How can I paint an MDI background image?

Applies To: C++Builder 1 or higher
Category: Knowledge Base

As shown in How can I paint a background image?, I can use the TCanvas object to draw on my form. This technique does not work for MDI applications. How can I solve this problem?

 

1.

  Create an MDI application. For more information, read

      How can I create MDI applications?

 

2.

  Edit the header file of your main form:

 

private:

    FARPROC Client, PrevClient; 

    void __fastcall ClientProc(TMessage &Message);

 

3.

  Insert in your source code:

 

void __fastcall TForm1::ClientProc(TMessage &Message)

{

    if(Message.Msg == WM_ERASEBKGND) 

    { 

        TCanvas *canvas = new TCanvas(); 

        canvas->Handle = (HDC)Message.WParam; 

        // Now you have the canvas 

        canvas->Brush->Bitmap = Image1->Picture->Bitmap; 

        canvas->FillRect(Rect(0, 0, ClientWidth, ClientHeight)); 

        Message.Result = true; 

        delete canvas; 

    } 

    else 

        Message.Result = CallWindowProc(PrevClient, ClientHandle, 

            Message.Msg, Message.WParam, Message.LParam); 

}

 

4.

  Double-click the OnCreate event of your form:

 

void __fastcall TForm1::FormCreate(TObject *Sender)

{

    Client = (FARPROC)MakeObjectInstance(ClientProc); 

    PrevClient = (FARPROC)GetWindowLong(ClientHandle,

        GWL_WNDPROC); 

    SetWindowLong(ClientHandle, GWL_WNDPROC, (long)Client); 

}

 

5.

  Add an Image. Load the bitmap of your choice.

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