Question

Im trying to make my custom TTreeNode Class

for example

TCustomTreeNode = class(TTreeNode)
  private
  public
    Comment:string;
  end;

and i create and add the node in the tree view like this:

var
  NewCustomTreeNode:TCustomTreeNode;
begin
  NewCustomTreeNode:= TCustomTreeNode.Create(TreeView.Items);
  NewCustomTreeNode.Comment:='blqblq';
  TreeView.Items.AddChild(NewCustomTreeNode,'NodeText');

and when i try to access the custom created tree nodes error pops up. For example i do like this:

TCustomTreeNode(TreeNode).Comment:='asdadssadas';

plase help

Answer 1

The problem with your code is that the call to AddChild results in the tree view creating a new node. And since you didn’t tell the tree view to create a node of your sub-class it creates a plain TTreeNode. And then when you try to cast it to a TCustomTreeNode, the world ends.

You need to use the OnCreateNodeClass method to make sure that the tree view is able to create new nodes. Like this:

type
  TCustomTreeNode = class(TTreeNode)
  protected
    procedure Assign(Source: TPersistent); override;
  public
    Comment: string;
  end;

procedure TCustomTreeNode.Assign(Source: TPersistent);
begin
  if Source is TCustomTreeNode then
    Comment := TCustomTreeNode(Source).Comment;
  inherited;
end;


procedure TForm1.FormCreate(Sender: TObject);
var
  NewNode: TCustomTreeNode;
begin
  NewNode := TreeView1.Items.Add(nil, 'Node1') as TCustomTreeNode;
  NewNode.Comment := 'A comment';
  NewNode := TreeView1.Items.Add(nil, 'Node2') as TCustomTreeNode;
  NewNode.Comment := 'Another comment';
end;

procedure TForm1.TreeView1Click(Sender: TObject);
var
  Node: TCustomTreeNode;
begin
  Node := TreeView1.Selected as TCustomTreeNode;
  if Assigned(Node) then
    ShowMessage(Node.Comment);
end;

procedure TForm1.TreeView1CreateNodeClass(Sender: TCustomTreeView; var NodeClass: TTreeNodeClass);
begin
  NodeClass := TCustomTreeNode;
end;

I can’t claim to being the world’s greatest expert on Delphi tree views but in my experience you never create a tree node yourself. You should always call one of the AddXXX methods on TTreeView.Items to create new nodes.