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Android ViewBinding

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一、kotlin-android-extensions

在使用ViewBinding之前，我们一直使用的是kotlin-android-extensions，使用kotlin-android-extensions可以节约很多写findViewById的时间。不过这个kotlin-android-extensions插件已经废弃了，简单说一下kotlin-android-extensions存在的问题：

1.通过反编译kotlin-android-extensions的代码，发现会创建一个HashMap，用来存放所有的id和对应的View的缓存，如果缓存中没有View，那么就通过findViewById去创建并存入缓存，否则就直接获取。所以会存在内存问题。

private HashMap _$_findViewCache;
public View _$_findCachedViewById(int var1) {
   if (this._$_findViewCache == null) {
      this._$_findViewCache = new HashMap();
   }
   View var2 = (View)this._$_findViewCache.get(var1);
   if (var2 == null) {
      View var10000 = this.getView();
      if (var10000 == null) {
         return null;
      }
      var2 = var10000.findViewById(var1);
      this._$_findViewCache.put(var1, var2);
   }
   return var2;
}
public void _$_clearFindViewByIdCache() {
   if (this._$_findViewCache != null) {
      this._$_findViewCache.clear();
   }
}
// $FF: synthetic method
public void onDestroyView() {
   super.onDestroyView();
   this._$_clearFindViewByIdCache();
}

2.由于kotlin-android-extensions是通过view的id名直接引用的，如果多个布局间的同名id，就需要手动对import进行重命名处理，如果引用错误的布局文件，就会出现crash。所以存在资源重名的问题。

3.只有Kotlin才可以使用。

所以ViewBinding优势有：java,kotlin都可以使用，可以有效避免NullPointerException。

二、ViewBinding使用

1.gradle配置

buildFeatures {
    viewBinding true
}

开启ViewBinding之后，在编译时，AGP会自动帮我们给每个xml布局创建一个Binding类，位于build/generated/data_binding_base_class_source_out/目录下。

public final class FragmentLoginBinding implements ViewBinding {
  @NonNull
  private final ConstraintLayout rootView;
  @NonNull
  public final ConstraintLayout container;
  @NonNull
  public final ProgressBar loading;
  @NonNull
  public final Button login;
  @NonNull
  public final EditText password;
  @NonNull
  public final EditText username;
  private FragmentLoginBinding(@NonNull ConstraintLayout rootView,
      @NonNull ConstraintLayout container, @NonNull ProgressBar loading, @NonNull Button login,
      @NonNull EditText password, @NonNull EditText username) {
    this.rootView = rootView;
    this.container = container;
    this.loading = loading;
    this.login = login;
    this.password = password;
    this.username = username;
  }
  @Override
  @NonNull
  public ConstraintLayout getRoot() {
    return rootView;
  }
  @NonNull
  public static FragmentLoginBinding inflate(@NonNull LayoutInflater inflater) {
    return inflate(inflater, null, false);
  }
  @NonNull
  public static FragmentLoginBinding inflate(@NonNull LayoutInflater inflater,
      @Nullable ViewGroup parent, boolean attachToParent) {
    View root = inflater.inflate(R.layout.fragment_login, parent, false);
    if (attachToParent) {
      parent.addView(root);
    }
    return bind(root);
  }
  @NonNull
  public static FragmentLoginBinding bind(@NonNull View rootView) {
    // The body of this method is generated in a way you would not otherwise write.
    // This is done to optimize the compiled bytecode for size and performance.
    int id;
    missingId: {
      ConstraintLayout container = (ConstraintLayout) rootView;
      id = R.id.loading;
      ProgressBar loading = rootView.findViewById(id);
      if (loading == null) {
        break missingId;
      }
      id = R.id.login;
      Button login = rootView.findViewById(id);
      if (login == null) {
        break missingId;
      }
      id = R.id.password;
      EditText password = rootView.findViewById(id);
      if (password == null) {
        break missingId;
      }
      id = R.id.username;
      EditText username = rootView.findViewById(id);
      if (username == null) {
        break missingId;
      }
      return new FragmentLoginBinding((ConstraintLayout) rootView, container, loading, login,
          password, username);
    }
    String missingId = rootView.getResources().getResourceName(id);
    throw new NullPointerException("Missing required view with ID: ".concat(missingId));
  }
}

注意：

1.因为这些类编译时就生成了，就不会占用运行时内存。

2.未使用的Binding文件会在混淆时被删除，所以对包大小影响很小。

3.编译器生成Binding文件是增量更新的。

那么如何不生成Binding类呢？tools:viewBindingIgnore="true"

<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:viewBindingIgnore="true"
    tools:context=".MainActivity">

2.在Activity 使用

class TestViewBindingActivity : AppCompatActivity() {
    private lateinit var bindding: ActivityTestViewBindingBinding
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        bindding = ActivityTestViewBindingBinding.inflate(layoutInflater)
        setContentView(bindding.root)
        changeText()
    }
    private fun changeText() {
        bindding.titleTv.text = "哈哈，在Activity中使用ViewBinding了"
    }
}
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".testviewbinding.TestViewBindingActivity">
    <TextView
        android:id="@+id/titleTv"
        android:layout_width="match_parent"
        android:layout_height="100dp"
        android:gravity="center"
        android:text="在Activity中使用ViewBinding"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

