Question

I have an TImage on a TPanel, and an other (empty) TPanels. I want to drag
the image from the first to the second panel using the drag and drop.

I actually want to see the image while it’s moving from one panel to the
other (semi-transparent).

I think I should use TDragObject.GetDragImages but I can’t figure out how to construct the whole magic.

procedure TForm1.Image1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  Image1.ControlStyle := Image1.ControlStyle + [csDisplayDragImage]; // ???
  TImage(Sender).BeginDrag(False);
end;

procedure TForm1.Image1StartDrag(Sender: TObject; var DragObject: TDragObject);
begin
  // ???
end;

procedure TForm1.Panel1DragOver(Sender, Source: TObject; X, Y: Integer;
  State: TDragState; var Accept: Boolean);
begin
  if (Source is TImage) then
    Accept := TImage(Source).Parent <> Sender;
end;

procedure TForm1.Panel1DragDrop(Sender, Source: TObject; X, Y: Integer);
begin
  if (Source is TImage) then
  begin
    TImage(Source).Parent := TPanel(Sender);
    TImage(Source).Align := alClient;
  end;
end;

---

Update – I found a useful article: Implementing Professional Drag & Drop In VCL/CLX Applications

Answer 1

unit Unit3;

interface
// 2012 Thomas Wassermann - demo
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs,  ExtCtrls;

type


  TMyDragObject = class(TDragControlObject)
  private
    FImageList:TImageList;
    FDragSource:TControl;
  protected
    function GetDragImages: TDragImageList; override;
  public
    Procedure StartDrag(G:TGraphic;p:TPoint;DragSource:TControl);
    Constructor Create(AControl: TControl); override;
    Destructor Destroy;override;
    Property DragSource:TControl read FDragSource;
  end;



  TForm3 = class(TForm)
    Panel1: TPanel;
    Panel2: TPanel;
    Image1: TImage;
    procedure Image1StartDrag(Sender: TObject; var DragObject: TDragObject);
    procedure FormCreate(Sender: TObject);
    procedure Panel1DragOver(Sender, Source: TObject; X, Y: Integer; State: TDragState; var Accept: Boolean);
    procedure FormDestroy(Sender: TObject);
    procedure Panel1DragDrop(Sender, Source: TObject; X, Y: Integer);
  private
    { Private-Deklarationen }
    FDragObject:TMyDragObject;
  public
    { Public-Deklarationen }
  end;

var
  Form3: TForm3;

implementation

constructor TMyDragObject.Create(AControl: TControl);
begin
  inherited;
  FImageList:=TImageList.Create(nil);
end;

destructor TMyDragObject.Destroy;
begin
  FImageList.Free;
  inherited;
end;

function TMyDragObject.GetDragImages: TDragImageList;
begin
   Result := FImageList;
end;

{$R *.dfm}

procedure TMyDragObject.StartDrag(G: TGraphic;p:TPoint;DragSource:TControl);
var
 bmp:TBitMap;
begin
   FDragSource := DragSource;
   bmp:=TBitMap.Create;
   try
   FImageList.Width := g.Width;
   FImageList.Height := g.Height;
   bmp.Width := g.Width;
   bmp.Height := g.Height;
   bmp.Canvas.Draw(0,0,g);
   FImageList.Add(bmp,nil);
   finally
     bmp.Free;
   end;
    FImageList.SetDragImage(0,p.x,p.y)
end;

procedure TForm3.FormCreate(Sender: TObject);
var
 i:Integer;
begin
  ControlStyle := ControlStyle + [csDisplayDragImage];
  for I := 0 to ControlCount -1  do
      if Controls[i] is TPanel then
         TPanel(Controls[i]).ControlStyle := TPanel(Controls[i]).ControlStyle + [csDisplayDragImage];
  ReportMemoryLeaksOnShutDown := True;
end;

procedure TForm3.FormDestroy(Sender: TObject);
begin
  if Assigned(FDragObject) then FDragObject.Free;
end;

procedure TForm3.Image1StartDrag(Sender: TObject; var DragObject: TDragObject);
var
 p:TPoint;

begin
    p:=TImage(Sender).ScreenToClient(mouse.cursorpos);
    if Assigned(FDragObject) then FDragObject.Free;
    FDragObject := TMyDragObject.Create(TImage(Sender));
    FDragObject.StartDrag(TImage(Sender).Picture.Graphic,p,TImage(Sender));
    DragObject := FDragObject;
end;

procedure TForm3.Panel1DragDrop(Sender, Source: TObject; X, Y: Integer);
begin
  if FDragObject.DragSource is TImage then
    TImage(FDragObject.DragSource).Parent := TPanel(Sender);
end;

procedure TForm3.Panel1DragOver(Sender, Source: TObject; X, Y: Integer; State: TDragState; var Accept: Boolean);
begin
    Accept := Source is TMyDragObject;
end;

end.