onMessageExternal API to allow NextPlease to be invoked from other addons

[marklieberman]

I have Foxy Gestures user who would like to invoke NextPlease commands via gestures. As I understand, in the pre-FF57 era (when FireGestures still worked) people used to dispatch key events to invoke NextPlease via shortcuts as demonstrated by the script in this issue. However, this doesn't work with web extensions. I don't believe that commands defined in manifest.json will respond to untrusted events dispatched from user scripts.

I've had other addons add or investigate support for an onMessageExternal API such as in: marklieberman/foxygestures#223 and marklieberman/foxygestures#146. In this way I can invoke NextPlease commands using code like runtime.sendMessage(*NextPlease-Extension-ID*, { command: ... }). Would you consider adding support for this type of API or taking a PR?

[alexeyr]

I'll happily take a PR. Adding it myself would have to wait until I am not under stress.

[gaberad]

This is the most simple solution, all you need to do is add this to nextplease_background.js

browser.runtime.onMessageExternal.addListener(function(message, sender) {
	nextplease.sendMessageToActiveTab({ direction: message.direction });
});

then in FoxyGestures, the userscript will look like this

browser.runtime.sendMessage('{57B65ABB-F4E3-4358-8472-15AEE0833E11}', {
    direction: 'Next'
});

'direction' being either Next, Prev, First, or Last.