3.在Fragment使用

class TextViewBindingFragment : Fragment() {
    private var param1: String? = null
    private var param2: String? = null
    private var _binding: FragmentTextViewBindingBinding? = null
    private val binding get() = _binding!!
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        arguments?.let {
            param1 = it.getString(ARG_PARAM1)
            param2 = it.getString(ARG_PARAM2)
        }
    }
    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        _binding = FragmentTextViewBindingBinding.inflate(layoutInflater, container, false)
        return binding.root
    }
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        changeText()
    }
    private fun changeText() {
        binding.tvTitle.text = "哈哈，在Fragment中使用ViewBinding"
    }
    override fun onDestroyView() {
        super.onDestroyView()
        _binding = null
    }
    companion object {
        @JvmStatic
        fun newInstance(param1: String, param2: String) =
            TextViewBindingFragment().apply {
                arguments = Bundle().apply {
                    putString(ARG_PARAM1, param1)
                    putString(ARG_PARAM2, param2)
                }
            }
        @JvmStatic
        fun newInstance() = TextViewBindingFragment()
    }
}
class TestViewBindingActivity : AppCompatActivity() {
    private lateinit var bindding: ActivityTestViewBindingBinding
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        bindding = ActivityTestViewBindingBinding.inflate(layoutInflater)
        setContentView(bindding.root)
        val newInstance = TextViewBindingFragment.newInstance()
        addFragment(
            supportFragmentManager,
            newInstance,
            isAllowStateLoss = true,
            frameId = R.id.fragmentFrame
        )
    }
}
<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".testviewbinding.TextViewBindingFragment">
    <TextView
        android:id="@+id/tvTitle"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:text="在Fragment中" />
</FrameLayout>

4.在Adapter中使用

class TestAdapterActivity : AppCompatActivity() {
    private lateinit var binding: ActivityTestAdapterBinding
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityTestAdapterBinding.inflate(layoutInflater)
        setContentView(binding.root)
        initView()
    }
    companion object {
        val ITEMS = mutableListOf<String>("1", "2", "3", "4", "5", "6")
    }
    private fun initView() {
        with(binding.contentRcycler) {
            layoutManager = GridLayoutManager(context, 4)
            adapter = TestRecyclerViewAdapter(ITEMS)
        }
    }
}
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".testviewbinding.TestAdapterActivity">
    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/contentRcycler"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</androidx.constraintlayout.widget.ConstraintLayout>
class TestRecyclerViewAdapter(private val values: List<String>) :
    RecyclerView.Adapter<TestRecyclerViewAdapter.ViewHolder>() {
    inner class ViewHolder(binding: RecyclerItemLayoutBinding) :
        RecyclerView.ViewHolder(binding.root) {
        val textTv = binding.contentTv
    }
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
        return ViewHolder(
            RecyclerItemLayoutBinding.inflate(
                LayoutInflater.from(parent.context),
                parent,
                false
            )
        )
    }
    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        val item = values[position]
        holder.textTv.text = item
    }
    override fun getItemCount(): Int = values.size
}
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="200dp">
    <TextView
        android:id="@+id/contentTv"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:textSize="16dp"
        tools:text="99" />
</androidx.constraintlayout.widget.ConstraintLayout>

5.在Dialog中使用

class CommonDialog(context: Context) : Dialog(context) {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(DialogLayoutBinding.inflate(layoutInflater).root)
    }
}
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <TextView
        android:id="@+id/dialogContent"
        android:layout_width="200dp"
        android:layout_height="200dp"
        android:text="This is Dialog"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

6.Include中使用

class TestIncludeActivity : AppCompatActivity() {
    private lateinit var binding: ActivityTestIncludeBinding
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityTestIncludeBinding.inflate(layoutInflater)
        setContentView(binding.root)
        initView()
    }
    private fun initView() {
        binding.itemInclude.itemContentTv.text = "哈哈， this is include"
    }
}
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".testviewbinding.TestIncludeActivity">
    <include
        android:id="@+id/itemInclude"
        layout="@layout/item_layout" />
</androidx.constraintlayout.widget.ConstraintLayout>
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <TextView
        android:id="@+id/itemContentTv"
        android:layout_width="match_parent"
        android:layout_height="200dp"
        android:text="Test include"
        android:textSize="30sp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

三、ViewBinding封装

1.在BaseActivity中封装

abstract class BaseViewBindingActivity<T : ViewBinding> : AppCompatActivity() {
    protected val binding by lazy {
        getViewBinding()
    }
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(binding.root)
    }
    protected abstract fun getViewBinding(): T
}
class ChildViewBindingMainActivity :
    BaseViewBindingActivity<ActivityChildViewBindingMainBinding>() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding.titleTv.text = "哈哈，this is child binding activity"
    }
    override fun getViewBinding(): ActivityChildViewBindingMainBinding {
        return ActivityChildViewBindingMainBinding.inflate(layoutInflater)
    }
}
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".encapsulatviewbinding.ChildViewBindingMainActivity">
    <TextView
        android:id="@+id/titleTv"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:text="36sp" />
</androidx.constraintlayout.widget.ConstraintLayout>

