如何从中心角度改变顶部和左侧视角zoomlayout android

Question

我想在布局中拖动和缩放视图我已经实现了拖动和缩放但我的问题是，当我缩放视图时它以非常快的速度缩放我已经尝试了很多次我认为我正在计算错误的因素或给出错误左上角和上边距

public class ZoomLayout extends RelativeLayout  {

  private static final int NONE = 0;
  private static final int DRAG = 1;
  private static final int ZOOM = 2;
  private int mode = NONE;
  // remember some things for zooming
  private PointF start = new PointF();
  private PointF mid = new PointF();
  private float oldDist = 1f;
  private float d = 0f;
  private static final String TAG = "ZoomLayout";
  private static final float MIN_ZOOM = 1.0f;
  private static final float MAX_ZOOM = 4.0f;


  private float scale = 1.0f;
  private float lastScaleFactor = 0f;

  // Where the finger first  touches the screen
  private float startX = 0f;
  private float startY = 0f;

  private int _xDelta;
  private int _yDelta;

  // How much to translate the canvas
  private float dx = 0f;
  private float dy = 0f;
  private float prevDx = 0f;
  private float prevDy = 0f;

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

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

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

  private void init(Context context) {

    this.setOnTouchListener(new View.OnTouchListener() {
      @Override
      public boolean onTouch(View view, MotionEvent event) {
          int dx = 0,dy = 0;
          switch (event.getAction() & MotionEvent.ACTION_MASK) {
          case MotionEvent.ACTION_DOWN:
              savedMatrix.set(matrix);
              start.set(event.getX(), event.getY());
              mode = DRAG;
              RelativeLayout.LayoutParams lParams = (RelativeLayout.LayoutParams) child().getLayoutParams();
                _xDelta =(int) (event.getX()  - lParams.leftMargin);
                _yDelta = (int) (event.getY() - lParams.topMargin);
              lastEvent = null;
              break;
          case MotionEvent.ACTION_POINTER_DOWN:
              oldDist = spacing(event);
              if (oldDist > 10f) {
                  RelativeLayout.LayoutParams l0Params = (RelativeLayout.LayoutParams) child().getLayoutParams();
                    dx = (int) (l0Params.leftMargin);
                    dy = (int) (l0Params.topMargin);
                  mode = ZOOM;
              }

              break;
          case MotionEvent.ACTION_UP:
          case MotionEvent.ACTION_POINTER_UP:
              mode = NONE;
              lastEvent = null;
              break;
          case MotionEvent.ACTION_MOVE:
              if (mode == DRAG) {

                RelativeLayout.LayoutParams layoutParams = (RelativeLayout.LayoutParams) child()
                .getLayoutParams();
                layoutParams.leftMargin = (int) (event.getX() - _xDelta);
                layoutParams.topMargin =(int) (event.getY() - _yDelta);
                layoutParams.rightMargin = -250;
                layoutParams.bottomMargin = -250;
                child().setLayoutParams(layoutParams);

              } else if (mode == ZOOM) {
                  float newDist = spacing(event);
                  if (newDist > 10f) {
                     // matrix.set(savedMatrix);
                      float scale = (newDist / oldDist);
                     // matrix.pos
                      //matrix.postScale(scale, scale, mid.x, mid.y);
                      LayoutParams layoutParams=(RelativeLayout.LayoutParams) child().getLayoutParams();
                      layoutParams.leftMargin= (int) (child().getWidth()*scale*layoutParams.leftMargin)/child().getWidth();//(layoutParams.leftMargin - dx);
                      layoutParams.topMargin = (int) (child().getHeight()*scale*layoutParams.topMargin)/child().getWidth();//(layoutParams.topMargin - dy);
                      layoutParams.height=(int) (child().getHeight()*scale);
                      layoutParams.width=(int) (child().getWidth()*scale);
                      child().setLayoutParams(layoutParams);
                  }

              }
              break;
      }

      //gifDrawable.setMatrix(matrix); 
      return true;
  }

    });
  }

  /**
   * Determine the space between the first two fingers
   */
  private float spacing(MotionEvent event) {
      float x = event.getX(0) - event.getX(1);
      float y = event.getY(0) - event.getY(1);
      return (float)Math.sqrt(x * x + y * y);
  }

  /**
   * Calculate the mid point of the first two fingers
   */
  private void midPoint(PointF point, MotionEvent event) {
      float x = event.getX(0) + event.getX(1);
      float y = event.getY(0) + event.getY(1);
      point.set(x / 2, y / 2);
  }

  /**
   * Calculate the degree to be rotated by.
   *
   * @param event
   * @return Degrees
   */
  private float rotation(MotionEvent event) {
      double delta_x = (event.getX(0) - event.getX(1));
      double delta_y = (event.getY(0) - event.getY(1));
      double radians = Math.atan2(delta_y, delta_x);
      return (float) Math.toDegrees(radians);
  }
  // ScaleGestureDetector

  @Override
  public boolean onScaleBegin(ScaleGestureDetector scaleDetector) {
    Log.i(TAG, "onScaleBegin");
    return true;
  }

  @Override
  public boolean onScale(ScaleGestureDetector scaleDetector) {
        return true;
  }

  @Override
  public void onScaleEnd(ScaleGestureDetector scaleDetector) {
    Log.i(TAG, "onScaleEnd");
  }


  private View child() {
    return getChildAt(0);
  }
}

我尝试了很多东西，但每当我缩放视图时它会变焦非常快，所以请一些人建议我如何在视图上实现缩放效果