View binding
is a feature that makes it easier to write code that interacts
with views. Once view binding is enabled in a module, it generates a
binding
class
for each XML layout file present in that module. An instance of a binding
class contains direct references to all views that have an ID in the
corresponding layout.
In most cases, view binding replaces
findViewById
.
Setup
View binding is enabled on a module-by-module basis. To enable view binding in a
module, set the
viewBinding
build option to
true
in the module-level
build.gradle
file, as shown in the following example:
Groovy
android {
...
buildFeatures {
viewBinding true
}
}
Kotlin
android {
...
buildFeatures {
viewBinding = true
}
}
If you want a layout file to be ignored while generating binding classes, add
the
tools:viewBindingIgnore="true"
attribute to the root view of that layout
file:
<LinearLayout
...
tools:viewBindingIgnore="true" >
...
</LinearLayout>
Usage
If view binding is enabled for a module, a binding class is generated for each
XML layout file that the module contains. Each binding class contains references
to the root view and all views that have an ID. The name of the binding class is
generated by converting the name of the XML file to Pascal case and adding the
word "Binding" to the end.
For example, consider a layout file called
result_profile.xml
that contains
the following:
<LinearLayout ... >
<TextView android:id="@+id/name" />
<ImageView android:cropToPadding="true" />
<Button android:id="@+id/button"
android:background="@drawable/rounded_button" />
</LinearLayout>
The generated binding class is called
ResultProfileBinding
. This class has two
fields: a
TextView
called
name
and a
Button
called
button
. The
ImageView
in the layout has no ID, so there is no reference to it in the
binding class.
Every binding class also includes a
getRoot()
method, providing a direct
reference for the root view of the corresponding layout file. In this example,
the
getRoot()
method in the
ResultProfileBinding
class returns the
LinearLayout
root view.
The following sections demonstrate the use of generated binding classes in
activities and fragments.
Use view binding in activities
To set up an instance of the binding class for use with an activity, perform the
following steps in the activity's
onCreate()
method:
- Call the static
inflate()
method included in the generated binding class.
This creates an instance of the binding class for the activity to use.
- Get a reference to the root view by either calling the
getRoot()
method or
using
Kotlin property
syntax
.
- Pass the root view to
setContentView()
to make it the active view on the screen.
These steps are shown in the following example:
Kotlin
private lateinit var binding: ResultProfileBinding
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = ResultProfileBinding.inflate(layoutInflater)
val view = binding.root
setContentView(view)
}
Java
private ResultProfileBinding binding;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
binding = ResultProfileBinding.inflate(getLayoutInflater());
View view = binding.getRoot();
setContentView(view);
}
You can now use the instance of the binding class to reference any of the views:
Kotlin
binding.name.text = viewModel.name
binding.button.setOnClickListener { viewModel.userClicked() }
Java
binding.name.setText(viewModel.getName());
binding.button.setOnClickListener(new View.OnClickListener() {
viewModel.userClicked()
});
Use view binding in fragments
To set up an instance of the binding class for use with a fragment, perform the
following steps in the fragment's
onCreateView()
method:
- Call the static
inflate()
method included in the generated binding class.
This creates an instance of the binding class for the fragment to use.
- Get a reference to the root view by either calling the
getRoot()
method or
using
Kotlin property
syntax
.
- Return the root view from the
onCreateView()
method to make it the
active view on the screen.
