Data Binding Guide

官方MVVM，先拷贝过来留待以后研究

Data Binding Guide

In this document:

Build Environment
Data Binding Layout Files

Writing your first data binding
expressions
Data Object
Binding Data
Binding Events

Layout Details

Imports
Variables
Custom Binding Class Names
Includes
Expression Language

Data Objects

Observable Objects
ObservableFields
Observable Collections

Generated Binding

Creating
Views With IDs
Variables
ViewStubs
Advanced Binding

Attribute Setters

Automatic Setters
Renamed Setters
Custom Setters

Converters

Object Conversions
Custom Conversions

This document explains how to use the Data Binding Library to write declarative layouts and minimize the glue code necessary to bind your application logic and layouts.
The Data Binding Library offers both flexibility and broad compatibility — it's a support library, so you can use it with all Android platform versions back to Android 2.1 (API level 7+).
To use data binding, Android Plugin for Gradle 1.5.0-alpha1 or higher is required.

Beta release

Please note that the Data Binding library is a beta release. While Data Binding is in beta, developers should be aware of the following caveats:

This is a beta release of the feature intended to generate developer feedback. It might contain bugs, and it might not work for your use case, so use it at your own risk. That said, we do want your feedback! Please let us know
20000

what is or isn’t working for you using the issue tracker.
The Data Binding library beta release is subject to significant changes, including those which are not source code compatible with your app. That is, significant rework may be required to take updates to the library in the future.
Developers should feel free to publish apps built with the Data Binding library beta release, with the caveats that the standard Android SDK and Google Play terms of service apply, and it’s always a great idea to test your app
thoroughly when adopting new libraries or tools.
We’re just getting started with Android Studio support at this time. Further Android Studio support will come in the future.
By using the Data Binding library beta release, you acknowledge these caveats.

Build Environment

To get started with Data Binding, download the library from the Support repository in the Android SDK manager.
To configure your app to use data binding, add the

dataBinding

element to your

build.gradle

file in the app module.
Use the following code snippet to configure data binding:

android {
....
dataBinding {
enabled = true
}
}

If you have an app module that depends on a library which uses data binding, your app module must configure data binding in its

build.gradle

file as well.
Also, make sure you are using a compatible version of Android Studio. Android Studio 1.3 adds the code-completion and layout-preview support for data binding.

Data Binding Layout Files

Writing your first data binding expressions

Data-binding layout files are slightly different and start with a root tag of layout followed by adata element and a view root element. This view element is what
your root would be in a non-binding layout file. A sample file looks like this:

<?xml version="1.0" encoding="utf-8"?>
<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}"/>
<TextView android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@{user.lastName}"/>
</LinearLayout>
</layout>

The user variable within data describes a property that may be used within this layout.

<variable name="user" type="com.example.User"/>

Expressions within the layout are written in the attribute properties using the “

@{}

” syntax. Here, the TextView’s text is set to the firstName property of user:

<TextView android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@{user.firstName}"/>

Data Object

Let’s assume for now that you have a plain-old Java object (POJO) for User:

public class User {
public final String firstName;
public final String lastName;
public User(String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}
}

This type of object has data that never changes. It is common in applications to have data that is read once and never changes thereafter. It is also possible to use a JavaBeans objects:

public class User {
private final String firstName;
private final String lastName;
public User(String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}
public String getFirstName() {
return this.firstName;
}
public String getLastName() {
return this.lastName;
}
}

From the perspective of data binding, these two classes are equivalent. The expression

@{user.firstName}

used for the TextView’s

android:text

attribute
will access the

firstName

field in the former class and the

getFirstName()

method in the latter class. Alternatively, it will also be resolved to

firstName()

if that method
exists.

Binding Data

By default, a Binding class will be generated based on the name of the layout file, converting it to Pascal case and suffixing “Binding” to it. The above layout file was

main_activity.xml

so
the generate class was

MainActivityBinding

. This class holds all the bindings from the layout properties (e.g. the

user

variable) to the layout’s Views and knows how to assign values for the binding expressions.The
easiest means for creating the bindings is to do it while inflating:

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
MainActivityBinding binding = DataBindingUtil.setContentView(this, R.layout.main_activity);
User user = new User("Test", "User");
binding.setUser(user);
}

You’re done! Run the application and you’ll see Test User in the UI. Alternatively, you can get the view via:

