How to modify sensitivity of viewpager

Question

Does anyone know how to modify sensitivity of ViewPager?

Currently I use a ViewPager from android v4 support library to implement my own photo viewer.

However, default ViewPager is too sensitive, when my finger touch down on screen and moves by a very short distinct(maybe just 0.5cm). Then ViewPager will be toggled to scroll to next/previous photo view.

Is there any way to overwrite default behavior of ViewPager to decrease sensitivity?

Thanks to any response.

Answer 1

For ViewPager2 extract and increase mTouchSlop using reflection.

    private fun ViewPager2.reduceDragSensitivity() {
        val recyclerViewField = ViewPager2::class.java.getDeclaredField("mRecyclerView")
        recyclerViewField.isAccessible = true
        val recyclerView = recyclerViewField.get(this) as RecyclerView

        val touchSlopField = RecyclerView::class.java.getDeclaredField("mTouchSlop")
        touchSlopField.isAccessible = true
        val touchSlop = touchSlopField.get(recyclerView) as Int
        touchSlopField.set(recyclerView, touchSlop*8)       // "8" was obtained experimentally
    }

Call this function from your view pager

val viewPager: ViewPager2 = view.findViewById(R.id.pager)
viewPager.reduceDragSensitivity()

Java version

private void reduceDragSensitivity() {
    try {
        Field ff = ViewPager2.class.getDeclaredField("mRecyclerView") ;
        ff.setAccessible(true);
        RecyclerView recyclerView =  (RecyclerView) ff.get(viewPager);
        Field touchSlopField = RecyclerView.class.getDeclaredField("mTouchSlop") ;
        touchSlopField.setAccessible(true);
        int touchSlop = (int) touchSlopField.get(recyclerView);
        touchSlopField.set(recyclerView,touchSlop*4);
    } catch (NoSuchFieldException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    }
}

All credits to Alexander Shevelev and Mahmoud Shabat.

Answer 2

While Pethő Jonatán has a good pointer to achieve better sensitivity, along with fling distance you may want to reduce minimumVelocity as well.

You may want to do this after initializing viewpager in onCreate:

 viewPager = findViewById(R.id.viewPager);
 Field mFlingDistance;
 Field mMinimumVelocity;

 try {
     mFlingDistance=ViewPager.class.getDeclaredField("mFlingDistance");
     mMinimumVelocity=ViewPager.class.getDeclaredField("mMinimumVelocity");

     mFlingDistance.setAccessible(true);
     mMinimumVelocity.setAccessible(true);

     //customize the values
     mFlingDistance.set(viewPager, 10);
     mMinimumVelocity.set(viewPager, 400);
    } catch (Exception e) {
      e.printStackTrace();
    }

Answer 3

I've checked out ViewPager.java from src of compatibility library and seems there's not way to do it by xml or some setter. ViewPager has code like the following:

final float density = context.getResources().getDisplayMetrics().density;
mFlingDistance = (int) (MIN_DISTANCE_FOR_FLING * density);
mCloseEnough = (int) (CLOSE_ENOUGH * density);
mDefaultGutterSize = (int) (DEFAULT_GUTTER_SIZE * density);

As You can see, it uses it's internal constants for determination of fling distance and etc.

Good news are: You can copy ViewPager (here licence of it should be considered) to Your code and extend it e.g. to get fling distance from xml attribute.

Answer 4

You can reflect the mFlingDistance field like this:

Field mFlingDistance;
mFlingDistance = ViewPager.class.getDeclaredField("mFlingDistance");
mFlingDistance.setAccessible(true);

// Set custom value:
mFlingDistance.set(mPager, 30);