ViewBinding 封装

Android 基础|Android|字数 766|阅读时长≈ 2 分钟

前言

ViewBinding 视图绑定功能可让您更轻松地编写与视图交互的代码。在模块中启用视图绑定后，它会为该模块中显示的每个 XML 布局文件生成一个绑定类。绑定类的实例包含对在相应布局中具有 ID 的所有视图的直接引用。ViewBinding 优点和配置方式见于视图绑定。

基本使用

在 Activity 中使用视图绑定

Code

1private lateinit var binding: ResultProfileBinding2 3override fun onCreate(savedInstanceState: Bundle?) {4    super.onCreate(savedInstanceState)5    binding = ResultProfileBinding.inflate(layoutInflater)6    val view = binding.root7    setContentView(view)8}

在 Fragment 中使用视图绑定

Code

1private var _binding: ResultProfileBinding? = null2// This property is only valid between onCreateView and3// onDestroyView.4private val binding get() = _binding!!5 6override fun onCreateView(7    inflater: LayoutInflater,8    container: ViewGroup?,9    savedInstanceState: Bundle?10): View? {11    _binding = ResultProfileBinding.inflate(inflater, container, false)12    val view = binding.root13    return view14}15 16override fun onDestroyView() {17    super.onDestroyView()18    _binding = null19}

如上代码所示，如果有很多 Fragment，每一个都要拷贝一份相同的代码，不符合 Don't repeat yourself 的原则，所以尝试对其进行封装。

几种封装方式

不使用反射

BaseActivity，使用泛型， ActivityMainBinding::inflate 函数作为参数传递给 BaseActivity。

Code

1abstract class BaseActivity<VB : ViewBinding>(private val inflate: (LayoutInflater) -> VB) : AppCompatActivity() {2    lateinit var binding: VB3 4    override fun onCreate(savedInstanceState: Bundle?) {5        super.onCreate(savedInstanceState)6        binding = inflate(layoutInflater)7        setContentView(binding.root)8    }9}

BaseFragment，onDestroyView() 时 _binding = null 以避免内存泄露。

Code

1abstract class BaseFragment<VB : ViewBinding>(private val inflate: (LayoutInflater, ViewGroup?, Boolean) -> VB) : Fragment() {2    private var _binding: VB? = null3    protected val binding: VB get() = _binding!!4 5    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {6        _binding = inflate(inflater, container, false)7        return binding.root8    }9 10    override fun onDestroyView() {11        super.onDestroyView()12        _binding = null13    }14}

Activity 中使用示例

Code

1class MainActivity : BaseActivity<ActivityMainBinding>(ActivityMainBinding::inflate) {}

Fragment 中使用示例

Code

1class TestFragment : BaseFragment<FragmentTestBinding>(FragmentTestBinding::inflate){}

使用反射

在基类中通过反射调用 ActivityMainBinding.java （ViewBinding 编译后生成的类）中的 inflate 方法获取视图 View。

Code

1import android.view.LayoutInflater2import android.view.ViewGroup3import androidx.appcompat.app.AppCompatActivity4import androidx.fragment.app.Fragment5import androidx.viewbinding.ViewBinding6import java.lang.reflect.ParameterizedType7 8// 扩展函数，通过反射 inflate 方法获取 view9@JvmName("inflateWithGeneric")10fun <VB : ViewBinding> AppCompatActivity.inflateBindingWithGeneric(layoutInflater: LayoutInflater): VB =11    withGenericBindingClass<VB>(this) { clazz ->12        clazz.getMethod("inflate", LayoutInflater::class.java).invoke(null, layoutInflater) as VB13    }14 15@JvmName("inflateWithGeneric")16fun <VB : ViewBinding> Fragment.inflateBindingWithGeneric(17    layoutInflater: LayoutInflater,18    parent: ViewGroup?,19    attachToParent: Boolean20): VB =21    withGenericBindingClass<VB>(this) { clazz ->22        clazz.getMethod("inflate", LayoutInflater::class.java, ViewGroup::class.java, Boolean::class.java)23            .invoke(null, layoutInflater, parent, attachToParent) as VB24    }25 26private fun <VB : ViewBinding> withGenericBindingClass(any: Any, block: (Class<VB>) -> VB): VB {27    var genericSuperclass = any.javaClass.genericSuperclass28    var superclass = any.javaClass.superclass29    // 多继承时的递归处理30    while (superclass != null) {31        if (genericSuperclass is ParameterizedType) {32            try {33                return block.invoke(genericSuperclass.actualTypeArguments[0] as Class<VB>)34            } catch (e: Exception) {35                throw e36            }37        }38        genericSuperclass = superclass.genericSuperclass39        superclass = superclass.superclass40    }41    throw IllegalArgumentException("There is no generic of ViewBinding.")42}

