How to change gallery's scrollview

MyGallery gallery = (MyGallery)findViewById(R.id.gallery_photo);
 PhonePhotoViewerAdapter = new PhonePhotoViewerAdapter(this, FilePath);
 gallery.setAdapter(PhonePhotoViewerAdapter);
 gallery.setSelection(0);
 

Above is my code to show photo with scrollview.

But if I want to focus on next, I have to move finger on screen with a larger distance.

I want to let it change focus while a smaller distance moved

How can I do it?

I had modify class Gallery as below:

@Override
 public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
 if (e1.getX() - e2.getX() > 50  && Math.abs(velocityX) > 100) {    
 // Fling left 
 } else if (e2.getX() - e1.getX() > 50 && Math.abs(velocityX) > 100) {    
 // Fling right 
 }  
 return false;
 }
 

Below code is my modify:

gallery.setLongClickable(false);
 gallery.setOnTouchListener(new OnTouchListener() {
         @Override
         public boolean onTouch(View v, MotionEvent event) {
             // TODO Auto-generated method stub
             int action = event.getAction();
             if((action == MotionEvent.ACTION_DOWN) && !Clicking) {
                 StartX = event.getX();
                 StartIndex = gallery.getSelectedItemPosition();
                 Clicking = true;
             }
             else if((action == MotionEvent.ACTION_UP) && Clicking) {
                 EndX = event.getX();
                 EndIndex = gallery.getSelectedItemPosition();
                 Clicking = false;
                 if(((EndX - StartX) > 50) && (StartIndex == EndIndex)) {
                     if(EndIndex > 0) {
                         gallery.setSelection(EndIndex - 1);
                     }
                 }
                 else if(((StartX - EndX) > 50) && (StartIndex == EndIndex)) {
                     if(EndIndex < count - 1) {
                         gallery.setSelection(EndIndex + 1);
                     }
                 }               
             }
             return false;
         }
     });
 

But there are some abnormal in showing.

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