MainActivityBinding binding = MainActivityBinding.inflate(getLayoutInflater());

If you are using data binding items inside a ListView or RecyclerView adapter, you may prefer to use:

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

Binding Events

Events may be bound to handler methods directly, similar to the way

android:onClick

can be assigned to a method in the Activity. Event attribute names are governed by the name
of the listener method with a few exceptions. For example,

View.OnLongClickListener

has
a method

onLongClick()

, so the attribute
for this event is

android:onLongClick

.
To assign an event to its handler, use a normal binding expression, with the value being the method name to call. For example, if your data object has two methods:

public class MyHandlers {
public void onClickFriend(View view) { ... }
public void onClickEnemy(View view) { ... }
}

The binding expression can assign the click listener for a View:

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android">
<data>
<variable name="handlers" type="com.example.Handlers"/>
<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:onClick="@{user.isFriend ? handlers.onClickFriend : handlers.onClickEnemy}"/>
<TextView android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@{user.lastName}"
android:onClick="@{user.isFriend ? handlers.onClickFriend : handlers.onClickEnemy}"/>
</LinearLayout>
</layout>

Layout Details

Imports

Zero or more

import

elements may be used inside the

data

element. These allow easy reference to classes inside your layout file, just
like in Java.

<data>
<import type="android.view.View"/>
</data>

Now, View may be used within your binding expression:

<TextView
android:text="@{user.lastName}"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:visibility="@{user.isAdult ? View.VISIBLE : View.GONE}"/>

When there are class name conflicts, one of the classes may be renamed to an “alias:”

<import type="android.view.View"/>
<import type="com.example.real.estate.View"
alias="Vista"/>

Now,

Vista

may be used to reference the

com.example.real.estate.View

and

View

may be used to reference

android.view.View

within
the layout file. Imported types may be used as type references in variables and expressions:

<data>
<import type="com.example.User"/>
<import type="java.util.List"/>
<variable name="user" type="User"/>
<variable name="userList" type="List<User>"/>
</data>

Note: Android Studio does not yet handle imports so the autocomplete for imported variables may not work in your IDE. Your application will still compile fine and you can work around the IDE issue by using fully qualified names in your variable
definitions.

<TextView
android:text="@{((User)(user.connection)).lastName}"
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>

Imported types may also be used when referencing static fields and methods in expressions:

<data>
<import type="com.example.MyStringUtils"/>
<variable name="user" type="com.example.User"/>
</data>
…
<TextView
android:text="@{MyStringUtils.capitalize(user.lastName)}"
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>

Just as in Java,

java.lang.*

is imported automatically.

Variables

Any number of

variable

elements may be used inside the

data

element. Each

variable

element describes
a property that may be set on the layout to be used in binding expressions within the layout file.

<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>

The variable types are inspected at compile time, so if a variable implements

Observable

or
is anobservable collection, that should be reflected in the type. If the variable is a base class
or interface that does not implement the Observable* interface, the variables will not be observed!
When there are different layout files for various configurations (e.g. landscape or portrait), the variables will be combined. There must not be conflicting variable definitions between these layout files.
The generated binding class will have a setter and getter for each of the described variables. The variables will take the default Java values until the setter is called —

null

for reference
types,

0

for

int

,

false

for

boolean

, etc.

Custom Binding Class Names

By default, a Binding class is generated based on the name of the layout file, starting it with upper-case, removing underscores ( _ ) and capitalizing the following letter and then suffixing “Binding”. This class
will be placed in a databinding package under the module package. For example, the layout file

contact_item.xml

will generate

ContactItemBinding

. If the module package is

com.example.my.app

,
then it will be placed in

com.example.my.app.databinding

.
Binding classes may be renamed or placed in different packages by adjusting the

class

attribute of the

data

element. For example:

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

This generates the binding class as

ContactItem

in the databinding package in the module package. If the class should be generated in a different package within the module package, it may be
prefixed with “.”:

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

In this case,

ContactItem

is generated in the module package directly. Any package may be used if the full package is provided:

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

Includes

Variables may be passed into an included layout's binding from the containing layout by using the application namespace and the variable name in an attribute:

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:bind="http://schemas.android.com/apk/res-auto">
<data>
<variable name="user" type="com.example.User"/>
</data>
<LinearLayout
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">
<include layout="@layout/name"
bind:user="@{user}"/>
<include layout="@layout/contact"
bind:user="@{user}"/>
</LinearLayout>
</layout>

