Android : Draw line progressively in android

I found some code to draw line and now i wand drawing line progressively so that i cloud see it being drawn.

This is the code

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.view.View;

public class DrawView extends View {
    Paint paint = new Paint();

    public DrawView(Context context) {
        super(context);
        paint.setColor(Color.BLACK);
    }

    @Override
    public void onDraw(Canvas canvas) {
            canvas.drawLine(0, 0, 20, 20, paint);
            canvas.drawLine(20, 0, 0, 20, paint);
    }

}

How can i do that? Tnx