Kotlin
private var _binding: ResultProfileBinding? = null
// This property is only valid between onCreateView and
// onDestroyView.
private val binding get() = _binding!!
override fun onCreateView(
inflater: LayoutInflater,
container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
_binding = ResultProfileBinding.inflate(inflater, container, false)
val view = binding.root
return view
}
override fun onDestroyView() {
super.onDestroyView()
_binding = null
}
Java
private ResultProfileBinding binding;
@Override
public View onCreateView (LayoutInflater inflater,
ViewGroup container,
Bundle savedInstanceState) {
binding = ResultProfileBinding.inflate(inflater, container, false);
View view = binding.getRoot();
return view;
}
@Override
public void onDestroyView() {
super.onDestroyView();
binding = null;
}
You can now use the instance of the binding class to reference any of the views:
Kotlin
binding.name.text = viewModel.name
binding.button.setOnClickListener { viewModel.userClicked() }
Java
binding.name.setText(viewModel.getName());
binding.button.setOnClickListener(new View.OnClickListener() {
viewModel.userClicked()
});
Provide hints for different configurations
When you declare views across multiple configurations, it occasionally makes
sense to use a different view type depending on the particular layout.
The following code snippet shows an example of this:
# in res/layout/example.xml
<TextView android:id="@+id/user_bio" />
# in res/layout-land/example.xml
<EditText android:id="@+id/user_bio" />
In this case, you might expect the generated class to expose a field
userBio
of type
TextView
, because
TextView
is the common base class. Due to
technical limitations, the view binding code generator can't determine this and
generates a
View
field instead. This requires casting the field later with
binding.userBio as TextView
.
To work around this limitation, view binding supports a
tools:viewBindingType
attribute, letting you tell the compiler what type to use in the generated code.
In the previous example, you can use this attribute to make the compiler
generate the field as a
TextView
:
# in res/layout/example.xml (unchanged)
<TextView android:id="@+id/user_bio" />
# in res/layout-land/example.xml
<EditText android:id="@+id/user_bio" tools:viewBindingType="TextView" />
In another example, suppose you have two layouts, one that contains a
BottomNavigationView
and another that contains a
NavigationRailView
. Both
classes extend
NavigationBarView
, which contains most of the implementation
details. If your code doesn't need to know exactly which subclass is present in
the current layout, you can use
tools:viewBindingType
to set the generated
type to
NavigationBarView
in both layouts:
# in res/layout/navigation_example.xml
<BottomNavigationView android:id="@+id/navigation" tools:viewBindingType="NavigationBarView" />
# in res/layout-w720/navigation_example.xml
<NavigationRailView android:id="@+id/navigation" tools:viewBindingType="NavigationBarView" />
View binding can't validate the value of this attribute when generating code. To
avoid compile-time and runtime errors, the value must meet the following
conditions:
- The value must be a class that inherits from
android.view.View
.
The value must be a superclass of the tag it is placed on. For example, the
following values don't work:
<TextView tools:viewBindingType="ImageView" /> <!-- ImageView is not related to TextView. -->
<TextView tools:viewBindingType="Button" /> <!-- Button is not a superclass of TextView. -->
The final type must resolve consistently across all configurations.
Differences from findViewById
View binding has important advantages over using
findViewById
:
- Null safety:
since view binding creates direct references to views,
there's no risk of a null pointer exception due to an invalid view ID.
Additionally, when a view is only present in some configurations of a
layout, the field containing its reference in the binding class is marked
with
@Nullable
.
- Type safety:
the fields in each binding class have types matching the
views they reference in the XML file. This means there's no risk of a class
cast exception.
These differences mean incompatibilities between your layout and your code
result in your build failing at compile time rather than at runtime.
Comparison with data binding
View binding and
data binding
both generate
binding classes that you can use to reference views directly. However, view
binding is intended to handle simpler use cases and provides the following
benefits over data binding:
- Faster compilation:
view binding requires no annotation processing, so
compile times are faster.
- Ease of use:
view binding doesn't require specially tagged XML layout
files, so it's faster to adopt in your apps. Once you enable view binding in
a module, it applies to all of that module's layouts automatically.
On the other hand, view binding has the following limitations compared to data
binding:
Because of these considerations, in some cases it's best to use both view
binding and data binding in a project. You can use data binding in layouts that
require advanced features and use view binding in layouts that don't.
Additional resources
To learn more about view binding, see the following additional resources:
Samples
Blogs
Videos
Recommended for you