Here, there must be a

user

variable in both the

name.xml

and

contact.xml

layout files.
Data binding does not support include as a direct child of a merge element. For example, the following layout is not supported:

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:bind="http://schemas.android.com/apk/res-auto">
<data>
<variable name="user" type="com.example.User"/>
</data>
<merge>
<include layout="@layout/name"
bind:user="@{user}"/>
<include layout="@layout/contact"
bind:user="@{user}"/>
</merge>
</layout>

Expression Language

Common Features

The expression language looks a lot like a Java expression. These are the same:

Mathematical

+ - / * %

String concatenation

+

Logical

&& ||

Binary

& | ^

Unary

+ - ! ~

Shift

>> >>> <<

Comparison

== > < >= <=

instanceof

Grouping

()

Literals - character, String, numeric,

null

Cast
Method calls
Field access
Array access

[]

Ternary operator

?:

Examples:

android:text="@{String.valueOf(index + 1)}"
android:visibility="@{age < 13 ? View.GONE : View.VISIBLE}"
android:transitionName='@{"image_" + id}'

Missing Operations

A few operations are missing from the expression syntax that you can use in Java.

this

super

new

Explicit generic invocation

Null Coalescing Operator

The null coalescing operator (

??

) chooses the left operand if it is not null or the right if it is null.

android:text="@{user.displayName ?? user.lastName}"

This is functionally equivalent to:

android:text="@{user.displayName != null ? user.displayName : user.lastName}"

Property Reference

The first was already discussed in the Writing
your first data binding expressions above: short form JavaBean references. When an expression references a property on a class, it uses the same format for fields, getters, and ObservableFields.

android:text="@{user.lastName}"

Avoiding NullPointerException

Generated data binding code automatically checks for nulls and avoid null pointer exceptions. For example, in the expression

@{user.name}

, if

user

is null,

user.name

will
be assigned its default value (null). If you were referencing

user.age

, where age is an

int

, then it would default to 0.

Collections

Common collections: arrays, lists, sparse lists, and maps, may be accessed using the

[]

operator for convenience.

<data>
<import type="android.util.SparseArray"/>
<import type="java.util.Map"/>
<import type="java.util.List"/>
<variable name="list" type="List<String>"/>
<variable name="sparse" type="SparseArray<String>"/>
<variable name="map" type="Map<String, String>"/>
<variable name="index" type="int"/>
<variable name="key" type="String"/>
</data>
…
android:text="@{list[index]}"
…
android:text="@{sparse[index]}"
…
android:text="@{map[key]}"

String Literals

When using single quotes around the attribute value, it is easy to use double quotes in the expression:

android:text='@{map["firstName"]}'

It is also possible to use double quotes to surround the attribute value. When doing so, String literals should either use the " or back quote (`).

android:text="@{map[`firstName`}"
android:text="@{map["firstName"]}"

Resources

It is possible to access resources as part of expressions using the normal syntax:

android:padding="@{large? @dimen/largePadding : @dimen/smallPadding}"

Format strings and plurals may be evaluated by providing parameters:

android:text="@{@string/nameFormat(firstName, lastName)}"
android:text="@{@plurals/banana(bananaCount)}"

When a plural takes multiple parameters, all parameters should be passed:

Have an orange
Have %d oranges

android:text="@{@plurals/orange(orangeCount, orangeCount)}"

Some resources require explicit type evaluation.

Type	Normal Reference	Expression Reference
String[]	@array	@stringArray
int[]	@array	@intArray
TypedArray	@array	@typedArray
Animator	@animator	@animator
StateListAnimator	@animator	@stateListAnimator
color int	@color	@color
ColorStateList	@color	@colorStateList

Data Objects

Any plain old Java object (POJO) may be used for data binding, but modifying a POJO will not cause the UI to update. The real power of data binding can be used by giving your data objects the ability to notify when
data changes. There are three different data change notification mechanisms, Observable objects, observable
fields, and observable collections.
When one of these observable data object is bound to the UI and a property of the data object changes, the UI will be updated automatically.

