为什么没有为我设置OnTouchListener？

Question

I am working on a custom view called CanvasView . This is a view that allows me to draw stuff on it outside of the onDraw method. It is something like this:

public class CanvasView extends View {
    private ArrayList<Shape> shapes;
    private Paint paint;

    public CanvasView (Context c) {
        super(c);
        init ();
    }

    public CanvasView(Context context, AttributeSet attrs) {
        super (context, attrs);
        init ();
    }

    public CanvasView(Context context, AttributeSet attrs, int defStyleAttr) {
        super (context, attrs, defStyleAttr);
        init ();
    }
    private void init () {
        shapes = new ArrayList<> ();
        paint = new Paint ();
        paint.setStrokeWidth (5);
        paint.setColor (Color.BLACK);
    }

    //focus on this method, I think the others are irrelevant
    @Override
    public void setOnTouchListener (final OnTouchListener listener) {
        final OnTouchListener baseListener = new OnTouchListener () {
            @Override
            public boolean onTouch(View v, MotionEvent event) {
                float x = event.getX ();
                float y = event.getY ();
                if (x < 18 || x > getWidth () - 18 || y < 18 ||
                        y > getHeight () - 18)
                    return false;

                return true;
            }
        };

        super.setOnTouchListener (new OnTouchListener () {
            @Override
            public boolean onTouch(View v, MotionEvent event) {
                if (baseListener.onTouch (v, event)) {
                    if (listener != null) {
                        return listener.onTouch (v, event);
                    } else {
                        return true;
                    }
                }
                return false;
            }
        });
    }

    @Override
    protected void onDraw (Canvas c) {
        super.onDraw (c);
        for (Shape s : shapes) {
            s.draw (c);
        }
        //draw the border. irrelevant
        c.drawLine (3, 3, getWidth () - 3, 3, paint);
        c.drawLine (3, getHeight () - 3, getWidth () - 3, getHeight () - 3, paint);
        c.drawLine (3, 3, 3, getHeight () - 3, paint);
        c.drawLine (getWidth () - 3, 3, getWidth () - 3, getHeight () - 3, paint);

        //draw the inner border
        c.drawLine (18, 18, getWidth () - 18, 18, paint);
        c.drawLine (18, getHeight () - 18, getWidth () - 18, getHeight () - 18, paint);
        c.drawLine (18, 18, 18, getHeight () - 18, paint);
        c.drawLine (getWidth () - 18, 18, getWidth () - 18, getHeight () - 18, paint);
    }

    public void addShape (Shape s) {
        shapes.add (s);
    }

    public void clear () {
        shapes.clear ();
    }
}

Explanation:

Shape is an interface with a method:

public void draw (Canvas c);

Focus on the setOnTouchListener override. I overrode this method because I want to limit the user not to touch the borders of the CanvasView . As you can see, I first invoke the baseListener to check whether the touch is in bounds. And then I invoke the listener passed in the constructor. Is this a good practice?

Anyway, I set the OnTouchListener of a CanvasView in the onCreate method:

canvas.setOnTouchListener (new View.OnTouchListener () {
        @Override
        public boolean onTouch(View v, final MotionEvent event) {
            if (point1 != null && point2 != null) {
                throw new IllegalStateException ("Both point1 and point2 are not null");
            }

            if (point1 == null) {
                point1 = new PointF (event.getX (), event.getY ());
            } else { //point2 is null
                point2 = new PointF (event.getX (), event.getY ());
                canvas.addShape (new Shape () {
                    @Override
                    public void draw(Canvas c) {
                        c.drawLine (point1.x, point1.y, point2.x, point2.y, paint);
                    }
                });
                canvas.setOnTouchListener (null);
            }

            canvas.addShape (new Shape () {
                @Override
                public void draw(Canvas c) {
                    c.drawCircle (event.getX (), event.getY (), 13, paint);
                }
            });

            return true;
        }
    });

Explanation:

point1 and point2 are fields declared in the activity class. When the user touches the screen, one of them will be instantiated and a little circle is drawn at the point. When the user touches the second time, point2 is instantiated and a line is drawn between the two points.

When I run my app and touch the screen, nothing is drawn! I think the setOnClickListener override is not written correctly. Can anyone tell me why?

Answer 1

The onDraw method of a View is only called when the View is first created. To cause it to be called again, you need to mark the View as “dirty” (changes have been made) using the invalidate() function on the View, every time the contents of the View change.

Thus, a convenient place to call this function would be just after adding a shape to the View in your addShape function.

 public void addShape (Shape s) {
        shapes.add (s);
        this.invalidate();
    }