How to populate a tree view based on a flat list with “levels”?

Question

I have a list of objects populated from a third-party project file. The way this file was designed is so each item is on a \"level\" of hierarchy. So the very first item is

Answer 1

Create a list that will contain the latest parent node for each level. Initially this list will be empty.

Now walk the linear list. Whenever you add an item at level N to the list, you do the following:

• Add it as a child of the latest parent at level N-1, and
• set the latest parent at level N to be the item you just added.

You'll need to handle the case where N=0 for which the above algo would require you to come up with the latest parent at level -1, whatever that means. Does your tree have an overall root node? If so, then by definition, the latest parent at level -1 is the root node. Otherwise, if there's no root node, you'll need do whatever your tree view component requires to add a node at the top level, as opposed to as a child of another node.

Assuming that there is no root node, then the code would look like this:

var
  Item: TItem;
  LatestParents: TList;
  Parent, NewNode: TTreeNode;
begin
  LatestParents := TList.Create;
  try
    LatestParents.Add(nil);
    for Item in Items do
    begin
      Parent := LatestParents[Item.Level];
      NewNode := TreeView.Items.AddChild(Parent, Item.Text);
      LatestParents.Count := Max(LatestParents.Count, Item.Level+2);
      LatestParents[Item.Level+1] := NewNode;
    end;
  finally
    LatestParents.Free;
  end;
end;

You may want to put some error checking into the code if there's a possibility of your code encountering a malformed description of the tree. For instance if the first node that you encounter does not have level 0, then this code will fail.