Add Streaming Response Feature

[ferjif]

Use Cases
When I want to send notifications to the client or when i want to stream webcam images to the client I need to use streaming responses. if you look at EventSource and also server-sent-events in the mozilla developer network you can see some of the use cases for streaming responses specially for server-sent-events. also for video streaming and webcam image streaming there are many examples. if you search for webcam streaming or http live stream (HLS) you'll find many examples specially in python.

Solution

drogon::app().registerHandler([](const drogon::HttpRequestPtr& req, std::function<void(const drogon::ResponsePtr&)>&& callback)
{
    //....
    while(true)
    {
        callback(resp);
    }
}

or maybe

drogon::app().registerHandler([](const drogon::HttpRequestPtr& req, std::function<void(const drogon::ResponsePtr&)>&& callback)
{
    //....
    callback(resp);
    while(true)
    {
        resp.append(content);
    }
}

Alternatives in different languages

• the examples and doc for every framework is provided using a hyperlink.
  in c++ the wt and oat++ frameworks have this concept implemented. in python django, sanic and other frameworks have implamented this concept. in Java there is jooby and other frameworks which have implemented this feature. I haven't implemented any streaming web servers in other languages but since webcam (mjpeg) streaming and server-sent-events are standards and are used industry wide other languages and frameworks have probably implemented them.

[AmissLake]

It's an important improvement !!! I was wondering if drogon had it. I really need this implementation

[louwers]

Django doesn't really support streaming...

Interesting take on SSE vs Websockets: https://stackoverflow.com/questions/28582935/does-http-2-make-websockets-obsolete

Probably better to watch this first: #349

[hwc0919]

See #1156