Why dataset's state changes to dsEdit after suppressing WM_PASTE? - delphi

Question

I'm intercepting and suppressing the WM_PASTE message for a TDBEdit by assigning its WindowProc property, as described in this answer.
After pressing Ctrl+V, despite the WM_PASTE is intercepted, the dataset's state changes from dsBrowse to dsEdit.
Why is this happening and how could I avoid that?
unit Unit1;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, ExtCtrls, DBCtrls, StdCtrls, Mask, DB, DBClient;
type
TForm1 = class(TForm)
procedure FormCreate(Sender: TObject);
private
FPrevWindowProc : TWndMethod;
procedure MyWindowProc(var AMessage: TMessage);
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.FormCreate(Sender: TObject);
var
Dst : TClientDataSet;
Dsc : TDataSource;
Fld : TField;
Nav : TDBNavigator;
Edt : TDBEdit;
begin
//dataset
Dst := TClientDataSet.Create(Self);
Dst.FieldDefs.Add('TEST', ftString, 20);
Dst.CreateDataSet();
Dst.Active := True;
Fld := Dst.Fields[0];
Dst.Append();
Fld.AsString := 'test';
Dst.Post();
//datasource
Dsc := TDataSource.Create(Self);
Dsc.DataSet := Dst;
//navigator
Nav := TDBNavigator.Create(Self);
Nav.DataSource := Dsc;
Nav.Top := 3;
Nav.Left := 3;
Nav.Parent := Self;
//editor
Edt := TDBEdit.Create(Self);
Edt.DataSource := Dsc;
Edt.DataField := Fld.FieldName;
Edt.Top := 31;
Edt.Left := 3;
Edt.Parent := Self;
FPrevWindowProc := Edt.WindowProc;
Edt.WindowProc := MyWindowProc;
end;
procedure TForm1.MyWindowProc(var AMessage: TMessage);
begin
if(AMessage.Msg = WM_PASTE) then
begin
ShowMessage('WM_PASTE, exit!');
Exit;
end;
FPrevWindowProc(AMessage);
end;
end.

Answer 1

Using the interposer class solution in Remy's answer to your linked question, if you create a BeforeEdit handler for your DataSet and put a breakpoint in it, you will find that the breakpoint trips before the interposer's WMPaste() method is entered.
If you then trace out of the BeforeEdit handler, you will eventually arrive in TDBEdit.KeyPress() which (in D7) contains the code below:
procedure TDBEdit.KeyPress(var Key: Char);
begin
inherited KeyPress(Key);
if (Key in [#32..#255]) and (FDataLink.Field <> nil) and
not FDataLink.Field.IsValidChar(Key) then
begin
MessageBeep(0);
Key := #0;
end;
case Key of
^H, ^V, ^X, #32..#255:
FDataLink.Edit;
#27:
begin
FDataLink.Reset;
SelectAll;
Key := #0;
end;
end;
end;
So, the DataSet is put into dsEdit state by the call to FDataLink.Edit() as a result of KeyPress() seeing the ^V character.
You could achieve the behavior you want by also overriding KeyPress() in the interposer class. The following will prevent pressing ^V from having any effect:
type // This can be in your Form's unit but must go before your Form's type declaration
TDBEdit = class(DBCtrls.TDBEdit)
procedure WMPaste(var Message: TMessage); message WM_PASTE;
procedure KeyPress(var Key: Char); override;
end;
[...]
procedure TDBEdit.WMPaste(var Message: TMessage);
begin
if not (Message.Msg = WM_PASTE) then
inherited;
end;
procedure TDBEdit.KeyPress(var Key: Char);
begin
case Key of
^V : Key := #0;
end; { case }
inherited;
end;