Metals: Don't compile projects until VS Code is focused

[jvican]

It is a common scenario to run git operations in the terminal while having VS Code open in another window. For example, myself I usually check out many different branches, rebase and cherry-pick commits before coming back to the editor and making changes to the sources.

Currently, Metals compiles in the background whenever there is a change in a source file. This is problematic because most of the checkouts/rebases I do are not meant to be compiled and triggering a compilation takes time and CPU resources.

I'd like Metals to hold off compilations until I focus again on VS Code's window. Such behavior would be the same as the one VS Code has whenever sources that are open in buffers change in the file system (only when the user focuses in VS Code they are updated).

[olafurpg]

This is a good idea! I believe IntelliJ also buffers up CPU intensive tasks and releases them only once the IntelliJ application window is back in focus. It seems Visual Studio Code exposes an event for when the window state changes

I propose we introduce a new Metals JSON-RPC notification metals/windowStateDidChange with the following parameter

interface WindowStateDidChangeParams {
  focused: boolean;
}

By default, we assume the window is in focus so the behavior remains unchanged for editors that don't support this notification. For editors that send this notification, Metals will buffer up file watching events while the editor window is unfocused and only release those events when the editor window regains focus.

[olafurpg]

Following up from an offline discussion with @agajek, who is going to look into this issue.

[agajek]

Do we have VS Code part implemented? As I've read the event responsible for that functionality
is onDidChangeWindowState: Event<WindowState> and I don't see it in extentions.ts.

[olafurpg]

The VS Code part is not implemented. That window state event sounds like what we’re looking for 👍

[jvican]

I'm not sure #709 fixes this issue. Doing any git operation such as a rebase, a checkout or a stash inside the VS Code terminal still triggers a compilation by Metals. Maybe it works if you use an external terminal such as iTerm.

[olafurpg]

@jvican this feature is still blocked by changes in the vscode extension https://github.com/scalameta/metals-vscode/pull/104

[olafurpg]

Once fully implemented, I don't expect compilations will pause if you do git checkout through the VS Code built-in terminal. Metals will only pause compilations when you focus on another application window like iTerm.

[jvican]

Once fully implemented, I don't expect compilations will pause if you do git checkout through the VS Code built-in terminal. Metals will only pause compilations when you focus on another application window like iTerm.

Is there an API Metals can use to know if the user is focused on the terminal?

It's quite common to use the VS Code terminal to run this kind of operations (at least, I use it all the time in my macOS machines). It would be good to have a fix for it, I would expect many people use it too.

[olafurpg]

@agajek are you aware of VS Code APIs to know when the built-in terminal is focused?

[olafurpg]

I wouldn't be surprised if many people use the built-in git support in VS Code as well..

[agajek]

@olafurpg, unfortunately, I've found only this general event for the whole vscode. I didn't see any distinction for the focus on editor area and others.

[olafurpg]

It might be possible with onDidChangeActiveTextEditor

    window.onDidChangeActiveTextEditor(e => {
      if (e) outputChannel.appendLine("focus: " + e.document.uri.toString());
      else outputChannel.appendLine("focus: undefined");
    });

I struggled to validate what happens when the focus is moved to the terminal since I can't have the output panel open while the terminal is focused.

[jvican]

That looks promising!

[agajek]

Nice, so I'll replace the previous event with that one. Thanks.

[agajek]

Unfortunately it looks like it treats Terminal as a TextEditor. When I changed to the Terminal and switched back to the Output panel a new line was appended indicating that Editor was changed and it prints path for currently opened file/