How to raise Event in HostedService and consume it in a Blazor Component? - multithreading

Question

i am building a BlazorServer-Side App und i try to Update one of my Components when a Event occurs in a HostedService.
The Problem is solved! Check the Comments for the Solution.
This is how i try to do it:
Configuration in Startup.cs:
public void ConfigureServices(IServiceCollection services)
{
services.AddRazorPages();
services.AddServerSideBlazor();
services.AddSingleton<WeatherForecastService>();
services.AddHostedService<WeatherForecastService>();
}
WeatherForecastService:
public class WeatherForecastService : IHostedService
{
private System.Timers.Timer timer = new System.Timers.Timer(10000);
public delegate void EventHandler(object sender, EventArgs e);
public event EventHandler NewWeather;
private static readonly string[] Summaries = new[]
{
"Freezing", "Bracing", "Chilly", "Cool", "Mild", "Warm", "Balmy", "Hot", "Sweltering", "Scorching"
};
private void TimeOver(object sender, ElapsedEventArgs e)
{
NewWeather?.Invoke(this, null);
}
public Task<WeatherForecast[]> GetForecastAsync(DateTime startDate)
{
var rng = new Random();
return Task.FromResult(Enumerable.Range(1, 5).Select(index => new WeatherForecast
{
Date = startDate.AddDays(index),
TemperatureC = rng.Next(-20, 55),
Summary = Summaries[rng.Next(Summaries.Length)]
}).ToArray());
}
public Task StartAsync(CancellationToken cancellationToken)
{
timer.Elapsed += TimeOver;
timer.AutoReset = true;
timer.Start();
return Task.CompletedTask;
}
public Task StopAsync(CancellationToken cancellationToken)
{
return Task.CompletedTask;
}
}
FetchData.razor:
#code {
private WeatherForecast[] forecasts;
protected override async Task OnInitializedAsync()
{
forecasts = await ForecastService.GetForecastAsync(DateTime.Now);
ForecastService.NewWeather += UpdateWeather;
}
private async void UpdateWeather(object sender, object args)
{
forecasts = await ForecastService.GetForecastAsync(DateTime.Now);
StateHasChanged();
}
}
UpdateWeather() on the FetchData.razor never gets called.
I am guessing the FetchData Component does not notice the event is fired because of some threading issue. But I dont konw yet what to do about it...
Your Help is appreciated.
Best Regards

Answer 1

The Code above generates two Instances of the WeatherForecastService!
The Solution is to change the registration of the HostedService
from:
services.AddHostedService();
to:
services.AddHostedService(sp => sp.GetRequiredService());