Explain how native language calls a Dart component

[xster]

https://docs.flutter.dev/platform-integration/platform-channels currently has an example of calling native via Dart. We should document the reverse as well.

For instance, how do I return an exception?

[matthew-carroll]

@xster I'm a little confused about the goal of this ticket. The first statement seems to want documentation for native invoking a method in Dart. But the second statement about returning an exception seems independent of the direction of communication. Also, I think the Result object is already demonstrated in the docs, right?

[xster]

I meant how does native invoke something in Dart and Dart answering with a result or error.

Then how does Dart invoke something in native and native answering with a result or error.

[matthew-carroll]

Isn't the Dart to native communication already documented?
https://flutter.dev/docs/development/platform-integration/platform-channels#example-calling-platform-specific-ios-and-android-code-using-platform-channels

[xster]

Right, and it looks like the reverse isn't. Invoke a thing from native, have Dart handle the method call and read its args etc.

[jmagman]

Related to (or maybe a duplicate of) flutter/flutter#32942

[xster]

related but not the same issue

[xster]

would be nice to have both directions described for pigeon on that page too @gaaclarke