From: tnomad@digital.net

Subject: Drag/Drop and Delphi Information

Date: 28 Jun 1995 12:43:55 GMT

Hey all,

I though you would be interested in a recent discovery I made regarding

Drag and Drop in Delphi. My requirement was to implement a similar

interface to Dragging and Dropping as is found in File Manager, with

respect to using the CTRL key to specify copying.

My first (and very unsuccessful) pass was to use GetKeyboardState to

examine the keyboard state and see if the CTRL key was depressed.

There were two problems with this approach:

1) This function does not seem to return consistent values.

2) When dragging and dropping, mouse capture is set on the

control. The OnKeyDown/OnKeyUp methods are not triggered during

dragging.

My second approach was to come up with a way to intercept the Windows

message when the dragging was taking place. The only way to do this is

to subclass the control, and override the WndProc method. I created a

new property (WndProcFunction), that will be called when the property

is assigned. In my subclass, I override the WndProc, call

WndProcFunction if it is assigned, then call the inherited version:

type

TMessageEvent = procedure(Sender: TObject; Message: TMessage) of object;

...

MyControl = class(subcontrol)

private

FWndProcFunction: TMessageEvent;

...

published

property WndProcFunction: TMessageEvent

read FWndProcFunction

write FWndProcFunction;

...

public

procedure WndProc(var Message: TMessage); override;

end

...

procedure MyControl.WndProc(var Message: TMessage);

begin

if Assigned(FWndProcFunction) then

WndProcFunction(self, Message);

inherited WndProc(Message);

end;

In my application (where I implement this class), I use the object

browser to create the following function:

procedure TViewRetrieval.ViewGridWndProcFunction(Sender: TObject;

Message: TMessage);

begin

case Message.Msg of

WM_KEYDOWN, WM_KEYUP: DoProcessKey(Sender, TWMKey(Message));

end;

end;

I then created DoProcessKey as follows (note: crDragCopy is my own

cursor):

procedure TViewRetrieval.DoProcessKey(Sender: TObject; var Message: TWMKey);

var

ShiftState: TShiftState;

pt: TPoint;

begin

ShiftState := KeyDataToShiftState(Message.KeyData);

if (ShiftState = [ssCtrl]) then

ViewGrid.DragCursor := crDragCopy

else

ViewGrid.DragCursor := crDrag;

if ViewGrid.Dragging then

begin

GetCursorPos(pt);

SetCursorPos(pt.x, pt.y); {Causes the cursor to be redrawn}

end;

end;

************************************

* The Nomad *

* tnomad@digital.net *

************************************

-------------------------------------------------------------------------------

From: mduncan@wpo.borland.com (Mark Duncan)

Subject: Re: Drag/Drop and Delphi Information

Date: 28 Jun 1995 21:01:31 GMT

Here's how I would do it:

Initiate the drag (call BeginDrag method of the dragged control) in that control's

OnMouseDown event, which passes in information about the button and keyboard states.

Procedure TForm1.SourceFileListBoxMouseDown(Sender: TObject; Button: tMouseButton;

Shift : TShiftState; X, Y: Integer);

begin

if (Button = mbLeft) and (ssCtrl in Shift)

then with (Sender as TWinControl) do begin

BeginDrag(True);

....... {init information about what files are selected, etc}

FCopying := True; {a global boolean you declare to keep track of what kind of

end; dragging is going on}

then in the OnDragDrop of the accepting control you would have something like:

TForm1.DestFileListBoxDragDrop(Sender, Source : TObject; X, Y : integer);

begin

if (Source = SourceFileListBox) and (FCopying = True)

then ...... {do the copy}

end;

Mark