Data Binding Library Part of Android Jetpack

The Data Binding Library is a support library that allows you to bind UI components in your layouts to data sources in your app using a declarative format rather than programmatically.

Layouts are often defined in activities with code that calls UI framework methods. For example, the code below calls findViewById() to find a TextView widget and bind it to the userName property of the viewModel variable:

Kotlin findViewById<TextView>(R.id.sample_text).apply { text = viewModel.userName } Java TextView textView = findViewById(R.id.sample_text); textView.setText(viewModel.getUserName());

The following example shows how to use the Data Binding Library to assign text to the widget directly in the layout file. This removes the need to call any of the Java code shown above. Note the use of @{} syntax in the assignment expression:

<TextView android:text="@{viewmodel.userName}" />

Binding components in the layout file lets you remove many UI framework calls in your activities, making them simpler and easier to maintain. This can also improve your app's performance and help prevent memory leaks and null pointer exceptions.

Note: findViewById() calls, consider using view binding instead. In many cases, view binding can provide the same benefits as data binding with simpler implementation and better performance. If you are using data binding primarily to replacecalls, consider using view binding instead.

Using the Data Binding Library

Use the following pages to learn how to use the Data Binding Library in your Android apps.

Get started Learn how to get your development environment ready to work with the Data Binding Library, including support for data binding code in Android Studio.

Layouts and binding expressions

The expression language allows you to write expressions that connect variables to the views in the layout. The Data Binding Library automatically generates the classes required to bind the views in the layout with your data objects. The library provides features such as imports, variables, and includes that you can use in your layouts. These features of the library coexist seamlessly with your existing layouts. For example, the binding variables that can be used in expressions are defined inside a data element that is a sibling of the UI layout's root element. Both elements are wrapped in a layout tag, as shown in the following example: <layout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto"> <data> <variable name="viewmodel" type="com.myapp.data.ViewModel" /> </data> <ConstraintLayout... /> <!-- UI layout's root element --> </layout>

Work with observable data objects The Data Binding Library provides classes and methods to easily observe data for changes. You don't have to worry about refreshing the UI when the underlying data source changes. You can make your variables or their properties observable. The library allows you to make objects, fields, or collections observable. Generated binding classes The Data Binding Library generates binding classes that are used to access the layout's variables and views. This page shows you how to use and customize generated binding classes. Binding adapters For every layout expression, there is a binding adapter that makes the framework calls required to set the corresponding properties or listeners. For example, the binding adapter can take care of calling the setText() method to set the text property or call the setOnClickListener() method to add a listener to the click event. The most common binding adapters, such as the adapters for the android:text property used in the examples in this page, are available for you to use in the android.databinding.adapters package. For a list of the common binding adapters, see adapters. You can also create custom adapters, as shown in the following example:

Kotlin @BindingAdapter("app:goneUnless") fun goneUnless(view: View, visible: Boolean) { view.visibility = if (visible) View.VISIBLE else View.GONE } Java @BindingAdapter("app:goneUnless") public static void goneUnless(View view, Boolean visible) { view.visibility = visible ? View.VISIBLE : View.GONE; }