title: Flutter Engine Custom Embedder Platform Messages Not Working
body: <p>I have successfully embedded Flutter in my OpenGL app using software renderer, using the official 2.0.1 build (using official artifacts from <a href="https://github.com/flutter/flutter/wiki/Custom-Flutter-Engine-Embedders" rel="nofollow noreferrer">the wiki page</a>. My project is similar to <a href="https://github.com/jwinarske/flutter_wayland" rel="nofollow noreferrer">https://github.com/jwinarske/flutter_wayland</a> for your reference.</p>
  <p>Rendering &amp; mouse/pointer events works fine. However, I'm not able to send or receive any platform messages to/from the flutter engine:</p>
  <ul>
  <li>The <code>FlutterProjectArgs.platform_message_callback</code> is never called, and</li>
  <li>sending keyboard events via <code>flutter/keyevent</code> has no effect (I implement something similar to the <a href="https://github.com/flutter/engine/blob/master/shell/platform/glfw/key_event_handler.cc" rel="nofollow noreferrer">glfw key handler</a>).</li>
  <li>Also if I use a native plugin, and if I invoke <code>MethodChannel</code> from Flutter/Dart side, it never returns (the native plugin works if we build the flutter app for desktop, for example Windows)).</li>
  </ul>
  <p>For the project args, I only filled <code>assets_path</code>, <code>icu_data_path</code> &amp; <code>platform_message_callback</code>, and the rest I leave to the default values.</p>
  <p>Are there other prerequisites for communication with flutter using platform messages?</p>
  <p>Thanks!</p>