Generated binding classes

The Data Binding Library generates binding classes that are used to access the layout's variables and views. This page shows how to create and customize generated binding classes.

The generated binding class links the layout variables with the views within the layout. The name and package of the binding class can be customized. All generated binding classes inherit from the ViewDataBinding class.

A binding class is generated for each layout file. By default, the name of the class is based on the name of the layout file, converting it to Pascal case and adding the Binding suffix to it. The above layout filename is activity_main.xml so the corresponding generated class is ActivityMainBinding. This class holds all the bindings from the layout properties (for example, the user variable) to the layout's views and knows how to assign values for the binding expressions.

Create a binding object

The binding object should be created soon after inflating the layout to ensure that the view hierarchy isn't modified before it binds to the views with expressions within the layout. The most common method to bind the object to a layout is to use the static methods on the binding class. You can inflate the view hierarchy and bind the object to it by using the inflate() method of the binding class, as shown in the following example:

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)

    val binding: MyLayoutBinding = MyLayoutBinding.inflate(layoutInflater)
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    MyLayoutBinding binding = MyLayoutBinding.inflate(getLayoutInflater());
}

There is an alternate version of the inflate() method that takes a ViewGroup object in addition to the LayoutInflater object , as shown in the following example:

val binding: MyLayoutBinding = MyLayoutBinding.inflate(getLayoutInflater(), viewGroup, false)

MyLayoutBinding binding = MyLayoutBinding.inflate(getLayoutInflater(), viewGroup, false);

If the layout was inflated using a different mechanism, it can be bound separately, as follows:

val binding: MyLayoutBinding = MyLayoutBinding.bind(viewRoot)

MyLayoutBinding binding = MyLayoutBinding.bind(viewRoot);

Sometimes the binding type cannot be known in advance. In such cases, the binding can be created using the DataBindingUtil class, as demonstrated in the following code snippet:

val rootView = LayoutInflater.from(this).inflate(layoutId, parent, attachToParent)
val binding: ViewDataBinding? = DataBindingUtil.bind(viewRoot)

View rootView = LayoutInflater.from(this).inflate(layoutId, parent, attachToParent);
ViewDataBinding binding = DataBindingUtil.bind(viewRoot);

If you are using data binding items inside a Fragment, ListView, or RecyclerView adapter, you may prefer to use the inflate() methods of the bindings classes or the DataBindingUtil class, as shown in the following code example:

val listItemBinding = ListItemBinding.inflate(layoutInflater, viewGroup, false)
// or
val listItemBinding = DataBindingUtil.inflate(layoutInflater, R.layout.list_item, viewGroup, false)

ListItemBinding binding = ListItemBinding.inflate(layoutInflater, viewGroup, false);
// or
ListItemBinding binding = DataBindingUtil.inflate(layoutInflater, R.layout.list_item, viewGroup, false);

Views with IDs

The Data Binding Library creates a immutable field in the binding class for each view that has an ID in the layout. For example, the Data Binding Library creates the firstName and lastName fields of type TextView from the following layout:

<layout xmlns:android="http://schemas.android.com/apk/res/android">
   <data>
       <variable name="user" type="com.example.User"/>
   </data>
   <LinearLayout
       android:orientation="vertical"
       android:layout_width="match_parent"
       android:layout_height="match_parent">
       <TextView android:layout_width="wrap_content"
           android:layout_height="wrap_content"
           android:text="@{user.firstName}"
   android:id="@+id/firstName"/>
       <TextView android:layout_width="wrap_content"
           android:layout_height="wrap_content"
           android:text="@{user.lastName}"
  android:id="@+id/lastName"/>
   </LinearLayout>
</layout>

The library extracts the views including the IDs from the view hierarchy in a single pass. This mechanism can be faster than calling the findViewById() method for every view in the layout.

IDs aren't as necessary as they are without data binding, but there are still some instances where access to views is still necessary from code.

Variables

The Data Binding Library generates accessor methods for each variable declared in the layout. For example, the following layout generates setter and getter methods in the binding class for the user, image, and note variables:

<data>
   <import type="android.graphics.drawable.Drawable"/>
   <variable name="user" type="com.example.User"/>
   <variable name="image" type="Drawable"/>
   <variable name="note" type="String"/>
</data>

ViewStubs

Unlike normal views, ViewStub objects start off as an invisible view. When they either are made visible or are explicitly told to inflate, they replace themselves in the layout by inflating another layout.

Because the ViewStub essentially disappears from the view hierarchy, the view in the binding object must also disappear to allow to be claimed by garbage collection. Because the views are final, a ViewStubProxy object takes the place of the ViewStub in the generated binding class, giving you access to the ViewStub when it exists and also access to the inflated view hierarchy when the ViewStub has been inflated.

When inflating another layout, a binding must be established for the new layout. Therefore, the ViewStubProxy must listen to the ViewStub OnInflateListener and establish the binding when required. Since only one listener can exist at a given time, the ViewStubProxy allows you to set an OnInflateListener, which it calls after establishing the binding.

Immediate Binding

When a variable or observable object changes, the binding is scheduled to change before the next frame. There are times, however, when binding must be executed immediately. To force execution, use the executePendingBindings() method.

Advanced Binding

Dynamic Variables

At times, the specific binding class isn't known. For example, a RecyclerView.Adapter operating against arbitrary layouts doesn't know the specific binding class. It still must assign the binding value during the call to the onBindViewHolder() method.

In the following example, all layouts that the RecyclerView binds to have an item variable. The BindingHolder object has a getBinding() method returning the ViewDataBinding base class.

override fun onBindViewHolder(holder: BindingHolder, position: Int) {
    item: T = mItems.get(position)
    holder.binding.setVariable(BR.item, item);
    holder.binding.executePendingBindings();
}

public void onBindViewHolder(BindingHolder holder, int position) {
    final T item = mItems.get(position);
    holder.getBinding().setVariable(BR.item, item);
    holder.getBinding().executePendingBindings();
}

Background Thread

You can change your data model in a background thread as long as it isn't a collection. Data binding localizes each variable / field during evaluation to avoid any concurrency issues.

Custom binding class names

By default, a binding class is generated based on the name of the layout file, starting with an uppercase letter, removing underscores ( _ ), capitalizing the following letter, and suffixing the word Binding. The class is placed in a databinding package under the module package. For example, the layout file contact_item.xml generates the ContactItemBinding class. If the module package is com.example.my.app, then the binding class is placed in the com.example.my.app.databinding package.

Binding classes may be renamed or placed in different packages by adjusting the class attribute of the data element. For example, the following layout generates the ContactItem binding class in the databinding package in the current module:

<data class="ContactItem">
    …
</data>

You can generate the binding class in a different package by prefixing the class name with a period. The following example generates the binding class in the module package:

<data class=".ContactItem">
    …
</data>

You can also use the full package name where you want the binding class to be generated. The following example creates the ContactItem binding class in the com.example package:

<data class="com.example.ContactItem">
    …
</data>