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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 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:background="#303F9F" 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: