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ViewBinding 与 Kotlin 委托双剑合璧

2022-09-08  今日头条  彭旭锐
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前言

大家好，我是小彭。

过去两年，我们在掘金平台上发表过一些文章，小彭也收到了大家的意见和鼓励。最近，我会陆续搬运到公众号上。

ViewBinding 是 Android Gradle Plugin 3.6 中新增的特性，用于更加轻量地实现视图绑定（即视图与变量的绑定），可以理解为轻量版本的 DataBinding。 在这篇文章里，我将总结 ViewBinding 使用方法 & 原理，示例程序 AndroidFamilyDemo · KotlinDelegate 有用请记得给 Star ，给小彭一点创作的动力。

前置知识：

学习路线图

 

1. 认识 ViewBinding

1.1 ViewBinding 用于解决什么问题？

ViewBinding 是 Android Gradle Plugin 3.6 中新增的特性，用于更加轻量地实现视图绑定（即视图与变量的绑定），可以理解为轻量版本的 DataBinding。

1.2 ViewBinding 与其他视图绑定方案对比

在 ViewBinding 之前，业界已经有过几种视图绑定方案了，想必你也用过。那么，ViewBinding 作为后起之秀就一定比前者香吗？我从多个维度对比它们的区别：

可以看到，并没有一种绝对优势的方法，但越往后整体的效果是有提升的。另外，❓是什么呢？

1.3ViewBinding 的实现原理

AGP 插件会为每个 XML 布局文件创建一个绑定类文件 xxxBinding ，绑定类中会持有布局文件中所有带 android:id 属性的 View 引用。例如，有布局文件为 fragment_test.xml ，则插件会生成绑定类 FragmentTestBinding.java 。

那么，所有 XML 布局文件都生成 Java 类，会不会导致包体积瞬间增大？不会的， 未使用的类会在混淆时被压缩。

2. ViewBinding 的基本用法

这一节我们来介绍 ViewBinding 的使用方法，内容不多。

提示： ViewBinding 要求在 Android Gradle Plugin 版本在至少在 3.6 以上。

2.1 添加配置

视图绑定功能按模块级别启用，启用的模块需要在模块级 build.gralde 中添加配置。例如：

build.gradle

android {
    ...
    viewBinding {
        enabled = true
    }
}

对于不需要生成绑定类的布局文件，可以在根节点声明 tools:viewBindingIgnore="true" 。例如：

<LinearLayout
    ...
    tools:viewBindingIgnore="true" >
    ...
</LinearLayout>

2.2 视图绑定

绑定类中提供了 3 个视图绑定 API：

// 绑定到视图 view 上
fun <T> bind(view : View) : T

// 使用 inflater 解析布局，再绑定到 View 上
fun <T> inflate(inflater : LayoutInflater) : T

// 使用 inflater 解析布局，再绑定到 View 上
fun <T> inflate(inflater : LayoutInflater, parent : ViewGroup?, attachToParent : Boolean) : T

MainActivity.kt

class TestActivity: AppCompatActivity(R.layout.activity_test) {

    private lateinit var binding: ActivityTestBinding

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

        binding = ActivityTestBinding.inflate(layoutInflater)
        setContentView(binding.root)
        binding.tvDisplay.text = "Hello World."
    }
}

TestFragment.kt

class TestFragment : Fragment(R.layout.fragment_test) {

    private var _binding: FragmentTestBinding? = null
    private val binding get() = _binding!!

    override fun onViewCreated(root: View, savedInstanceState: Bundle?) {
        _binding = FragmentTestBinding.bind(root)

        binding.tvDisplay.text = "Hello World."
    }

    override fun onDestroyView() {
        super.onDestroyView()

        // 置空
        _binding = null
    }
}

2.3 避免内存泄露

这里有一个隐藏的内存泄露问题，你需要理解清楚（严格来说这并不是 ViewBinding 的问题，即使你采用其它视图绑定方案也要考虑这个问题）。

问题：为什么 Fragment#onDestroyView() 里需要置空绑定类对象，而 Activity 里不需要？ 答：Activity 实例和 Activity 视图的生命周期是同步的，而 Fragment 实例和 Fragment 视图的生命周期并不是完全同步的，因此需要在 Fragment 视图销毁时，手动回收绑定类对象，否则造成内存泄露。例如：detach Fragment，或者 remove Fragment 并且事务进入返回栈，此时 Fragment 视图销毁但 Fragment 实例存在。关于 Fragment 生命周期和事务在我之前的一篇文章里讨论过：Android | Fragment 核心原理 & 面试题 (AndroidX 版本)

