我如何在不同的应用程序状态下管理我的计时器，我正在使用 MVVM 方法？

Question

I have a timer that works fine, the problem I'm having is to keep the timer running when the app is on sleep mode or minimized or when I hit the back button, the timer should only stop when I hit the stop button of which I have done.

/// <summary>
/// Starts the timer
/// </summary>
private void StartCommandAction()
{
    CancellationTokenSource cts = _cancellationTokenSource; // safe copy
    Device.StartTimer(TimeSpan.FromSeconds(1),() =>
    {
        if (cts.IsCancellationRequested)
        {
            return false;
        }
        else
        {
            Device.BeginInvokeOnMainThread(() =>
            {
                var totalTimeInt = string.IsNullOrEmpty(TxtTotalTime.Value) ? 0  : int.Parse(TxtTotalTime.Value);
                var totalSec = (int)TotalSeconds.TotalSeconds;
                TimeSpan _TimeSpan = new TimeSpan(totalTimeInt, 0, totalSec); //TimeSpan.FromSeconds(TotalSeconds.TotalSeconds);
                LblTime = string.Format("{0:00}:{1:00}:{2:00}", _TimeSpan.Hours, _TimeSpan.Minutes, _TimeSpan.Seconds);
                IsVisibleTimerLabel = true;
                Count();
            });

            return true;
        }
    });

    IsVisibleButtonStart = false;
    IsVisibleButton = true;
}

Answer 1

Without knowing the rest of your source code, the following struck me: You are expecting the timer event to be raised exactly once each second and taking the textual representation to calculate a total time. This may work for the current timer implementation, but this is not guaranteed. Even worse, your implementation is not robust against different timer implementations.

When you sum up your times each iteration, the error of the total time will grow and grow. Depending on your use-case this may be irrelevant, but luckily, the remedy to this issue is the remedy to the issue you're trying to solve, too.

My suggestion is: Refrain from summing up the times, but introduce a fixed reference. In the first order this might be a DateTime (if precision was a matter to you, your solution would look different, hence the precision of DateTime.Now will do), but a Stopwatch would do the trick either.

When starting the timer the first time, store the current DateTime.Now value in a member variable and use this to calculate the elapsed time

CancellationTokenSource cts = _cancellationTokenSource; // safe copy
this._startedAt = DateTime.Now;

Device.StartTimer(TimeSpan.FromSeconds(1),() =>
{
    if (cts.IsCancellationRequested)
    {
        return false;
    }
    else
    {
        Device.BeginInvokeOnMainThread(() =>
        {
            TimeSpan _TimeSpan = DateTime.Now - _startedAt;
            LblTime = _TimeSpan.ToString("hh:mm:ss);
            IsVisibleTimerLabel = true;
            Count();
        });

        return true;
    }
});

( Please note: To format a TimeSpan you can use the ToString method with a format string. Please see the documentation of TimeSpan.ToString on how to format TimeSpan values according to your needs)

This way, when returning to the page, you can just restart the timer ( without setting _startedAt , though). Because you've already set _startedAt , the timer will continue to run and show the correct time.