Emit a signal when a setting is changed in the Project Settings

[andy-noisyduck]

Describe the project you are working on

Plugin

Describe the problem or limitation you are having in your project

There's no way for a plugin to know if a project's settings have been changed. Currently the best way to use project settings is to re-fetch them at the point the setting is used. It's not feasible to do this for all types of settings, for example, a toggle for an operation that happens in the background. In my plugins I'm getting around this by polling settings on a timer, but that solution feels a bit ugly.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

An extra signal called emitted through EditorPlugin (?) called settings_changed. Optionally we could include the key of the setting that was changed, but having the signal at all would be a great start.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

myPlugin.connect("settings_changed", self, "on_settings_changed")

If this enhancement will not be used often, can it be worked around with a few lines of script?

You could work around this by implementing polling yourself. It's reasonably trivial, but this very much feels like functionality that should already be there. We already have methods for plugins to get and set the settings.

Is there a reason why this should be core and not an add-on in the asset library?

Fixes a gap in existing core functionality.

[me2beats]

What about ProjectSettings.settings_changed signal (instead of EditorPlugin one)?
I also think some info on "what setting was changed" would be useful as well, so it could be setting_changed instead of settings_changed:
String ProjectSetings.setting_changed()
This would allow to do something like:

func on_setting_changed(setting:String):
    ...

[YuriSizov]

Definitely should be a signal on the ProjectSettings singleton. It is exposed, no reason not to use it.

[YuriSizov]

Apparently, godotengine/godot#45956 added a project_settings_changed signal to EditorPlugin API. Should probably be manually backported to 3.x, because I don't think that this PR itself can be backported.

[akien-mga]

Implemented by godotengine/godot#53296.