什么是View Binding

View Binding是Android Studio 3.6推出的新特性，目的是为了替代findViewById(内部实现还是使用findViewById)。。在启动视图绑定后，系统会为改模块中的每个xml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。

View Binding 的优点

Null 安全：由于视图绑定会创建对视图的直接引用，因此不存在因视图 ID 无效而引发 Null 指针异常的风险。此外，如果视图仅出现在布局的某些配置中，则绑定类中包含其引用的字段会使用 @Nullable 标记。
类型安全：每个绑定类中的字段均具有与它们在 XML 文件中引用的视图相匹配的类型。这意味着不存在发生类转换异常的风险。

如何启用View Binding 功能

android {buildFeatures {viewBinding true}
}

如果想在生成绑定类时忽略某个布局文件，将 tools:viewBindingIgnore="true" 属性添加到相应布局文件的根视图中：

<?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:viewBindingIgnore="true"tools:context=".MainActivity"><TextViewandroid:layout_width="wrap_content"android:layout_height="wrap_content"android:text="Hello World!"app:layout_constraintBottom_toBottomOf="parent"app:layout_constraintLeft_toLeftOf="parent"app:layout_constraintRight_toRightOf="parent"app:layout_constraintTop_toTopOf="parent" /></androidx.constraintlayout.widget.ConstraintLayout>

怎么去使用View Binding

为用视图绑定功能后，系统会为该模块中包含的每个 XML 布局文件生成一个绑定类。这个类的类名是以xml布局文件名去掉下换线后，单词首字母大写加上Binding命名的。如 activity_main.xml生成的类 ActivityMainBinding.

Activity 绑定视图

第一步：调用生成的绑定类中包含的静态 inflate() 方法。此操作会创建该绑定类的实例以供 Activity 使用。
第二步：通过调用 getRoot() 方法或使用 Kotlin 属性语法获取对根视图的引用。
第三步：将根视图传递到 setContentView()，使其成为屏幕上的活动视图。

<?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=".MainActivity"><TextViewandroid:id="@+id/tv"android:layout_width="100dp"android:layout_height="100dp"android:background="#77f"android:text="赵彦军"app:layout_constraintEnd_toEndOf="parent"app:layout_constraintStart_toStartOf="parent"app:layout_constraintTop_toTopOf="parent" /></androidx.constraintlayout.widget.ConstraintLayout>

使用：

class MainActivity : AppCompatActivity() {lateinit var binding: ActivityMainBindingoverride fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)//关键代码binding = ActivityMainBinding.inflate(layoutInflater)setContentView(binding.root)//获取控件binding.tv.setOnClickListener {Toast.makeText(this, "hahha", Toast.LENGTH_SHORT).show()}}
}

Fragment 绑定视图

如何在 Fragment 中使用视图绑定请在 Fragment 的 onCreateView()方法中执行以下步骤(注意：Fragment 的存在时间比其视图长。请务必在 Fragment 的 onDestroyView() 方法中清除对绑定类实例的所有引用。)

调用生成的绑定类中包含的静态 inflate() 方法。此操作会创建该绑定类的实例以供 Fragment 使用。
通过调用 getRoot() 方法或使用 Kotlin 属性语法获取对根视图的引用。
从 onCreateView() 方法返回根视图，使其成为屏幕上的活动视图

//fragment_my.xml
<?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=".MainActivity"><Buttonandroid:id="@+id/button"android:layout_width="wrap_content"android:layout_height="40dp"android:text="这是Fragment按钮"app:layout_constraintBottom_toBottomOf="parent"app:layout_constraintLeft_toLeftOf="parent"app:layout_constraintRight_toRightOf="parent"app:layout_constraintTop_toBottomOf="@+id/textView" /><TextViewandroid:id="@+id/textView"android:layout_width="wrap_content"android:layout_height="40dp"android:gravity="center"android:text="这是FragmentTextView"app:layout_constraintBottom_toTopOf="@+id/button"app:layout_constraintLeft_toLeftOf="parent"app:layout_constraintRight_toRightOf="parent"app:layout_constraintTop_toTopOf="parent" /></androidx.constraintlayout.widget.ConstraintLayout>

public class MyFragment extends Fragment {private FragmentMyBinding binding;public MyFragment() {}@Overridepublic void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);}@Overridepublic View onCreateView(LayoutInflater inflater, ViewGroup container,Bundle savedInstanceState) {binding = FragmentMyBinding.inflate(inflater, container, false);return binding.getRoot();}@Overridepublic void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {super.onViewCreated(view, savedInstanceState);binding.textView.setText("这是Fragment");binding.button.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View view) {Log.d("Fragment", "点击了按钮");}});}@Overridepublic void onDestroy() {super.onDestroy();binding = null;}

在onViewCreated中使用View Binding

//在onViewCreated中使用View Binding
public class MyFragment extends Fragment {private FragmentMyBinding binding;public MyFragment() {}@Overridepublic void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);}@Overridepublic View onCreateView(LayoutInflater inflater, ViewGroup container,Bundle savedInstanceState) {return inflater.inflate(R.layout.fragment_my,container,false);}@Overridepublic void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {super.onViewCreated(view, savedInstanceState);FragmentMyBinding binding = FragmentMyBinding.bind(view);this.binding = binding;binding.textView.setText("这是Fragment");binding.button.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View view) {Log.d("Fragment", "点击了按钮");}});}@Overridepublic void onDestroy() {super.onDestroy();binding = null;}

