How to drag a thumbnail from JvtThumbview? - delphi

Question

I am writing a WYSIWYG type of editor program in which the user can drag image thumbnails onto an editor surface (TPanel) and then create a PDF by rendering the editor surface onto the PDF.
On my TPanel, I have a TImage which the user can resize and move. I am using TSizeCtrl for this.
I have a TJvThumbview which is being loaded with images from a disk folder.
I want to accomplish drag-drop from the JvThumbview onto the TImage - but cannot do this.
Please can someone detail how I would accomplish this?
Thanks so much in advance.

Answer 1

I cannot resist.
My demo project consists of:
one TJvThumbView and
one TImage
Dragging is achieved by:
starting the drag operation when the user mouse-downs on the thumb view,
managing the dragged image by a TDragObject derivative,
drawing the dragged image when the drag object says the drag operation ended on the TImage.
This is how it could look like:
unit Unit1;
interface
uses
Classes, Graphics, Controls, Forms, JvExForms, JvBaseThumbnail, JvThumbViews,
ExtCtrls;
type
TMyDragObject = class(TDragControlObjectEx)
private
FDragImages: TDragImageList;
FPictureToDrag: TPicture;
protected
function GetDragImages: TDragImageList; override;
procedure Finished(Target: TObject; X, Y: Integer;
Accepted: Boolean); override;
public
constructor CreateFromThumbView(ThumbView: TJvThumbView);
destructor Destroy; override;
end;
TForm1 = class(TForm)
JvThumbView1: TJvThumbView;
Image1: TImage;
procedure FormCreate(Sender: TObject);
procedure JvThumbView1MouseDown(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
procedure JvThumbView1StartDrag(Sender: TObject;
var DragObject: TDragObject);
procedure Image1DragOver(Sender, Source: TObject; X, Y: Integer;
State: TDragState; var Accept: Boolean);
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.FormCreate(Sender: TObject);
begin
// Fill our image list with arbitrary images
if JvThumbView1.Directory = '' then
JvThumbView1.Directory := 'C:\Users\Public\Pictures\Sample Pictures';
// Style all controls for showing the drag image if Delphi version is D7 or
// lower. See also comment in TMyDragObject.CreateFromThumbView
JvThumbView1.ControlStyle := JvThumbView1.ControlStyle +
[csDisplayDragImage];
Image1.ControlStyle := Image1.ControlStyle + [csDisplayDragImage];
ControlStyle := ControlStyle + [csDisplayDragImage];
end;
procedure TForm1.Image1DragOver(Sender, Source: TObject; X, Y: Integer;
State: TDragState; var Accept: Boolean);
begin
// The destination image component accepts all drag operations
Accept := True;
end;
procedure TForm1.JvThumbView1MouseDown(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
// If mouse down on a thumb...
if JvThumbView1.SelectedFile <> '' then
// then let's start dragging
JvThumbView1.BeginDrag(False, Mouse.DragThreshold);
end;
procedure TForm1.JvThumbView1StartDrag(Sender: TObject;
var DragObject: TDragObject);
begin
// DragObject will automatically be destroyed when necessary when it's
// derived from TDragControlObjectEx
DragObject := TMyDragObject.CreateFromThumbView(JvThumbView1);
end;
{ TMyDragObject }
const
DragImageSize = 100;
constructor TMyDragObject.CreateFromThumbView(ThumbView: TJvThumbView);
begin
inherited Create(ThumbView);
// This is the picture the user will drag around
FPictureToDrag := TPicture.Create;
FPictureToDrag.LoadFromFile(ThumbView.SelectedFile);
// We want a nice drag image, but this property is only available in >D7
{ AlwaysShowDragImages := True; }
end;
destructor TMyDragObject.Destroy;
begin
FDragImages.Free;
FPictureToDrag.Free;
inherited Destroy;
end;
procedure TMyDragObject.Finished(Target: TObject; X, Y: Integer;
Accepted: Boolean);
begin
// Finished dragging
inherited Finished(Target, X, Y, Accepted);
// If we are over an Image component, then draw the picture
if Accepted and (Target is TImage) then
TImage(Target).Canvas.StretchDraw(Bounds(X, Y, DragImageSize,
DragImageSize), FPictureToDrag.Graphic);
end;
function TMyDragObject.GetDragImages: TDragImageList;
var
DragImage: TBitmap;
begin
if FDragImages = nil then
begin
FDragImages := TDragImageList.Create(nil);
// Set dimensions of drag image list
FDragImages.Width := DragImageSize;
FDragImages.Height := DragImageSize;
// Prepare drag image
DragImage:= TBitmap.Create;
try
DragImage.Width := DragImageSize;
DragImage.Height := DragImageSize;
DragImage.Canvas.StretchDraw(Rect(0, 0, DragImage.Width,
DragImage.Height), FPictureToDrag.Graphic);
FDragImages.AddMasked(DragImage, clWhite);
finally
DragImage.Free;
end;
end;
Result := FDragImages;
end;
end.