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Kotlin Android 扩展

作者 Yan Zhulanow
本教程介绍如何使用 Kotlin Android 扩展来改进对 Android 开发的支持。

在本章教程中,我们将逐步介绍如何使用 Kotlin 安卓扩展插件提升安卓的开发体验。

View Binding


相信每一位安卓开发人员对 findViewById() 这个方法再熟悉不过了,毫无疑问,潜在的 bug 和脏乱的代码令后续开发无从下手的。尽管存在一系列的开源库能够为这个问题带来解决方案,those libraries require annotating fields for each exposed View.

现在 Kotlin 安卓扩展插件能够提供与这些开源库功能相同的体验,不需要添加任何额外代码。

In essence, this allows for the following code:

// Using R.layout.activity_main from the 'main' source set
import kotlinx.android.synthetic.main.activity_main.*

class MyActivity : Activity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        // Instead of findViewById<TextView>(R.id.textView)
        textView.setText("Hello, world!")

textView 是对 Activity 的一项扩展属性,与在 activity_main.xml 中的声明具有同样类型 (so it is a TextView)。

使用 Kotlin 安卓扩展


In this tutorial we're going to be using Gradle but the same can be accomplished using either IntelliJ IDEA project structure or Maven. For details on setting up Gradle to work with Kotlin, see Using Gradle.

安卓扩展是 IntelliJ IDEA 与 Android Studio 的 Kotlin 插件的组成之一,因此不需要再单独安装额外插件。

开发者仅需要在模块的 build.gradle 文件中启用 Gradle 安卓扩展插件即可:

apply plugin: 'kotlin-android-extensions'



import kotlinx.android.synthetic.main.<布局>.*

假设当前布局文件是 activity_main.xml,我们只需要引入 kotlinx.android.synthetic.main.activity_main.*

若需要调用 View 的合成属性,同时还应该导入 kotlinx.android.synthetic.main.activity_main.view.*



将有一个名为 hello 的属性:

activity.hello.text = "Hello World!"

Experimental Mode

Android Extensions plugin includes several experimental features such as LayoutContainer support and a Parcelable implementation generator. These features are not considered production ready yet, so you need to turn on the experimental mode in build.gradle in order to use them:

androidExtensions {
    experimental = true

LayoutContainer Support

Android Extensions plugin supports different kinds of containers. The most basic ones are Activity, Fragment and View, but you can turn (virtually) any class to an Android Extensions container by implementing the LayoutContainer interface, e.g.:

import kotlinx.android.extensions.LayoutContainer

class ViewHolder(override val containerView: View) : ViewHolder(containerView), LayoutContainer {
    fun setup(title: String) {
        itemTitle.text = "Hello World!"

Note that you need to turn on the experimental flag to use LayoutContainer.


安卓扩展插件现已支持安卓多渠道。假设当前在 build.gradle 文件中指定一个名为 free 的渠道:

android {
    productFlavors {
        free {
            versionName "1.0-free"

所以现在只需要添加一行导入语句即可从 free/res/layout/activity_free.xml 布局中导入所有的合成属性:

import kotlinx.android.synthetic.free.activity_free.*

In the experimental mode, you can specify any variant name (not only flavor), e.g. freeDebug or freeRelease will work as well.

View Caching

Invoking findViewById() can be slow, especially in case of huge view hierarchies, so Android Extensions tries to minimize findViewById() calls by caching views in containers.

By default, Android Extensions adds a hidden cache function and a storage field to each container (Activity, Fragment, View or a LayoutContainer implementation) written in Kotlin. The method is pretty small so it does not increase the size of APK much.

In the following example, findViewById() is only invoked once:

class MyActivity : Activity()

fun MyActivity.a() { 
    textView.text = "Hidden view"
    textView.visibility = View.INVISIBLE


fun Activity.b() { 
    textView.text = "Hidden view"
    textView.visibility = View.INVISIBLE

We wouldn't know if this function would be invoked on only activities from our sources or on plain Java activities also. Because of this, we don’t use caching there, even if MyActivity instance from the previous example is passed as a receiver.

Changing View Caching Strategy

You can change the caching strategy globally or per container. This also requires switching on the experimental mode.

Project-global caching strategy is set in the build.gradle file:

androidExtensions {
    defaultCacheImplementation = "HASH_MAP" // also SPARSE_ARRAY, NONE

By default, Android Extensions plugin uses HashMap as a backing storage, but you can switch to the SparseArray implementation, or just switch off caching. The latter is especially useful when you use only the Parcelable part of Android Extensions.

Also, you can annotate a container with @ContainerOptions to change its caching strategy:

import kotlinx.android.extensions.ContainerOptions

@ContainerOptions(cache = CacheImplementation.NO_CACHE)
class MyActivity : Activity()

fun MyActivity.a() { 
    // findViewById() will be called twice
    textView.text = "Hidden view"
    textView.visibility = View.INVISIBLE


Starting from Kotlin 1.1.4, Android Extensions plugin provides Parcelable implementation generator as an experimental feature.

Enabling Parcelable support

Apply the kotlin-android-extensions Gradle plugin as described above and turn on the experimental flag.

How to use

Annotate the class with @Parcelize, and a Parcelable implementation will be generated automatically.

import kotlinx.android.parcel.Parcelize

class User(val firstName: String, val lastName: String, val age: Int): Parcelable

@Parcelize requires all serialized properties to be declared in the primary constructor. Android Extensions will issue a warning on each property with a backing field declared in the class body. Also, @Parcelize can't be applied if some of the primary constructor parameters are not properties.

If your class requires more advanced serialization logic, you can write it inside a companion class:

data class User(val firstName: String, val lastName: String, val age: Int) : Parcelable {
    private companion object : Parceler<User> {
        override fun User.write(parcel: Parcel, flags: Int) {
            // Custom write implementation

        override fun create(parcel: Parcel): User {
            // Custom read implementation

Supported Types

@Parcelize supports a wide range of types:

  • Primitive types (and its boxed versions);
  • Objects and enums;
  • String, CharSequence;
  • Exception;
  • Size, SizeF, Bundle, IBinder, IInterface, FileDescriptor;
  • SparseArray, SparseIntArray, SparseLongArray, SparseBooleanArray;
  • All Serializable (yes, Date is supported too) and Parcelable implementations;
  • Collections of all supported types: List (mapped to ArrayList), Set (mapped to LinkedHashSet), Map (mapped to LinkedHashMap);
    • Also a number of concrete implementations: ArrayList, LinkedList, SortedSet, NavigableSet, HashSet, LinkedHashSet, TreeSet, SortedMap, NavigableMap, HashMap, LinkedHashMap, TreeMap, ConcurrentHashMap;
  • Arrays of all supported types;
  • Nullable versions of all supported types.

Custom Parcelers

Even if your type is not supported directly, you can write a Parceler mapping object for it.

class ExternalClass(val value: Int)

object ExternalClassParceler : Parceler<ExternalClass> {
    override fun create(parcel: Parcel) = ExternalClass(parcel.readInt())

    override fun ExternalClass.write(parcel: Parcel, flags: Int) {

External parcelers can be applied using @TypeParceler or @WriteWith annotations:

// Class-local parceler
@TypeParceler<ExternalClass, ExternalClassParceler>()
class MyClass(val external: ExternalClass)

// Property-local parceler
class MyClass(@TypeParceler<ExternalClass, ExternalClassParceler>() val external: ExternalClass)

// Type-local parceler
class MyClass(val external: @WriteWith<ExternalClassParceler>() ExternalClass)