Android深入学习之LayoutInflater类和ViewBinding

在build.gradle(Module)中添加viewBinding元素后，Android会自动给模块中的每个XML布局文件生成一个相应的Binding类，该Binding类名称为XML布局文件驼峰式大写+Binding后缀。以如下所示的activity_welcome.xml文件为例，对应的ActivityWelcomeBinding.java的源代码如下所示。

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/welcome_container"
    android:orientation="vertical"
    android:gravity="center"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Please choose an item"
        android:textSize="30dp"
        android:gravity="center"/>
    <LinearLayout
        android:id="@+id/button_container"
        android:layout_marginTop="30dp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="vertical">
        <Button
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="GEOGRAPHY"
            android:id="@+id/btn_geo"/>
        <Button
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="MATHEMATICS"
            android:id="@+id/btn_math"/>
    </LinearLayout>
</LinearLayout>

// Generated by view binder compiler. Do not edit!
package com.larissa.android.quiz.databinding;

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.LinearLayout;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.viewbinding.ViewBinding;
import androidx.viewbinding.ViewBindings;
import com.larissa.android.quiz.R;
import java.lang.NullPointerException;
import java.lang.Override;
import java.lang.String;

// ActivityWelcomeBinding是final类，实现ViewBinding接口
public final class ActivityWelcomeBinding implements ViewBinding {
  // XML布局文件中的根元素被定义为一个私有的常量字段
  @NonNull
  private final LinearLayout rootView;
  // XML布局文件中有id的控件被定义为公有的常量字段
  @NonNull
  public final Button btnGeo;

  @NonNull
  public final Button btnMath;

  @NonNull
  public final LinearLayout buttonContainer;

  @NonNull
  public final LinearLayout welcomeContainer;

  // ActivityWelcomeBinding的构造函数为私有的，即ActivityWelcomeBinding不能在类的外部通过new实例化
  // 构造函数的参数为上方所定义的常量字段
  private ActivityWelcomeBinding(@NonNull LinearLayout rootView, @NonNull Button btnGeo,
      @NonNull Button btnMath, @NonNull LinearLayout buttonContainer,
      @NonNull LinearLayout welcomeContainer) {
    this.rootView = rootView;
    this.btnGeo = btnGeo;
    this.btnMath = btnMath;
    this.buttonContainer = buttonContainer;
    this.welcomeContainer = welcomeContainer;
  }

  // 实现ViewBinding接口中定义的getRoot()方法，其实这就是rootView字段的getter方法
  @Override
  @NonNull
  public LinearLayout getRoot() {
    return rootView;
  }

  // 返回ActivityWelcomeBinding实例的两个inflate()方法。可见,inflate(LayoutInflater)方法
  // 其实是调用了inflate(LayoutInflater, ViewGroup, boolean)方法
  @NonNull
  public static ActivityWelcomeBinding inflate(@NonNull LayoutInflater inflater) {
    return inflate(inflater, null, false);
  }

  @NonNull
  public static ActivityWelcomeBinding inflate(@NonNull LayoutInflater inflater,
      @Nullable ViewGroup parent, boolean attachToParent) {
    // 调用LayoutInflater.inflate()方法，返回解析XML布局文件得到的View对象/View树
    View root = inflater.inflate(R.layout.activity_welcome, parent, false);
    // 如果attachToParent为true，则将View对象贴parent上
    if (attachToParent) {
      parent.addView(root);
    }
    return bind(root);
  }

  // bind()方法其实是获取ActivityWelcomeBinding实例的最终方法
  @NonNull
  public static ActivityWelcomeBinding 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: {
      id = R.id.btn_geo;
      // 从rootView树中查找指定id的控件
      Button btnGeo = ViewBindings.findChildViewById(rootView, id);
      // 如果从rootView树中没有找到指定id的控件，则跳出missingId label
      if (btnGeo == null) {
        break missingId;
      }

      id = R.id.btn_math;
      Button btnMath = ViewBindings.findChildViewById(rootView, id);
      if (btnMath == null) {
        break missingId;
      }

      id = R.id.button_container;
      LinearLayout buttonContainer = ViewBindings.findChildViewById(rootView, id);
      if (buttonContainer == null) {
        break missingId;
      }

      LinearLayout welcomeContainer = (LinearLayout) rootView;
      // 所有的控件都找到了，则实例化ActivityWelcomeBinding对象
      return new ActivityWelcomeBinding((LinearLayout) rootView, btnGeo, btnMath, buttonContainer,
          welcomeContainer);
    }
    // 有控件没有找到的话，则抛出错误
    String missingId = rootView.getResources().getResourceName(id);
    throw new NullPointerException("Missing required view with ID: ".concat(missingId));
  }
}

---

可见，在ActivityWelcomeBinding中用到了LayoutInflater类。LayoutInflater类中也定义了inflate()方法，该方法的主要作用就是将XML布局文件解析为相应的View对象或者称为View树，可实现动态换肤、视图转换、属性转换等需求。activity_welcome.xml对应的View树如下图所示。

