How to fix 'Design assumption violated' error in ViewPager2?

I'm using ViewPager2, Kotlin and AndroidX.
When the adapter is not at index 0 and I change the adapter list and set current item to index 0 the exception is thrown.

```
java.lang.IllegalStateException: Design assumption violated.
at androidx.viewpager2.widget.ViewPager2.updateCurrentItem(ViewPager2.java:538)
at androidx.viewpager2.widget.ViewPager2$4.onAnimationsFinished(ViewPager2.java:518)
at androidx.recyclerview.widget.RecyclerView$ItemAnimator.isRunning(RecyclerView.java:13244)
at androidx.viewpager2.widget.ViewPager2.onLayout(ViewPager2.java:515)
at android.view.View.layout(View.java:15596)
```

In line 537 of ViewPager2 there is an if which causes the exception:
```
// Extra checks verifying assumptions
// TODO: remove after testing
View snapView1 = mPagerSnapHelper.findSnapView(mLayoutManager);
View snapView2 = mLayoutManager.findViewByPosition(snapPosition);
if (snapView1 != snapView2) {
throw new IllegalStateException("Design assumption violated.");
}
```

This is how I'm updating the adapter list:
```
adapter.list = newList
adapter.notifyDataSetChanged()
viewpager.setCurrentItem(0, true)
```
It happens only when `smoothScroll` is set to `true`

What am I missing?

**Edit :**
I'm using `androidx.viewpager2.widget.ViewPager2` available in `com.google.android.material:material:1.1.0-alpha08`

I had the same problem but the bug seems to be fixed (at least for me) in the current version `androidx.viewpager2:viewpager2:1.0.0-beta01`.

See more in [ViewPager2 1.0.0-beta01 Release Notes][1]



[1]:

[To see links please register here]

I had this same problem with **ViewPager2** on configuration change. I'm using:

implementation 'com.google.android.material:material:1.0.0'
implementation 'androidx.viewpager2:viewpager2:1.0.0-beta03'

In my case, I had overridden the `getItemId()` method in my **FragmentStateAdapter** class. When I got rid of my `getItemId()` method, the "*IllegalStateException: Design assumption violated*" error was no more! :-)

I'm using `androidx.viewpager2:viewpager2:1.0.0`

For whoever still stuck at this, and need to implement `getItemId()` `containsItem()`. Please chek your `getItemId()` and `containsItem()` implementation. My problem is when I serve 10 item with **possibility several item have same value**, this error occurs.

Media have several data with same value.

private var mediaId = media.map { it.hashCode().toLong() }

override fun getItemId(position: Int): Long = mediaId[position]

override fun containsItem(itemId: Long): Boolean = mediaId.contains(itemId)

while you updating the data or even simply swiping, the viewpager is confuse because you implement `getItemId()` with non unique id. The solution is just **make sure your data is unique or have their own id**. *My media datas dont have id.

> If you open the implementation of `getItemId()` notes this part :
>
> * @return stable item id {@link RecyclerView.Adapter#hasStableIds()}
>
> you need to have unique id for each of item.

My issue was that I was customizing `containsItem()` to have some conditions under which I remove Fragments from the `ViewPager`, and then return `false`;

I encounter `Design assumption violated` after rotating the phone (and then swipe the `ViewPager2`).

I solved that by just returning the super class method: `return super.containsItem(itemId);`

@Override
public boolean containsItem(long itemId) {
// Your code
return super.containsItem(itemId);
}

UPDATE:

> The containsItem method does not have the "CallSuper" annotation and is therefore not required to be called

This is right, no need to call the super; and I didn't call it explicitly before returning a boolean; just returning it solving this issue

Here is the solution for a straightforward use case

import androidx.fragment.app.Fragment
import androidx.fragment.app.FragmentActivity
import androidx.viewpager2.adapter.FragmentStateAdapter

class ViewPager2Adapter(
val dataList: List<Item> = listOf(),
fragmentActivity: FragmentActivity
) :
FragmentStateAdapter(fragmentActivity) {

val pageIds= dataList.map { it.hashCode().toLong() }.toMutableList()

override fun createFragment(position: Int): Fragment {
return MyFragment.newInstance(dataList[position])
}

override fun getItemCount(): Int {
return dataList.size
}

override fun getItemId(position: Int): Long {
return pageIds[position]
}

override fun containsItem(itemId: Long): Boolean {
return pageIds.contains(itemId)
}
}

Then I encountered a specific use case when I was replacing my object at the position with a new one then I got the exception.

**Solution:**
Now in case you are making data changes to the adapter i.e. with replacing an object in dataList with a new one, then make sure to replace id in **pageIds** of the old object with the new one.

(viewPager2.adapter as? ViewPager2Adapter)?.apply {
pageIds[oldIndex] = (NewItemObject).hashCode().toLong()
notifyItemChanged(oldIndex)
}