[REQ] [typescript-rxjs] Add cancel event to Middleware

Is your feature request related to a problem? Please describe.

Using a Middleware is a great way to enhance the requests or responses. However, we use it to

• enhance the request with a unique id header: progress-key
• show a progress bar on the UI while at least one request is processed by the server
• the progress bar queries the status of the requests frequently based on the progress key
• when all requests are finished, the progress bar is not shown anymore

To hide the progressbar, we use the post(...) function of the middleware. However, sometimes requests are cancelled (e.g.: due to a user interaction) and we cannot catch that event.

Describe the solution you'd like

Add a new function to the middleware:

cancel(requestArgs: RequestArgs);

In case the request is cancelled, this function would be called.

Describe alternatives you've considered

Implement it on the RXJS level. However, in this case the header cannot be added to the request headers automatically.

Start by locating the typescript-rxjs middleware implementation and the RequestArgs definition. Trace how request, response, and post middleware hooks are invoked during cancellation. Done means a cancelled request invokes the new cancel hook and the middleware behavior is covered by the relevant existing tests.