总之，在视图销毁但是控制类对象实例还存活的时机，你就需要手动回收绑定类对象，否则造成内存泄露。

2.4 ViewBinding 绑定类源码

反编译如下：

ActivityTestBinding.java

public final class ActivityTestBinding implements ViewBinding {
    private final ConstraintLayout rootView;
    public final TextView tvDisplay;

    private ActivityTestBinding (ConstraintLayout paramConstraintLayout1, TextView paramTextView)
        this.rootView = paramConstraintLayout1;
        this.tvDisplay = paramTextView;
    }

    public static ActivityTestBinding bind(View paramView) {
        TextView localTextView = (TextView)paramView.findViewById(2131165363);
        if (localTextView != null) {
            return new ActivityMainBinding((ConstraintLayout)paramView, localTextView);
        }else {
          paramView = "tvDisplay";
        }
        throw new NullPointerException("Missing required view with ID: ".concat(paramView));
    }

    public static ActivityMainBinding inflate(LayoutInflater paramLayoutInflater) {
        return inflate(paramLayoutInflater, null, false);
    }

    public static ActivityMainBinding inflate(LayoutInflater paramLayoutInflater, ViewGroup paramViewGroup, boolean paramBoolean) {
        paramLayoutInflater = paramLayoutInflater.inflate(2131361821, paramViewGroup, false);
        if (paramBoolean) {
            paramViewGroup.addView(paramLayoutInflater);
        }
        return bind(paramLayoutInflater);
    }

    public ConstraintLayout getRoot() {
        return this.rootView;
    }
}

3. ViewBinding 与 Kotlin 委托双剑合璧

到这里，ViewBinding 的使用教程已经说完了。但是回过头看，有没有发现一些局限性呢？

那么，有没有可优化的方案呢？我们想起了 Kotlin 属性委托，关于 Kotlin 委托机制在我之前的一篇文章里讨论过：Kotlin | 委托机制 & 原理。如果你还不太了解 Kotlin 委托，下面的内容对你会有些难度。下面，我将带你一步步封装 ViewBinding 属性委托工具。首先，我们梳理一下我们要委托的内容与需求，以及相应的解决办法：

需求解决办法需要委托 ViewBinding#bind() 的调用反射需要委托 binding = null 的调用监听 Fragment 视图生命周期期望 binding 属性声明为非空不可变变量ReadOnlyProperty<F, V>

3.1 ViewBinding + Kotlin 委托 1.0

我们现在较复杂的 Fragment 中尝试使用 Kotlin 委托优化：

FragmentViewBindingPropertyV1.kt

private const val TAG = "ViewBindingProperty"

public inline fun <reified V : ViewBinding> viewBindingV1() = viewBindingV1(V::class.java)

public inline fun <reified T : ViewBinding> viewBindingV1(clazz: Class<T>): FragmentViewBindingPropertyV1<Fragment, T> {
    val bindMethod = clazz.getMethod("bind", View::class.java)
    return FragmentViewBindingPropertyV1 {
        bindMethod(null, it.requireView()) as T
    }
}

/**
 * @param viewBinder 创建绑定类对象
 */
class FragmentViewBindingPropertyV1<in F : Fragment, out V : ViewBinding>(
    private val viewBinder: (F) -> V
) : ReadOnlyProperty<F, V> {

    private var viewBinding: V? = null

    @MainThread
    override fun getValue(thisRef: F, property: KProperty<*>): V {
        // 已经绑定，直接返回
        viewBinding?.let { return it }

        // Use viewLifecycleOwner.lifecycle other than lifecycle
        val lifecycle = thisRef.viewLifecycleOwner.lifecycle
        val viewBinding = viewBinder(thisRef)
        if (lifecycle.currentState == Lifecycle.State.DESTROYED) {
            Log.w(
                TAG, "Access to viewBinding after Lifecycle is destroyed or hasn't created yet. " +
                        "The instance of viewBinding will be not cached."
            )
            // We can access to ViewBinding after Fragment.onDestroyView(), but don't save it to prevent memory leak
        } else {
            lifecycle.addObserver(ClearOnDestroyLifecycleObserver())
            this.viewBinding = viewBinding
        }
        return viewBinding
    }

