Add support for event callbacks

[jni]

Just creating this issue to discuss a way for plugins to register events that happen on specific callbacks. For example, in napari/napari#4361, @psobolewskiPhD wants to be able to reset the dims sliders whenever new images are added.

Adding an on: argument for commands that would trigger function calls whenever specific events happen seems like a pretty useful thing for plugins to do!

Of course there's still questions about when these things get activated... I might argue that if I install a plugin that provides specific callbacks, I want it activated every time I launch napari. Or maybe this goes in plugin preferences?

[Czaki]

I might argue that if I install a plugin that provides specific callbacks, I want it activated every time I launch napari

I cannot agree. You may install a plugin for only one widget and do not want to use all others things from the plugin.

[jni]

Agreed, I think a preference like "startup items", similar to an OS, would be useful for this.

[tlambert03]

just going through some issues here. I'm not sure this particular use case makes as much sense in the declarative schema. I kinda picture an API where a plugin would receive an object that looked very much like viewer, but which was capable of tracking event connections, such that they could be disconnected upon deactivation:

# pass plugin an object, that can track what gets connected
event_subscriptions = pass_plugin_the_viewer_proxy(plugin, viewer_proxy)
# add a callback that can undo those event connections to the plugin context
plugin_context.register_disposable(subscriptions)
# when plugin is deactivated, `context._dispose()` gets called that will call all "undo" things