Preventing a dialog from closing while autocomplete is active

Delphi, dzLib 2 Minutes

In an older blog post I wrote about AutoComplete for TEdits using SHAutoComplete.

I just actually tried to use that function in one of my applications and found that there is a quite annoying problem with it: If you have set the OK button’s Default property to true (so it gets “clicked” when you press return), selecting an entry from the autocomplete list with the return key also closes the form, which is usually not what the user wants.

I turns out that I am not the first to stumble upon that problem.

The suggestion posted there by mghie is a bit ugly because it hooks the Application.OnMessage event which might conflict with other code that uses it.

I had another problem anyway (see below) so I extended a class that hooks a TEdit’s WindowProc method instead. Here is the code:

procedure TAutoCompleteActivator.NewWindowProc(var _Msg: TMessage);
begin
  if (_Msg.Msg = CM_WANTSPECIALKEY) then begin
    if (_Msg.wParam = VK_RETURN) or (_Msg.wParam = VK_ESCAPE) then begin
      if IsAutoSuggestDropdownVisible then begin
        _Msg.Result := 1;
        Exit; //==>
      end;
    end;
  end;
  inherited NewWindowProc(_Msg);
end;

The IsAutoSuggestDropdownVisible function is directly taken from mghie’s answer:

function EnumThreadWindowsProc(AWnd: HWnd; AParam: LParam): BOOL; stdcall;
var
  WndClassName: string;
  FoundAndVisiblePtr: PInteger;
begin
  SetLength(WndClassName, 1024);
  GetClassName(AWnd, PChar(WndClassName), Length(WndClassName));
  WndClassName := PChar(WndClassName);
  if WndClassName = 'Auto-Suggest Dropdown' then begin // do not translate
    FoundAndVisiblePtr := PInteger(AParam);
    FoundAndVisiblePtr^ := Ord(IsWindowVisible(AWnd));
    Result := False;
  end else
    Result := True;
end;

function IsAutoSuggestDropdownVisible: Boolean;
var
  FoundAndVisible: Integer;
begin
  FoundAndVisible := 0;
  EnumThreadWindows(GetCurrentThreadId, @EnumThreadWindowsProc,
    LParam(@FoundAndVisible));
  Result := FoundAndVisible > 0;
end;

This works fine in my program compiled with Delphi 2010 and running on Windows 8.1 (your mileage may vary).

Edit: It also works fine compiled with Delphi 2007 and running onWindows 7.

Now to the other problem mentioned above:
In the old blog post I published a TEdit_SetAutocomplete function that activates autocomplete for a TEdit control. This function works fine as as long as you don’t try to call it in the form’s constructor. If you do, it does nothing. The reason is that the TEdit’s handle gets destroyed and recreated after the form’s constructor was called, which results in autocomplete being turned off again. One option would have been to put the function call into the form’s OnShow handler, but I am no fan of distributing code that in my opinion belongs into the constructor to these event handlers, so I wanted a different solution.

It turned out that I already had one in my dzlib.u_dzVclUtils unit: TWinControl_ActivateDropFiles returns a TObject that hooks the TWinControl’s WindowProc and handles the WM_NCCREATE and WM_NCDESTROY messages. I refactored that class a bit to create a generic TWindowProcHook ancestor and derived TAutoCompleteActivator from it. Its WmNcCreate method now looks like this:

procedure TAutoCompleteActivator.WmNcCreate;
begin
  inherited;
  SetAutoComplete;
end;

procedure TAutoCompleteActivator.SetAutoComplete;
begin
  TEdit_SetAutocomplete(FCtrl as TCustomEdit, FSource, FType);
end;

So every time the window handle gets created anew, it activates autocomplete for it again.

The full code can be found in my dzlib library on OSDN. It’s in the u_dzVclUtils unit (the preview only shows the first 3000 lines of the unit).

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