java - 为什么没有为我设置 OnTouchListener？

Question

我正在开发一个名为 CanvasView 的自定义 View 。这个 View 允许我在 onDraw 方法之外在其上绘制内容。它是这样的:

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 ();
    }
}

说明:

Shape 是一个带有方法的接口(interface):

public void draw (Canvas c);

重点关注 setOnTouchListener 覆盖。我重写了这个方法，因为我想限制用户不触摸 CanvasView 的边框。如您所见，我首先调用 baseListener 来检查触摸是否在范围内。然后我调用构造函数中传递的监听器。这是一个好的做法吗？

无论如何，我在onCreate方法中设置了CanvasView的OnTouchListener:

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;
        }
    });

说明:

point1 和 point2 是在 Activity 类中声明的字段。当用户触摸屏幕时，其中一个将被实例化，并在该点绘制一个小圆圈。当用户第二次触摸时，point2 被实例化，并在两点之间绘制一条线。

当我运行我的应用程序并触摸屏幕时，没有绘制任何内容!我认为 setOnClickListener 重写编写不正确。谁能告诉我为什么？

Answer 1

View 的 onDraw 方法仅在首次创建 View 时调用。要使其再次被调用，每次 View 的内容发生更改时，您都需要使用 View 上的 invalidate() 函数将 View 标记为“脏”(已进行更改)。

因此，调用此函数的一个方便位置是在 addShape 函数中向 View 添加形状之后。

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