Observable Objects

A class implementing the

Observable

interface
will allow the binding to attach a single listener to a bound object to listen for changes of all properties on that object.
The

Observable

interface has a mechanism
to add and remove listeners, but notifying is up to the developer. To make development easier, a base class,

BaseObservable

,
was created to implement the listener registration mechanism. The data class implementer is still responsible for notifying when the properties change. This is done by assigning a

Bindable

annotation
to the getter and notifying in the setter.

private static class User extends BaseObservable {
private String firstName;
private String lastName;
@Bindable
public String getFirstName() {
return this.firstName;
}
@Bindable
public String getLastName() {
return this.lastName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
notifyPropertyChanged(BR.firstName);
}
public void setLastName(String lastName) {
this.lastName = lastName;
notifyPropertyChanged(BR.lastName);
}
}

The

Bindable

annotation generates an
entry in the BR class file during compilation. The BR class file will be generated in the module package. If the base class for data classes cannot be changed, the

Observable

interface
may be implemented using the convenient

PropertyChangeRegistry

to
store and notify listeners efficiently.

ObservableFields

A little work is involved in creating

Observable

classes,
so developers who want to save time or have few properties may use

ObservableField

and
its siblings

ObservableBoolean

,

ObservableByte

,

ObservableChar

,

ObservableShort

,

ObservableInt

,

ObservableLong

,

ObservableFloat

,

ObservableDouble

,
and

ObservableParcelable

.

ObservableFields

are
self-contained observable objects that have a single field. The primitive versions avoid boxing and unboxing during access operations. To use, create a public final field in the data class:

private static class User {
public final ObservableField<String> firstName =
new ObservableField<>();
public final ObservableField<String> lastName =
new ObservableField<>();
public final ObservableInt age = new ObservableInt();
}

That's it! To access the value, use the set and get accessor methods:

user.firstName.set("Google");
int age = user.age.get();

Observable Collections

Some applications use more dynamic structures to hold data. Observable collections allow keyed access to these data objects.

ObservableArrayMap

is
useful when the key is a reference type, such as String.

ObservableArrayMap<String, Object> user = new ObservableArrayMap<>();
user.put("firstName", "Google");
user.put("lastName", "Inc.");
user.put("age", 17);

In the layout, the map may be accessed through the String keys:

<data>
<import type="android.databinding.ObservableMap"/>
<variable name="user" type="ObservableMap<String, Object>"/>
</data>
…
<TextView
android:text='@{user["lastName"]}'
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>
<TextView
android:text='@{String.valueOf(1 + (Integer)user["age"])}'
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>

ObservableArrayList

is useful
when the key is an integer:

ObservableArrayList<Object> user = new ObservableArrayList<>();
user.add("Google");
user.add("Inc.");
user.add(17);

In the layout, the list may be accessed through the indices:

<data>
<import type="android.databinding.ObservableList"/>
<import type="com.example.my.app.Fields"/>
<variable name="user" type="ObservableList<Object>"/>
</data>
…
<TextView
android:text='@{user[Fields.LAST_NAME]}'
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>
<TextView
android:text='@{String.valueOf(1 + (Integer)user[Fields.AGE])}'
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>

Generated Binding

The generated binding class links the layout variables with the Views within the layout. As discussed earlier, the name and package of the Binding may be customized.
The Generated binding classes all extend

ViewDataBinding

.

Creating

The binding should be created soon after inflation to ensure that the View hierarchy is not disturbed prior to binding to the Views with expressions within the layout. There are a few ways to bind to a layout. The
most common is to use the static methods on the Binding class.The inflate method inflates the View hierarchy and binds to it all it one step. There is a simpler version that only takes a

LayoutInflater

and
one that takes a

ViewGroup

as well:

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

If the layout was inflated using a different mechanism, it may be bound separately:

MyLayoutBinding binding = MyLayoutBinding.bind(viewRoot);

Sometimes the binding cannot be known in advance. In such cases, the binding can be created using the

DataBindingUtil

class:

ViewDataBinding binding = DataBindingUtil.inflate(LayoutInflater, layoutId,
parent, attachToParent);
ViewDataBinding binding = DataBindingUtil.bindTo(viewRoot, layoutId);

Views With IDs

A public final field will be generated for each View with an ID in the layout. The binding does a single pass on the View hierarchy, extracting the Views with IDs. This mechanism can be faster than calling findViewById
for several Views. For example:

<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>

Will generate a binding class with:

public final TextView firstName;
public final TextView lastName;

IDs are not nearly as necessary as without data binding, but there are still some instances where access to Views are still necessary from code.

Variables

Each variable will be given accessor methods.

