How to set RecyclerView app:layoutManager="" from XML?

As you can check in the doc:

Class name of the Layout Manager to be used.

The class must extend androidx.recyclerview.widget.RecyclerViewView$LayoutManager and have either a default constructor or constructor with the signature (android.content.Context, android.util.AttributeSet, int, int)

If the name starts with a '.', application package is prefixed. Else, if the name contains a '.', the classname is assumed to be a full class name. Else, the recycler view package (androidx.appcompat.widget) is prefixed

With androidx you can use:

<androidx.recyclerview.widget.RecyclerView
     xmlns:app="http://schemas.android.com/apk/res-auto"
     app:layoutManager="androidx.recyclerview.widget.GridLayoutManager">

With the support libraries you can use:

<android.support.v7.widget.RecyclerView
    xmlns:app="http://schemas.android.com/apk/res-auto"
    app:layoutManager="android.support.v7.widget.GridLayoutManager" >

Also you can add these attributes:

• android:orientation = "horizontal|vertical": to control the orientation of the LayoutManager (eg:LinearLayoutManager)
• app:spanCount: to set the number of columns for GridLayoutManager

Example:

<androidx.recyclerview.widget.RecyclerView
    app:layoutManager="androidx.recyclerview.widget.GridLayoutManager"
    app:spanCount="2"
    ...>

or:

<androidx.recyclerview.widget.RecyclerView
    app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager"
    android:orientation="vertical"
    ...>

You can also add them using the tools namespace (i.e. tools:orientation and tools:layoutManager) and then it only impacts the IDE preview and you can continue setting those values in code.

if you want use it with LinearLayoutManager

<android.support.v7.widget.RecyclerView
    xmlns:app="http://schemas.android.com/apk/res-auto"
    app:layoutManager="android.support.v7.widget.LinearLayoutManager" >

that equivalent to

LinearLayoutManager mLayoutManager = new LinearLayoutManager(getApplicationContext());
mRecyclerView.setLayoutManager(mLayoutManager);

And I came here looking for androidx version though it was pretty easy to figure out, here it is

LinearLayoutManager:

app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager"

Example:

<androidx.recyclerview.widget.RecyclerView
    android:id="@+id/recyclerView"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager"/>

GridLayoutManager:

app:layoutManager="androidx.recyclerview.widget.GridLayoutManager"

Example:

<androidx.recyclerview.widget.RecyclerView
    android:id="@+id/recyclerView"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    app:spanCount="2"
    app:layoutManager="androidx.recyclerview.widget.GridLayoutManager"/>

As you can see in examples above you can control the orientation from within xml using

android:orientation="vertical"

and

android:orientation="horizontal"

And to set the number of columns for GridLayoutManager using

app:spanCount="2"