Android 照片选择区域功能实现示例

/* 裁剪区域,
0， 左上 -> LeftTop, 
1， 右上 -> RightTop，
2， 右下 -> RightBottom, 
3， 左下 -> LeftBottom
*/
Point[] mCropPoints;
// 4 条边的中点
Point[] mEdgeMidPoints;

    protected void onDrawCropPoint(Canvas canvas) {
        //绘制蒙版
        onDrawMask(canvas);
        //绘制辅助线
        onDrawGuideLine(canvas);
        //绘制选区线
        onDrawLines(canvas);
        //绘制锚点
        onDrawPoints(canvas);
        //绘制放大镜
        // ...
    }

protected void onDrawPoints(Canvas canvas) {
        if (!checkPoints(mCropPoints)) {
            return;
        }
        // 绘制 4 个角
        for (Point point : mCropPoints) {
            canvas.drawCircle(getViewPointX(point), getViewPointY(point), dp2px(POINT_RADIUS), mPointFillPaint);
            canvas.drawCircle(getViewPointX(point), getViewPointY(point), dp2px(POINT_RADIUS), mPointPaint);
        }
        if (mShowEdgeMidPoint) {
            setEdgeMidPoints();
            // 中间锚点
            // 绘制 4 条边上的中点
            for (Point point : mEdgeMidPoints){
                canvas.drawCircle(getViewPointX(point), getViewPointY(point), dp2px(POINT_RADIUS), mPointFillPaint);
                canvas.drawCircle(getViewPointX(point), getViewPointY(point), dp2px(POINT_RADIUS), mPointPaint);
            }
        }
    }

     public void setEdgeMidPoints(){
        // 中点不存在，就新建
        if (mEdgeMidPoints == null){
            mEdgeMidPoints = new Point[4];
            for (int i = 0; i < mEdgeMidPoints.length; i++){
                mEdgeMidPoints[i] = new Point();
            }
        }
        // 维护 4 个顶点的位置，
        // 通过顶点的位置，算出边上中点的位置
        int len = mCropPoints.length;
        for (int i = 0; i < len; i++){
            // 为了避免极端情况，
            // 采用  （   坐标 + 距离的一半   ）   的方式
            mEdgeMidPoints[i].set(mCropPoints[i].x + (mCropPoints[(i+1)%len].x - mCropPoints[i].x)/2,
                                    mCropPoints[i].y + (mCropPoints[(i+1)%len].y - mCropPoints[i].y)/2);
        }
    }

enum DragPointType{
        LEFT_TOP,
        RIGHT_TOP,
        RIGHT_BOTTOM,
        LEFT_BOTTOM,
        TOP,
        RIGHT,
        BOTTOM,
        LEFT;
        // 判断是角点拖拽，不是中点平移
        public static boolean isEdgePoint(DragPointType type){
            return type == TOP || type == RIGHT || type == BOTTOM || type == LEFT;
        }
    }

@Override
    public boolean onTouchEvent(MotionEvent event) {
        int action = event.getAction();
        boolean handle = true;
        switch (action) {
            case MotionEvent.ACTION_DOWN:
                // 识别到，当前点
                mDraggingPoint = getNearbyPoint(event);
                if (mDraggingPoint == null) {
                    handle = false;
                }
                break;
            case MotionEvent.ACTION_MOVE:
                // 移动
                toImagePointSize(mDraggingPoint, event);
                break;
            case MotionEvent.ACTION_UP:
                // 手指抬起，
                // 操作取消
                mDraggingPoint = null;
                break;
        }
        // 绘制
        // 更新完位置后，刷新绘制
        invalidate();
        return handle || super.onTouchEvent(event);
    }

private Point getNearbyPoint(MotionEvent event) {
        // 判断 4 个角点，可用
        if (checkPoints(mCropPoints)) {
            for (Point p : mCropPoints) {
                // 找出当前的点
                if (isTouchPoint(p, event)) return p;
            }
        }
        // 判断 4 个中点可用
        if (checkPoints(mEdgeMidPoints)) {
            for (Point p : mEdgeMidPoints){
                // 找出当前的点
                if (isTouchPoint(p, event)) return p;
            }
        }
        return null;
    }

