Is there a way to show a preview of a RecyclerView's contents in the Android Studio editor?

@oRRs is right !

I'm using Android Studio 1.4 RC2 and you can now specify any custom layout.

I tried a custom CardView and it works.

tools:listitem="@android:layout/simple_list_item_checked"

tools namespace enables design-time features (such as which layout to show in a fragment) or compile-time behaviors (such as which shrinking mode to apply to your XML resources) It is really powerful feature that is developing and allows you not compile code every time to see changes

AndroidX^[About] and GridLayoutManager

implementation 'androidx.recyclerview:recyclerview:1.1.0'

<androidx.recyclerview.widget.RecyclerView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"

    android:layout_width="match_parent"
    android:layout_height="match_parent"

    tools:layoutManager="androidx.recyclerview.widget.GridLayoutManager"
    tools:listitem="@layout/item"
    tools:itemCount="10"
    tools:orientation="vertical"
    tools:scrollbars="vertical"
    tools:spanCount="3"/>

Support and LinearLayoutManager

implementation 'com.android.support:recyclerview-v7:28.0.0'

<android.support.v7.widget.RecyclerView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"

    android:layout_width="match_parent"
    android:layout_height="wrap_content"

    tools:layoutManager="android.support.v7.widget.LinearLayoutManager"
    tools:listitem="@layout/item"
    tools:itemCount="3"
    tools:orientation="horizontal"
    tools:scrollbars="horizontal" />

Another cool feature that was introduced in Android studio 3.0 is predefining a data through the tools attributes, to visualised easily your layout structure using @tools:sample/* resources

item.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="100dp"
    android:layout_height="150dp"
    android:layout_marginRight="15dp"
    android:layout_marginBottom="10dp"
    android:orientation="vertical"
    tools:background="@tools:sample/backgrounds/scenic">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textColor="@color/colorWhite"
        tools:text="@tools:sample/first_names" />

</FrameLayout>

Results:

First, add the following line in your item XML to made a preview of your list while you edit your item:

tools:showIn="@layout/activity_my_recyclerview_item"

And them, add the following line in your RecyclerView XML to preview how your item will look in your list:

tools:listitem="@layout/adapter_item"