How do I make a RecyclerView work in an Android PopupWindow?

Question

I have read several similar questions but I couldn't make any of the answers work. The sticking point is that

val popupMenu = findViewById<RecyclerView>(R.id.popmenu)

always returns null.

Here is activity_popup.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/popupactivity"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".PopupActivity">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/popmenu"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

Here is the code to set up the PopupWindow:

val inflater = getSystemService(Context.LAYOUT_INFLATER_SERVICE) as LayoutInflater
        val popupView = inflater.inflate(R.layout.activity_popup, null)
        popupWin = PopupWindow(popupView, popupWidth, popupHeight, true)
        val layoutMgr = LinearLayoutManager(applicationContext)
        val popupMenu = findViewById<RecyclerView>(R.id.popmenu)

What am I doing wrong?

Answer 1

What am I doing wrong?

You're trying to find a view in the current view hierachy. You need to be looking in the PopupWindow's hierarchy.

// Inflates a layout that's not attached to anything
val popupView = inflater.inflate(R.layout.activity_popup, null)

// Creates a PopupWinow with the given popupView that's not attached to anything
popupWin = PopupWindow(popupView, popupWidth, popupHeight, true)

val layoutMgr = LinearLayoutManager(applicationContext)

// Attempts to find the recycler view in *this* view hierchy, which does not exist
val popupMenu = findViewById<RecyclerView>(R.id.popmenu)

To Fix: Change the last line to popupView.findViewById<RecyclerView>(r.id.popmenu) so you're searching within the correct view hierarchy.