a simple countdown timer with dotnet Blazor

Countdown.razor

<div class = "clock">
    <h1>@Time</h1>
</div>

Countdown.razor.cs

using System.Timers;
using Microsoft.AspNetCore.Components;

public partial class Countdown : ComponentBase, IDisposable
{
    private System.Timers.Timer _timer = null!;
    private int _secondsToRun = 0;

    protected string Time { get; set; } = "00:00";

    [Parameter]
    public EventCallback TimerOut { get; set; }

    public void Start(int secondsToRun)
    {
        _secondsToRun = secondsToRun;

        if (_secondsToRun > 0)
        {
            Time = TimeSpan.FromSeconds(_secondsToRun).ToString(@"mm\:ss");
            StateHasChanged();
            _timer.Start();
        }
    }

    public void Stop()
    {
        _timer.Stop();
    }

    protected override void OnInitialized()
    {
        _timer = new System.Timers.Timer(1000);
        _timer.Elapsed += OnTimedEvent;
        _timer.AutoReset = true;
    }

    private async void OnTimedEvent(object? sender, ElapsedEventArgs e)
    {
        _secondsToRun--;

        await InvokeAsync(() =>
        {
            Time = TimeSpan.FromSeconds(_secondsToRun).ToString(@"mm\:ss");
            StateHasChanged();
        });
        
        if (_secondsToRun <= 0)
        {
            _timer.Stop();
            await TimerOut.InvokeAsync();
        }
    }

    public void Dispose()
    {
        _timer.Dispose();
    }
}

Countdown.razor.css

.clock {
    color: var(--color-tone-1);
    font-family: sans-serif;
    display: flex;
    justify-content: center;
    align-items: center;
    flex-grow: 1;
    overflow: hidden;
}

I have a question! What does the public EventCallback TimerOut { get; set; } here?

I have a question! What does the public EventCallback TimerOut { get; set; } here?

it allows you to do some actions on the timer completion
when used like this

<Countdown @ref="timer" TimerOut="TimerOutCallback"/>
...
    protected override void OnAfterRender(bool firstRender)
    {
        if (firstRender)
        {
            timer.Start(10);
        }
    }

    protected Shared.Countdown timer;

    private void TimerOutCallback()
    {
        ...
    }

This is really neat! Thanks for sharing it.