BaseActivity 定义

Code

1abstract class BaseActivity<VB : ViewBinding> : AppCompatActivity() {2    lateinit var binding: VB3 4    override fun onCreate(savedInstanceState: Bundle?) {5        super.onCreate(savedInstanceState)6        binding = inflateBindingWithGeneric(layoutInflater)7        setContentView(binding.root)8    }9}

BaseFragment 定义

Code

1abstract class BaseFragment<VB : ViewBinding> : Fragment() {2    private var _binding: VB? = null3    protected val binding: VB get() = _binding!!4 5    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {6        _binding = inflateBindingWithGeneric(inflater, container, false)7        return binding.root8    }9    10    override fun onDestroyView() {11        super.onDestroyView()12        _binding = null13    }    14}

ViewBindingPropertyDelegate 框架

使用 ViewBindingPropertyDelegate 开源框架，在 module 下的 build.gradle 文件里引入

Code

1implementation 'com.github.kirich1409:viewbindingpropertydelegate:1.5.6'

基类中使用

BaseActivity 定义

Code

1abstract class BaseActivity(@LayoutRes contentLayoutId: Int) : AppCompatActivity(contentLayoutId) {2    protected abstract val binding: ViewBinding3}

BaseFragment 定义

Code

1abstract class BaseFragment(@LayoutRes contentLayoutId: Int) : Fragment(contentLayoutId) {2    protected abstract val binding: ViewBinding3}

Activity 中使用示例

Code

1class MainActivity : BaseActivity(R.layout.activity_main) {2    override val binding: ActivityMainBinding by viewBinding()3}

Fragment 中使用示例

Code

1class TestFragment : BaseFragment(R.layout.fragment_test) {2    override val binding: FragmentTestBinding by viewBinding()3}

直接在 Activity、Fragment 里面使用

Code

1class TestActivity : AppCompatActivity(R.layout.activity_test) {2    private val binding: ActivityTestBinding by viewBinding()3}

ViewBindingPropertyDelegate 原理简析

kotlin 的委托

本质上使用了 kotlin 的委托，关于 kotlin 委托，这篇一文彻底搞懂Kotlin中的委托博客写的很好。拿对 Fragment 处理举例

Code

1/**2* 使用 ReadOnlyProperty 属性委托，3* 添加 fragment 声明周期的监听，以便 onDestoryView 时 viewBinding 置空4* 通过反射调用 xxxBinding.java 里面的 bind、inflate 方法（混淆时要 keep）5*/6@PublishedApi7internal class FragmentViewBindingProperty<T : ViewBinding>(8    private val viewBinder: ViewBinder<T>9) : ReadOnlyProperty<Fragment, T> {10    internal var viewBinding: T? = null11    private val lifecycleObserver = BindingLifecycleObserver()12 13    @MainThread14    override fun getValue(thisRef: Fragment, property: KProperty<*>): T {15        checkIsMainThread()16        this.viewBinding?.let { return it }17        val view = thisRef.requireView()18        thisRef.viewLifecycleOwner.lifecycle.addObserver(lifecycleObserver)19        return viewBinder.bind(view).also { vb -> this.viewBinding = vb }20    }21 22    private inner class BindingLifecycleObserver : DefaultLifecycleObserver {23        private val mainHandler = Handler(Looper.getMainLooper())24 25        @MainThread26        override fun onDestroy(owner: LifecycleOwner) {27            owner.lifecycle.removeObserver(this)28            // Fragment.viewLifecycleOwner call LifecycleObserver.onDestroy() before Fragment.onDestroyView().29            // That's why we need to postpone reset of the viewBinding30            mainHandler.post {31                viewBinding = null32            }33        }34    }35}36 37/**38 * Create new [ViewBinding] associated with the [Fragment][this]39 */40@Suppress("unused")41inline fun <reified T : ViewBinding> Fragment.viewBinding(): ReadOnlyProperty<Fragment, T> {42    return FragmentViewBindingProperty(DefaultViewBinder(T::class.java))43}44 45/**46 * Create new [ViewBinding] associated with the [Fragment][this] and allow customize how47 * a [View] will be bounded to the view binding.48 */49@Suppress("unused")50inline fun <T : ViewBinding> Fragment.viewBinding(51    crossinline bindView: (View) -> T52): ReadOnlyProperty<Fragment, T> {53    return FragmentViewBindingProperty(viewBinder(bindView))54}55 56 57 58@RestrictTo(RestrictTo.Scope.LIBRARY)59@PublishedApi60internal class DefaultViewBinder<T : ViewBinding>(61    private val viewBindingClass: Class<T>62) : ViewBinder<T> {63 64    /**65     * Cache static method `ViewBinding.bind(View)`66     */67    private val bindViewMethod by lazy(LazyThreadSafetyMode.NONE) {68        viewBindingClass.getMethod("bind", View::class.java)69    }70 71    /**72     * Create new [ViewBinding] instance73     */74    @Suppress("UNCHECKED_CAST")75    override fun bind(view: View): T {76        return bindViewMethod(null, view) as T77    }78}79 80 81 82/**83 * Create instance of [ViewBinding] from a [View]84 */85interface ViewBinder<T : ViewBinding> {86    fun bind(view: View): T87}88 89@PublishedApi90@RestrictTo(RestrictTo.Scope.LIBRARY)91internal inline fun <T : ViewBinding> viewBinder(crossinline bindView: (View) -> T): ViewBinder<T> {92    return object : ViewBinder<T> {93        override fun bind(view: View) = bindView(view)94    }95}

