Access violation after TStringEditLink get destroyed (TVirtualStringTree) - Lazarus example

Question

I've try to implement an editor for a VirtualStringTree based on example of Lazarius

Can you tell me why did I get an Access Violation after TStringEditLink gets destroyed?

It's weired that error appear only when i press ESCAPE or ENTER. If i click from one cell to another there is no error.

Like an observation, I sow that if I remove the FEdit.Free code from destructor TStringEditLink.Destroy the error disappear.

Do you have a solution for this?

Bellow the full code:

unit Unit2;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, VirtualTrees, EditorLink, Vcl.StdCtrls,
  Vcl.ExtCtrls, Vcl.Imaging.jpeg;

type
  TTreeData = record
    Fields: array of String;
  end;
  PTreeData = ^TTreeData;

const
  SizeVirtualTree = SizeOf(TTreeData);

type
  TForm2 = class(TForm)
    VirtualTree: TVirtualStringTree;
    procedure FormCreate(Sender: TObject);
    procedure VirtualTreeClick(Sender: TObject);
    procedure VirtualTreeCreateEditor(Sender: TBaseVirtualTree;
      Node: PVirtualNode; Column: TColumnIndex; out EditLink: IVTEditLink);
    procedure VirtualTreeEditing(Sender: TBaseVirtualTree; Node: PVirtualNode;
      Column: TColumnIndex; var Allowed: Boolean);
    procedure VirtualTreeFreeNode(Sender: TBaseVirtualTree; Node: PVirtualNode);
    procedure VirtualTreeGetNodeDataSize(Sender: TBaseVirtualTree;
      var NodeDataSize: Integer);
    procedure VirtualTreeGetText(Sender: TBaseVirtualTree; Node: PVirtualNode;
      Column: TColumnIndex; TextType: TVSTTextType; var CellText: string);
    procedure VirtualTreeNewText(Sender: TBaseVirtualTree; Node: PVirtualNode;
      Column: TColumnIndex; NewText: string);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form2: TForm2;

implementation

{$R *.dfm}

procedure TForm2.FormCreate(Sender: TObject);
var
  Node: PVirtualNode;
  LTreeData: PTreeData;
begin
  VirtualTree.Clear;
  VirtualTree.BeginUpdate;

  //node 1
  Node:= VirtualTree.AddChild(nil,nil);
  VirtualTree.ValidateNode(Node,False);

  LTreeData:= VirtualTree.GetNodeData(Node);
  SetLength(LTreeData^.Fields,3);

  LTreeData^.Fields[0]:= 'John';
  LTreeData^.Fields[1]:= '2500';
  LTreeData^.Fields[2]:= 'Production';

  //node 2
  Node:= VirtualTree.AddChild(nil,nil);
  VirtualTree.ValidateNode(Node,False);

  LTreeData:= VirtualTree.GetNodeData(Node);
  SetLength(LTreeData^.Fields,3);

  LTreeData^.Fields[0]:= 'Mary';
  LTreeData^.Fields[1]:= '2100';
  LTreeData^.Fields[2]:= 'HR';

  VirtualTree.EndUpdate;
end;

procedure TForm2.VirtualTreeClick(Sender: TObject);
var
  VT: TVirtualStringTree;
  Click: THitInfo;
begin
  VT:= Sender as TVirtualStringTree;
  VT.GetHitTestInfoAt(Mouse.CursorPos.X-VT.ClientOrigin.X, Mouse.CursorPos.Y-VT.ClientOrigin.Y, True, Click);
  VT.EditNode(Click.HitNode,Click.HitColumn);
end;

procedure TForm2.VirtualTreeCreateEditor(Sender: TBaseVirtualTree;
  Node: PVirtualNode; Column: TColumnIndex; out EditLink: IVTEditLink);
begin
  EditLink := TStringEditLink.Create;
end;

procedure TForm2.VirtualTreeEditing(Sender: TBaseVirtualTree;
  Node: PVirtualNode; Column: TColumnIndex; var Allowed: Boolean);
begin
  Allowed:= True;
end;

procedure TForm2.VirtualTreeFreeNode(Sender: TBaseVirtualTree;
  Node: PVirtualNode);
var
  LTreeData: PTreeData;
begin
  LTreeData:= Sender.GetNodeData(Node);
  Finalize(LTreeData^);
end;

procedure TForm2.VirtualTreeGetNodeDataSize(Sender: TBaseVirtualTree;
  var NodeDataSize: Integer);
begin
  NodeDataSize:= SizeVirtualTree;
end;

procedure TForm2.VirtualTreeGetText(Sender: TBaseVirtualTree;
  Node: PVirtualNode; Column: TColumnIndex; TextType: TVSTTextType;
  var CellText: string);
var
  LTreeData: PTreeData;
begin
  if Assigned(Node) and (Column > NoColumn) then
    begin
      LTreeData:= Sender.GetNodeData(Node);
      CellText:= LTreeData^.Fields[Column];
    end;
end;

procedure TForm2.VirtualTreeNewText(Sender: TBaseVirtualTree;
  Node: PVirtualNode; Column: TColumnIndex; NewText: string);
var
  LTreeData: PTreeData;
begin
  LTreeData:= Sender.GetNodeData(Node);
  LTreeData^.Fields[Column]:= NewText;
end;

end.

