TTreeView.md

Using TTreeView in Lazarus

The TTreeView component in Lazarus is a standard component that allows you to create hierarchical lists. This guide will walk you through its usage and common operations.

Adding TTreeView to Your Form

1. In the Lazarus IDE, go to the "Standard" tab of the component palette.
2. Find and double-click on TTreeView to add it to your form.

Basic Usage in Code

First, ensure you have the necessary unit in your uses clause:

uses
  ComCtrls; // Make sure this is in your uses clause

Here's a basic example of adding nodes to the TTreeView:

procedure TForm1.FormCreate(Sender: TObject);
var
  RootNode, ChildNode: TTreeNode;
begin
  // Add a root node
  RootNode := TreeView1.Items.Add(nil, 'Root Node');
  
  // Add child nodes
  ChildNode := TreeView1.Items.AddChild(RootNode, 'Child 1');
  TreeView1.Items.AddChild(RootNode, 'Child 2');
  
  // Add a grandchild
  TreeView1.Items.AddChild(ChildNode, 'Grandchild');
  
  // Expand the root node
  RootNode.Expand(False);
end;

Handling Node Selection

procedure TForm1.TreeView1SelectionChanged(Sender: TObject);
begin
  if TreeView1.Selected <> nil then
    ShowMessage('Selected: ' + TreeView1.Selected.Text);
end;

Adding Nodes Dynamically

procedure TForm1.AddNodeButtonClick(Sender: TObject);
var
  NewNode: TTreeNode;
begin
  if TreeView1.Selected <> nil then
    NewNode := TreeView1.Items.AddChild(TreeView1.Selected, 'New Node')
  else
    NewNode := TreeView1.Items.Add(nil, 'New Root Node');
  TreeView1.Selected := NewNode;
end;

Deleting Nodes

procedure TForm1.DeleteNodeButtonClick(Sender: TObject);
begin
  if TreeView1.Selected <> nil then
    TreeView1.Items.Delete(TreeView1.Selected);
end;

Customizing the Appearance

TreeView1.ShowRoot := False; // Hide the root node
TreeView1.ShowLines := False; // Hide the lines connecting nodes
TreeView1.ReadOnly := True; // Make the tree read-only

Working with Node Data

You can associate custom data with each node using the Data property:

type
  PMyData = ^TMyData;
  TMyData = record
    ID: Integer;
    Description: string;
  end;

procedure TForm1.AddNodeWithData;
var
  NewNode: TTreeNode;
  NodeData: PMyData;
begin
  New(NodeData);
  NodeData^.ID := 1;
  NodeData^.Description := 'Some description';
  NewNode := TreeView1.Items.AddChild(nil, 'Node with Data');
  NewNode.Data := NodeData;
end;

// Don't forget to free the data when deleting nodes or closing the form
procedure TForm1.FreeNodeData(Node: TTreeNode);
begin
  if Node.Data <> nil then
    Dispose(PMyData(Node.Data));
end;

Handling Keyboard Input

procedure TForm1.TreeView1KeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
begin
  case Key of
    VK_DELETE:
      if TreeView1.Selected <> nil then
        TreeView1.Items.Delete(TreeView1.Selected);
    VK_INSERT:
      AddNodeButtonClick(nil); // Reuse the add node method
  end;
end;

Remember to set up event handlers in the Object Inspector for events like OnSelectionChanged, OnKeyDown, etc., as needed for your application.

This guide provides a good starting point for working with TTreeView in Lazarus. You can further customize its behavior and appearance based on your specific requirements.