使用折叠的工具栏滚动到recyclerview的最后一项

Question

I've got a CoordinatorLayout which contains a CollapsingToolbarLayout and a RecyclerView . Everything looks the way it's supposed to, except that when I try to scroll to the last item programmatically, it doesn't go all the way to the bottom. Instead, it does this:

I don't think this is a clipping problem, since the bottom item is fully there if I scroll down:

Here's the main layout:

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout 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:fitsSystemWindows="true">
    <android.support.design.widget.AppBarLayout
            android:id="@+id/app_bar"
            android:layout_width="match_parent"
            android:layout_height="150dp"
            android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.design.widget.CollapsingToolbarLayout
                android:id="@+id/toolbar_layout"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:fitsSystemWindows="true"
                app:contentScrim="?attr/colorPrimary"
                app:expandedTitleMarginStart="16dp"
                app:layout_scrollFlags="scroll|exitUntilCollapsed"
                app:toolbarId="@+id/toolbar">

            <android.support.v7.widget.Toolbar
                    android:id="@+id/toolbar"
                    android:layout_width="match_parent"
                    android:layout_height="?attr/actionBarSize"
                    app:contentInsetStart="72dp"
                    app:popupTheme="@style/AppTheme.PopupOverlay"/>

        </android.support.design.widget.CollapsingToolbarLayout>
    </android.support.design.widget.AppBarLayout>

    <android.support.v7.widget.RecyclerView
            android:id="@+id/recycler_view"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_marginBottom="@dimen/recyclerview_bottom_margin"
            app:layout_behavior="@string/appbar_scrolling_view_behavior"/>
</android.support.design.widget.CoordinatorLayout>

And here's the code that goes with the screencaps above:

class TestActivity : AppCompatActivity() {

    private val itemNames = listOf("top item", "next item", "yada", "yada yada", "yada yada yada", "second last item", "last item")

    private val selectedPosition = itemNames.size - 1

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.recyclerview_with_collapsing_toolbar)
        setSupportActionBar(toolbar)
        supportActionBar?.setTitle(R.string.some_title)

        val recyclerView  = findViewById<RecyclerView>(R.id.recycler_view)
        recyclerView.setHasFixedSize(true)
        val layoutManager = LinearLayoutManager(this)
        recyclerView.layoutManager = layoutManager
        recyclerView.adapter = MyAdapter()

        // try to scroll to the initial selected position

        recyclerView.scrollToPosition(selectedPosition)

//        layoutManager.scrollToPosition(selectedPosition)

//        layoutManager.scrollToPositionWithOffset(selectedPosition, resources.getDimensionPixelOffset(R.dimen.item_height))

//            recyclerView.post {
//                recyclerView.smoothScrollToPosition(selectedPosition)
//            }

    }

    inner class MyAdapter: RecyclerView.Adapter<MyViewHolder>() {
        override fun onCreateViewHolder(parent: ViewGroup, itemType: Int): MyViewHolder {
            val view = LayoutInflater.from(parent.context).inflate(R.layout.item_layout, parent, false)
            return MyViewHolder(view)
        }

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

        override fun onBindViewHolder(vh: MyViewHolder, position: Int) {
            vh.words.text = itemNames[position]
            if (selectedPosition == position) {
                vh.parent.setBackgroundColor(Color.MAGENTA)
            } else {
                vh.parent.setBackgroundColor(Color.BLACK)
            }
        }
    }

    inner class MyViewHolder(itemView: View): RecyclerView.ViewHolder(itemView) {
        val parent = itemView
        val words: TextView = itemView.findViewById(R.id.some_text)
    }
}

Additional notes:

• If I get rid of the CollapsingToolbarLayout then it does show the entire last item.

• I've left some of my other attempts in the code above (commented out). None of them worked.

• This example just involves a short static list and always scrolls to the same item, but the code I'm actually working on is a bit more complicated.

• The designer really wants everything to look exactly as designed, and I'm not free to change the visual design.

How can I scroll to the last item in a RecyclerView that's inside a layout with a collapsing toolbar?

Answer 1

The problem is that getNestedScrollingParentForType(type) in NestedScrollingChildHelper#dispatchNestedScroll returns null for the non-touch scroll, so scrolling is not dispatched when it is done programmatically.

So we need to enable that before scrolling programmatically:

if (!recyclerView.hasNestedScrollingParent(ViewCompat.TYPE_NON_TOUCH)) {
    recyclerView.startNestedScroll(View.SCROLL_AXIS_VERTICAL,ViewCompat.TYPE_NON_TOUCH);
}
// now smooth scroll your recycler view programmatically here. 

Answer 2

A fix for this problem would be to collapse the toolbar before scrolling to the given position. This can be done by adding app_bar_layout.setExpanded(false) before scrollToPosition.