[Feature request] Watch for changes to relevant project files

[NSExceptional]

I originally opened this as an issue on the sourcekit-lsp repo, at the time not really sure where this functionality belonged. I was told it'd be part of the BSP by this comment here.

To rehash my use case outlined in my original issue:

Consider the following scenario:

1. I'm editing a CXX project in VS Code and I see a warning that I don't want enabled
2. I go into Xcode and disable the warning
3. I observe the warning does not disappear from VS Code, so I run the "Reload Window" command to restart the language server
4. I observe the warning is gone now
5. The reverse holds true as well and the warning reappears when re-enabled and the server restarted

And to summarize the advice from the linked comment:

• The BSP server can watch for file changes that affect the build settings by specifying watchers in the build/initialize response
• It can then notify SourceKit-LSP about the build settings change by sending a buildTarget/didChange notification

I suspect this might address #52 and #89

I'm happy to help and contribute, but I may need some guidance; I'm not even sure where to begin. I don't see how specific messages are handled for example. Looks like some runtime magic?

[SolaWing]

When the setting changes, xcode-build-server does automatically notify LSP. However, the compilation flags of xcode-build-server come from the build log. Therefore, if you do not trigger a build after changing the flags, xcode-build-server will not have a new log to read the new compilation flags.

In order to enhance the immediacy of this feedback, some degree of automatic build or indexing is needed. However, I haven't found a good way to do it yet. If you simply call xcodebuild, there may be performance issues (xcodebuild is triggered very slowly).

If you are interested in implementing this feature, I would appreciate your contribution and welcome to communicate with me at any time.

PS: the BSP server entrance is

xcode-build-server/server.py

Line 472 in 937bc9c

def serve():

, and the most importantant infomation provide to LSP is the flags:

xcode-build-server/server.py

Line 160 in 937bc9c

def notify_option_changed(self, uri):