2.通过反射的方式封装

class TestViewBindingMainActivity : AppCompatActivity() {
    private val binding by inflate<ActivityTestViewBindingMainBinding>()
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding.titleTv.text = "哈哈，通过反射封装ViewBinding"
    }
}
inline fun <reified T : ViewBinding> inflateByViewBinding(layoutInflater: LayoutInflater) =
    T::class.java.getMethod("inflate", LayoutInflater::class.java).invoke(null, layoutInflater) as T
inline fun <reified T : ViewBinding> Activity.inflate() = lazy {
    inflateByViewBinding<T>(layoutInflater).apply {
        setContentView(root)
    }
}
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".encapsulatviewbinding.TestViewBindingMainActivity">
    <TextView
        android:id="@+id/titleTv"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:textSize="36sp" />
</androidx.constraintlayout.widget.ConstraintLayout>

3.反射+基类

1.在Activity 中使用

abstract class BaseBindingMainActivity2<T : ViewBinding> : AppCompatActivity() {
    protected lateinit var binding: T
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val type = javaClass.genericSuperclass
        if (type is ParameterizedType) {
            val clazz = type.actualTypeArguments[0] as Class<T>
            val method = clazz.getMethod("inflate", LayoutInflater::class.java)
            binding = method.invoke(null, layoutInflater) as T
        }
        setContentView(binding.root)
    }
}
class ChildViewBindingMainActivity2 :
    BaseBindingMainActivity2<ActivityChildViewBindingMain2Binding>() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding.titleTv.text = "哈哈，这是反射+基类的方式"
    }
}
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".encapsulatviewbinding.ChildViewBindingMainActivity2">
    <TextView
        android:id="@+id/titleTv"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

2.在Fragment中使用

abstract class BaseBindingViewFragment<T : ViewBinding> : Fragment() {
    private var _binding: T? = null
    protected val binding get() = _binding!!
    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        val type = javaClass.genericSuperclass
        val clazz = (type as ParameterizedType).actualTypeArguments[0] as Class<T>
        val method = clazz.getMethod(
            "inflate",
            LayoutInflater::class.java,
            ViewGroup::class.java,
            Boolean::class.java
        )
        _binding = method.invoke(null, layoutInflater, container, false) as T
        this.viewLifecycleOwner.lifecycle.addObserver(object : LifecycleEventObserver {
            override fun onStateChanged(source: LifecycleOwner, event: Lifecycle.Event) {
                if (event == Lifecycle.Event.ON_DESTROY) {
                    Log.v(TAG, "onDestroy binding be null")
                    _binding = null
                }
            }
        })
        return binding.root
    }
    companion object {
        const val TAG = "BaseBindingViewFragment"
    }
}
class ChildBindingFragment : BaseBindingViewFragment<FragmentChildBindingBinding>() {
    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return super.onCreateView(inflater, container, savedInstanceState)
    }
    companion object {
        @JvmStatic
        fun newInstance() = ChildBindingFragment()
    }
}
class TestBindingMainActivity3 : BaseBindingMainActivity2<ActivityTestBindingMain3Binding>() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val newInstance = ChildBindingFragment.newInstance()
        addFragment(
            supportFragmentManager,
            newInstance,
            isAllowStateLoss = true,
            frameId = R.id.frame
        )
    }
}
<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".encapsulatviewbinding.ChildBindingFragment">
    <TextView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:text="@string/hello_blank_fragment" />
</FrameLayout>

4.委托的方式

class TestViewBindingFragment2 : Fragment(R.layout.fragment_test_view_binding2) {
    private val binding by inflate<FragmentTestViewBinding2Binding>()
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        binding.root
    }
    companion object {
        @JvmStatic
        fun newInstance() = TestViewBindingFragment2()
    }
}
inline fun <reified T : ViewBinding> Fragment.inflate() =
    FragmentViewBindingDelegate(T::class.java)
class FragmentViewBindingDelegate<T : ViewBinding>(private val clazz: Class<T>) :
    ReadOnlyProperty<Fragment, T> {
    private var binding: T? = null
    override fun getValue(thisRef: Fragment, property: KProperty<*>): T {
        if (binding == null) {
            binding =
                clazz.getMethod("bind", View::class.java).invoke(null, thisRef.requireView()) as T
            thisRef.viewLifecycleOwner.lifecycle.addObserver(object : LifecycleEventObserver {
                override fun onStateChanged(source: LifecycleOwner, event: Lifecycle.Event) {
                    if (event == Lifecycle.Event.ON_DESTROY) {
                        binding = null
                    }
                }
            })
        }
        return binding!!
    }
}
<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".encapsulatviewbinding.TestViewBindingFragment2">
    <TextView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:text="8888888" />
</FrameLayout>

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