用手指画一条衰落的道路

Question

我正在尝试实现以下目标：

（摘自《水果忍者》）

目前，我可以用手指画一条路径，但无法完成以下两项任务：

1. 清除路径的“尾部”。我的意思是，现在，只要用户的手指在移动，路径就不会清除他的尾巴，这将是我不希望的长线。

2. 我还想获得缩放效果，这意味着路径的最旧部分应该最小，而最新部分应该最大。

这是我所能做到的：

public class DrawableGridLayout extends GridLayout {

    private Paint mPaint;
    public int width;
    public int height;
    private Bitmap mBitmap;
    private Canvas mCanvas;
    private Path mPath;
    private Paint mBitmapPaint;
    private Paint circlePaint;
    private Path circlePath;


    public DrawableGridLayout(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public DrawableGridLayout(Context context) {
        super(context);
        init();
    }

    public DrawableGridLayout(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        init();
    }

    private void init() {
        mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
        mPath = new Path();
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setPathEffect(new CornerPathEffect(1000));
        mPaint.setDither(true);
        mPaint.setColor(Color.WHITE);
        mPaint.setStrokeJoin(Paint.Join.MITER);
        mPaint.setStrokeCap(Paint.Cap.ROUND);
        mPaint.setStrokeWidth(16);

        mBitmapPaint = new Paint(Paint.DITHER_FLAG);

        circlePaint = new Paint();
        circlePath = new Path();
        circlePaint.setAntiAlias(true);
        circlePaint.setColor(Color.WHITE);
        circlePaint.setStyle(Paint.Style.STROKE);
        circlePaint.setStrokeJoin(Paint.Join.MITER);
        circlePaint.setStrokeWidth(4f);
    }

    public void clearDrawing() {

        setDrawingCacheEnabled(false);

        onSizeChanged(width, height, width, height);
        invalidate();

        setDrawingCacheEnabled(true);
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);

        width = w;      // don't forget these
        height = h;

        if (width > 0 && height > 0) {
            mBitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
            mCanvas = new Canvas(mBitmap);
        }
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);
        canvas.drawPath(mPath, mPaint);
        canvas.drawPath(circlePath, circlePaint);
    }

    private float mX, mY;
    private static final float TOUCH_TOLERANCE = 4;

    private void touch_start(float x, float y) {
        mPath.reset();
        mPath.moveTo(x, y);
        mX = x;
        mY = y;
    }

    private void touch_move(float x, float y) {
        float dx = Math.abs(x - mX);
        float dy = Math.abs(y - mY);
        if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
            mPath.quadTo(mX, mY, (x + mX) / 2, (y + mY) / 2);
            mX = x;
            mY = y;

            circlePath.reset();
            circlePath.addCircle(mX, mY, 30, Path.Direction.CW);
        }
    }

    private void touch_up() {
        mPath.lineTo(mX, mY);
        circlePath.reset();
        // commit the path to our offscreen
        mCanvas.drawPath(mPath, mPaint);
        // kill this so we don't double draw
        mPath.reset();
        clearDrawing();
    }


    @Override
    public boolean onTouchEvent(MotionEvent event) {
        float x = event.getX();
        float y = event.getY();

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                touch_start(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_MOVE:
                touch_move(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_UP:
                touch_up();
                invalidate();
                break;
            case MotionEvent.ACTION_CANCEL:
                clearDrawing();
                break;
        }
        return true;
    }