From what I can see, currently the only way to set the expanded items for a TreeList is through the state.

The main problem is that the state doesn't fire the OnExpand event, meaning that if you have a server loaded tree you need to make sure to load the data before each item is expanded. It also allows for duplicates in the 'ExpandedItems' if you don't check for duplicates before adding to it.

The TreeView has an 'ExpandedItems' parameter that can be bound which is a bit nicer than using the state, but from my quick test this also doesn't fire the OnExpand event.

My request is for one of the following to be added:

1. Similar to how you can set the 'HasChildrenField' or 'ItemsField' on the TreeList, allow a 'IsExpandedField' value to be bound (Should fire the OnExpand when changed if possible). This would be the simplest way allow bindable control over the expansion of items while also making it so we don't have to manage duplicates in the 'ExpandedItems' collection.
   
2. Expose a method to expand/unexpand rows. This would be different to manually changing the state as it would also fire the OnExpand event for each row expanded.
   
3. Similar to the TreeView, allow an 'ExpandedItems' to be bound. Preferably this would also fire the OnExpand event.