布局与 Binding 对象建立关联
CreateMethod.BIND

例如下面这种使用方式，viewBinding() 默认通过反射 ActivityMainBinding.java 的 bind 方法绑定到 View，怎么获取这个 View 呢？

Code

1class TestActivity : AppCompatActivity(R.layout.activity_test) {2    private val binding: ActivityTestBinding by viewBinding()3}

Activity 中获取 setContentView() 所对应的这个 View 如下

Code

1@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)2fun findRootView(activity: Activity): View {3    val contentView = activity.findViewById<ViewGroup>(android.R.id.content)4    checkNotNull(contentView) { "Activity has no content view" }5    return when (contentView.childCount) {6        1 -> contentView.getChildAt(0)7        0 -> error("Content view has no children. Provide root view explicitly")8        else -> error("More than one child view found in Activity content view")9    }10}

Fragment 中绑定的 View 是直接 Fragment::getView() 获取。

CreateMethod.INFLATE

这种方式跟上面使用反射小节一样。

问题

内存泄露

Fragment 中常见的一种内存泄露，例如下面这个例子，回退栈持有 Fragment 的引用，Fragment 生命周期 onDestoryView() 调用后，view 已经被移除，而且对应的 binding 对象还被 Fragment 持有引用，因此造成内存泄露。引用链条为 BackStack→Fragment→binding

示例

Code

1// 演示内存泄露2abstract class BaseFragment<VB : ViewBinding>(private val inflate: (LayoutInflater, ViewGroup?, Boolean) -> VB) : Fragment() {3    protected lateinit var binding: VB4 5    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {6        binding = inflate(inflater, container, false)7        return binding.root8    }9 10    override fun onDestroyView() {11        super.onDestroyView()12    }13}

Test01Fragment、Test02Fragment 是 BaseFragment 子类，这样在切换后，Test01Fragment 的 binding 成员便引发了内存泄露

Code

1class MainActivity : BaseActivity<ActivityMainBinding>(ActivityMainBinding::inflate) {2 3    override fun bindListener() {4        binding.btnReplace1.setOnClickListener {5            supportFragmentManager.beginTransaction()6                .replace(R.id.fl_content, Test01Fragment())7                .addToBackStack(null) // fragment 对象不会被释放8                .commit()9        }10 11        binding.btnReplace2.setOnClickListener {12            supportFragmentManager.beginTransaction()13                .replace(R.id.fl_content, Test02Fragment())14                .addToBackStack(null)15                .commit()16        }17    }18 19}

解决

在 onDestoryView() 时将 _binding = null 变量置空

Code

1abstract class BaseFragment<VB : ViewBinding>(private val inflate: (LayoutInflater, ViewGroup?, Boolean) -> VB) : Fragment() {2    private var _binding: VB? = null3    protected val binding: VB get() = _binding!!4 5    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {6        _binding = inflate(inflater, container, false)7        return binding.root8    }9 10    override fun onDestroyView() {11        super.onDestroyView()12        _binding = null13    }14}

