TPanel.OnDragDrop Event

Occurs when the user drops an object being dragged.

Pascal

property OnDragDrop: TDragDropEvent;

C++

__property TDragDropEvent OnDragDrop;

Description

Use the OnDragDrop event handler to specify what happens when the user drops an object. The Source parameter of the OnDragDrop event is the object being dropped, and the Sender is the control the object is being dropped on. The X and Y parameters are the coordinates of the mouse positioned over the control.

C++ Examples:

Copy Code

/*
This code is an OnDragOver event handler that won't allow a
label control to be dropped on a panel control and stops the
dragging of the label as soon as the user drags the label
onto the panel:
Note: Set the label's DragMode property to dmAutomatic.
*/
void __fastcall TForm1::FormDragDrop(TObject *Sender, TObject *Source, int X, int Y)

{
  reinterpret_cast<TForm1 *>(Source)->Left = X;
  reinterpret_cast<TForm1 *>(Source)->Top = Y;
}

void __fastcall TForm1::FormDragOver(TObject *Sender, TObject *Source, int X, int Y,
          TDragState State, bool &Accept)
{
  Accept = Source->ClassNameIs("TLabel");
}

void __fastcall TForm1::Panel1DragOver(TObject *Sender, TObject *Source, int X, int Y,
          TDragState State, bool &Accept)
{
  Accept = false;
  if ((Source->ClassNameIs("TLabel")) && (State == dsDragEnter))
    dynamic_cast<TLabel *>(Source)->EndDrag(false);
}

Copy Code

/*
This code comes from an application that contains a list box
and three labels, each with a different font and color.  The
DragMode property for each of the labels is dmAutomatic.
The user can select a label and drag it to a list box and
drop it. When the label is dropped, the items in the list
box assume the color and font of the dropped label.
*/

// This OnDragOver event handler permits the list box to
// accept a dropped label:
void __fastcall TForm1::ListBox1DragOver(TObject *Sender, TObject *Source, int X, int Y, TDragState State, bool &Accept)
{
   Accept = Source->ClassNameIs("TLabel");
}

/*
This OnDragDrop event handler implements the drop behavior.
*/ 
void __fastcall TForm1::ListBox1DragDrop(TObject *Sender, TObject *Source, int X, int Y)
{
  if (Sender->ClassNameIs("TListBox") && Source->ClassNameIs("TLabel"))
  {
    TListBox *DestList = dynamic_cast<TListBox *>(Sender);
    DestList->Font = (dynamic_cast<TLabel *>(Source))->Font;
    DestList->Color = (dynamic_cast<TLabel *>(Source))->Color;
    DestList->DoubleBuffered = true;
    DestList->Color = clWindow;
  }
}

void __fastcall TForm1::FormCreate(TObject *Sender)
{
  ListBox1->Items->Add("Not");
  ListBox1->Items->Add("In");
  ListBox1->Items->Add("Alphabetical");
  ListBox1->Items->Add("Order");

}

Delphi Examples:

Copy Code

{
This code is an OnDragOver event handler that won't allow a
label control to be dropped on a panel control and stops the
dragging of the label as soon as the user drags the label
onto the panel:
Note: Set the label's DragMode property to dmAutomatic.
}
type
  TForm1 = class(TForm)
    Panel1: TPanel;
    Label1: TLabel;
    procedure Panel1DragOver(Sender, Source: TObject; X, Y: Integer;
      State: TDragState; var Accept: Boolean);
    procedure FormDragOver(Sender, Source: TObject; X, Y: Integer;
      State: TDragState; var Accept: Boolean);
    procedure FormDragDrop(Sender, Source: TObject; X, Y: Integer);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormDragDrop(Sender, Source: TObject; X, Y: Integer);
begin
  TForm1(Source).Left := X;
  TForm1(Source).Top := Y;
end;

procedure TForm1.FormDragOver(Sender, Source: TObject; X, Y: Integer;
  State: TDragState; var Accept: Boolean);
begin
  Accept := Source is TLabel;
end;

procedure TForm1.Panel1DragOver(Sender, Source: TObject; X, Y: Integer;
  State: TDragState; var Accept: Boolean);
begin
  Accept := False;
  if (Source is TLabel) and (State = dsDragEnter) then
    (Source as TLabel).EndDrag(False);
end;

Copy Code

{
This code requires a list box and three labels, each with a
different font and color.  The DragMode property for each of
the labels is dmAutomatic.  The user can select a label and
drag it to a list box and drop it. When the label is dropped,
the items in the list box assume the color and font of the
dropped label.
}

procedure TForm1.FormCreate(Sender: TObject);
begin
  ListBox1.Items.Add('Not');
  ListBox1.Items.Add('In');
  ListBox1.Items.Add('Alphabetical');
  ListBox1.Items.Add('Order');
end;

// This OnDragOver event handler permits the list box to
// accept a dropped label:

procedure TForm1.ListBox1DragOver(Sender, Source: TObject; X, Y: Integer;
  State: TDragState; var Accept: Boolean);
begin
  Accept := Source is TLabel;
end;

// This OnDragDrop event handler implements the drop behavior.

procedure TForm1.ListBox1DragDrop(Sender, Source: TObject; X, Y: Integer);
begin
  if (Sender is TListBox) and (Source is TLabel) then
  begin
    with Sender as TListBox do
    begin
      Font := (Source as TLabel).Font;
      Color := (Source as TLabel).Color;
      DoubleBuffered := true;
      Color := clWindow;
    end;
  end;
end;