Delphi: TOleControl puts ActiveControl in wrong state?

Question

In Mike Lischke's now defunct Virtual Treeview, there was workaround code added to fix a bug when using a TWebBrowser control on the same form.

The problem was that if the user tries to interact with a TOleControl (from which TWebBrowser descends), the first mouse click is eaten. They have to then click again to give the control focus. Then they can interact with the control.

He has comments to explain:

procedure TBaseVirtualTree.WMKillFocus(var Msg: TWMKillFocus);
var
  Form: TCustomForm;
  Control: TWinControl;
  Pos: TSmallPoint;
  Unknown: IUnknown;
begin
  inherited;

  [snip]

  {
    Workaround for wrapped non-VCL controls (like TWebBrowser), 
    which do not use VCL mechanisms and 
    leave the ActiveControl property in the wrong state, 
    which causes trouble when the control is refocused.
  }
  Form := GetParentForm(Self);
  if Assigned(Form) and (Form.ActiveControl = Self) then
  begin
    Cardinal(Pos) := GetMessagePos;
    Control := FindVCLWindow(SmallPointToPoint(Pos));
    {
      Every control derived from TOleControl has potentially 
      the focus problem. In order to avoid including 
      the OleCtrls unit (which will, among others, include Variants),  
      which would allow to test for the TOleControl
      class, the IOleClientSite interface is used for the test, 
      which is supported by TOleControl and a good indicator.
    }
    if Assigned(Control) and Control.GetInterface(IOleClientSite, Unknown) then
      Form.ActiveControl := nil;

    // For other classes the active control should not be modified. Otherwise you need two clicks to select it.
  end;
end;

Problem is that the workaround is no longer working for me. And to be honest i have no idea what the problem really was, and how his solution fixed it.

Is there anyone who knows what his comments understand what he's talking about, could explain what the problem is, and how what he's doing was supposed to fix it?

Workaround for wrapped non-VCL controls (like TWebBrowser), which do not use VCL mechanisms and leave the ActiveControl property in the wrong state, which causes trouble when the control is refocused. Every control derived from TOleControl has potentially the focus problem.

The code is reaching the intended

Form.ActiveControl := nil; 

statement, but it just isn't doing the trick.

i'd fix it, but i have no idea how he found it, or how it can come about that TOleControl doesn't "use VCL mechanisms and leaves the ActiveControl property in the wrong state."

---

Google archive of original question.

Question on Soft-Gems forum

Bump: 20110515 (12 months later)

Answer 1

I have overcome this issue by using TEmbeddedWB (which is much better than the standard TWebBrowser) and then I had to add this OnShowUI event:

function THtmlFrame.webBrowserShowUI(const dwID: Cardinal;
  const pActiveObject: IOleInPlaceActiveObject;
  const pCommandTarget: IOleCommandTarget; const pFrame: IOleInPlaceFrame;
  const pDoc: IOleInPlaceUIWindow): HRESULT;
begin
  try
    if WebBrowser.CanFocus then
      WebBrowser.SetFocus; // tell the VCL that the web-browser is focused
  except
    on E: EInvalidOperation do
      ; // ignore "Cannot focus inactive or invisible control"
  end;
  Result := S_FALSE;
end;

But if you must use TWebBrowser you need to write more code:

type
  IDocHostUIHandler = interface(IUnknown)
    ['{bd3f23c0-d43e-11cf-893b-00aa00bdce1a}']
    function ShowContextMenu(const dwID: DWORD; const ppt: PPOINT;
      const CommandTarget: IUnknown; const Context: IDispatch): HRESULT; stdcall;
    function GetHostInfo(var pInfo: TDOCHOSTUIINFO): HRESULT; stdcall;
    function ShowUI(const dwID: DWORD; const pActiveObject: IOleInPlaceActiveObject;
      const pCommandTarget: IOleCommandTarget; const pFrame: IOleInPlaceFrame;
      const pDoc: IOleInPlaceUIWindow): HRESULT; stdcall;
    function HideUI: HRESULT; stdcall;
    function UpdateUI: HRESULT; stdcall;
    function EnableModeless(const fEnable: BOOL): HRESULT; stdcall;
    function OnDocWindowActivate(const fActivate: BOOL): HRESULT; stdcall;
    function OnFrameWindowActivate(const fActivate: BOOL): HRESULT; stdcall;
    function ResizeBorder(const prcBorder: PRECT; const pUIWindow: IOleInPlaceUIWindow; const fRameWindow: BOOL): HRESULT; stdcall;
    function TranslateAccelerator(const lpMsg: PMSG; const pguidCmdGroup: PGUID; const nCmdID: DWORD): HRESULT; stdcall;
    function GetOptionKeyPath(out pchKey: POLESTR; const dw: DWORD): HRESULT; stdcall;
    function GetDropTarget(const pDropTarget: IDropTarget; out ppDropTarget: IDropTarget): HRESULT; stdcall;
    function GetExternal(out ppDispatch: IDispatch): HRESULT; stdcall;
    function TranslateUrl(const dwTranslate: DWORD; const pchURLIn: POLESTR; out ppchURLOut: POLESTR): HRESULT; stdcall;
    function FilterDataObject(const pDO: IDataObject; out ppDORet: IDataObject): HRESULT; stdcall;
  end; // IDocHostUIHandler

  ICustomDoc = interface(IUnknown)
    ['{3050f3f0-98b5-11cf-bb82-00aa00bdce0b}']
    function SetUIHandler(const pUIHandler: IDocHostUIHandler): HResult; stdcall;
  end;