<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>

will generate setters and getters in the binding:

public abstract com.example.User getUser();
public abstract void setUser(com.example.User user);
public abstract Drawable getImage();
public abstract void setImage(Drawable image);
public abstract String getNote();
public abstract void setNote(String note);

ViewStubs

ViewStub

s are a little different from normal
Views. They start off invisible and 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 collection. Because the Views are final, a

ViewStubProxy

object
takes the place of the

ViewStub

, giving the developer 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

's

ViewStub.OnInflateListener

and
establish the binding at that time. Since only one can exist, the

ViewStubProxy

allows the developer to set an

OnInflateListener

on it that it will call after establishing the binding.

Advanced Binding

Dynamic Variables

At times, the specific binding class won't be known. For example, a

RecyclerView.Adapter

operating
against arbitrary layouts won't know the specific binding class. It still must assign the binding value during the

onBindViewHolder(VH,
int)

.
In this example, all layouts that the RecyclerView binds to have an "item" variable. The

BindingHolder

has a

getBinding

method returning the

ViewDataBinding

base.

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

Immediate Binding

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

executePendingBindings()

method.

Background Thread

You can change your data model in a background thread as long as it is not a collection. Data binding will localize each variable / field while evaluating to avoid any concurrency issues.

Attribute Setters

Whenever a bound value changes, the generated binding class must call a setter method on the View with the binding expression. The data binding framework has ways to customize which method to call to set the value.

Automatic Setters

For an attribute, data binding tries to find the method setAttribute. The namespace for the attribute does not matter, only the attribute name itself.
For example, an expression associated with TextView's attribute

android:text

will look for a setText(String). If the expression returns an int, data binding will search for
a setText(int) method. Be careful to have the expression return the correct type, casting if necessary. Note that data binding will work even if no attribute exists with the given name. You can then easily "create" attributes for any setter by using data binding.
For example, support DrawerLayout doesn't have any attributes, but plenty of setters. You can use the automatic setters to use one of these.

<android.support.v4.widget.DrawerLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
app:scrimColor="@{@color/scrim}"
app:drawerListener="@{fragment.drawerListener}"/>

Renamed Setters

Some attributes have setters that don't match by name. For these methods, an attribute may be associated with the setter through

BindingMethods

annotation.
This must be associated with a class and contains

BindingMethod

annotations,
one for each renamed method. For example, the

android:tint

attribute is really associated with

setImageTintList(ColorStateList)

,
not

setTint

.

@BindingMethods({
@BindingMethod(type = "android.widget.ImageView",
attribute = "android:tint",
method = "setImageTintList"),
})

It is unlikely that developers will need to rename setters; the android framework attributes have already been implemented.

Custom Setters

Some attributes need custom binding logic. For example, there is no associated setter for the

android:paddingLeft

attribute. Instead,

setPadding(left, top, right,
bottom)

exists. A static binding adapter method with the

BindingAdapter

annotation
allows the developer to customize how a setter for an attribute is called.
The android attributes have already had

BindingAdapter

s created. For example, here is the one for

paddingLeft

:

@BindingAdapter("android:paddingLeft")
public static void setPaddingLeft(View view, int padding) {
view.setPadding(padding,
view.getPaddingTop(),
view.getPaddingRight(),
view.getPaddingBottom());
}

Binding adapters are useful for other types of customization. For example, a custom loader can be called off-thread to load an image.
Developer-created binding adapters will override the data binding default adapters when there is a conflict.
You can also have adapters that receive multiple parameters.

@BindingAdapter({"bind:imageUrl", "bind:error"})
public static void loadImage(ImageView view, String url, Drawable error) {
Picasso.with(view.getContext()).load(url).error(error).into(view);
}

<ImageView app:imageUrl=“@{venue.imageUrl}”
app:error=“@{@drawable/venueError}”/>

This adapter will be called if both imageUrl and error are used for an ImageView and imageUrl is a string and error is a drawable.

Custom namespaces are ignored during matching.
You can also write adapters for android namespace.

