View Binding

895 阅读3分钟

View Binding 是Google推出的视图绑定组件,启动View Binding之后,系统会为我们添加每个xml布局文件都自动生成一个对应的绑定类。绑定类的实例包含对在相应布局中具有ID的所有的view的直接引用。下面文章会介绍使用,以及分析原理。

注意:View Binding只能在Android Studio3.6 Canary 11及更高版本中可用

一:依赖

在app的build.gradle文件里添加如下:

android {
    ...
    buildFeatures {
        viewBinding true
    }
}

二:根据布局文件自动生成XxxBinding类

创建一个布局,并且重新编译,比如布局文件为activity_main.xml。那么系统会自动在app->build->generated->data_binding_base_class_source_out->debug->out->包名->databinding 下自动创建一个ActivityMainBinding.java文件。
ActivityMainBinding.java的文件命名由来是activity_main 改成头峰的形式命名。代码如下:

<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=".ui.activity.MainActivity">

    <TextView
        android:id="@+id/tv_name"
        android:layout_width="wrap_content"
        android:layout_height="40dp"
        android:gravity="center"
        android:text="Hello World!"
        android:textColor="@color/colorPrimary"
        android:textSize="15sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/btn_commit"
        android:layout_width="wrap_content"
        android:layout_height="40dp"
        android:text="完成"
        android:textSize="15sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/tv_name" />

    <FrameLayout
        android:id="@+id/layout_content"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/btn_commit" />

</androidx.constraintlayout.widget.ConstraintLayout>

布局里最外层是ConstraintLayout,包含的View有 TextView,Button,FramLayout。来看看对应的自动生成的ActivityMainBinding.java类的代码

public final class ActivityMainBinding implements ViewBinding {
  @NonNull
  private final ConstraintLayout rootView;

  @NonNull
  public final Button btnCommit;

  @NonNull
  public final FrameLayout layoutContent;

  @NonNull
  public final TextView tvName;

  private ActivityMainBinding(@NonNull ConstraintLayout rootView, @NonNull Button btnCommit,
      @NonNull FrameLayout layoutContent, @NonNull TextView tvName) {
    this.rootView = rootView;
    this.btnCommit = btnCommit;
    this.layoutContent = layoutContent;
    this.tvName = tvName;
  }

  @Override
  @NonNull
  public ConstraintLayout getRoot() {
    return rootView;
  }

  @NonNull
  public static ActivityMainBinding inflate(@NonNull LayoutInflater inflater) {
    return inflate(inflater, null, false);
  }

  @NonNull
  public static ActivityMainBinding inflate(@NonNull LayoutInflater inflater,
      @Nullable ViewGroup parent, boolean attachToParent) {
    View root = inflater.inflate(R.layout.activity_main, parent, false);
    if (attachToParent) {
      parent.addView(root);
    }
    return bind(root);
  }

  @NonNull
  public static ActivityMainBinding 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.
    String missingId;
    missingId: {
      Button btnCommit = rootView.findViewById(R.id.btn_commit);
      if (btnCommit == null) {
        missingId = "btnCommit";
        break missingId;
      }
      FrameLayout layoutContent = rootView.findViewById(R.id.layout_content);
      if (layoutContent == null) {
        missingId = "layoutContent";
        break missingId;
      }
      TextView tvName = rootView.findViewById(R.id.tv_name);
      if (tvName == null) {
        missingId = "tvName";
        break missingId;
      }
      return new ActivityMainBinding((ConstraintLayout) rootView, btnCommit, layoutContent, tvName);
    }
    throw new NullPointerException("Missing required view with ID: ".concat(missingId));
  }
}
  • ActivityMainBinding里4个成员变量对应布局文件activity_main.xml里的4个View
  • ActivityMainBinding里构造器是传入4个控件
  • ActivityMainBinding的getRoot()方法返回的最外层控件
  • ActivityMainBinding的inflate方法是 View root = inflater.inflate(R.layout.activity_main, parent, false);根据activity_main布局创建出view,并执行bind方法
  • ActivityMainBinding的bind方法是fingViewById绑定控件,并生成ActivityMainBinding实例

