java - How to maintain the focus point in a Pan/Zoom Android Layout while zooming? -

i have custom layout extends framelayout , supports need. scrolling , zooming child view or layout works well. 1 thing having trouble maintaining focus point while zooming. center point between pinching fingers moves off screen, work google maps app focus(center) point maintained.

here's code.

public class betterzoomlayout extends framelayout {  // state objects , values related gesture tracking. private scalegesturedetector mscalegesturedetector; private gesturedetectorcompat mgesturedetector; 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; // how translate canvas private float distancex = 0f; private float distancey = 0f;  public betterzoomlayout(context context)     {     super(context);     init(context);     }  public betterzoomlayout(context context, attributeset attrs)     {     super(context, attrs);     init(context);     }  public betterzoomlayout(context context, attributeset attrs, int defstyle)     {     super(context, attrs, defstyle);     init(context);     }  private void init(context context)     {     // sets interactions     mscalegesturedetector = new scalegesturedetector(context, mscalegesturelistener);     mgesturedetector = new gesturedetectorcompat(context, mgesturelistener);     }  @override public boolean ontouchevent(motionevent event)     {     boolean retval = mscalegesturedetector.ontouchevent(event);     retval = mgesturedetector.ontouchevent(event) || retval;     applyscaleandtranslation();     return retval || super.ontouchevent(event);     }  /**  * scale listener, used handling multi-finger scale gestures.  */ private final scalegesturedetector.onscalegesturelistener mscalegesturelistener         = new scalegesturedetector.simpleonscalegesturelistener()     {     /**      * active focal point in terms of viewport. local      * variable kept here minimize per-frame allocations.      */      @override     public boolean onscalebegin(scalegesturedetector scalegesturedetector)         {         lastscalefactor = 0f;         return true;         }      @override     public boolean onscale(scalegesturedetector scalegesturedetector)         {         float scalefactor = scalegesturedetector.getscalefactor();          if (lastscalefactor == 0 || (math.signum(scalefactor) == math.signum(lastscalefactor)))             {             scale *= scalefactor;             scale = math.max(min_zoom, math.min(scale, max_zoom));             lastscalefactor = scalefactor;             }         else             {             lastscalefactor = 0;             }          return true;         }     };  /**  * gesture listener, used handling simple gestures such double touches, scrolls,  * , flings.  */ private final gesturedetector.simpleongesturelistener mgesturelistener         = new gesturedetector.simpleongesturelistener()     {     @override     public boolean ondown(motionevent e)         {         return true;         }      @override     public boolean onscroll(motionevent e1, motionevent e2, float dx, float dy)         {         setuptranslation(dx, dy);         return true;         }      @override     public boolean onfling(motionevent e1, motionevent e2, float velocityx, float velocityy)         {         //fling((int) -velocityx, (int) -velocityy);         return true;         }     };    private void setuptranslation(float dx, float dy)     {     distancex = -1 * dx + distancex;     distancey = -1 * dy + distancey;     getparent().requestdisallowintercepttouchevent(true);     rect viewablerect = new rect();     betterzoomlayout.this.getdrawingrect(viewablerect);     float offscreenwidth = child().getwidth() - (viewablerect.right - viewablerect.left);     float offscreenheight = child().getheight() - (viewablerect.bottom - viewablerect.top);     float maxdx = (child().getwidth() - (child().getwidth() / scale)) / 2 * scale;     float maxdy = (child().getheight() - (child().getheight() / scale)) / 2 * scale;     distancex = math.min(math.max(distancex, -(maxdx + offscreenwidth)), maxdx);     distancey = math.min(math.max(distancey, -(maxdy + offscreenheight)), maxdy);     }  private void applyscaleandtranslation()     {     child().setscalex(scale);     child().setscaley(scale);     child().settranslationx(distancex);     child().settranslationy(distancey);     }  private view child()     {     return getchildat(0);     } }