C# Blazor: Countdown Timer

Question

I'm new to C# and trying to create a simple countdown timer using System.Timer.Timers. It didn't work as expected and I searched the internet for a solution but it didn't really fix my problem. What I want is when the user clicks the start button, it begins and displays the countdown. But although the timer kinda worked, it didn't continuously display the timer when I clicked the button once, instead, I need to click the start button many times to see the countdown number or the timer display will not change. Here's the code.

@page "/"

<h1>Timer</h1>

<p>@counter</p>
<button @onclick="StartTimer">Start</button>


@code {
    private static System.Timers.Timer aTimer;
    private int counter = 60;
    public void StartTimer()
    {
        aTimer = new System.Timers.Timer(1000);
        aTimer.Elapsed += CountDownTimer;
        aTimer.Enabled = true;
    }

    public void CountDownTimer(Object source, System.Timers.ElapsedEventArgs e)
    {
        if (counter > 0)
        {
            counter -= 1;
        }
        else
        {
            aTimer.Enabled = false;
        }
    }
}

Answer 1

Call StateHasChanged() when updating the counter so that the UI elements are updated.

Because your callback will run on a separate thread you will need to use InvokeAsync to call StateHasChanged().

public void CountDownTimer(Object source, ElapsedEventArgs e)
{
    if (counter > 0)
    {
        counter -= 1;
    }
    else
    {
        aTimer.Enabled = false;
    }
    InvokeAsync(StateHasChanged);
}