3

嘗試類似

[NSTimer scheduledTimerWithTimeInterval:1 
                                       target:self
                                     selector:@selector(methodNameUsedToupdateButtonText)
                                     userInfo:nil
                                      repeats:YES];

並在倒數達到0時（並且如果視圖卸載）使計時器無效

檢查NSTimer類參考