Android ViewBinding 使用详解

什么是View Binding

官方文档

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

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android {
        ...
        viewBinding {
            enabled = 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：

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<?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">

    <TextView
        android: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.

• 如何在Activity中设置绑定，请在 Activity 的 onCreate() 方法中执行以下步骤：

  1. 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。
  2. 通过调用 getRoot() 方法或使用 Kotlin 属性语法获取对根视图的引用。
  3. 将根视图传递到 setContentView()，使其成为屏幕上的活动视图。

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  //activity_main.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"> <Button android:id="@+id/button" android:layout_width="100dp" android:layout_height="40dp" android:text="这是按钮" app:layout_constraintBottom_toBottomOf="parent" app:layout_constraintLeft_toLeftOf="parent" app:layout_constraintRight_toRightOf="parent" app:layout_constraintTop_toBottomOf="@+id/textView" /> <TextView android:id="@+id/textView" android:layout_width="100dp" android:layout_height="40dp" android:gravity="center" android:text="Hello World!" 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>

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  //MainActivity.java public class MainActivity extends AppCompatActivity { private ActivityMainBinding binding; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); //关键代码 binding = ActivityMainBinding.inflate(getLayoutInflater()); View rootView = binding.getRoot(); setContentView(rootView); //如何使用 binding.textView.setText("这是修改的"); binding.button.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View view) { Log.d("Main","点击了按钮"); } }); } }

• 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。)

  1. 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。

  2. 通过调用 getRoot() 方法或使用 Kotlin 属性语法获取对根视图的引用。

  3. 从 onCreateView() 方法返回根视图，使其成为屏幕上的活动视图。

     如果view已经创建可以在onViewCreated()中使用bind(view)方法。官方示例

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  //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"> <Button android: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" /> <TextView android: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>

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

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  //在onViewCreated中使用View Binding public class MyFragment extends Fragment { private FragmentMyBinding binding; public MyFragment() { } @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); } @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { return inflater.inflate(R.layout.fragment_my,container,false); } @Override public 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() { @Override public void onClick(View view) { Log.d("Fragment", "点击了按钮"); } }); } @Override public void onDestroy() { super.onDestroy(); binding = null; }

  使用View Binding 写的基类

  1. 通过反射的方式

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  //Java public class BaseActivity<T extends ViewBinding> extends AppCompatActivity { protected T viewBinding; @Override protected 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> { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); viewBinding.button.setText("这是 MainActivity ViewBinding"); viewBinding.button.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View view) { Log.d("MainView","点击按钮"); } }); } }

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

class MainActivity : BaseActivity<ActivityMainBinding>() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding.textView.text = "这是MainActivity"
    }
}

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//Java
public class BaseFragment<T extends ViewBinding> extends Fragment {
    protected T viewBinding;

    @Nullable
    @Override
    public 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>{
    @Override
    public 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() {
            @Override
            public void onClick(View view) {
                Log.d("MainView","点击按钮");
            }
        });
    }
}

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//Kotlin
open class BaseFragment<T:ViewBinding>:Fragment(){
    lateinit var binding: T
    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        val type = javaClass.genericSuperclass as ParameterizedType
        val 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 T
        return 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

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abstract class BaseActivity<T : ViewBinding> : AppCompatActivity() {
    private lateinit var _binding: T
    protected 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)
}

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abstract class BaseFragment<T : ViewBinding> : Fragment() {
    private lateinit var _binding: T
    protected 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)
}