Use view binding to replace findViewById

Sean McQuillan

·

Follow

Published in

Android Developers

·

6 min read

·

Feb 13, 2020

--

29

Listen

Share

New in Android Studio 3.6, view binding gives you the ability to replace findViewById with generated binding objects to simplify code, remove bugs, and avoid all the boilerplate of findViewById.

TL;DR

• Enable view binding in build.gradle (no libraries dependencies)
• View binding generates a binding object for every layout in your module (activity_awesome.xml → ActivityAwesomeBinding.java)
• Binding object contains one property for every view with an id in the layout — with the correct type and null-safety
• Full support for both the Java programming language and Kotlin

Update build.gradle to enable view binding

You don’t need to include any extra libraries to enable view binding. It’s built into the Android Gradle Plugin starting with the versions shipped in Android Studio 3.6. To enable view binding, configure viewBinding in your module-level build.gradle file.

// Available in Android Gradle Plugin 3.6.0
android {
    viewBinding {
        enabled = true
    }
}

In Android Studio 4.0, viewBinding has been moved into buildFeatures [release notes] and you should use:

// Android Studio 4.0
android {
    buildFeatures {
        viewBinding = true
    }
}

Once enabled for a project, view binding will generate a binding class for all of your layouts automatically. You don’t have to make changes to your XML — it’ll automatically work with your existing layouts.

View binding works with your existing XML, and will generate a binding object for each layout in a module.

You can use the binding class whenever you inflate layouts such as Fragment, Activity, or even a RecyclerView Adapter (or ViewHolder).

Use view binding in an Activity

If you have a layout called activity_awesome.xml, which contains a button and two text views, view binding generates a small class called ActivityAwesomeBinding that contains a property for every view with an ID in the layout.

You don’t have to call findViewById when using view binding — instead just use the properties provided to reference any view in the layout with an id.

The root element of the layout is always stored in a property called root which is generated automatically for you. In an Activity’s onCreate method you pass root to setContentView to tell the Activity to use the layout from the binding object.

Easy Mistake: Calling setContentView(…) with the layout resource id instead of the inflated binding object is an easy mistake to make. This causes the layout to be inflated twice and listeners to be installed on the wrong layout object.

Solution: When using view binding in an Activity, you should always pass the layout from the binding object with setContentView(binding.root).

Safe code using binding objects

findViewById is the source of many user-facing bugs in Android. It’s easy to pass an id that’s not in the current layout — producing null and a crash. And, since it doesn’t have any type-safety built in it’s easy to ship code that calls findViewById<TextView>(R.id.image). View binding replaces findViewById with a concise, safe alternative.

View bindings are…

• Type-safe because properties are always correctly typed based on the views in the layout. So if you put a TextView in the layout, view binding will expose a TextView property.
• Null-safe for layouts defined in multiple configurations. View binding will detect if a view is only present in some configurations and create a @Nullable property.

And since the generated binding classes are regular Java classes with Kotlin-friendly annotations, you can use view binding from both the Java programming language and Kotlin.

What code does it generate?

View binding generates a Java class that replaces the need for findViewById in your code. It will generate one binding object for every XML layout in your module while mapping names so activity_awesome.xml maps toActivityAwesomeBinding.java.

When editing an XML layout in Android Studio, code generation will be optimized to only update the binding object related to that XML file, and it will do so in memory to make things fast. This means that changes to the binding object are available immediately in the editor and you don’t have to wait for a full rebuild.

Android Studio is optimized to update the binding objects immediately when editing XML layouts.

Let’s step through the generated code for the example XML layout from earlier in this post to learn what view binding generates.

View binding will generate one correctly-typed property for each view with a specified id. It will also generate a property called rootView that’s exposed via a getter getRoot. View binding doesn’t do any logic– it just exposes your views in a binding object so you can wire them up without error-prone calls to findViewById. This keeps the generated file simple (and avoids slowing down builds).

If you’re using Kotlin, this class is optimized for interoperability. Since all properties are annotated with @Nullable or @NonNull Kotlin knows how to expose them as null-safe types. To learn more about interop between the languages, check out the documentation for calling Java from Kotlin.

In ActivityAwesomeBinding.java, view binding generates a public inflate method. The one argument version passes null as the parent view and doesn’t attach to parent. View binding also exposes a three argument version of inflate that lets you pass the parent and attachToParent parameters when needed.

The call to bind is where the magic happens. It will take the inflated layout and bind all of the properties, with some error checking added to generate readable error messages.

The bind method is the most complex code in the generated binding object, with a call to findViewById for each view to bind. And here you can see the magic happen – since the compiler can check the types and potential nullability of each property directly from the XML layouts it can safely call findViewById.

Note, the actual generated code for the bind method is longer and uses a labeled break to optimize bytecode. Check out this post by Jake Wharton to learn more about the optimizations applied.

On each binding class, view binding exposes three public static functions to create a binding an object, here’s a quick guide for when to use each:

• inflate(inflater) – Use this in an Activity onCreate where there is no parent view to pass to the binding object.
• inflate(inflater, parent, attachToParent) – Use this in a Fragment or a RecyclerView Adapter (or ViewHolder) where you need to pass the parent ViewGroup to the binding object.
• bind(rootView) – Use this when you’ve already inflated the view and you just want to use view binding to avoid findViewById. This is useful for fitting view binding into your existing infrastructure and when refactoring code to use ViewBinding.

What about included layouts

One binding object will be generated for each layout.xml in a module. This is true even when another layout <include>s this this layout.

In the case of included layouts, view binding will create a reference to the included layout’s binding object.

Note that the <include> tag has an id: android:id="@+id/includes". This is required for view binding to generate a property (just like a normal view).

Include tags must have an id to generate a binding property.

View binding will generate a reference to the IncludedButtonsBinding object in ActivityAwesomeBinding.

Using view binding and data binding

View binding is only a replacement for findViewById. If you also want to automatically bind views in XML you can use the data binding library. Both libraries can be applied to the same module and they’ll work together.

When both are enabled, layouts that use a <layout> tag will use data binding to generate binding objects. All other layouts will use view binding to generate binding objects.

You can use data binding and view binding in the same module.

We developed view binding in addition to data binding because many developers provided feedback that they wanted a lighter weight solution to replace findViewById without the rest of the data binding library – and view binding provides that solution.

View binding and Kotlin synthetics or ButterKnife

One of the most common questions asked about view binding is, “Should I use view binding instead of Kotlin synthetics or ButterKnife?” Both of these libraries are used successfully by many apps and solve the same problem.

For most apps we recommend trying out view binding instead of these libraries because view binding provides safer, more concise view lookup.

While ButterKnife validates nullable/non-null at runtime, the compiler does not check that you’ve correctly matched what’s in your layouts

We recommend trying out view binding for safe, concise, view lookup.

Learn more

To learn more about view binding check out the official documentation.

And we’d love to hear your experiences with #ViewBinding library on twitter!