Blazor Webassembly - StateAsChanged not fire using IAsyncEnumerable · Issue #43098 · dotnet/aspnetcore (original) (raw)

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

Hello,

I am trying to use a controller that returns an IAsyncEnumerable from a webassembly Razor application.

In this repo https://github.com/GLuca74/TestBlazorWebAssembyIAsyncEnumerable there is a simply Web Api Application with the WeatherForecastController that have the Get method that resutrns 1 milion of int as IAsyncEnuverable

    [ApiController]
    [Route("[controller]")]
    public class WeatherForecastController : ControllerBase
    {
        [HttpGet]
        public async IAsyncEnumerable<int> Get()
        {
            for (int i = 0; i < 1000000; i++)
            {
                yield return i;
            }
        }
    }

In the Balzor webassembly application, the counter Page contain the code that use the controller :

<h1>Counter</h1>
<span>@Count</span>
<table class="table">
        <thead>
            <tr>
                <th>Title</th>
            </tr>
        </thead>
        <tbody>
            @foreach (var n in Source) 
            {
                <tr>
                    <td>@n</td>
                </tr>
            }
        </tbody>
    </table>


@code {
    private int Count;
    public List<int> Source;

    protected async override Task OnInitializedAsync()
    {
        await base.OnInitializedAsync();
        Source = new List<int>();
        await foreach (int n in GetData())
        {
            Count = Count + 1;
            Source.Add(n);
            if ((Count % 10) == 0)
            {
                //await Task.Delay(1);
                InvokeAsync(StateHasChanged); // StateHasChanged();
            }
        }
    }

    public async IAsyncEnumerable<int> GetData(CancellationToken cancellationToken = default)
    {
        using (HttpClient httpClient = new HttpClient())
        {
            httpClient.BaseAddress = new Uri(@"https://localhost:7034/");
            httpClient.DefaultRequestHeaders.Accept.Clear();
            httpClient.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"));
            httpClient.Timeout = new TimeSpan(1, 0, 0);
            HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Get, "weatherforecast");
            request.SetBrowserResponseStreamingEnabled(true);
            using (var response = await httpClient.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, cancellationToken).ConfigureAwait(false))
            {
                if (!response.IsSuccessStatusCode)
                {
                    HttpRequestException ex = new HttpRequestException(response.Content.ReadAsStringAsync().GetAwaiter().GetResult(), null, response.StatusCode);
                    throw ex;
                }
                Stream responseStream = await response.Content.ReadAsStreamAsync().ConfigureAwait(false);
                IAsyncEnumerable<int> intResponse = JsonSerializer.DeserializeAsyncEnumerable<int>(responseStream, new JsonSerializerOptions{PropertyNameCaseInsensitive = true}, cancellationToken);
                await foreach (int n in intResponse)
                {
                    yield return n;
                }
            }
        }
    }
}

When the data is loaded, only one update of the UI is performed and the page remain locked. But if I call await Task.Delay(1) before call StateHasChanged all works and any update of the UI is performed.

Can I ask why the Delay is needed to let it works or wath is wrong with my code?

Expected Behavior

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Steps To Reproduce

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Exceptions (if any)

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.NET Version

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Anything else?

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