Binding adapter methods may optionally take the old values in their handlers. A method taking old and new values should have all old values for the attributes come first, followed by the new values:

@BindingAdapter("android:paddingLeft")
public static void setPaddingLeft(View view, int oldPadding, int newPadding) {
if (oldPadding != newPadding) {
view.setPadding(newPadding,
view.getPaddingTop(),
view.getPaddingRight(),
view.getPaddingBottom());
}
}

Event handlers may only be used with interfaces or abstract classes with one abstract method. For example:

@BindingAdapter("android:onLayoutChange")
public static void setOnLayoutChangeListener(View view, View.OnLayoutChangeListener oldValue,
View.OnLayoutChangeListener newValue) {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
if (oldValue != null) {
view.removeOnLayoutChangeListener(oldValue);
}
if (newValue != null) {
view.addOnLayoutChangeListener(newValue);
}
}
}

When a listener has multiple methods, it must be split into multiple listeners. For example,

View.OnAttachStateChangeListener

has
two methods:

onViewAttachedToWindow()

and

onViewDetachedFromWindow()

.
We must then create two interfaces to differentiate the attributes and handlers for them.

@TargetApi(VERSION_CODES.HONEYCOMB_MR1)
public interface OnViewDetachedFromWindow {
void onViewDetachedFromWindow(View v);
}

@TargetApi(VERSION_CODES.HONEYCOMB_MR1)
public interface OnViewAttachedToWindow {
void onViewAttachedToWindow(View v);
}

Because changing one listener will also affect the other, we must have three different binding adapters, one for each attribute and one for both, should they both be set.

@BindingAdapter("android:onViewAttachedToWindow")
public static void setListener(View view, OnViewAttachedToWindow attached) {
setListener(view, null, attached);
}

@BindingAdapter("android:onViewDetachedFromWindow")
public static void setListener(View view, OnViewDetachedFromWindow detached) {
setListener(view, detached, null);
}

@BindingAdapter({"android:onViewDetachedFromWindow", "android:onViewAttachedToWindow"})
public static void setListener(View view, final OnViewDetachedFromWindow detach,
final OnViewAttachedToWindow attach) {
if (VERSION.SDK_INT >= VERSION_CODES.HONEYCOMB_MR1) {
final OnAttachStateChangeListener newListener;
if (detach == null && attach == null) {
newListener = null;
} else {
newListener = new OnAttachStateChangeListener() {
@Override
public void onViewAttachedToWindow(View v) {
if (attach != null) {
attach.onViewAttachedToWindow(v);
}
}

@Override
public void onViewDetachedFromWindow(View v) {
if (detach != null) {
detach.onViewDetachedFromWindow(v);
}
}
};
}
final OnAttachStateChangeListener oldListener = ListenerUtil.trackListener(view,
newListener, R.id.onAttachStateChangeListener);
if (oldListener != null) {
view.removeOnAttachStateChangeListener(oldListener);
}
if (newListener != null) {
view.addOnAttachStateChangeListener(newListener);
}
}
}

The above example is slightly more complicated than normal because View uses add and remove for the listener instead of a set method for

View.OnAttachStateChangeListener

.
The

android.databinding.adapters.ListenerUtil

class helps keep track of the previous listeners so that they may be removed in the Binding Adaper.
By annotating the interfaces

OnViewDetachedFromWindow

and

OnViewAttachedToWindow

with

@TargetApi(VERSION_CODES.HONEYCOMB_MR1)

, the data binding code
generator knows that the listener should only be generated when running on Honeycomb MR1 and new devices, the same version supported by

addOnAttachStateChangeListener(View.OnAttachStateChangeListener)

.

Converters

Object Conversions

When an Object is returned from a binding expression, a setter will be chosen from the automatic, renamed, and custom setters. The Object will be cast to a parameter type of the chosen setter.
This is a convenience for those using ObservableMaps to hold data. for example:

<TextView
android:text='@{userMap["lastName"]}'
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>

The

userMap

returns an Object and that Object will be automatically cast to parameter type found in the setter

setText(CharSequence)

. When there may be confusion about
the parameter type, the developer will need to cast in the expression.

Custom Conversions

Sometimes conversions should be automatic between specific types. For example, when setting the background:

<View
android:background="@{isError ? @color/red : @color/white}"
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>

Here, the background takes a

Drawable

, but the color is an integer. Whenever a

Drawable

is expected and an integer is returned, the

int

should be
converted to a

ColorDrawable

. This conversion is done using a static method with a BindingConversion annotation:

@BindingConversion
public static ColorDrawable convertColorToDrawable(int color) {
return new ColorDrawable(color);
}

Note that conversions only happen at the setter level, so it is not allowed to mix types like this:

<View
android:background="@{isError ? @drawable/error : @color/white}"
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>