代码混淆

ViewBinding 基于 .xml 生成的视图绑定类如下

Code

1// Generated by view binder compiler. Do not edit!2package com.dafay.demo.viewbinding.databinding;3 4import android.view.LayoutInflater;5import android.view.View;6import android.view.ViewGroup;7import android.widget.Button;8import android.widget.FrameLayout;9import androidx.annotation.NonNull;10import androidx.annotation.Nullable;11import androidx.viewbinding.ViewBinding;12import androidx.viewbinding.ViewBindings;13import com.dafay.demo.viewbinding.R;14import java.lang.NullPointerException;15import java.lang.Override;16import java.lang.String;17 18public final class ActivityMainBinding implements ViewBinding {19  @NonNull20  private final FrameLayout rootView;21 22  @NonNull23  public final Button btnReplace1;24 25  @NonNull26  public final Button btnReplace2;27 28  @NonNull29  public final FrameLayout flContent;30 31  private ActivityMainBinding(@NonNull FrameLayout rootView, @NonNull Button btnReplace1,32      @NonNull Button btnReplace2, @NonNull FrameLayout flContent) {33    this.rootView = rootView;34    this.btnReplace1 = btnReplace1;35    this.btnReplace2 = btnReplace2;36    this.flContent = flContent;37  }38 39  @Override40  @NonNull41  public FrameLayout getRoot() {42    return rootView;43  }44 45  @NonNull46  public static ActivityMainBinding inflate(@NonNull LayoutInflater inflater) {47    return inflate(inflater, null, false);48  }49 50  @NonNull51  public static ActivityMainBinding inflate(@NonNull LayoutInflater inflater,52      @Nullable ViewGroup parent, boolean attachToParent) {53    View root = inflater.inflate(R.layout.activity_main, parent, false);54    if (attachToParent) {55      parent.addView(root);56    }57    return bind(root);58  }59 60  @NonNull61  public static ActivityMainBinding bind(@NonNull View rootView) {62    // The body of this method is generated in a way you would not otherwise write.63    // This is done to optimize the compiled bytecode for size and performance.64    int id;65    missingId: {66      id = R.id.btn_replace_1;67      Button btnReplace1 = ViewBindings.findChildViewById(rootView, id);68      if (btnReplace1 == null) {69        break missingId;70      }71 72      id = R.id.btn_replace_2;73      Button btnReplace2 = ViewBindings.findChildViewById(rootView, id);74      if (btnReplace2 == null) {75        break missingId;76      }77 78      id = R.id.fl_content;79      FrameLayout flContent = ViewBindings.findChildViewById(rootView, id);80      if (flContent == null) {81        break missingId;82      }83 84      return new ActivityMainBinding((FrameLayout) rootView, btnReplace1, btnReplace2, flContent);85    }86    String missingId = rootView.getResources().getResourceName(id);87    throw new NullPointerException("Missing required view with ID: ".concat(missingId));88  }89}90

当开启代码混淆后，其中的 bind、inflate 等方法因为混淆改变了方法名，使用反射的方式便有找不到对应方法的异常。

Code

1FATAL EXCEPTION: main2Process: com.dafay.demo.viewbinding, PID: 3093java.lang.RuntimeException: Unable to start activity ComponentInfo{com.dafay.demo.viewbinding/com.dafay.demo.viewbinding.MainActivity}: java.lang.IllegalArgumentException: There is no generic of ViewBinding.4at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3676)5at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3813)6at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:101)7at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)8at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)9at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2308)10at android.os.Handler.dispatchMessage(Handler.java:106)11at android.os.Looper.loopOnce(Looper.java:201)12at android.os.Looper.loop(Looper.java:288)13at android.app.ActivityThread.main(ActivityThread.java:7898)14at java.lang.reflect.Method.invoke(Native Method)15at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548)16at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:936)17Caused by: java.lang.IllegalArgumentException: There is no generic of ViewBinding.18at com.dafay.demo.viewbinding.utils.ViewBindingUtilsKt.genericViewBindingClass(SourceFile:42)19at com.dafay.demo.viewbinding.utils.ViewBindingUtilsKt.inflateWithGeneric(SourceFile:13)20at com.example.demo.lab.base.base.BaseActivity.onCreate(SourceFile:15)21at android.app.Activity.performCreate(Activity.java:8290)22at android.app.Activity.performCreate(Activity.java:8269)23at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1384)24at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3657)25at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3813) 26at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:101) 27at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 28at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 29at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2308) 30at android.os.Handler.dispatchMessage(Handler.java:106) 31at android.os.Looper.loopOnce(Looper.java:201) 32at android.os.Looper.loop(Looper.java:288) 33at android.app.ActivityThread.main(ActivityThread.java:7898) 34at java.lang.reflect.Method.invoke(Native Method) 35at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:548) 36at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:936)

解决

在混淆 proguard-rules.pro 配置文件里面，keep 对应的类

Code

1# keep 所有生成的视图绑定类2-keep class * implements androidx.viewbinding.ViewBinding { *; }3-keepclassmembers class * { public <init>(android.view.View); }

更细粒度的配置，keep 对应的方法

Code

1-keep,allowoptimization class * implements androidx.viewbinding.ViewBinding {2    public static *** bind(android.view.View);3    public static *** inflate(...);4}

参考文档

视图绑定（developer.android.com）

View Binding 与Kotlin委托属性的巧妙结合，告别垃圾代码！

一文彻底搞懂Kotlin中的委托（深入简出，博客结构很好）

ViewBindingPropertyDelegate

示例代码