Question

Im trying to learn Android graphics & event handeling for a multiplayer game.

As a building block Im developing some code that will draw path of points based on touch/motion events, however Im unable to draw, nothing gets drawn on the Emulator

I have 3 simple classes (MyPoint,GameCanvas,Game):

1) MyPoint class encapsulates x & y positions , draw method draws a point based on those positions

public class MyPoint  {

    private float x; 
    private float y; 
    Paint pWhite = new Paint(R.color.white);

    public MyPoint(float x, float y) {

           this.x = x;
           this.y = y;
    }//end const
    public void draw(Canvas canvas) {
        canvas.drawPoint(this.x, this.y, pWhite);
    }//end method
}//end MyPoint Class

2) GameCanvas is the View that will be drawn on , this class is responsible for
its on event handling by implementing OnTouchListener ,onThouch() method

    public class GameCanvas extends View implements OnTouchListener {

        //keep track on points created by touch events
        java.util.List<MyPoint> pointsList = null;

        public GameCanvas(Context context) {
            super(context);
            setFocusable(true);
            setFocusableInTouchMode(true);

            setOnTouchListener(this);
            pointsList = new java.util.Stack();

        }//end const 

//onDraw iterates through points that were added during motion/touch events and calls the MyPoint.draw(canvas) 

        @Override
        protected void onDraw(Canvas canvas) {

            MyPoint p = null;
            for (int i = 0; i < pointsList.size(); i++) {
                p = (MyPoint) pointsList.remove(i);
                p.draw(canvas);
            }

        }//end onDraw

        //implemented in order to handel touch events 
        public boolean onTouch(View arg0, MotionEvent event) {
            int action = event.getAction();
            float currentX = event.getX();
            float currentY = event.getY();

            //log x and y coordinates
            Log.v(this.getClass().getName().toString(), "X=" + currentX);
            //log x and y coordinates
            Log.v(this.getClass().getName().toString(), "Y=" + currentY);

            if (action == MotionEvent.ACTION_DOWN) {

                // log action down 
                Log.v(this.getClass().getName().toString(), "MotionEvent = ACTION_DOWN");

                pointsList.add(new MyPoint(currentX, currentY));
                this.invalidate();
            } else if (action == MotionEvent.ACTION_MOVE) {

                //log action move 
                Log.v(this.getClass().getName().toString(), "MotionEvent = ACTION_MOVE");

                pointsList.add(new MyPoint(currentX, currentY));
                this.invalidate();
            } else if (action == MotionEvent.ACTION_UP) {

                //log action move 
                Log.v(this.getClass().getName().toString(), "MotionEvent = ACTION_UP");

                pointsList.add(new MyPoint(currentX, currentY));
                this.invalidate();
            }
            //call invalidate in order to call trigger onDraw()

            return true;
        }//end onTouch() 
    }//end GameCanvas class

3) Game class is the Activity launches GameCanvas View

public class Game extends Activity {

GameCanvas newGameCanvas = null;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        //create GameCanvas object 
        newGameCanvas = new GameCanvas(this);
        super.onCreate(savedInstanceState);
        setContentView(newGameCanvas);
        //
        newGameCanvas.requestFocus();

    }//end OnCreate 
}//end Game class

Answer 1

Here is the actual working code. I only changed a few lines at most & also made the View a inner class of the Activity, so I dont have to look at different screens when testing/debugging.

1. First change was to make View inner class , no change in logic
   just for convenience
2. I set the canvas background color to white,
   in the onCreate()
3. I changed my logic in onDraw() from
   pointsQ.removw(i), to just pointsQ.get(i), with remove(i) only the
   last point was being drawn

4. Finally you get a canvas that you can
   use to draw/trace based on touch events

   public class Game extends Activity {
   
   private gameView newGameView = null;
   private MyPoint newMyPoint = null;
   
   private java.util.List<MyPoint> pointsQ = new ArrayList<MyPoint>();
   
   @Override
   public void onCreate(Bundle savedInstanceState) {
       try {
   
           newGameView = new gameView(this);
           newGameView.setBackgroundColor(Color.WHITE);
           super.onCreate(savedInstanceState);
           setContentView(newGameView);
           //initially requestFocus
           newGameView.requestFocus();
   
   
       } catch (NullPointerException exc) {
           Log.d(this.getClass().getName().toString(), exc.getMessage());
       }
   }
   
   //inner class that represents View 
   public class gameView extends View {
   
   
   private Paint wPaint = new Paint(Color.BLACK);
   
   
   public gameView(Context context) {
       super(context);
       setFocusable(true); 
       setFocusableInTouchMode(true); 
       //
       wPaint.setStyle(Paint.Style.FILL);
       wPaint.setStrokeWidth(3);
   
   }
   
   //implemented in order to handle touch events 
   @Override
   public boolean onTouchEvent(MotionEvent event) {
       int action = event.getAction();
       float currentX = event.getX();
       float currentY = event.getY();
   
       //log x and y coordinates
       Log.d(this.getClass().getName().toString(), "X=" + currentX);
       //log x and y coordinates
       Log.d(this.getClass().getName().toString(), "Y=" + currentY);
   
       if (action == MotionEvent.ACTION_DOWN) {
   
           // log action down 
           Log.d(this.getClass().getName().toString(), "MotionEvent = ACTION_DOWN");
   
           newMyPoint = new MyPoint(currentX, currentY);
           pointsQ.add(newMyPoint);
   
   
       } else if (action == MotionEvent.ACTION_MOVE) {
   
           //log action move 
           Log.d(this.getClass().getName().toString(), "MotionEvent = ACTION_MOVE");
   
           newMyPoint = new MyPoint(currentX, currentY);
           pointsQ.add(newMyPoint);
   
       } else if (action == MotionEvent.ACTION_UP) {
   
           //log action move 
           Log.d(this.getClass().getName().toString(), "MotionEvent = ACTION_UP");
   
           newMyPoint = new MyPoint(currentX, currentY);
           pointsQ.add(newMyPoint);
   
   
       }
   
       //I moved the invalidate to the end of the method se=ince all action called it anyways 
       this.invalidate(); 
       return true;
   }//end onTouch() 
   
   @Override
   protected void onDraw(Canvas canvas) {
       MyPoint p = null;
       for (int i = 0; i < pointsQ.size(); i++) {
           //get MyPoint & draw based on its x,y attributes 
           p = (MyPoint) pointsQ.get(i);
                  //
           canvas.drawCircle(p.getX(),p.getY(),2,wPaint);
           //
           Log.d(this.getClass().getName().toString(), "onDraw X= "+p.getX()+" Y= "+p.getY());    
           }
   
       } 
   }//end inner class gameView
   }//end Game activity class