1. @override
Future<RequestOrResponse> processRequest(Request request)

Overridden by subclasses to modify or respond to an incoming request.

Subclasses override this method to provide their specific handling of a request. A RequestController should either modify or respond to the request. For concrete subclasses of RequestController - like HTTPController - this method has already been implemented.

RequestControllers should return a Response from this method if they responded to the request. If a RequestController does not respond to the request, but instead modifies it, this method must return the same Request.

Source 

@override
Future<RequestOrResponse> processRequest(Request request) async {
  if (request.innerRequest.method.toLowerCase() != "get") {
    return new Response(HttpStatus.METHOD_NOT_ALLOWED, null, null);
  }

  var relativePath = request.path.remainingPath;
  var fileUri = _servingDirectory.resolve(relativePath);
  File file;
  if (FileSystemEntity.isDirectorySync(fileUri.toFilePath())) {
    file = new File.fromUri(fileUri.resolve("index.html"));
  } else {
    file = new File.fromUri(fileUri);
  }

  if (!(await file.exists())) {
    return new Response.notFound();
  }

  var lastModifiedDate = await file.lastModified();
  var ifModifiedSince = request.innerRequest.headers.value(HttpHeaders.IF_MODIFIED_SINCE);
  if (ifModifiedSince != null) {
    var date = HttpDate.parse(ifModifiedSince);
    if (!lastModifiedDate.isAfter(date)) {
      return new Response.notModified(lastModifiedDate, _policyForFile(file));
    }
  }

  var lastModifiedDateStringValue = HttpDate.format(lastModifiedDate);
  var contentType = contentTypeForExtension(path.extension(file.path))
      ?? new ContentType("application", "octet-stream");
  var byteStream = file.openRead();

  return new Response.ok(byteStream,
      headers: {HttpHeaders.LAST_MODIFIED: lastModifiedDateStringValue})
    ..cachePolicy = _policyForFile(file)
    ..encodeBody = false
    ..contentType = contentType;
}