How do I draw on TCanvas from inside a function call ?

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3 total messages Started by Joseph Wed, 05 Mar 2003 06:03

Author: Joseph
Date: Wed, 05 Mar 2003 06:03

41 lines
2367 bytes

Hello All,

Is it possible to draw on TControlCanvas from inside a function call ?

I mean ...

   void __fastcall TForm1::ShowTabSheet (Object *Sender) {
   
        TControlCanvas *pCanvas = new TControlCanvas ;
        
        pCanvas->Control = Tabsheet ;  // ie., TForm1::Tabsheet. Need to draw
                                       // on TabSheet, not on the main Form.

        DrawOnCanvas ( pCanvas ) ;  //** I need to draw on pCanvas from here 

        delete pCanvas ; 
   };



   void TForm1::DrawOnCanvas ( TControlCanvas *CANVAS ) {

       CANVAS->Pen->Color = clBlue ;
       CANVAS->MoveTo(0,0);
       CANVAS->LineTo(100,200);

   };


 ... This compiles and Links OK, but when I run it I get an error
message saying that "Canvas does not allow drawing". If I do the
drawing from inside the calling function, it's ok. But if done via a
function call, it won't draw.

Is the above the appropriate way of drawing on TControlCanvas from
inside a function ?



Regards,
Joseph 76

Re: How do I draw on TCanvas from inside a function call ?

Author: Maynard Philbrook
Date: Wed, 05 Mar 2003 07:14

41 lines
2708 bytes

pCanvas->Canvas->?????


Joseph wrote:

> Hello All,
>
> Is it possible to draw on TControlCanvas from inside a function call ?
>
> I mean ...
>
>    void __fastcall TForm1::ShowTabSheet (Object *Sender) {
>
>         TControlCanvas *pCanvas = new TControlCanvas ;
>
>         pCanvas->Control = Tabsheet ;  // ie., TForm1::Tabsheet. Need to draw
>                                        // on TabSheet, not on the main Form.
>
>         DrawOnCanvas ( pCanvas ) ;  //** I need to draw on pCanvas from here
>
>         delete pCanvas ;
>    };
>
>    void TForm1::DrawOnCanvas ( TControlCanvas *CANVAS ) {
>
>        CANVAS->Pen->Color = clBlue ;
>        CANVAS->MoveTo(0,0);
>        CANVAS->LineTo(100,200);
>
>    };
>
>  ... This compiles and Links OK, but when I run it I get an error
> message saying that "Canvas does not allow drawing". If I do the
> drawing from inside the calling function, it's ok. But if done via a
> function call, it won't draw.
>
> Is the above the appropriate way of drawing on TControlCanvas from
> inside a function ?
>
> Regards,
> Joseph 76

Re: How do I draw on TCanvas from inside a function call ?

Author: Joseph
Date: Thu, 06 Mar 2003 23:34

81 lines
3622 bytes

Ok I worked it out.  The reason why I was getting "Cannot Draw on
Canvas" runtime message is that CANVAS parameter inside the function
points to a TControlCanvas "interface" but nothing to draw on (ie., to
real device context). So this is the fix ...

    void TForm1::DrawOnCanvas ( TControlCanvas *CANVAS ) {

        CANVAS = new TControlCanvas ;
        CANVAS->Control = Tabsheet ;   // draw on Tabsheet        

        CANVAS->Pen->Color = clBlue ;
        CANVAS->MoveTo(0,0);
        CANVAS->LineTo(100,200);

        delete CANVAS ;
    };

Actually, I don't even need to pass in a TControlCanvas parameter, so
the function can be simplified to this ...



    void TForm1::DrawOnCanvas ( ) {
        TControlCanvas *CANVAS = new TControlCanvas;
        CANVAS->Control = Tabsheet ;   // draw on Tabsheet        

        CANVAS->Pen->Color = clBlue ;
        CANVAS->MoveTo(0,0);
        CANVAS->LineTo(100,200);

        delete CANVAS ;
    };

 
Cheers,
Joseph 76.



joseph.redimerio@baesystems.com (Joseph) wrote in message news:<7b424d28.0303042203.21e36935@posting.google.com>...
> Hello All,
> 
> Is it possible to draw on TControlCanvas from inside a function call ?
> 
> I mean ...
> 
>    void __fastcall TForm1::ShowTabSheet (Object *Sender) {
>    
>         TControlCanvas *pCanvas = new TControlCanvas ;
>         
>         pCanvas->Control = Tabsheet ;  // ie., TForm1::Tabsheet. Need to draw
>                                        // on TabSheet, not on the main Form.
> 
>         DrawOnCanvas ( pCanvas ) ;  //** I need to draw on pCanvas from here 
> 
>         delete pCanvas ; 
>    };
> 
> 
> 
>    void TForm1::DrawOnCanvas ( TControlCanvas *CANVAS ) {
> 
>        CANVAS->Pen->Color = clBlue ;
>        CANVAS->MoveTo(0,0);
>        CANVAS->LineTo(100,200);
> 
>    };
> 
> 
>  ... This compiles and Links OK, but when I run it I get an error
> message saying that "Canvas does not allow drawing". If I do the
> drawing from inside the calling function, it's ok. But if done via a
> function call, it won't draw.
> 
> Is the above the appropriate way of drawing on TControlCanvas from
> inside a function ?
> 
> 
> 
> Regards,
> Joseph 76

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