Set selection, expansion and tick props for ui.tree only when needed (

#3902)

This PR solves a problem mentioned in #3840: The `ui.tree` element
should only be in "selection mode" if there's a selection handler.
Otherwise clicking on a node label should expand/collapse it rather than
toggling the selection.

Click on label expands/collapses node:
```py
ui.tree([
    {'id': 'numbers', 'children': [{'id': '1'}, {'id': '2'}]},
    {'id': 'letters', 'children': [{'id': 'A'}, {'id': 'B'}]},
], label_key='id')
```

Click on label selects/deselects node:
```py
ui.tree([
    {'id': 'numbers', 'children': [{'id': '1'}, {'id': '2'}]},
    {'id': 'letters', 'children': [{'id': 'A'}, {'id': 'B'}]},
], label_key='id', on_select=ui.notify)
```

While at it, I also improved expansion and tick props, so that they are
only defined if event handlers or a tick strategy is given.

nicegui/elements/tree.py

@@ -40,11 +40,13 @@ def __init__(self,
  self._props['node-key'] = node_key
  self._props['label-key'] = label_key
  self._props['children-key'] = children_key
- self._props['selected'] = None
- self._props['expanded'] = []
- self._props['ticked'] = []
- if tick_strategy is not None:
- self._props['tick-strategy'] = tick_strategy
+ if on_select:
+ self._props['selected'] = None
+ if on_expand:
+ self._props['expanded'] = []
+ if on_tick or tick_strategy:
+ self._props['ticked'] = []
+ self._props['tick-strategy'] = tick_strategy or 'leaf'
  self._select_handlers = [on_select] if on_select else []
  self._expand_handlers = [on_expand] if on_expand else []
  self._tick_handlers = [on_tick] if on_tick else []
@@ -79,6 +81,7 @@ def handle_ticked(e: GenericEventArguments) -> None:
 
  def on_select(self, callback: Handler[ValueChangeEventArguments]) -> Self:
  """Add a callback to be invoked when the selection changes."""
+ self._props.setdefault('selected', None)
  self._select_handlers.append(callback)
  return self
 
@@ -87,6 +90,7 @@ def select(self, node_key: Optional[str]) -> Self:
 
  :param node_key: node key to select
  """
+ self._props.setdefault('selected', None)
  if self._props['selected'] != node_key:
  self._props['selected'] = node_key
  self.update()
@@ -98,11 +102,14 @@ def deselect(self) -> Self:
 
  def on_expand(self, callback: Handler[ValueChangeEventArguments]) -> Self:
  """Add a callback to be invoked when the expansion changes."""
+ self._props.setdefault('expanded', [])
  self._expand_handlers.append(callback)
  return self
 
  def on_tick(self, callback: Handler[ValueChangeEventArguments]) -> Self:
  """Add a callback to be invoked when a node is ticked or unticked."""
+ self._props.setdefault('ticked', [])
+ self._props.setdefault('tick-strategy', 'leaf')
  self._tick_handlers.append(callback)
  return self
 
@@ -111,6 +118,7 @@ def tick(self, node_keys: Optional[List[str]] = None) -> Self:
 
  :param node_keys: list of node keys to tick or ``None`` to tick all nodes (default: ``None``)
  """
+ self._props.setdefault('ticked', [])
  self._props['ticked'][:] = self._find_node_keys(node_keys).union(self._props['ticked'])
  self.update()
  return self
@@ -120,6 +128,7 @@ def untick(self, node_keys: Optional[List[str]] = None) -> Self:
 
  :param node_keys: list of node keys to untick or ``None`` to untick all nodes (default: ``None``)
  """
+ self._props.setdefault('ticked', [])
  self._props['ticked'][:] = set(self._props['ticked']).difference(self._find_node_keys(node_keys))
  self.update()
  return self
@@ -129,6 +138,7 @@ def expand(self, node_keys: Optional[List[str]] = None) -> Self:
 
  :param node_keys: list of node keys to expand (default: all nodes)
  """
+ self._props.setdefault('expanded', [])
  self._props['expanded'][:] = self._find_node_keys(node_keys).union(self._props['expanded'])
  self.update()
  return self
@@ -138,6 +148,7 @@ def collapse(self, node_keys: Optional[List[str]] = None) -> Self:
 
  :param node_keys: list of node keys to collapse (default: all nodes)
  """
+ self._props.setdefault('expanded', [])
  self._props['expanded'][:] = set(self._props['expanded']).difference(self._find_node_keys(node_keys))
  self.update()
  return self