Delphi: Can't Draw On Panel's Canvas - image

Question

I'm trying to draw an image on a TPanel's Canvas in a procedure of this Panel. When I do this in Paint Method of the Panel it works just fine, but when I try to draw on the Canvas in another procedure or in the constructor nothing happens.
This is how I draw on the Panel's Canvas in the Paint Procedure:
procedure TFeld.Paint;
var bmp : TBitmap;
begin
inherited;
bmp := TBitmap.Create;
try
bmp.LoadFromFile('textures\ground.bmp');
self.Canvas.Draw(0,0,bmp);
finally
bmp.Free;
end;
end;
And this is how I try to draw on it in another procedure:
procedure TFeld.setUnsichtbar;
var bmp : TBitmap;
begin
bmp := TBitmap.Create;
try
bmp.LoadFromFile('textures\invisible.bmp');
self.Canvas.Draw(0,0,bmp);
finally
bmp.Free;
end;
end;
But the Panel's Canvas is still the image I applied in the Paint procedure.
I already tried to move the drawing from the Paint procedure to the Constructor which didn't work.
The path is also correct, switched the paths and now the Panels have the 'invisible.bmp' image.
Whole Class/Unit: http://pastebin.com/YhhDr1F9
Any idea why this doesn't work?

Answer 1

Looking at your whole class I asume you desire to controll which image is being shown at which time based on certain condition. Right?
If that is the case first thing that you need is for your class to have a field for storing the image data. In your example above you are only using bmp files so TBitmap would suffice. But if you are using other picture types you might want to use TPicture field instead as this one alows loading of all supported picture images as TImage that you also tried to use component can.
Then you change your component's Paint method to use the above mentioned field for getting picture data instead of creating local picture data variable every time as you do it now.
In fact what you are doing now is terrible as you are forcing your application to read the image data from file into memory every time your component is rendered. This could cause terrible performance.
And finally when you want to change the picture that is shown on your component just load different picture into your picture field.
So with above changes your class should look something like this:
type
TFeld=class(TPanel)
protected
procedure Paint;override;
procedure BitmapChange(Sender: TObject);
private
zSichtbar : Boolean;
zPosHor,zPosVer : Integer; // Position im Array
zEinheit : TEinheit;
Bitmap: TBitmap; //Field for storing picture data
public
// hImage : TImage;
constructor Create(pX,pPosHor,pY,pPosVer,pHoehe,pBreite:integer;pImgPath:String;pForm:Tform); virtual;
destructor Destroy;
procedure ChangeImage(pImgPath: String);
end;
...
implementation
constructor TFeld.Create(pX,pPosHor,pY,pPosVer,pHoehe,pBreite:integer;pImgPath:String;pForm:Tform);
begin
inherited create(pForm);
...
//Creater internal component for storing image data
Bitmap := TBitmap.Create;
//Assign proper method to Bitmaps OnChange event
Bitmap.OnChange := BitmapChange;
//Load initial image data
Bitmap.LoadFromFile(pImgPath);
....
end;
destructor Destroy;
begin
//We need to destroy the internal component for storing image data before
//we destory our own component in order to avoid memory leaks
Bitmap.Free;
end;
procedure TFeld.Paint;
begin
inherited;
//Use local image field to tell the Paint method of what to render
self.Canvas.Draw(0,0,Bitmap);
end;
procedure TFeld.BitmapChange(Sender: TObject);
begin
//Force redrawing of the component on bitmap change
self.Invalidate;
end;
procedure TFeld.ChangeImage(pImgPath: String);
begin
//Load different image into image field
Bitmap.LoadFromFile(pImgPath);
end;
EDIT: Adding necessary code to force component redrawing after bitmap has been changed.