Support cell-level custom context from extensions

[roblourens]

Notebook extensions often need to set a cell-level context for actions that are different per-cell. Currently there is no way to do that, except for workarounds like changing the global context based on the currently selected cell.

Examples: Run by line, #146353

Currently extensions can use the setContext command to set global context values. Custom trees allow setting a single context value on individual items with the contextValue property. I think that only the tree provider can set this. In notebooks we would ideally allow different extensions to set different context key/values on a cell.

[roblourens]

We could do a low-impact thing like extending the setContext command to allow setting a value within a certain cell sub-context.

commands.executeCommand('setContext', 'jupyter.runByLineInProgress', true, { cellUri: cell.uri });

Or real API on NotebookCell

export interface NotebookCell {
	setContext(key: string, value: any): void;
}

[roblourens]

It was pointed out that we can pretty much do this without new API, maybe using an array context key and the "in" operator: https://code.visualstudio.com/api/references/when-clause-contexts#in-conditional-operator

We have a set of 'resource' context keys which are bound to the editor resource URI, but not the cell URI. Cell URIs are the document URI plus some fragment. I could bind those keys to the cell URI, or add a new set (like resource -> cellResource) if it might interfere, like if cell URIs diverge more from the document URI.

Also, the fragment is not exposed. The resource key is a URI but I can't use this with a custom context key from setContext because it can only do object-equality. That can be fixed.

So to give an example, one scenario is

• VS Code binds the existing resource keys to the cell URI in the cell's CKS
• the extension contributes a menu item with "when": "resource in jupyter.runByLineCells"
• the extension runs something like
  executeCommand('setContext', 'jupyter.runByLineCells', [cell.uri])
• and the menu item shows up in the cell toolbar

cc @jrieken @rebornix

[jrieken]

@roblourens Do you want this to be something per notebook document or per notebook editor?

[roblourens]

Per document. So the same cell in split editors gets the same menu items.