TCustomControl.Canvas Property

Specifies the TCanvas object that presents a drawing surface for the control.

Pascal

property Canvas: TCanvas;

C++

__property TCanvas Canvas;

Description

Use the properties of the TCanvas object to draw or paint on the surface of the control. Canvas encapsulates a Windows device context, providing all the tools and methods needed for drawing and painting.

Canvas is a protected property that is usually redeclared as public in descendants of TCustomControl.

C++ Examples:

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/*
The following code uses the bitmaps in an image list
component to draw the contents of each cell in a draw grid.
It draws a focus rectangle around the cell that has focus.
goDrawFocusSelect in the DrawGrid Options parameter must be
True to set focus on a cell.  The ImageList Draw method must
be called after DrawFocusRect. The OnSelectCell event handler
must implemented to return true.
*/
void __fastcall TForm1::DrawGrid1DrawCell(
  TObject *Sender, int ACol, int ARow, TRect &Rect, TGridDrawState State)
{
  long index = ARow * DrawGrid1->ColCount + ACol;
  DrawGrid1->Canvas->Brush->Color = clBackground;
  DrawGrid1->Canvas->FillRect(Rect);
  if (State.Contains(gdFocused))
  {
    DrawGrid1->Canvas->DrawFocusRect(Rect);
    Label1->Caption = "Cell " + IntToStr(int(index)) + " has the focus.";
  }
  ImageList1->Draw(
    DrawGrid1->Canvas, Rect.Left, Rect.Top, index, True);
}

void __fastcall TForm1::DrawGrid1SelectCell(TObject *Sender, int ACol, int ARow,
      bool &CanSelect)
{
  CanSelect= True;
}

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/*
The following code uses ClientRect to find and draw a line
from the top left to the bottom right of the current
control.  Select a TControl in the ListBox list.  This
example assumes that the current object is a descendent of
TControl and has a public Canvas property.
*/
{
  TPoint APoint = Point(X, Y);
  int Index = ListBox1->ItemAtPos(APoint, True);
  TStringGrid *myStringGrid;
  TClass ClassRef = ListBox1->Items->Objects[Index]->ClassType();
  if (String(ClassRef->ClassName()) == "TStringGrid")
  {
    myStringGrid = dynamic_cast<TStringGrid *>(ListBox1->Items->Objects[Index]);
    // make sure we are using the TControls canvas and not the from's!
    myStringGrid->Canvas->MoveTo(myStringGrid->ClientRect.Left, myStringGrid->ClientRect.Top);
    myStringGrid->Canvas->LineTo(myStringGrid->ClientRect.Right, myStringGrid->ClientRect.Bottom);
    myStringGrid = NULL;
  }
}

Delphi Examples:

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{
The following code uses the bitmaps in an image list
component to draw the contents of each cell in a draw grid.
It draws a focus rectangle around the cell that has focus.
goDrawFocusSelect in the DrawGrid Options parameter must be
True to set focus on a cell.  The ImageList Draw method must
be called after DrawFocusRect. The OnSelectCell event handler
must implemented to return true.
}
procedure TForm1.DrawGrid1DrawCell(Sender: TObject; ACol, ARow: Integer;
  Rect: TRect; State: TGridDrawState);
var
  index: Integer;
begin
  index := ARow * DrawGrid1.ColCount + ACol;
  DrawGrid1.Canvas.Brush.Color := clWhite;
  DrawGrid1.Canvas.FillRect(Rect);
  if (gdFocused in State) then
  begin
    DrawGrid1.Canvas.DrawFocusRect(Rect);
    Label1.Caption:= 'Cell ' + InttoStr(index) + ' has the focus.';
  end;
  ImageList1.Draw(DrawGrid1.Canvas,Rect.Left,Rect.Top,index, True);
end;

procedure TForm1.DrawGrid1SelectCell(Sender: TObject; ACol, ARow: Integer;
  var CanSelect: Boolean);
begin
  CanSelect:= True;
end;

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{
The following code uses ClientRect to find and draw a line
from the top left to the bottom right of the current
control.  Select a TControl in the ListBox list.  This
example assumes that the current object is a descendent of
TControl and has a public Canvas property.
}
  if (ListBox1.Items.Objects[Index] is TStringGrid) then
  begin
    myStringGrid:= (ListBox1.Items.Objects[Index] as TStringGrid);
    with myStringGrid.ClientRect do
    begin // make sure we are using the TControl's canvas and not the form's!
      myStringGrid.Canvas.MoveTo(Left, Top);
      myStringGrid.Canvas.LineTo(Right, Bottom);
    end;
  myStringGrid:= nil;
  end;