How do I allow dragging files for specific control(s) in Delphi - windows

Question

I would like to accept files as soon as someone drops a file to a specific control (e.g. TMemo). I started with this example: http://delphi.about.com/od/windowsshellapi/a/accept-filedrop.htm and modified it like this:
procedure TForm1.FormCreate(Sender: TObject);
begin
DragAcceptFiles( Memo1.Handle, True ) ;
end;
This allows the control to show the dragging icon but the proper WM_DROPFILES message is not getting called because DragAcceptFiles needs a (Parent?)windowhandle. I could determine the MemoHandle in the WMDROPFILES procedure but I don't how, plus the dragging cursor applies for all the controls now. How do I allow dragging for a specific control (and block other controls from dragging)?

Answer 1

You should indeed pass the window handle of the memo control, but then you also need to listen to the WM_DROPFILES message sent to it:
unit Unit5;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls, ShellAPI;
type
TMemo = class(StdCtrls.TMemo)
protected
procedure WMDropFiles(var Message: TWMDropFiles); message WM_DROPFILES;
procedure CreateWnd; override;
procedure DestroyWnd; override;
end;
TForm5 = class(TForm)
Memo1: TMemo;
procedure FormCreate(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form5: TForm5;
implementation
{$R *.dfm}
procedure TForm5.FormCreate(Sender: TObject);
begin
end;
{ TMemo }
procedure TMemo.CreateWnd;
begin
inherited;
DragAcceptFiles(Handle, true);
end;
procedure TMemo.DestroyWnd;
begin
DragAcceptFiles(Handle, false);
inherited;
end;
procedure TMemo.WMDropFiles(var Message: TWMDropFiles);
var
c: integer;
fn: array[0..MAX_PATH-1] of char;
begin
c := DragQueryFile(Message.Drop, $FFFFFFFF, fn, MAX_PATH);
if c <> 1 then
begin
MessageBox(Handle, 'Too many files.', 'Drag and drop error', MB_ICONERROR);
Exit;
end;
if DragQueryFile(Message.Drop, 0, fn, MAX_PATH) = 0 then Exit;
Text := fn;
end;
end.
The example above only accept a single file dropped. The file name will be put in the memo control. But you can also allow a multiple selection to be dropped:
var
c: integer;
fn: array[0..MAX_PATH-1] of char;
i: Integer;
begin
c := DragQueryFile(Message.Drop, $FFFFFFFF, fn, MAX_PATH);
Clear;
for i := 0 to c - 1 do
begin
if DragQueryFile(Message.Drop, i, fn, MAX_PATH) = 0 then Exit;
Lines.Add(fn);
end;