LayoutInflater是一个抽象类，在Activity中获得与其相关联的LayoutInflater实例一般是通过调用Activity.getLayoutInflater()方法。关于Activity.getLayoutInflater()方法是如何得到与Activity相关联的LayoutInflater的实例请参见：https://www.jianshu.com/p/a4dd4892c84e

LayoutInflater类中定义了4个inflate()方法：

1.public View inflate (int resource, ViewGroup root)

2.public View inflate(XmlPullParser parser, ViewGroup root)

3.public View inflate(XmlPullParser parser, ViewGroup root, boolean attachToRoot)

4.public View inflate(int resource, ViewGroup root, boolean attachToRoot)

通过源代码可以发现，这些inflate()其实最终都调用了第3个inflate()方法。本文并不打算对LayoutInflater.inflate()方法的业务逻辑进行详细分析，只是通过源代码其实可以发现，在inflate()中其实会调用到LayoutInflater.createView()方法，而在createView()方法中其实是用反射reflection来动态地生成View对象的。

另外，值得一提的是，在第3个和第4个inflate()方法中,当attachToRoot为true时，返回的是ViewGroup root对象；当attachToRoot为false时，返回的是解析resource得到的View对象。这里可以看个例子：

public class WelcomeActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_welcome);
        View rootView=getLayoutInflater().inflate(R.layout.activity_welcome,null,false);
        getAllViews(rootView);
    }
    private void getAllViews(View view){
        if(view instanceof ViewGroup){
            Log.d(TAG,"ViewGroup:".concat(view.getClass().getTypeName()));
            int count=((ViewGroup) view).getChildCount();
            for(int i=0;i<count;i++){
                getAllViews(((ViewGroup) view).getChildAt(i));
            }
        }
        else{
            Log.d(TAG,"View:".concat(view.getClass().getTypeName()));
        }
    }
}

当ViewGroup为null，且attachToRoot为false时，日志输出为：

而当指定了一个FrameLayout作为ViewGroup，且attachToRoot为true时，日志输出为：

public class WelcomeActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_welcome);
        FrameLayout viewGroup=new FrameLayout(this);
        View rootView=getLayoutInflater().inflate(R.layout.activity_welcome,viewGroup,true);
        getAllViews(rootView);
    }
    private void getAllViews(View view){
        if(view instanceof ViewGroup){
            Log.d(TAG,"ViewGroup:".concat(view.getClass().getTypeName()));
            int count=((ViewGroup) view).getChildCount();
            for(int i=0;i<count;i++){
                getAllViews(((ViewGroup) view).getChildAt(i));
            }
        }
        else{
            Log.d(TAG,"View:".concat(view.getClass().getTypeName()));
        }
    }
}

故而可以发现，ViewBinding.inflate()和LayoutInflater.inflate()是不同的。在ViewBinding.inflate()中，即便指定了ViewGroup且attachToParent为true，但是返回的仍然是解析XML布局文件得到的View树。只不过当attachToParent为true时，解析XML布局文件得到的View树被贴在了ViewGroup parent(形参)上，那么对应的实参也会被修改。

---

通过分析ActivityWelcomeBinding源代码可知，在WelcomeActivity中使用ViewBinding首先是调用inflate()方法得到ActivityWelcomeBinding的实例，然后调用ActivityWelcomeBinding.getRoot()方法得到rootView，并通过setContentView()注入rootView。进而在WelcomeActivity中就可以通过ActivityWelcomeBinding的实例.属性名的方式得到相应的控件并使用控件。如下方代码所示：

public class WelcomeActivity extends AppCompatActivity {
    public static final String CLASS="class";
    public static final int GEOGRAPHY=0;
    public static final int MATHEMATICS=1;
    private final String TAG="Welcome";
    private ActivityWelcomeBinding binding;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        getSupportActionBar().setTitle("Welcome to Quiz");
        binding=ActivityWelcomeBinding.inflate(getLayoutInflater());
        setContentView(binding.getRoot());
        // btnGeo是ActivityWelcomeBinding中定义的公有字段
        binding.btnGeo.setOnClickListener(v->btnClick(GEOGRAPHY));
        binding.btnMath.setOnClickListener(v->btnClick(MATHEMATICS));
    }

    private void btnClick(int flag){
        Intent intent= new Intent(this,MainActivity.class);
        intent.putExtra(CLASS,flag);
        startActivity(intent);
    }
}

 那么如果代码改成下方这样，WelcomeActivity的屏幕则如下图所示。ActivityWelcomeBinding.inflate()返回的仍然是ActivityWelcomeBinding的实例，但是改变了FrameLayout parent这个实参。

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        getSupportActionBar().setTitle("Welcome to Quiz");
        FrameLayout parent=new FrameLayout(this);
        TextView msg=new TextView(this);
        msg.setText("FrameLayout is the parent");
        msg.setTextSize(30);
        parent.addView(msg);
        binding=ActivityWelcomeBinding.inflate(getLayoutInflater(),parent,true);
        setContentView(parent);
        // btnGeo是ActivityWelcomeBinding中定义的公有字段
        binding.btnGeo.setOnClickListener(v->btnClick(GEOGRAPHY));
        binding.btnMath.setOnClickListener(v->btnClick(MATHEMATICS));
    }