Expose the collapsible state of the tree item

[chrmarti]

That would allow the TreeDataProvider to preserve the collapsibleState on updates.

[weinand]

For me it would even be sufficient to preserve the expand/collapse state that the user has changed. So the TreeItem.collapsibleState would be only used for the initial state.

[chrisdias]

This would let me run an expensive refresh operation in the docker extension only when the parent node is expanded. right now it runs even if the parent node is collapsed because I can't see the state of the node.

[eamodio]

Same for me with GitLens. Though ideally I'd want an state change event so that I could hook/unhook other handlers (file system watchers) only if needed.

[hbenl]

+1
Right now it is impossible to preserve the visible state on updates (which can be very annoying for the user). Even when I'm reusing the old TreeItems, their state is replaced with the initial collapsibleState.

[weinand]

I'm bumping the severity of this since it makes the tree very difficult to use for users.
In addition this is a bug not a feature request.

[GordonSmith]

@weinand If you also made label + collapsibleState refresh their UI when they are written to, then it would allow a simple way to refresh a single node.

Or support: tree._onDidChangeTreeData.fire(<T>); to trigger a refresh of single node?

[sandy081]

Looks like this issue contains multiple requests

1. Expose collapsible state of Tree item (Original request)
2. Preserve the collapsible state of the Tree item on updates - Retain the tree expansion state after tree refresh #39505
3. Firing onDidChangeTreeData event on a node should also refresh itself - Firing TreeDataProvider.onDidChangeTreeData(item) doesn't fully update the item #34789

Instead of hijacking this for multiple issues, I have created issues for each.

Coming back to this issue, IMO, this is a feature request to expose the view state of the Tree. Issue #39505 is a bug and tagged important.

Please let me know if this feature is still needed after fixing issues #39505 and #34789. If so, please provide the use cases. Otherwise, I would prefer to close it.

@weinand FYI

[sandy081]

As I do not see any scenarios to support this request after other fixes/enhancements done to custom view, I am inclined to close this.

If there is still need of it, please reopen it with the supporting use cases.

Thanks.

[eamodio]

I would vote to re-open this. GitLens has certain nodes that when expanded, start file watchers and other event hooks, but currently I have no way to undo them if the node is collapsed. These leads to wasted performance, battery, etc

[chrisdias]

The Docker extension has Images and Containers nodes. When expanded, I poll Docker for status updates (start, stopped, creation date/time). This is expensive and when the Images or Containers nodes are collapsed, I would like to get an event and be able to test the state so I can stop polling.

[sandy081]

@eamodio @chrmarti Your use cases makes sense to me.

[sandy081]

Added following events in the Treeview

/**
 * Event that is fired when an element is expanded
 */
readonly onDidExpandElement: Event<T>;

/**
 * Event that is fired when an element is collapsed
 */
readonly onDidCollapseElement: Event<T>;

[sandy081]

All, please note that there is a slight change done to the events

export interface TreeViewExpansionEvent<T> {

	/**
	 * Element that is expanded or collapsed.
	 */
	element: T;

}

/**
 * Event that is fired when an element is expanded
 */
readonly onDidExpandElement: Event<TreeViewExpansionEvent<T>>;

/**
 * Event that is fired when an element is collapsed
 */
readonly onDidCollapseElement: Event<TreeViewExpansionEvent<T>>;