ItemDecoration in Android

ItemDecoration can be drawn to all four sides of RecyclerView items

First things first. So what is ItemDecoration? This is how official doc puts it.

An ItemDecoration allows the application to add a special drawing and layout offset to specific item views from the adapter’s data set. This can be useful for drawing dividers between items, highlights, visual grouping boundaries and more.

We can not simply say that ItemDecoration is just a divider with a fancy name. It’s much more than that. A divider, as the name suggests, can only be drawn between items. But ItemDecoration can be drawn on all four sides of the item. ItemDecoration gives full control to the developers in measuring and drawing the decorations. A decoration can be a divider or just an inset.

Don’t add dividers as views —It affects performance

I have personally seen some developers taking shortcuts to add dividers to the RecyclerView. The reason is obvious. ListView had a native way to add the divider. You can add the divider through xml itself.

<ListView
    android:id="@+id/activity_home_list_view"
    android:layout_width="match_parent" 
    android:layout_height="match_parent"
    android:divider="@android:color/black"
    android:dividerHeight="8dp"/>

But with RecyclerView, you can not directly add a divider. You need to add an ItemDecoration which can draw the divider. But developers find it difficult and directly add the dividers to the view, instead of using the item decoration.

<LinearLayout android:orientation="vertical">    <LinearLayout android:orientation="horizontal">        <ImageView />        <TextView />    </LinearLayout>    <View
        android:width="match_parent"
        android:height="1dp"
        android:background="#333" /></LinearLayout>

Whenever we are taking a shortcut it might have some adverse effects. In this case it affects performance.

When adding the divider to the layout, we are increasing the view count. We know that having less number of views in the layout is better for performance. Sometimes adding the divider as a view increases the layout hierarchy. Consider the above example, we really don’t need the outer linear layout. To add a divider we had to create an additional outer layout.

Don’t add dividers as views — It has side effects

During the item animation divider will be animated along with the animation, since divider is part of the view. To get more clarity, look at the following gif.

Divider moves along with item — looks bad in my opinion

Clearly dividers should not be animated along with the item view. It should be separate from the item view’s animation like this

Only the item moves — Divider fades

Don’t add dividers as views — It lacks flexibility

You don’t have control over the dividers if it’s part of the view. The only thing that you can do is based on the item’s position visibility of the divider can be changed. In case you want to do just more than visibility, item decorations are flexible.

Dividers with an inset for few items

In the above image for the last item in the group divider fills the entire width. Other dividers have a margin of 56dp to their left side. Here is the ItemDecorator’s onDraw code.

Don’t add dividers as views —Use ItemDecoration

Writing your own item decoration is simple. You just need to create a class which extends from ItemDecoration. Override the getItemOffsets() and onDraw() methods. For a sample implementation take a look here.

With the release of support lib’s 25.0.0 version, we have a new class “DividerItemDecoration”. This is a utility class with which you can add simple dividers through decoration.

Notes

1. Multiple ItemDecorations can be added to a single RecyclerView. Time to make your creativity run wild.
2. All decorations are drawn before drawing the items. In case you want to draw decorations after drawing the view, override onDrawOver() method instead of onDraw().

So next time when you want to add dividers to the RecyclerView, don’t use views inside layout. Instead, use ItemDecoration.