    @MainThread
    fun clear() {
        viewBinding = null
    }

    private inner class ClearOnDestroyLifecycleObserver : LifecycleObserver {

        private val mainHandler = Handler(Looper.getMainLooper())

        @MainThread
        @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
        fun onDestroy(owner: LifecycleOwner) {
            owner.lifecycle.removeObserver(this)
            mainHandler.post { clear() }
        }
    }
}

使用示例：

class TestFragment : Fragment(R.layout.fragment_test) {

    private val binding : FragmentTestBinding by viewBindingV1()

    override fun onViewCreated(root: View, savedInstanceState: Bundle?) {
        binding.tvDisplay.text = "Hello World."
    }
}

干净清爽！前面提出的三个需求也都实现了，现在我为你解答细节：

FragmentViewBindingPropertyV1.kt

@MainThread
override fun getValue(thisRef: F, property: KProperty<*>): V {
    // 1、viewBinding 不为空说明已经绑定，直接返回
    viewBinding?.let { return it }

    // 2、Fragment 视图的生命周期
    val lifecycle = thisRef.viewLifecycleOwner.lifecycle

    // 3、实例化绑定类对象
    val viewBinding = viewBinder(thisRef)

    if (lifecycle.currentState == Lifecycle.State.DESTROYED) {
        // 4.1 如果视图生命周期为 DESTROYED，说明视图被销毁，此时不缓存绑定类对象（避免内存泄漏）
    } else {
        // 4.2 定义视图生命周期监听者
        lifecycle.addObserver(ClearOnDestroyLifecycleObserver())
        // 4.3 缓存绑定类对象
        this.viewBinding = viewBinding
    }
    return viewBinding
}

3.2 ViewBinding + Kotlin 委托 2.0

V1.0 版本使用了反射，真的一定要反射吗？反射调用 bind 函数的目的就是获得一个 ViewBinding 绑定类对象，或许我们可以试试把创建对象的行为交给外部去定义，类似这样用一个 lambda 表达式实现工厂函数：

FragmentViewBindingPropertyV2.kt

inline fun <F : Fragment, V : ViewBinding> viewBindingV2(
    crossinline viewBinder: (View) -> V,
    // 类似于创建工厂
    crossinline viewProvider: (F) -> View = Fragment::requireView
) = FragmentViewBindingPropertyV2 { fragment: F ->
    viewBinder(viewProvider(fragment))
}

class FragmentViewBindingPropertyV2<in F : Fragment, out V : ViewBinding>(
    private val viewBinder: (F) -> V
) : ReadOnlyProperty<F, V> {
    // 以下源码相同 ...
}

使用示例：

class TestFragment : Fragment(R.layout.fragment_test) {

    private val binding by viewBindingV2(FragmentTestBinding::bind)

    override fun onViewCreated(root: View, savedInstanceState: Bundle?) {
        binding.tvDisplay.text = "Hello World."
    }
}

干净清爽！不使用反射也可以实现，现在我为你解答细节：

3.3 ViewBinding + Kotlin 委托最终版

V2.0 版本已经完成了针对 Fragment 的属性代理，但是实际场景中只会在 Fragment 中使用 ViewBinding 吗？显然并不是，我们还有其他一些场景：

所以，我们有必要将委托工具适当封装得更通用些，完整代码和演示工程你可以直接下载查看： AndroidFamilyDemo · KotlinDelegate

ViewBindingProperty.kt

// -------------------------------------------------------
// ViewBindingProperty for Activity
// -------------------------------------------------------

@JvmName("viewBindingActivity")
inline fun <V : ViewBinding> ComponentActivity.viewBinding(
    crossinline viewBinder: (View) -> V,
    crossinline viewProvider: (ComponentActivity) -> View = ::findRootView
): ViewBindingProperty<ComponentActivity, V> = ActivityViewBindingProperty { activity: ComponentActivity ->
    viewBinder(viewProvider(activity))
}

@JvmName("viewBindingActivity")
inline fun <V : ViewBinding> ComponentActivity.viewBinding(
    crossinline viewBinder: (View) -> V,
    @IdRes viewBindingRootId: Int
): ViewBindingProperty<ComponentActivity, V> = ActivityViewBindingProperty { activity: ComponentActivity ->
    viewBinder(activity.requireViewByIdCompat(viewBindingRootId))
}

// -------------------------------------------------------
// ViewBindingProperty for Fragment / DialogFragment
// -------------------------------------------------------

