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.
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 thesetOnClickListener()
method to add a listener to the click event. The most common binding adapters, such as the adapters for theandroid:text
property used in the examples in this page, are available for you to use in theandroid.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; }
- Bind layout views to Architecture Components
- The Android Support Library includes the Architecture Components, which you can use to design robust, testable, and maintainable apps. You can use the Architecture Components with the Data Binding Library to further simplify the development of your UI.
- Two-way data binding
- The Data Binding Library supports two-way data binding. The notation used for this type of binding supports the ability to receive data changes to a property and listen to user updates to that property at the same time.
Additional resources
To learn more about data binding, consult the following additional resources.
Samples
Codelabs
Blog posts
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