How to stop device.starttimer xamarin forms on button click?

Question

So I have two buttons Start/Stop and start works fine because it starts at the beginning each time start is clicked, which is what I want. But I'm new to xamarin forms and don't entirely understand how to stop device.starttimer.

This is what I have currently and it's not working. (don't worry about the sound stuff)

//timer
    bool shouldRun = false;
    private void timer()
    {
        Device.StartTimer(TimeSpan.FromSeconds(3), () =>
        {
            // Do something
            label.Text = "Time is up!";
            //shows start button instead of stop button
            startButton.IsVisible = true;
            //hides stop button
            stopButton.IsVisible = false;
            return shouldRun;
        });
    }

    private void STOPButton_Clicked(object sender, EventArgs e)
    {
        //shows start button instead of stop button
        startButton.IsVisible = true;
        //hides stop button
        stopButton.IsVisible = false;
        //stops timer
        shouldRun = false;
        //stops sound
    }

    private void STARTButton_Clicked(object sender, EventArgs e)
    {
        //hides start button
        startButton.IsVisible = false;
        //shows stop button instead of start button
        stopButton.IsVisible = true;

        //starts timer from beginning

        timer();
        //starts sound from beginning
    }

When you return false from the Func<bool> the timer is stopped. — SushiHangover
– SushiHangover, Commented Oct 15, 2018 at 13:04
That's what I thought but the Stop button isn't working when I declare shouldRun = false. — Pinpaho
– Pinpaho, Commented Oct 15, 2018 at 13:24
After you declare shouldRun = false in the stop button, the timer delegate will run one more time at the end of its 3 second timer, if you do not want the label being set to "Time is up!", then you would need to perform a check of shouldRun before that and jump over that assignment (or perform an early exit, return false). — SushiHangover
– SushiHangover, Commented Oct 15, 2018 at 13:30

Markus Michel · Accepted Answer · 2018-10-15 14:18:50Z

You add a cancellation token source to the view running the timer

private CancellationTokenSource cancellation;

Adapt your StopButton code as follows:

private void STOPButton_Clicked(object sender, EventArgs e)
{
    startButton.IsVisible = true;
    //hides stop button
    stopButton.IsVisible = false;
    //stops timer
    if (this.cancellation != null)
        Interlocked.Exchange(ref this.cancellation, new CancellationTokenSource()).Cancel();
    shouldRun = false;
}

Finally in your timer delegate you create the cancellation token source

CancellationTokenSource cts = this.cancellation = new CancellationTokenSource();
Device.StartTimer(TimeSpan.FromSeconds(3), () =>
{
    if (this.cancellation != null)
        Interlocked.Exchange(ref this.cancellation, new CancellationTokenSource()).Cancel();
    // Do something
    label.Text = "Time is up!";
    //shows start button instead of stop button
    startButton.IsVisible = true;
    //hides stop button
    stopButton.IsVisible = false;
    return shouldRun;
});

Basically it is quite similar to the bool flag method, SushiHangover mentioned in his comment. However a cancellation source is thread safe, so you don't run into nasty race conditions when stopping your timer from a different thread.

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