@Suppress("UNCHECKED_CAST")
@JvmName("viewBindingFragment")
inline fun <F : Fragment, V : ViewBinding> Fragment.viewBinding(
    crossinline viewBinder: (View) -> V,
    crossinline viewProvider: (F) -> View = Fragment::requireView
): ViewBindingProperty<F, V> = when (this) {
    is DialogFragment -> DialogFragmentViewBindingProperty { fragment: F ->
        viewBinder(viewProvider(fragment))
    } as ViewBindingProperty<F, V>
    else -> FragmentViewBindingProperty { fragment: F ->
        viewBinder(viewProvider(fragment))
    }
}

@Suppress("UNCHECKED_CAST")
@JvmName("viewBindingFragment")
inline fun <F : Fragment, V : ViewBinding> Fragment.viewBinding(
    crossinline viewBinder: (View) -> V,
    @IdRes viewBindingRootId: Int
): ViewBindingProperty<F, V> = when (this) {
    is DialogFragment -> viewBinding(viewBinder) { fragment: DialogFragment ->
        fragment.getRootView(viewBindingRootId)
    } as ViewBindingProperty<F, V>
    else -> viewBinding(viewBinder) { fragment: F ->
        fragment.requireView().requireViewByIdCompat(viewBindingRootId)
    }
}

// -------------------------------------------------------
// ViewBindingProperty for ViewGroup
// -------------------------------------------------------

@JvmName("viewBindingViewGroup")
inline fun <V : ViewBinding> ViewGroup.viewBinding(
    crossinline viewBinder: (View) -> V,
    crossinline viewProvider: (ViewGroup) -> View = { this }
): ViewBindingProperty<ViewGroup, V> = LazyViewBindingProperty { viewGroup: ViewGroup ->
    viewBinder(viewProvider(viewGroup))
}

@JvmName("viewBindingViewGroup")
inline fun <V : ViewBinding> ViewGroup.viewBinding(
    crossinline viewBinder: (View) -> V,
    @IdRes viewBindingRootId: Int
): ViewBindingProperty<ViewGroup, V> = LazyViewBindingProperty { viewGroup: ViewGroup ->
    viewBinder(viewGroup.requireViewByIdCompat(viewBindingRootId))
}

// -------------------------------------------------------
// ViewBindingProperty for RecyclerView#ViewHolder
// -------------------------------------------------------

@JvmName("viewBindingViewHolder")
inline fun <V : ViewBinding> RecyclerView.ViewHolder.viewBinding(
    crossinline viewBinder: (View) -> V,
    crossinline viewProvider: (RecyclerView.ViewHolder) -> View = RecyclerView.ViewHolder::itemView
): ViewBindingProperty<RecyclerView.ViewHolder, V> = LazyViewBindingProperty { holder: RecyclerView.ViewHolder ->
    viewBinder(viewProvider(holder))
}

@JvmName("viewBindingViewHolder")
inline fun <V : ViewBinding> RecyclerView.ViewHolder.viewBinding(
    crossinline viewBinder: (View) -> V,
    @IdRes viewBindingRootId: Int
): ViewBindingProperty<RecyclerView.ViewHolder, V> = LazyViewBindingProperty { holder: RecyclerView.ViewHolder ->
    viewBinder(holder.itemView.requireViewByIdCompat(viewBindingRootId))
}

// -------------------------------------------------------
// ViewBindingProperty
// -------------------------------------------------------

private const val TAG = "ViewBindingProperty"

interface ViewBindingProperty<in R : Any, out V : ViewBinding> : ReadOnlyProperty<R, V> {
    @MainThread
    fun clear()
}

class LazyViewBindingProperty<in R : Any, out V : ViewBinding>(
    private val viewBinder: (R) -> V
) : ViewBindingProperty<R, V> {

    private var viewBinding: V? = null

    @Suppress("UNCHECKED_CAST")
    @MainThread
    override fun getValue(thisRef: R, property: KProperty<*>): V {
        // Already bound
        viewBinding?.let { return it }

        return viewBinder(thisRef).also {
            this.viewBinding = it
        }
    }

