如何从WPF中的项目容器中获取项目？

Question

Im currently trying to get drag/drop within a treeview working(using databinding and HierarchicalDataTemplate), and have the dragging working, but im running into a problem when trying to get the drop working, since i need to get the data item being dropped on and add it to its item collection of children nodes.

//treeitem is the name of my item data class

private void TreeViewItem_Drop(object sender, DragEventArgs e)
    {
        if (e.Data.GetDataPresent("DragableTreeViewItem"))
        {
     //the data of the treeitem i need to duplicate, provided by the drag operation
            TreeItem data = e.Data.GetData("DragableTreeViewItem") as TreeItem;
            TreeViewItem tvi = sender as TreeViewItem;
            //here im getting the treeviewitem, but i need the treeitem
        }
    }

My best idea on how to get around this was to assign the treeitem as the tag of the treeviewitem on initialization/loading of the tree, and to do so i need to get the itemcontainer of the item, which i already have a working function to get that which i use when initiating the DoDrag()

//returns the item container of the parent of the TreeItem given
private TreeViewItem GetParentContainerFromItem(TreeItem ti)
    {
        List<TreeItem> GetOrderedParents(TreeItem item)
        {
            TreeItem currentParent = item;
            List<TreeItem> items = new List<TreeItem>();

            int i = 0;
            do
            {
                if (currentParent.parentItem != null)
                {
                    items.Insert(0, currentParent);
                    currentParent = currentParent.parentItem;
                }
                else
                {
                    items.Insert(0, currentParent);
                    i++;
                    return items;
                }
            } while (i == 0);
            return null;
        }

        //the local tree in a list, ordered from the original item (in this case "ti") at 0, down to the root at the end of the list
        List<TreeItem> LocalHierarchy = GetOrderedParents(ti);
        if (LocalHierarchy != null)
        {
         //print out the names of each treeitem in it, in order from the root down
            string hierarchyString = "";
            foreach (TreeItem t in LocalHierarchy)
            {
                if (hierarchyString == "")
                {
                    hierarchyString = t.Title;
                }
                else
                {
                    hierarchyString = (hierarchyString + ", " + t.Title);
                }
            }
            System.Console.WriteLine(hierarchyString);

            TreeViewItem localCurrentParent = null;
            TreeViewItem finalContainer = null;

            //walk down the tree in order to get the container of the parent 
            foreach (TreeItem t in LocalHierarchy)
            {

                //if the parent of the item given is a root node, meaning we can return its container
                if (LocalHierarchy.IndexOf(t) == 0 && (LocalHierarchy.IndexOf(t) == (LocalHierarchy.Count - 2)))
                {
                    finalContainer = treeView.ItemContainerGenerator.ContainerFromItem(t) as TreeViewItem;
                    break;
                }
                else
                //if we're at a root node
                if (LocalHierarchy.IndexOf(t) == 0)
                {
                    localCurrentParent = treeView.ItemContainerGenerator.ContainerFromItem(t) as TreeViewItem;
                }
                else
                //if we're at the 2nd to last, AKA the parent of the item given
                if (LocalHierarchy.IndexOf(t) == (LocalHierarchy.Count - 2))
                {
                    finalContainer = localCurrentParent.ItemContainerGenerator.ContainerFromItem(t) as TreeViewItem;
                    break;
                }
                else
                {
                    localCurrentParent = localCurrentParent.ItemContainerGenerator.ContainerFromItem(t) as TreeViewItem;
                }
            }

            if (finalContainer == null)
            {
                System.Console.WriteLine("Final container is null");
            }
            return finalContainer;
        }
        else
        {
            System.Console.WriteLine("ERROR: LocalHierarchy is null");
            return null;
        }
    }

This seems to work perfectly when using to start the DoDrag()

private void DoDrag()
    {
        if(selectedItem != null)
        {
            TreeItem t = selectedItem;
            TreeViewItem tvi = null;
            if (t.parentItem == null)
            {
                //it has no parent, and is a root node
                tvi = treeView.ItemContainerGenerator.ContainerFromItem(t) as TreeViewItem;
            }
            else
            {
                //it has a parent, and i can get the container for the parent
                tvi = GetParentContainerFromItem(t).ItemContainerGenerator.ContainerFromItem(t) as TreeViewItem;
            }
            DragDrop.DoDragDrop(tvi, new DataObject("DragableTreeViewItem", t, true), DragDropEffects.Copy);
            dragNeeded = false;
        }
        else if(selectedItem == null)
        {
            Console.WriteLine("Selected item was null; cant drag");
        }
    }