  TDocHostUIHandler = class(TInterfacedObject, IDocHostUIHandler)
  private
    FWebBrowser: TWebBrowser;
  protected
    function EnableModeless(const fEnable: BOOL): HResult; stdcall;
    function FilterDataObject(const pDO: IDataObject; out ppDORet: IDataObject): HResult; stdcall;
    function GetDropTarget(const pDropTarget: IDropTarget; out ppDropTarget: IDropTarget): HResult; stdcall;
    function GetExternal(out ppDispatch: IDispatch): HResult; stdcall;
    function GetHostInfo(var pInfo: TDocHostUIInfo): HResult; stdcall;
    function GetOptionKeyPath(var pchKey: POLESTR; const dw: DWORD): HResult; stdcall;
    function HideUI: HResult; stdcall;
    function OnDocWindowActivate(const fActivate: BOOL): HResult; stdcall;
    function OnFrameWindowActivate(const fActivate: BOOL): HResult; stdcall;
    function ResizeBorder(const prcBorder: PRECT; const pUIWindow: IOleInPlaceUIWindow;
      const fFrameWindow: BOOL): HResult; stdcall;
    function ShowContextMenu(const dwID: DWORD; const ppt: PPOINT;
      const pcmdtReserved: IInterface; const pdispReserved: IDispatch): HResult; stdcall;
    function ShowUI(const dwID: DWORD; const pActiveObject: IOleInPlaceActiveObject;
      const pCommandTarget: IOleCommandTarget; const pFrame: IOleInPlaceFrame;
      const pDoc: IOleInPlaceUIWindow): HResult; stdcall;
    function TranslateAccelerator(const lpMsg: PMSG; const pguidCmdGroup: PGUID; const nCmdID: DWORD): HResult; stdcall;
    function TranslateUrl(const dwTranslate: DWORD; const pchURLIn: POLESTR; var ppchURLOut: POLESTR): HResult; stdcall;
    function UpdateUI: HResult; stdcall;
  public
    constructor Create(AWebBrowser: TWebBrowser);
    property WebBrowser: TWebBrowser read FWebBrowser;
  end;


{ TDocHostUIHandler }

function TDocHostUIHandler.EnableModeless(const fEnable: BOOL): HResult;
begin
  Result := S_OK;
end;

function TDocHostUIHandler.FilterDataObject(const pDO: IDataObject; out ppDORet: IDataObject): HResult;
begin
  ppDORet := nil;
  Result := S_FALSE;
end;

function TDocHostUIHandler.GetDropTarget(const pDropTarget: IDropTarget; out ppDropTarget: IDropTarget): HResult;
begin
  ppDropTarget := nil;
  Result := E_FAIL;
end;

function TDocHostUIHandler.GetExternal(out ppDispatch: IDispatch): HResult;
begin
  ppDispatch := nil;
  Result := E_FAIL;
end;

function TDocHostUIHandler.GetHostInfo(var pInfo: TDocHostUIInfo): HResult;
begin
  Result := S_OK;
end;

function TDocHostUIHandler.GetOptionKeyPath(var pchKey: POLESTR; const dw: DWORD): HResult;
begin
  Result := E_FAIL;
end;

function TDocHostUIHandler.HideUI: HResult;
begin
  Result := S_OK;
end;

function TDocHostUIHandler.OnDocWindowActivate(const fActivate: BOOL): HResult;
begin
  Result := S_OK;
end;

function TDocHostUIHandler.OnFrameWindowActivate(const fActivate: BOOL): HResult;
begin
  Result := S_OK;
end;

function TDocHostUIHandler.ResizeBorder(const prcBorder: PRECT; const pUIWindow: IOleInPlaceUIWindow; const fFrameWindow: BOOL): HResult;
begin
  Result := S_FALSE;
end;

function TDocHostUIHandler.ShowContextMenu(const dwID: DWORD; const ppt: PPOINT; const pcmdtReserved: IInterface; const pdispReserved: IDispatch): HResult;
begin
  Result := S_FALSE
end;

function TDocHostUIHandler.TranslateAccelerator(const lpMsg: PMSG; const pguidCmdGroup: PGUID; const nCmdID: DWORD): HResult;
begin
  Result := S_FALSE;
end;

function TDocHostUIHandler.TranslateUrl(const dwTranslate: DWORD; const pchURLIn: POLESTR; var ppchURLOut: POLESTR): HResult;
begin
  Result := E_FAIL;
end;

function TDocHostUIHandler.UpdateUI: HResult;
begin
  Result := S_OK;
end;

function TDocHostUIHandler.ShowUI(const dwID: DWORD; const pActiveObject: IOleInPlaceActiveObject; const pCommandTarget: IOleCommandTarget;
  const pFrame: IOleInPlaceFrame; const pDoc: IOleInPlaceUIWindow): HResult;
begin
  try
    if WebBrowser.CanFocus then
      WebBrowser.SetFocus; // tell the VCL that the web-browser is focused
  except
    on E: EInvalidOperation do
      ; // ignore "Cannot focus inactive or invisible control"
  end;
  Result := S_OK;
end;



// install the DocHostUIHandler into the WebBrowser
var
  CustomDoc: ICustomDoc;
begin
  if WebBrowser1.Document.QueryInterface(ICustomDoc, CustomDoc) = S_OK then
    CustomDoc.SetUIHandler(TDocHostUIHandler.Create(WebBrowser1));
end;