Add label for transaction commits for undo/redo grouping.

[daesunp]

Description

This PR exposes a user provided "label" object that can be accessed through SchematizingSimpleTreeView.currentTransactionLabel, which can be accessed from the event callback in commitApplied.

[github-actions]

🔗 No broken links found! ✅

Your attention to detail is admirable.

linkcheck output

> [email protected] ci:check-links /home/runner/work/FluidFramework/FluidFramework/docs
> start-server-and-test "npm run serve -- --no-open" 3000 check-links

1: starting server using command "npm run serve -- --no-open"
and when url "[ 'http://127.0.0.1:3000' ]" is responding with HTTP status code 200
running tests using command "npm run check-links"


> [email protected] serve
> docusaurus serve --no-open

[SUCCESS] Serving "build" directory at: http://localhost:3000/

> [email protected] check-links
> linkcheck http://localhost:3000 --skip-file skipped-urls.txt

Crawling...

Stats:
  243138 links
    1776 destination URLs
    2013 URLs ignored
       0 warnings
       0 errors

[noencke]

This is the first half of the feature - adding and setting the property. The second half is to pass the property to the changed and commitApplied event. I think that both halves are small enough changes that you should do them in the same PR. So next:

1. Create a CommitMetadataAlpha that extends CommitMetadata and has a label: unknown property.
2. Update both TreeBranch.commitApplied and TreeBranch.changed to use CommitMetadataAlpha.
3. Update the tests to consume the label from the event commit metadata rather than directly from the static.

[noencke]

Currently, there is no benefit to having a private property here and then making a getter and setter, because the getter and setter do no validation or transformation of the value.

I do think we would benefit from constraining the way that this field can be set. How about this:

private static transactionLabel?: unknown;

public static runWithTransactionLabel<T>(label: T, fn: (label: T) => void): void {
  this.transactionLabel = label;
  try {
    fn(label);
  } finally {
    this.transactionLabel = undefined;
  }
}

Then it's impossible for any code to ever accidentally set the label and forget to clear it, or forget to properly handle errors.

The passing of label to the fn and the resulting generic parameter is not strictly necessary but just a nice to have, IMO.

[noencke]

I think this property might work out even better if you put it on TreeCheckout - and it could be a member rather than be static. I know we discussed using a static because it's correct and it's easy - but if a non-static member is just as easy, then non-static is probably better style/convention.