But when i try to use it in my function to assign container tags it says that Container was null and that GetParentContainerFromItem returned null, yet my function does not log that it returned null

private void AssignContainerTag(TreeItem t)
    {
        TreeViewItem Container = null;

        if (t.parentItem == null)
        {
            //its a root node
            Container =  treeView.ItemContainerGenerator.ContainerFromItem(t) as TreeViewItem;
            Container.Tag = t;
        }
        else
        {
            //it is not a root node
             Container = GetParentContainerFromItem(t).ItemContainerGenerator.ContainerFromItem(t) as TreeViewItem;
            Container.Tag = t;
        }
    }

Ive spent days stumped on why this does not seem to work, so if someone could give me some pointers on what im doing wrong or another way i could get the item from its container it would be a lifesaver. Also please excuse if this is poorly written, i am exhausted and about to go to sleep.

Answer 1

Your question is a little bit confusing. But it looks like you are trying to get the data item of the TreeViewItem which is the drop target of the drag&drop operation.

This is pretty simple. All you need to know is that if the item container is auto-generated via data binding ( ItemsControl.ItemsSource ) the DataContext of the container is the data item itself.
This applies to all item containers of an ItemsControl (eg, ComboBoxItem , ListBoxItem , ListViewItem ).

So TreeViewItem.DataContext references the underlying TreeItem instance that is wrapped by the TreeViewItem :

private void TreeViewItem_Drop(object sender, DragEventArgs e)
{
  if (e.Data.GetDataPresent("DragableTreeViewItem"))
  {
    var sourceItem = e.Data.GetData("DragableTreeViewItem") as TreeItem;

    var dropTargetItemContainer = sender as TreeViewItem;
    var dropTargetItem = targetItemContainer.DataContext as TreeItem;
  }
}

Remark

It looks like you are using the ItemContainerGenerator wrong. TreeView.ItemContainerGenerator will only handle top level items (ie child items). But as a tree node can have child nodes, each TreeViewItem is itself an ItemsControl as it contains an ItemsPresenter to display child items.
Therefore you have to use the appropriate ItemContainerGenerator to retrieve the child container or ItemContainerGenerator will return null .

For top-level items use TreeView.ItemContainerGenerator .
For child items use the parent's TreeViewItem.ItemContainerGenerator .

Also, in case of UI virtualization is enabled not all containers are generated when the TreeView is loaded. They are generated when need eg, for display. Those containers (the TreeViewItem ) are also shared to save resources. So once you set the TreeViewItem.Tag property its value might get lost as a new TreeViewItem instance is generated later to wrap the data item.

So you start at the root node and get its generated container. Now perform a tree search by traversing the TreeViewItems using a specific algorithm until you found the node where the DataContext equals the data item you are looking for and eg, modify the Tag property.
You access the children of eg treeViewItemA by referencing the treeViewItemA.Items property and get their containers by calling treeVieItemA.ItemContainerGenerator.ContainerFromItem method for each child:

Example

public static class MyExtensions
{
  // Get item container of item from TreeView, TreeViewItem, ListView or any ItemsControl
  public static bool TryGetContainerOfChildItem<TItemContainer>(this ItemsControl itemsControl, object item, out TItemContainer itemContainer) where TItemContainer : DependencyObject
  {
    itemContainer = null;
    foreach (object childItem in itemsControl.Items)
    {
      if (childItem == item)
      {
        itemContainer = (TItemContainer) itemsControl.ItemContainerGenerator.ContainerFromItem(item);
        return true;
      }

      DependencyObject childItemContainer = itemsControl.ItemContainerGenerator.ContainerFromItem(childItem);
      if (childItemContainer is ItemsControl childItemsControl && childItemsControl.TryGetContainerOfChildItem(item, out itemContainer))
      {
        return true;
      }
    }

    return false;
  }
}

Usage

// Search whole TreeView
if (treeView.TryGetContainerOfChildItem(item, out TreeViewItem itemContainer)
{
  ...
}

// Search from a specific parent TreeViewItem node
if (treeViewItem.TryGetContainerOfChildItem(item, out TreeViewItem itemContainer)
{
  ...
}