Add Results.Stream overload that takes a Func<Stream, Task>

Background and Motivation

There are scenarios where APIs want to work directly on the response stream without buffering. Today the Stream overloads assume you have a stream that you have produced that is then copied to the underlying response stream, instead, we want to provide a result type that gives you access to the underlying response stream.

Example:

https://github.com/khalidabuhakmeh/dotnet-dramameter/blob/main/DramaMeter/Program.cs#L37-L43

Another is writing a blob storage results to the http response.

Proposed API

namespace Microsoft.AspNetCore.Http
{
    public static class Results
    {
+       public IResult Stream(Func<Stream, Task> streamWriterCallback, 
+                             string? contentType = null,
+                             string? fileDownloadName = null,
+                             DateTimeOffset? lastModified = null,
+                             EntityTagHeaderValue? entityTag = null,
+                             bool enableRangeProcessing = false);
    }
}

Usage Examples

app.MapGet("/", async (HttpContext http, string? level) => {
    level ??= "low";

    if (!levels.TryGetValue(level.Trim(), out var result))
        return Results.BadRequest("invalid level");

    var image = background.CloneAs<Rgba32>();
    image.Mutate(ctx => {
        ctx.Vignette(result.color); // match the background to the intensity
        ctx.DrawImage(foreground, new Point(0, 0), 1f);
        ctx.DrawImage(result.image, new Point(0, 0), opacity: 1f);
    });
    
    http.Response.Headers.CacheControl = $"public,max-age={FromHours(24).TotalSeconds}";
    return Results.Stream(stream => image.SaveAsync(stream, PngFormat.Instance), "image/png");
});

Risks

None

[pranavkm]

- Func<Stream, Task> streamFactory
+Func<Stream, Task> writeHttpResponseStreamAsync,

or something like that. I think we’d spoken about this API when reviewing the Results type but left it alone at that time to get feedback. Also I think we should try and keep some parity between this and the API in controllers to allow users to migrate from one to another without too much effort (in particular since it’s low cost for us to add support)

[rafikiassumani-msft]

Triage: We will keep this issue open to give us time to add these results to Web APIs in MVC.