Detecting changes in an editable TWebBrowser

Question

I'm loading an HTML local file into TWebBrowser as follows:

procedure TForm1.FormCreate(Sender: TObject);
begin
  WebBrowser1.Navigate('file:///C:\Tmp\input.html');
end;

In the TWebBrowser.OnDocumentComplete event handler I'm making it editable:

procedure TForm1.WebBrowser1DocumentComplete(ASender: TObject;
  const pDisp: IDispatch; const URL: OleVariant);
begin
  (WebBrowser1.Document as IHTMLDocument2).designMode := 'on';
end;

I need to be notified as soon as the user applies any changes through the TWebBrowser (i.e: he writes something...) but I can't see any OnChanged or similar event handler.

---

I've tried capturing WM_PASTE and WM_KEYDOWN but my code is never executed:

  TMyWebBrowser = class(TWebBrowser)
  public
    procedure WM_Paste(var Message: TWMPaste); message WM_PASTE;
    procedure WM_KeyDown(var Message: TWMKeyDown); message WM_KEYDOWN;
  end;

...

procedure TMyWebBrowser.WM_Paste(var Message: TWMPaste);
begin
  inherited;
  ShowMessage('Paste');
end;

procedure TMyWebBrowser.WM_KEYDOWN(var Message: TWMKeyDown);
begin
  inherited;
  ShowMessage('KeyDown');
end;

I've also tried setting the WindowProc property but without any success.

Answer 1

To capture changes to the document in design mode you should use its IMarkupContainer2 interface to register an IHTMLChangeSink via RegisterForDirtyRange method. The process is pretty simple - implement IHTMLChangeSink, obtain IMarkupContainer2 from WebBrowser1.Document and call its RegisterForDirtyRange method, but there's a catch.

When you change the designMode of IHTMLDocument2, TWebBrowser control reloads the current document and it loses all registered change sinks. Therefore you should register it after putting the document in design mode. After that you receive change notifications via IHTMLChangeSink.Notify method.

But there's another catch. Since entering the design mode causes reloading of the document and that in turn causes changing the readyState property of the document to 'loading' and then consecutively to 'complete'. Your change sink will receive those readyState change notifications. Note that TWebBrowser.OnDocumentComplete is not invoked after entering design mode. That's why you should ignore any notifications until the document is fully reloaded in design mode.

Another minor complication is that RegisterForDirtyRange creates a cookie that you need to maintain in order to unregister the change sink. Since you need a class to implement IHTMLChangeSink anyway, it could also encapsulate the design mode state and change registration.

uses
  System.SysUtils, SHDocVw, MSHTML;

const
  DesignMode: array[Boolean] of string = ('off', 'on');

type
  TWebBrowserDesign = class(TInterfacedObject, IHTMLChangeSink)
  private
    FDirtyRangeCookie: LongWord;
    FDocumentComplete: Boolean;
    FHTMLDocument2: IHTMLDocument2;
    FMarkupContainer2: IMarkupContainer2;
    FOnChange: TProc;
    { IHTMLChangeSink }
    function Notify: HResult; stdcall;
  public
    constructor Create(WebBrowser: TWebBrowser; const AOnChange: TProc);
    destructor Destroy; override;
  end;

constructor TWebBrowserDesign.Create(WebBrowser: TWebBrowser; const AOnChange: TProc);
begin
  inherited Create;
  if not Assigned(WebBrowser) then
    raise Exception.Create('Web browser control missing.');
  if not Supports(WebBrowser.Document, IHTMLDocument2, FHTMLDocument2) then
    raise Exception.Create('No HTML document loaded.');
  FHTMLDocument2.designMode := DesignMode[True];
  if Supports(WebBrowser.Document, IMarkupContainer2, FMarkupContainer2) then
  begin
    if FMarkupContainer2.RegisterForDirtyRange(Self, FDirtyRangeCookie) <> S_OK then
      FDirtyRangeCookie := 0
    else
      _Release;
  end;
  FOnChange := AOnChange;
end;

destructor TWebBrowserDesign.Destroy;
begin
  if Assigned(FMarkupContainer2) and (FDirtyRangeCookie <> 0) then
    FMarkupContainer2.UnRegisterForDirtyRange(FDirtyRangeCookie);
  if Assigned(FHTMLDocument2) then
    FHTMLDocument2.designMode := DesignMode[False];
  inherited;
end;

function TWebBrowserDesign.Notify: HResult;
begin
  Result := S_OK;
  if not FDocumentComplete then
    FDocumentComplete := FHTMLDocument2.readyState = 'complete'
  else if Assigned(FOnChange) then
    FOnChange();
end;

Note the call to _Release after registering the change sink. This is to "prevent" markup container from holding strong reference to TWebBrowserDesign instance. That allows you to control design mode using the lifetime of TWebBrowserDesign instance:

type
  TForm1 = class(TForm)
    { ... }
  private
    FWebBrowserDesign: IInterface;
    { ... }
  end;

procedure TForm1.WebBrowser1DocumentComplete(ASender: TObject;
  const pDisp: IDispatch; const URL: OleVariant);
begin
  { enter design mode }
  FWebBrowserDesign := TWebBrowserDesign.Create(WebBrowser1, procedure
    begin
      ButtonSave.Enabled := True;
    end);
end;

procedure TForm1.ButtonSave(Sender: TObject);
begin
  { exit design mode }
  FWebBrowserDesign := nil;
  ButtonSave.Enabled := False;
end;