三:在Activity中使用XXXBinding

上面我们已经自动生成了ActivitMainBinding类,看我们如何在MainActivity里使用它,如下:

class MainActivity : AppCompatActivity() ,View.OnClickListener{

    private lateinit var binding: ActivityMainBinding

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

        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        binding.tvName.text = "View Binding Text"
        binding.btnCommit.text = "View Binding Btn"
        binding.btnCommit.setOnClickListener(this)

        supportFragmentManager.beginTransaction().replace(R.id.layout_content,LoginFragment()).commitAllowingStateLoss()

    }

    override fun onClick(v: View?) {
        when(v){
            binding.btnCommit-> Toast.makeText(this,"click btn",Toast.LENGTH_SHORT).show()
        }
    }
}
  • binding = ActivityMainBinding.inflate(layoutInflater) 根据上面我们分析的ActivityMainBinding类的源码,可以知道inflate方法根据activity_main.xml去创建View,并把生成的View,通过构造器传入ActivityMainBinding,最终去创建出ActivityMainBinding实例
  • setContentView(binding.root) 把binding的最外层布局作为contentView
  • binding.tvName,binding.btnCommit 接着就可以通过binding去使用里面的view了

四:在Fragment中使用

新建一个fragment_login.xml布局

// fragment_login.xml的布局如下
<?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">

    <Button
        android:id="@+id/btn_login"
        android:layout_width="wrap_content"
        android:layout_height="40dp"
        android:gravity="center"
        android:text="登录"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

重新编译会自动帮我们生成FragmentMainBinding类

// 对应的自动生成的FragmentLoginBinding类
public final class FragmentLoginBinding implements ViewBinding {
  @NonNull
  private final ConstraintLayout rootView;

  @NonNull
  public final Button btnLogin;

  private FragmentLoginBinding(@NonNull ConstraintLayout rootView, @NonNull Button btnLogin) {
    this.rootView = rootView;
    this.btnLogin = btnLogin;
  }

  @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.
    String missingId;
    missingId: {
      Button btnLogin = rootView.findViewById(R.id.btn_login);
      if (btnLogin == null) {
        missingId = "btnLogin";
        break missingId;
      }
      return new FragmentLoginBinding((ConstraintLayout) rootView, btnLogin);
    }
    throw new NullPointerException("Missing required view with ID: ".concat(missingId));
  }
}

在Fragment中使用如下:

class LoginFragment :Fragment(),View.OnClickListener{

    private var loginBinding:FragmentLoginBinding?=null

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        loginBinding = FragmentLoginBinding.inflate(inflater,container,false)
        return loginBinding?.root
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        loginBinding?.btnLogin?.text = "登录完成"
        loginBinding?.btnLogin?.setOnClickListener(this)
    }

    override fun onClick(v: View?) {
        when(v){
            loginBinding?.btnLogin-> Toast.makeText(activity,"click login btn", Toast.LENGTH_SHORT).show()
        }
    }

    override fun onDestroyView() {
        loginBinding = null
        super.onDestroyView()
    }
}

我们新建了一个fragment_login.xml的布局,运行会自动帮我们生成FragmentLoginBinding类,在Fragment中使用,在onCreateView里调用FragmentLoginBinding.inflate(inflater,container,false),并把loginBinding的最外层布局返回。在onDestroyView里置空loginBinding。

五:不生成Binding类的配置

上面我们讲到,新建个布局,重新编译,会自动帮我们创建对应的Binding类,如果我们不想生成呢。那么只需要在跟布局添加上 tools:viewBindingIgnore="true" 即可。代码如下:

<FrameLayout
   tools:viewBindingIgnore="true">
</FrameLayout>