Blazor Timer call async API task to update UI

Question

I am setting up a timer in a Blazor server-side page. The goal is to call an API every x seconds and based on the return value, update the UI.

I got this code:

private string Time { get; set; }

protected override void OnInitialized()
{
    var timer = new System.Threading.Timer((_) =>
    {
        Time = DateTime.Now.ToString();
        InvokeAsync(() =>
        {
            StateHasChanged();
        });
    }, null, 0, 1000);
    base.OnInitialized();
}

This works beautifully. The UI was updated every second with the new time value. However, I can't figure out how to call an async task to get the value. I would like to replace the line:

Time = DateTime.Now.ToString();

with a line that calls the following function:

private async Task<string> GetValue()
{
    var result = await _api.GetAsync<StringDto>("/api/GetValue");
    return result.Text;
}

I've tried this line:

Time = GetValue().Result;

But I received this following error:

The current thread is not associated with the Dispatcher. Use InvokeAsync() to switch execution to the Dispatcher when triggering rendering or component state.

What do I need to do to call the async method?

Thanks a bunch!

Answer 1

You probably don't want to Invoke() the GetValue(), that would be rather pointless. You can implement the timer like this:

System.Threading.Timer timer;
protected override void OnInitialized()
{
    timer = new System.Threading.Timer(async _ =>  // async void
    {
        Time = await GetValue();
        // we need StateHasChanged() because this is an async void handler
        // we need to Invoke it because we could be on the wrong Thread          
        await InvokeAsync(StateHasChanged);
    }, null, 0, 1000);
}

I used a field to store the Timer because you should dispose it, add this to the Razor section:

@implements IDisposable

and this to the code:

public void Dispose()
{
    timer?.Dispose();
}