Alternatively you can implement change sink as a component.

type
  TWebBrowserDesign = class(TComponent, IHTMLChangeSink)
  private
    FDirtyRangeCookie: LongWord;
    FDocumentComplete: Boolean;
    FHTMLDocument2: IHTMLDocument2;
    FMarkupContainer2: IMarkupContainer2;
    FOnChange: TNotifyEvent;
    FWebBrowser: TWebBrowser;
    procedure EnterDesignMode;
    procedure ExitDesignMode;
    function GetActive: Boolean;
    procedure SetActive(const Value: Boolean);
    procedure SetWebBrowser(const Value: TWebBrowser);
    { IHTMLChangeSink }
    function Notify: HResult; stdcall;
  protected
    procedure Notification(AComponent: TComponent; Operation: TOperation); override;
  public
    destructor Destroy; override;
  published
    property Active: Boolean read GetActive write SetActive;
    property OnChange: TNotifyEvent read FOnChange write FOnChange;
    property WebBrowser: TWebBrowser read FWebBrowser write SetWebBrowser;
  end;

destructor TWebBrowserDesign.Destroy;
begin
  ExitDesignMode;
  inherited;
end;

procedure TWebBrowserDesign.EnterDesignMode;
begin
  if not Assigned(FWebBrowser) then
    raise Exception.Create('Web browser control missing.');
  if not Supports(FWebBrowser.Document, IHTMLDocument2, FHTMLDocument2) then
    raise Exception.Create('No HTML document loaded.');
  try
    FHTMLDocument2.designMode := DesignMode[True];
    if Supports(FWebBrowser.Document, IMarkupContainer2, FMarkupContainer2) then
    begin
      if FMarkupContainer2.RegisterForDirtyRange(Self, FDirtyRangeCookie) <> S_OK then
        FDirtyRangeCookie := 0;
    end;
  except
    ExitDesignMode;
    raise;
  end;
end;

procedure TWebBrowserDesign.ExitDesignMode;
begin
  if Assigned(FMarkupContainer2) then
  begin
    if FDirtyRangeCookie <> 0 then
    begin
      FMarkupContainer2.UnRegisterForDirtyRange(FDirtyRangeCookie);
      FDirtyRangeCookie := 0;
    end;
    FMarkupContainer2 := nil;
  end;
  if Assigned(FHTMLDocument2) then
  begin
    FHTMLDocument2.designMode := DesignMode[False];
    if not (csDestroying in ComponentState) then
      FHTMLDocument2 := nil; { causes AV when its hosting TWebBrowser component is destroying; I didn't dig into details }
  end;
  FDocumentComplete := False;
end;

function TWebBrowserDesign.GetActive: Boolean;
begin
  Result := Assigned(FHTMLDocument2);
end;

procedure TWebBrowserDesign.Notification(AComponent: TComponent;
  Operation: TOperation);
begin
  inherited;
  if (Operation = opRemove) and (AComponent = FWebBrowser) then
    WebBrowser := nil;
end;

function TWebBrowserDesign.Notify: HResult;
begin
  Result := S_OK;
  if not FDocumentComplete then
    FDocumentComplete := FHTMLDocument2.readyState = 'complete'
  else if Assigned(FOnChange) then
    FOnChange(Self);
end;

procedure TWebBrowserDesign.SetActive(const Value: Boolean);
begin
  if Active <> Value then
  begin
    if Value then
      EnterDesignMode
    else
      ExitDesignMode;
  end;
end;

procedure TWebBrowserDesign.SetWebBrowser(const Value: TWebBrowser);
begin
  if Assigned(FWebBrowser) then
  begin
    ExitDesignMode;
    FWebBrowser.RemoveFreeNotification(Self);
  end;
  FWebBrowser := Value;
  if Assigned(FWebBrowser) then
    FWebBrowser.FreeNotification(Self);
end;

If you put such a component in a design-time package and register it within the IDE, then you'll be able to link this component with TWebBrowser and assign OnChange event handler in the form designer. Use Active property in code to enter/exit the design mode.

type
  TForm1 = class(TForm)
    { ... }
    WebBrowserDesign1: TWebBrowserDesign;
    { ... }
  end;

procedure WebBrowserDesign1Change(Sender: TObject);
begin
  ButtonSave.Enabled := True;
end;

procedure TForm1.WebBrowser1DocumentComplete(ASender: TObject;
  const pDisp: IDispatch; const URL: OleVariant);
begin
  { enter design mode }
  WebBrowserDesign1.Active := True;
end;

procedure TForm1.ButtonSave(Sender: TObject);
begin
  { exit design mode }
  WebBrowserDesign1.Active := False;
  ButtonSave.Enabled := False;
end;

NB: Similar question has been asked regarding C#/WinForms - How do I detect when the content of a WebBrowser control has changed (in design mode)?

Final note: I'm not convinced that enabling save button after a change is the best UX design. If you think that the code above is worth to achieve your goal then go ahead. This is just a proof of concept and the code hasn't been thoroughly tested. Use it at your own risk.