获取 HTTP 流会阻塞渲染

Question

我正在从 WebAPI 下载一个流，我正在尝试运行在读取该流时进行编码。

下面的代码在 Blazor 服务器端或控制台应用程序上运行良好。但是在 blazor-client 端，流将在执行任何代码之前首先完全下载....即使只是 Console.WriteLine() 也会在流完成后开始写入控制台。我正在使用最新的 .Net Core 3.0 Preview 8...

public async IAsyncEnumerable<SomeClass> GetDataAsync()
{
    var serializer = new JsonSerializer();
    using (var stream = await Http.GetStreamAsync("https://localhost:44334/api/values/i"))
    {
        using (var sr = new StreamReader(stream))
        using (var jr = new JsonTextReader(sr))
        {
            while (await jr.ReadAsync()) //<-- Stream is fully download to browser before taking any actions.
            {
                if (jr.TokenType != JsonToken.StartArray && jr.TokenType != JsonToken.EndArray)
                {
                    Console.WriteLine(jsonReader.LinePosition);
                    yield return serializer.Deserialize<SomeClass>(jr);
                }
            };
        }
    }
}

我用 WireShark 对此进行了测试，以确认这种情况正在发生。

这是设计使然吗？

Answer 1

Is this by design?

我猜这是目前的设计 !

这不会仅仅因为客户端 Blazor 支持异步/等待模式而阻塞。在我看来，真正的问题在于 HttpClient.GetStreamAsync，因为 HttpClient 服务（客户端 Blazor）不是实际或真正的 HttpClient。它基于 JavaScript 获取 Api (https://github.com/aspnet/AspNetCore/blob/master/src/Components/Web.JS/src/Services/Http.ts):

responseData = await response.arrayBuffer();

But if they switch to the mono implementation (https://github.com/aspnet/AspNetCore/issues/10489), streaming will be support if the browser supports it: https://github.com/mono/mono/blob/254d6d37580a70fbb4fa072b0b2b16d2984b3bb5/sdks/wasm/framework/src/WebAssembly.Net.Http/WasmHttpMessageHandler.cs#L171-L173

A call to arrayBuffer() will never return from a pure stream. A different MessageHandler based on this example would have to be implemented, but I'm pretty sure it's not enough to just implement a new message handler in this case.

希望这对您有所帮助...

获取 HTTP 流会阻塞渲染

Getting HTTP stream is blocking rendering

blazor

blazor-client-side