README

CrossDragDrop

Native drag and drop for existing Lazarus controls. Two nonvisual components: point one at a control, handle one event.

Status: tested on Linux with GTK2 and GTK3 on X11, GTK3 on native Wayland, and Qt5 on X11 and native Wayland, and on Windows 10 (64-bit, Win32 widgetset). macOS is the next backend; it compiles today and reports that drag and drop is not available.

"Two programs" is the test probe: two separate processes, a plain click, a press below the drag threshold, Unicode text, two files with spaces, %, # and accented names, a fallback to text, a drop the target refuses and Escape, all checked against both programs' event logs. Details per platform are in the platform matrix.

// Drop a TCrossDragDropSource on the form, set Control to Button1 and
// write this handler. Dragging the button now carries the text.
procedure TForm1.Source1GetText(Sender: TObject; out AText: string);
begin
  AText := Edit1.Text;
end;

// Drop a TCrossDragDropTarget, set Control to Memo1 and write this one.
// Text dropped from any program lands here.
procedure TForm1.Target1DropText(Sender: TObject; const AText: string; X, Y: integer);
begin
  Memo1.Lines.Add(AText);
end;

A source offers UTF-8 text in OnGetText, existing files in OnGetFiles, or both; the receiver picks. Leave the text empty and the list empty, and no drag starts. A target takes text in OnDropText and files in OnDropFiles, as full paths. The control can be anything: a button, a label, an image, a panel, a list box. A label or an image works through its parent. The data is whatever the program hands over; the control itself is not transferred. Copy is the only operation.

While a drag is in progress the source control fades under a dashed outline and a snapshot of it travels under the pointer. A target that would take the drop gets a frame in the desktop highlight color; a target that would not stays as it is, and so does the cursor. Fade, Highlight and HighlightColor turn these off or recolor them. An ordinary click on a source button still clicks; a drag beyond the desktop's threshold does not.

OnDragEnd reports whether the data was dropped, canceled, rejected or failed. OnDragOver can refuse an offer by position or format before the drop. RejectDrop inside a drop event tells the other program the data was not used. OnError reports a second helper on the same control, a bad file list or a widgetset without support; nothing shows a dialog.

• Implementation roadmap
• Platform matrix
• Reference documentation

Demos

demo/a and demo/b are two separate programs. Run both and drag between them: a button that carries an edit box's text, a label that carries a file, a picture that is offered as a PNG file, a button with a line of text, and targets in a panel and a picture box. The picture box refuses a file that is not an image, and both sides say so.

Demo A	Demo B

Install

Open crossdragdrop.lpk in Lazarus, compile and install. The components are on the System page. The package needs only the LCL; the GTK and Qt backends use the libraries the LCL already loads, and the Windows backend uses the system's OLE drag and drop. Test automation uses LazHIDControl's hidctl, which is not a runtime dependency.

What differs between desktops

• Qt cannot tell a canceled drag from a rejected one: OnDragEnd reports Rejected for both. GTK on Wayland reports Failed for a cancel.
• On Wayland, GTK takes the highlight off a target before OnDropText or OnDropFiles runs; on X11 and Qt it is still on during the event.
• On Qt the fade and the highlight are drawn by a see-through label laid over the control; it never takes mouse or drop events. A control that is both a source and a target gets one for each.
• Whether the snapshot of the control appears under the pointer on Wayland depends on the compositor.
• Windows tells a canceled drag (Escape) from a rejected one, like X11. Text goes out as Unicode text and files as a file list (CF_HDROP), the formats Explorer, WordPad and most programs use; a target also reads the ANSI text of older programs. Files from a network share arrive as \\server\share\... paths.
• On Windows a target registers its control's window with OLE. A window that other code in the program already registered for drops is refused with an error.
• On GTK a memo's text view and an edit box's entry are drop targets of their own, set up by GTK for text. A target on such a control sits on that inner widget, so a file dropped on a memo reaches OnDropFiles instead of being finished unhandled by the text view; the widget's own text targets are given back when the component goes.

Origin

The simple control assignment and data-event experience is inspired by Bernd Kreuss's NativeDragDrop. CrossDragDrop is a new implementation. No source from that project has been included in this repository.

License

0BSD. Copyright (c) 2026 Tony Stone.