使用View Binding 写的基类

1、通过反射的方式（这种方式不推荐使用，会有混淆问题）

Activity 基类设计

//Java
public class BaseActivity<T extends ViewBinding> extends AppCompatActivity {protected T viewBinding;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);ParameterizedType type = (ParameterizedType) getClass().getGenericSuperclass();Class cls = (Class) type.getActualTypeArguments()[0];try {Method inflate = cls.getDeclaredMethod("inflate", LayoutInflater.class);viewBinding = (T) inflate.invoke(null, getLayoutInflater());setContentView(viewBinding.getRoot());} catch (NoSuchMethodException | IllegalAccessException| InvocationTargetException e) {e.printStackTrace();}}
}//使用public class MainActivity extends BaseActivity<ActivityMainBinding> {@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);viewBinding.button.setText("这是 MainActivity ViewBinding");viewBinding.button.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View view) {Log.d("MainView","点击按钮");}});}
}

//Kotlin
open class BaseActivity<T : ViewBinding> : AppCompatActivity() {protected lateinit var binding: Toverride fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)val type = javaClass.genericSuperclass as ParameterizedTypeval aClass = type.actualTypeArguments[0] as Class<*>val method = aClass.getDeclaredMethod("inflate", LayoutInflater::class.java)binding = method.invoke(null, layoutInflater) as TsetContentView(binding.root)}
}class MainActivity : BaseActivity<ActivityMainBinding>() {override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)binding.textView.text = "这是MainActivity"}
}

Fragment 基类设计

//Java
public class BaseFragment<T extends ViewBinding> extends Fragment {protected T viewBinding;@Nullable@Overridepublic View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {ParameterizedType type = (ParameterizedType) getClass().getGenericSuperclass();Class cls = (Class) type.getActualTypeArguments()[0];try {Method inflate = cls.getDeclaredMethod("inflate", LayoutInflater.class, ViewGroup.class, boolean.class);viewBinding = (T) inflate.invoke(null, inflater, container, false);}  catch (NoSuchMethodException | IllegalAccessException| InvocationTargetException e) {e.printStackTrace();}return viewBinding.getRoot();}
}//使用
public class MainFragment extends BaseFragment<FragmentMainBinding>{@Overridepublic void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {super.onViewCreated(view, savedInstanceState);viewBinding.button.setText("这是 MainFragment ViewBinding");viewBinding.button.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View view) {Log.d("MainView","点击按钮");}});}
}

//Kotlin
open class BaseFragment<T:ViewBinding>:Fragment(){lateinit var binding: Toverride fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {val type = javaClass.genericSuperclass as ParameterizedTypeval aClass = type.actualTypeArguments[0] as Class<*>val method = aClass.getDeclaredMethod("inflate", LayoutInflater::class.java,ViewGroup::class.java,Boolean::class.java)binding = method.invoke(null,layoutInflater,container,false) as Treturn binding.root}
}class FirstFragment : BaseFragment<FragmentFirstBinding>() {override fun onViewCreated(view: View, savedInstanceState: Bundle?) {super.onViewCreated(view, savedInstanceState)binding.textView.text = "这是FirstFragment"}
}

2、不通过反射的方式

Activity 基类设计

abstract class BaseActivity<T : ViewBinding> : AppCompatActivity() {private lateinit var _binding: Tprotected val binding get() = _binding;override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)_binding = getViewBinding()setContentView(_binding.root)}protected abstract fun getViewBinding(): T
}class MainActivity : BaseActivity<ActivityMainBinding>() {override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)binding.textView.text = "这是MainActivity"}override fun getViewBinding() = ActivityMainBinding.inflate(layoutInflater)
}

Fragment 基类设计

abstract class BaseFragment<T : ViewBinding> : Fragment() {private lateinit var _binding: Tprotected val binding get() = _binding;override fun onCreateView(inflater: LayoutInflater,container: ViewGroup?,savedInstanceState: Bundle?): View? {_binding = getViewBinding(inflater, container)return _binding.root}protected abstract fun getViewBinding(inflater: LayoutInflater, container: ViewGroup?): T
}class FirstFragment : BaseFragment<FragmentFirstBinding>() {override fun onViewCreated(view: View, savedInstanceState: Bundle?) {super.onViewCreated(view, savedInstanceState)binding.textView.text = "这是FirstFragment"}override fun getViewBinding(inflater: LayoutInflater,container: ViewGroup?) = FragmentFirstBinding.inflate(inflater, container, false)
}

RecyclerView ViewHolder 使用

方式一：

lass MyItemRecyclerViewAdapter(private val values: List<PlaceholderItem>
) : RecyclerView.Adapter<MyItemRecyclerViewAdapter.ViewHolder>() {override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {return ViewHolder(FragmentItemBinding.inflate(LayoutInflater.from(parent.context),parent,false))}override fun onBindViewHolder(holder: ViewHolder, position: Int) {val item = values[position]holder.idView.text = item.idholder.contentView.text = item.content}override fun getItemCount(): Int = values.sizeclass ViewHolder(binding: FragmentItemBinding) : RecyclerView.ViewHolder(binding.root) {val idView: TextView = binding.itemNumberval contentView: TextView = binding.content}}

方式二：

class MyItemRecyclerViewAdapter(private val values: List<PlaceholderItem>
) : RecyclerView.Adapter<MyItemRecyclerViewAdapter.ViewHolder>() {override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {return ViewHolder(FragmentItemBinding.inflate(LayoutInflater.from(parent.context),parent,false))}override fun onBindViewHolder(holder: ViewHolder, position: Int) {val item = values[position]holder.binding.itemNumber.text = item.id}override fun getItemCount(): Int = values.sizeclass ViewHolder(val binding: FragmentItemBinding) : RecyclerView.ViewHolder(binding.root)}

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