Paint in Android Studio in background

Question

Im trying to develop an App in android Studio and I want to use Paint and Canvas, basically paint something, when I use publishProgress.

It would be great if any of you could tell me what is wrong with my code and why this is not painting anything.

Note. This all is in a AsyncTaskActivity class

 @Override
    protected void doInBackground(Void... params) {

             publishProgress(); // Run onProgressUpdate() method
         }

    }

 @Override
    protected void onProgressUpdate(Void... params) {
        // Here you can access the UI thread
        Canvas canvas = new Canvas();
        Paint pa1= new Paint();
        pa1.setColor(Color.RED);
        pa1.setStyle(Paint.Style.FILL);
        pa1.setStrokeWidth(50);

        canvas.drawLine(0, 0, 100, 100, pa1);
    }

Answer 1

Your code does not draw because you draw on new canvas instance. For draw on screen you must create custom view and override onDraw method on it. In this method you have canvas. Try draw on it.

https://developer.android.com/training/custom-views/custom-drawing.html

Answer 2

A canvas needs to draw to either a bitmap or a view. To draw to a Bitmap, you can create an empty Bitmap then pass it to the Canvas in the constructor. To draw to a view, you would create a custom view and override the onDraw function which will be passed a Canvas.

Drawing to a Bitmap won't actually display to the screen unless some view draws that Bitmap to its own canvas.