# View binding
Part of [Android Jetpack](https://developer.android.com/jetpack).

*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

```groovy
android {
    ...
    buildFeatures {
        viewBinding true
    }
}
```

### Kotlin

```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()`](https://developer.android.com/reference/kotlin/android/app/Activity#oncreate) method:

1. Call the static `inflate()` method included in the generated binding class. This creates an instance of the binding class for the activity to use.
2. Get a reference to the root view by either calling the `getRoot()` method or using [Kotlin property
   syntax](https://kotlinlang.org/docs/reference/properties.html#declaring-properties).
3. Pass the root view to [`setContentView()`](https://developer.android.com/reference/kotlin/android/app/Activity#setcontentview_1) to make it the active view on the screen.

These steps are shown in the following example:  

### Kotlin

```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

```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

```kotlin
binding.name.text = viewModel.name
binding.button.setOnClickListener { viewModel.userClicked() }
```

### Java

```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()`](https://developer.android.com/reference/kotlin/androidx/fragment/app/Fragment#oncreateview)
method:

1. Call the static `inflate()` method included in the generated binding class. This creates an instance of the binding class for the fragment to use.
2. Get a reference to the root view by either calling the `getRoot()` method or using [Kotlin property
   syntax](https://kotlinlang.org/docs/reference/properties.html#declaring-properties).
3. Return the root view from the `onCreateView()` method to make it the active view on the screen.

**Note:** The `inflate()` method requires you to pass in a layout inflater. If the layout is already inflated, you can instead call the binding class's static `bind()` method.  

### Kotlin

```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

```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

```kotlin
binding.name.text = viewModel.name
binding.button.setOnClickListener { viewModel.userClicked() }
```

### Java

```java
binding.name.setText(viewModel.getName());
binding.button.setOnClickListener(new View.OnClickListener() {
    viewModel.userClicked()
});
```
| **Note:** Fragments outlive their views. Make sure you clean up any references to the binding class instance in the fragment's [`onDestroyView()`](https://developer.android.com/reference/kotlin/androidx/fragment/app/Fragment#ondestroyview) method.

### 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](https://developer.android.com/topic/libraries/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:

- View binding doesn't support [layout variables or layout
  expressions](https://developer.android.com/topic/libraries/data-binding/expressions), so it can't be used to declare dynamic UI content straight from XML layout files.
- View binding doesn't support [two-way data
  binding](https://developer.android.com/topic/libraries/data-binding/two-way).

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:

### Blogs

- [Use view binding to replace findViewById](https://medium.com/androiddevelopers/use-view-binding-to-replace-findviewbyid-c83942471fc)

### Videos

- [Android Jetpack: Replace findViewById with view binding](https://www.youtube.com/watch?v=W7uujFrljW0)

## Recommended for you

- Note: link text is displayed when JavaScript is off
- [Migrate from Kotlin synthetics to Jetpack view binding](https://developer.android.com/topic/libraries/view-binding/migration)
- [Layouts and binding expressions](https://developer.android.com/topic/libraries/data-binding/expressions)
- [App Architecture: UI Layer - Get Started - Android Developers](https://developer.android.com/topic/libraries/data-binding/start)