and the EditorLink unit

unit EditorLink;

interface

uses
  Classes, SysUtils, Forms, Controls, Graphics, Dialogs,
  VirtualTrees, Messages, Windows, StdCtrls, Vcl.ExtCtrls;

type

  TStringEditLink = class(TInterfacedObject, IVTEditLink)
  private
    FEdit: TWinControl;
    FTree: TVirtualStringTree;
    FNode: PVirtualNode;
    FColumn: Integer;
    FStopping: Boolean;
  protected
    procedure EditKeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
  public
    destructor Destroy; override;
    function BeginEdit: Boolean; stdcall;
    function CancelEdit: Boolean; stdcall;
    function EndEdit: Boolean; stdcall;
    function GetBounds: TRect; stdcall;
    function PrepareEdit(Tree: TBaseVirtualTree; Node: PVirtualNode; Column: TColumnIndex): Boolean; stdcall;
    procedure ProcessMessage(var Message: TMessage); stdcall;
    procedure SetBounds(R: TRect); stdcall;
  end;

implementation

uses unit2;

destructor TStringEditLink.Destroy;
begin
  FEdit.Free;  //--> seems that due to this I get the access violation
  inherited;
end;

procedure TStringEditLink.EditKeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
begin
  case Key of
    VK_ESCAPE:
      begin
        FTree.CancelEditNode;
        Key := 0;
        FTree.setfocus;
      end;
    VK_RETURN:
      begin
       PostMessage(FTree.Handle, WM_KEYDOWN, VK_DOWN, 0);
       Key := 0;
       FTree.EndEditNode;
       FTree.setfocus;
      end;
  end; //case
end;

function TStringEditLink.BeginEdit: Boolean;
begin
  Result := not FStopping;
  if Result then
    begin
      FEdit.Show;
      FEdit.SetFocus;
    end;
end;

function TStringEditLink.CancelEdit: Boolean;
begin
  Result := True;
  FEdit.Hide;
end;

function TStringEditLink.EndEdit: Boolean;
var
  s: String;
begin
  Result := True;
  s := TComboBox(FEdit).Text;
  FTree.Text[FNode, FColumn] := s;

  FTree.InvalidateNode(FNode);
  FEdit.Hide;
  FTree.SetFocus;
end;

function TStringEditLink.GetBounds: TRect;
begin
  Result := FEdit.BoundsRect;
end;

function TStringEditLink.PrepareEdit(Tree: TBaseVirtualTree; Node: PVirtualNode; Column: TColumnIndex): Boolean;
var
  FCellText: String;
  FCellTextBounds: TRect;
  FCellFont: TFont;
begin
  Result := True;
  FTree := Tree as TVirtualStringTree;

  FNode := Node;
  FColumn := Column;

  FCellFont:= TFont.Create;
  FTree.GetTextInfo(FNode, FColumn, FCellFont, FCellTextBounds, FCellText);

  FEdit := TComboBox.Create(nil);
  with FEdit as TComboBox do
    begin
      Visible := False;
      Parent := Tree;
      Items.Add('Google');
      Items.Add('Yahoo');
      Items.Add('Altavista');
      OnKeyDown := EditKeyDown;
      Text:= FCellText;
    end;
end;

procedure TStringEditLink.ProcessMessage(var Message: TMessage);
begin
  FEdit.WindowProc(Message);
end;

procedure TStringEditLink.SetBounds(R: TRect);
var
  Dummy: Integer;
begin
  FTree.Header.Columns.GetColumnBounds(FColumn, Dummy, R.Right);
  FEdit.BoundsRect := R;
end;

end.

Answer 1

The solution I used at the end is listed bellow:

TBasePanel = class(TPanel)
  private
    procedure CMRelease(var Message: TMessage); message CM_RELEASE;
  protected
  public
    procedure Release; virtual;
  end;

TStringEditLink = class(TInterfacedObject, IVTEditLink)
  private
    FBasePanel: TBasePanel;
    ...
  protected
    procedure EditKeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
  public
    destructor Destroy; override;
    function BeginEdit: Boolean; stdcall;
    function CancelEdit: Boolean; stdcall;
    function EndEdit: Boolean; stdcall;
    function GetBounds: TRect; stdcall;
    function PrepareEdit(Tree: TBaseVirtualTree; Node: PVirtualNode; Column: TColumnIndex): Boolean; stdcall;
    procedure ProcessMessage(var Message: TMessage); stdcall;
    procedure SetBounds(R: TRect); stdcall;
  end;

implementation

procedure TBasePanel.CMRelease(var Message: TMessage);
begin
  Free;
end;

procedure TBasePanel.Release;
begin
  if HandleAllocated then
    PostMessage(Handle, CM_RELEASE, 0, 0);
end;

destructor TStringEditLink.Destroy;
begin
  if Assigned(FBasePanel) then FBasePanel.Release;
  inherited;
end;

FBasePanel should be used as owner and as parent for as many component editors would like to be displayed in the same time.