ST should re-check TextChangeListener.is_applicable when a buffer's view's settings change

[Calvin-L]

Description of the bug

A common way to implement TextChangeListener.is_applicable is to consult the buffer's primary view's settings:

class MyViewEventListener(sublime_plugin.ViewEventListener):
    @classmethod
    def is_applicable(cls, settings):
        return # ... check some property of the settings ...

class MyTextChangeListener(sublime_plugin.TextChangeListener):
    @classmethod
    def is_applicable(cls, buffer):
        return MyViewEventListener.is_applicable(buffer.primary_view().settings())

For instance, this pattern was mentioned and upvoted here.

However, ST does not re-check TextChangeListener.is_applicable when the settings change, even though it does re-check ViewEventListener.is_applicable when the settings change.

This is a problem for plugins that only want text change events on certain kinds of syntax. Consider e.g. Calvin-L/sublime-coq-plugin#9, in which the following sequence of actions thwarts the plugin's attempts to listen to text changes in Coq files:

1. make a new file
2. set the syntax to Coq
3. interact with the plugin --- does not work properly since ST only checks TextChangeListener.is_applicable after step 1, when the file is not yet a Coq file

Steps to reproduce

Create text_change_plugin_example.py in <ST>/Packages/User/:

import sublime
import sublime_plugin


class MyViewEventListener(sublime_plugin.ViewEventListener):

    @classmethod
    def is_applicable(cls, settings):
        # only listen to events on Python3 files
        print(f"Checking applicability with syntax={settings.get('syntax')}")
        return settings.get("syntax", "").endswith("/Python.sublime-syntax")


class MyTextChangeListener(sublime_plugin.TextChangeListener):

    @classmethod
    def is_applicable(cls, buffer):
        return MyViewEventListener.is_applicable(buffer.primary_view().settings())

    def on_text_changed(self, changes):
        print(f"Handling {len(changes)} changes")

Then

1. Create a new file
2. Set the syntax to Python
3. Make some modifications

Expected behavior

There should be "Handling X changes" messages in the console.

Actual behavior

There are no "Handling X changes" messages, since MyTextChangeListener was not given a second chance to advertise its applicability after step 2.

Sublime Text build number

4152

Operating system & version

macOS 12.6.8

(Linux) Desktop environment and/or window manager

No response

Additional information

No response

OpenGL context information

No response

[BenjaminSchaaf]

This is intentional. TextChangeListener.is_applicable can change its output due to any number of things; you're expected to use TextChangeListener.attach and detach to manage the listener yourself if more complex behavior is needed.

[Calvin-L]

Ah, gotcha. If that's the official word then I'm prepared to close this and adopt attach/detach in my plugins.

But, if you have time to clarify a little:

In my current understanding, I don't see any correct implementation of TextChangeListener.is_applicable except return True or return False. Every interesting property of a buffer except its id() is mutable---if ST never re-checks is_applicable, then error-prone attach/detach code looks like the only correct way to restrict which buffers are being listened to.

Obviously ST can't re-check is_applicable for every change to every possible property of a buffer or its views, but re-checking on settings changes seems (to me) like a reasonable middle-ground. It would have nice symmetry with how ST already re-checks ViewEventListener.is_applicable on settings changes.