private static final float TOUCH_POINT_CATCH_DISTANCE = 15;
private boolean isTouchPoint(Point p, MotionEvent event){
        float x = event.getX();
        float y = event.getY();
        float px = getViewPointX(p);
        float py = getViewPointY(p);
        double distance =  Math.sqrt(Math.pow(x - px, 2) + Math.pow(y - py, 2));
        // 也就是，判断距离
        if (distance < dp2px(TOUCH_POINT_CATCH_DISTANCE)) {
            return true;
        }
        return false;
    }

private void toImagePointSize(Point dragPoint, MotionEvent event) {
        if (dragPoint == null) {
            return;
        }
        // 找出当前移动类型，
        // 是角点拖拽，还是中点平移
        DragPointType pointType = getPointType(dragPoint);
        int x = (int) ((Math.min(Math.max(event.getX(), mActLeft), mActLeft + mActWidth) - mActLeft) / mScaleX);
        int y = (int) ((Math.min(Math.max(event.getY(), mActTop), mActTop + mActHeight) - mActTop) / mScaleY);
        // 判断可以移动
        // ...
        if (DragPointType.isEdgePoint(pointType)){
            // ...
            // 中点平移
        } else {
            // 角点拖拽
            // 实现很简单，
            // 更新就好了
            dragPoint.y = y;
            dragPoint.x = x;
        }
    }

// 拿采集的点，找出对应的类型
private DragPointType getPointType(Point dragPoint){
        if (dragPoint == null) return null;
        DragPointType type;
        // 看，是不是顶点 / 角点
        if (checkPoints(mCropPoints)) {
            for (int i = 0; i < mCropPoints.length; i++) {
                if (dragPoint == mCropPoints[i]) {
                    // 找到了，直接返回
                    type = DragPointType.values()[i];
                    return type;
                }
            }
        }
        // 看，是不是中点
        if (checkPoints(mEdgeMidPoints)) {
            for (int i = 0; i < mEdgeMidPoints.length; i++){
                if (dragPoint == mEdgeMidPoints[i]){
                    // 找到了，直接返回
                    type = DragPointType.values()[4+i];
                    return type;
                }
            }
        }
        return null;
    }

private void toImagePointSize(Point dragPoint, MotionEvent event) {
        if (dragPoint == null) {
            return;
        }
        DragPointType pointType = getPointType(dragPoint);
        int x = // ...
        int y = // ...
        // 判断可以移动
        // ...
        if (DragPointType.isEdgePoint(pointType)){
            //  中点平移,
            //  拿到的是，一个偏移向量
            int xoff = x - dragPoint.x;
            int yoff = y - dragPoint.y;
            moveEdge(pointType, xoff, yoff);
        } else {
            // 角点拖拽
            // ...
        }
    }

private void moveEdge(DragPointType type, int xoff, int yoff){
        switch (type){
            case TOP:
                // 这边的平移，比较简单
                // 找到中点，旁边的两个焦点
                // 再移动位置
                movePoint(mCropPoints[P_LT], 0, yoff);
                movePoint(mCropPoints[P_RT], 0, yoff);
                break;
            case RIGHT:
                // 右移处理
                movePoint(mCropPoints[P_RT], xoff, 0);
                movePoint(mCropPoints[P_RB], xoff, 0);
                break;
            case BOTTOM:
                // 下移处理
                // ...
            case LEFT:
                // 左移处理
                // ...
            default: break;
        }
    }

private void movePoint(Point point, int xoff, int yoff){
        if (point == null) return;
        int x = point.x + xoff;
        int y = point.y + yoff;
        // 检查边界
        if (x < 0 || x > getDrawable().getIntrinsicWidth()) return;
        if (y < 0 || y > getDrawable().getIntrinsicHeight()) return;
        point.x = x;
        point.y = y;
    }