Add FlMethodChannel, FlMethodCodec, FlStandardMethodCodec and FlJsonMethodCodec

[robert-ancell]

No description provided.

[robert-ancell]

From review in #18118:

• "How do you send a not-implemented response?"
• "This, and/or some of the other methods in this class, would benefit from some high-level documentation about how method calls work (see other platforms for examples). Right now it seems like you already need to know how method channels work conceptually (E.g., that you'll get one of three possible outcomes to a method call) to use this class."
• Regarding response envelopes - "Another option would be to return a value here, and do something like DecodeAndProcessResponseEnvelope in the C++ code for the envelope decoding case. (I don't really love that solution either though.)"

[robert-ancell]

I think I've come up with a better solution for the response encoding, the usage is now something like this:

static void method_response_cb(GObject *object, GAsyncResult *result, gpointer user_data) {
  g_autoptr(GError) error = NULL;
  g_autoptr(FlMethodResponse) response = fl_method_channel_invoke_finish (FL_METHOD_CHANNEL(object), result, &error);
  if (response == NULL) {
    g_warning ("Failed to call method: %s", error->message);
    return;
  }

  FlValue *result = fl_method_response_get_result (response, &error);
  if (result == NULL) {
    g_warning ("Method call returned error %s", error->message);
    return;
  }
  
  do_something_with_result (result);
}

int main(int argc, char **argv)
{
  ...
  
  g_autoptr(FlMethodChannel) channel = fl_method_channel_new (fl_engine_get_binary_messenger (engine), "foo", FL_METHOD_CODEC(fl_standard_method_codec_new ()));

  g_autoptr(FlValue) args = fl_value_new_int(42);
  fl_method_channel_invoke (channel, "Foo", args, NULL, method_response_cb, NULL);
}

If you want to get the error details, the callback now looks like:

static void method_response_cb(GObject *object, GAsyncResult *result, gpointer user_data) {
  g_autoptr(GError) error = NULL;
  g_autoptr(FlMethodResponse) response = fl_method_channel_invoke_finish (FL_METHOD_CHANNEL(object), result, &error);
  if (response == NULL) {
    g_warning ("Failed to call method: %s", error->message);
    return;
  }

  if (FL_IS_METHOD_RESPONSE_SUCCESS (response) {
    do_something_with_result (fl_method_response_success_get_result (FL_METHOD_RESPONSE_SUCCESS (response)));
  } else if (FL_IS_METHOD_RESPONSE_ERROR (response)) {
    g_warning ("Method call returned error %s: %s",
               fl_method_response_error_get_code (FL_METHOD_RESPONSE_ERROR (response));
               fl_method_response_error_get_message (FL_METHOD_RESPONSE_ERROR (response)));
  }
}

I think this is a reasonable balance between making the simple case relatively easy to do, but still be able to access the detail if necessary.

[robert-ancell]

@stuartmorgan I haven't been able to work out what a "not implemented" response is. The standard codec has two codes for responses (success and error) and the JSON codec either returns "[<result>]" or "[<code>,<method>,<args>]". Is not implemented just an error code?

[robert-ancell]

Ah, is not implemented when you received a zero length response?

[robert-ancell]

I'm unsure if we want to implement FlOptionalMethodChannel - it seems like this is because different shells may have missing methods, but the Dart side should be more predictable.