RecyclerView changes its scroll position when inside a ConstraintLayout

问题

I encountered an issue with RecyclerView when it is a child of a ConstraintLayout. Originally, the RecyclerView was somewhere deep in the view hierarchy of Relative-, Frame- and LinearLeayouts, and everything worked well until I decided to flatten view tree using ConstraintLayout. I noticed that each time when there is a layout change, caused by anything (window resize, data set change notification, etc), the scroll position of the RecyclerView is changed.

For example, every time I show and hide soft keyboard, the content recycler view drifts by a certain amount of pixels.

I was able to reproduce this behavior on a very simple layout:

ConstraintLayout
|---RecyclerView
|---Button
|---EditText

If I set constraints such that RecyclerView is placed above any other child of ConstraintLayout (meaning RecyclerView has a bottom constraint), and if I use LinearLayoutManager with reverseLayout = true, I can reproduce the described above bahavior.

How should I fix this (I don't want the scroll position to change)? Maybe there are some flags on RecyclerView and/or ConstraintLayout that I don't know of...

Here is my layout.xml:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    >

    <android.support.v7.widget.RecyclerView
        android:id="@+id/recycler"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        app:layout_constraintBottom_toTopOf="@+id/button"
        app:layout_constraintTop_toTopOf="parent"
        />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Notify Items Changed"
        app:layout_constraintBottom_toTopOf="@+id/edit_text"
        app:layout_constraintEnd_toStartOf="parent"
        app:layout_constraintStart_toEndOf="parent"
        />

    <EditText
        android:id="@+id/edit_text"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        />

</android.support.constraint.ConstraintLayout>

And here is code to setup RecyclerView in Activity.onCreate():

setContentView(R.layout.activity_main)

val adapter = Adapter()

val layoutManager = LinearLayoutManager(this)
layoutManager.orientation = LinearLayoutManager.VERTICAL
layoutManager.reverseLayout = true

val recycler = findViewById<RecyclerView>(R.id.recycler)
recycler.layoutManager = layoutManager
recycler.adapter = adapter

recycler.addOnLayoutChangeListener { _, _, _, _, bottom, _, _, _, oldBottom ->
    Log.d("TEST", "bottom=$bottom, oldBottom=$oldBottom")
}

recycler.addOnScrollListener(object : RecyclerView.OnScrollListener() {
    override fun onScrolled(recyclerView: RecyclerView?, dx: Int, dy: Int) {
        Log.d("TEST", "Scrolled: dx=$dx, dy=$dy")
        Log.d("TEST", "offset = " + recycler.computeVerticalScrollOffset())
    }
})

findViewById<Button>(R.id.button)
        .setOnClickListener {
            adapter.notifyDataSetChanged()
        }

Here is a fragment of debug logs, which were collected after several clicks on the button:

D/TEST: Scrolled: dx=0, dy=0
D/TEST: offset = 4167
D/TEST: bottom=1253, oldBottom=1253
D/TEST: Scrolled: dx=0, dy=0
D/TEST: offset = 3887
D/TEST: bottom=1253, oldBottom=1253
D/TEST: Scrolled: dx=0, dy=0
D/TEST: offset = 3607
D/TEST: bottom=1253, oldBottom=1253
D/TEST: Scrolled: dx=0, dy=0
D/TEST: offset = 3327
D/TEST: bottom=1253, oldBottom=1253

The scroll positions is slowly changing and content is being scrolled towards the top. The shift is roughly amounts to the total height of widgets below RecyclerView.

I could reproduce this behavior only when all conditions are met:

• RecyclerView is positioned inside a ConstraintLayout
• There are other sibling widgets below RecyclerView
• LinearLayoutManager is used with either of flags reverseLayout or stackFromBottom being set to true.

I am using ConstraintLayout version is 1.1.2.

来源：https://stackoverflow.com/questions/52487038/recyclerview-changes-its-scroll-position-when-inside-a-constraintlayout