    @MainThread
    override fun clear() {
        viewBinding = null
    }
}

abstract class LifecycleViewBindingProperty<in R : Any, out V : ViewBinding>(
    private val viewBinder: (R) -> V
) : ViewBindingProperty<R, V> {

    private var viewBinding: V? = null

    protected abstract fun getLifecycleOwner(thisRef: R): LifecycleOwner

    @MainThread
    override fun getValue(thisRef: R, property: KProperty<*>): V {
        // Already bound
        viewBinding?.let { return it }

        val lifecycle = getLifecycleOwner(thisRef).lifecycle
        val viewBinding = viewBinder(thisRef)
        if (lifecycle.currentState == Lifecycle.State.DESTROYED) {
            Log.w(
                TAG, "Access to viewBinding after Lifecycle is destroyed or hasn't created yet. " +
                        "The instance of viewBinding will be not cached."
            )
            // We can access to ViewBinding after Fragment.onDestroyView(), but don't save it to prevent memory leak
        } else {
            lifecycle.addObserver(ClearOnDestroyLifecycleObserver(this))
            this.viewBinding = viewBinding
        }
        return viewBinding
    }

    @MainThread
    override fun clear() {
        viewBinding = null
    }

    private class ClearOnDestroyLifecycleObserver(
        private val property: LifecycleViewBindingProperty<*, *>
    ) : LifecycleObserver {

        private companion object {
            private val mainHandler = Handler(Looper.getMainLooper())
        }

        @MainThread
        @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
        fun onDestroy(owner: LifecycleOwner) {
            mainHandler.post { property.clear() }
        }
    }
}

class FragmentViewBindingProperty<in F : Fragment, out V : ViewBinding>(
    viewBinder: (F) -> V
) : LifecycleViewBindingProperty<F, V>(viewBinder) {

    override fun getLifecycleOwner(thisRef: F): LifecycleOwner {
        try {
            return thisRef.viewLifecycleOwner
        } catch (ignored: IllegalStateException) {
            error("Fragment doesn't have view associated with it or the view has been destroyed")
        }
    }
}

class DialogFragmentViewBindingProperty<in F : DialogFragment, out V : ViewBinding>(
    viewBinder: (F) -> V
) : LifecycleViewBindingProperty<F, V>(viewBinder) {

    override fun getLifecycleOwner(thisRef: F): LifecycleOwner {
        return if (thisRef.showsDialog) {
            thisRef
        } else {
            try {
                thisRef.viewLifecycleOwner
            } catch (ignored: IllegalStateException) {
                error("Fragment doesn't have view associated with it or the view has been destroyed")
            }
        }
    }
}

// -------------------------------------------------------
// Utils
// -------------------------------------------------------

@RestrictTo(RestrictTo.Scope.LIBRARY)
class ActivityViewBindingProperty<in A : ComponentActivity, out V : ViewBinding>(
    viewBinder: (A) -> V
) : LifecycleViewBindingProperty<A, V>(viewBinder) {

    override fun getLifecycleOwner(thisRef: A): LifecycleOwner {
        return thisRef
    }
}

fun <V : View> View.requireViewByIdCompat(@IdRes id: Int): V {
    return ViewCompat.requireViewById(this, id)
}

fun <V : View> Activity.requireViewByIdCompat(@IdRes id: Int): V {
    return ActivityCompat.requireViewById(this, id)
}

/**
 * Utility to find root view for ViewBinding in Activity
 */
fun findRootView(activity: Activity): View {
    val contentView = activity.findViewById<ViewGroup>(android.R.id.content)
    checkNotNull(contentView) { "Activity has no content view" }
    return when (contentView.childCount) {
        1 -> contentView.getChildAt(0)
        0 -> error("Content view has no children. Provide root view explicitly")
        else -> error("More than one child view found in Activity content view")
    }
}

fun DialogFragment.getRootView(viewBindingRootId: Int): View {
    val dialog = checkNotNull(dialog) {
        "DialogFragment doesn't have dialog. Use viewBinding delegate after onCreateDialog"
    }
    val window = checkNotNull(dialog.window) { "Fragment's Dialog has no window" }
    return with(window.decorView) {
        if (viewBindingRootId != 0) requireViewByIdCompat(
            viewBindingRootId
        ) else this
    }
}

4. 总结

ViewBinding 是一个轻量级的视图绑定方案，Android Gradle 插件会为每个 XML 布局文件创建一个绑定类。在 Fragment 中使用 ViewBinding 需要注意在 Fragment#onDestroyView() 里置空绑定类对象避免内存泄漏。但这会带来很多重复编写样板代码，使用属性委托可以收敛模板代码，保证调用方代码干净清爽。

参考资料

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