在 Kotlin 1.4.20-M2 中，JetBrains 废弃了 Kotlin Android Extensions 编译插件。

不要与Data Binding混淆。View Binding是一种功能，它允许您更容易地编写与视图交互的代码。
一旦在一个模块中启用了View Binding，它就会为该模块中存在的每个 XML 布局文件生成一个绑定类。

• 绑定类的实例包含对相应布局中具有ID的所有VIew的直接引用。
• View Binding对于在多个配置中定义的布局来说是Null-safe的。
• View Binding将检测视图是否只存在于某些配置中，并创建一个@Nullable属性。
• View Binding适用于Java和Kotlin。

禁用某个布局文件使用 ViewBinding，则需要在布局文件的根节点中添加 tools：viewBindingIgnore = "true"。

环境配置

1. Android Studio 3.6 及以上；
2. Android Gradle 插件 3.6.0 及以上；
3. Gradle 版本 5.6.4 及以上；
4. 在模块的 build.gradle 文件中添加 android { buildFeatures { viewBinding = true } }，android { viewBinding.enabled = true } 方式已废弃。
   DSL element 'android.viewBinding.enabled' is obsolete and has been replaced with 'android.buildFeatures.viewBinding'.

GradlePlugin 和 Gradle 要对应，否则会不兼容，出现同步失败的问题。查看 Android Gradle 插件版本说明。

使用

class MainActivity : AppCompatActivity() {
    private lateinit var binding: ActivityMainBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        binding.textView.text = "text"
    }
}

Activity 对应的 activity_main.xml 文件如下：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

</LinearLayout>

生成的 ActivityMainBinding 在 module/build/generated/data_binding_base_class_source_out/${buildTypes}/out/${包名}/databinding 目录，源码如下：

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

  @NonNull
  public final TextView textView;

  private ActivityMainBinding(@NonNull LinearLayout rootView, @NonNull TextView textView) {
    this.rootView = rootView;
    this.textView = textView;
  }

  @Override
  @NonNull
  public LinearLayout 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.
    int id;
    missingId: {
      id = R.id.textView;
      TextView textView = rootView.findViewById(id);
      if (textView == null) {
        break missingId;
      }

      return new ActivityMainBinding((LinearLayout) rootView, textView);
    }
    String missingId = rootView.getResources().getResourceName(id);
    throw new NullPointerException("Missing required view with ID: ".concat(missingId));
  }
}

Binding 类生成原理

由于 ViewBinding 类是通过 GradlePlugin 支持的，编译时包含有 DataBinding 相关的 task Task :app:dataBindingGenBaseClassesDebug。

com.android.tools.build:gradle:4.1.3 依赖 androidx.databinding:databinding-compiler-common:4.1.3，下载 databinding-compiler-common jar 包，解压到文件夹后使用 AS 打开查看源码。

解析 xml 文件

/**
 * Processes the layout XML, stripping the binding attributes and elements
 * and writes the information into an annotated class file for the annotation
 * processor to work with.
 */
public class LayoutXmlProcessor {
  public boolean processResources(ResourceInput input, boolean isViewBindingEnabled,
        boolean isDataBindingEnabled) {

        ProcessFileCallback callback = new ProcessFileCallback() {
            @Override
            public void processLayoutFile(File file) {
                processSingleFile(...);
            }

            // 其他回调方法
        };

        if (input.isIncremental()) {
            processIncrementalInputFiles(input, callback);
        } else {
            processAllInputFiles(input, callback);
        }
    }

    private static void processAllInputFiles(ResourceInput input, ProcessFileCallback callback) {
        for (File firstLevel : input.getRootInputFolder().listFiles()) {
            if (firstLevel.isDirectory()) {
                // 判断文件名是否已 layout 开头
                if (LAYOUT_FOLDER_FILTER.accept(firstLevel, firstLevel.getName())) {
                    callback.processLayoutFolder(firstLevel);
                    // 过滤以 “.xml” 结尾的文件
                    for (File xmlFile : firstLevel.listFiles(XML_FILE_FILTER)) {
                        callback.processLayoutFile(xmlFile);
                    }
                }
            }
        }
    }

    public boolean processSingleFile(RelativizableFile input, File output,
                        boolean isViewBindingEnabled,
                        boolean isDataBindingEnabled) {
        // 解析 xml
        final ResourceBundle.LayoutFileBundle bindingLayout = LayoutFileParser.parseXml(input, output, ...);
        // 缓存解析的 xml
        mResourceBundle.addLayoutBundle(bindingLayout, true);
    }
}

public final class LayoutFileParser {

    public static ResourceBundle.LayoutFileBundle parseXml(final RelativizableFile input,
            final File outputFile, ...) {
        return parseOriginalXml(
                RelativizableFile.fromAbsoluteFile(originalFile, input.getBaseDir()),...);
    }

    private static ResourceBundle.LayoutFileBundle parseOriginalXml(
            final RelativizableFile originalFile, ...) {
        File original = originalFile.getAbsoluteFile();
        // 是否是 DataBinding
        if (isBindingData) {
            data = getDataNode(root);
            rootView = getViewNode(original, root);
        } else if (isViewBindingEnabled) {
            // 排除不生成 Binding 类的 xml
            if ("true".equalsIgnoreCase(attributeMap(root).get("tools:viewBindingIgnore"))) {
                L.d("Ignoring %s for view binding", originalFile);
                return null;
            }
            data = null;
            rootView = root;
        } else {
            return null;
        }

        // 判断是否是 merge 节点
        boolean isMerge = "merge".equals(rootView.elmName.getText());
        if (isBindingData && isMerge && !filter(rootView, "include").isEmpty()) {
            L.e(ErrorMessages.INCLUDE_INSIDE_MERGE);
            return null;
        }

        String rootViewType = getViewName(rootView);
        // 获取 View ID
        String rootViewId = attributeMap(rootView).get("android:id");
        // 创建 LayoutFileBundle 对象
        ResourceBundle.LayoutFileBundle bundle =
            new ResourceBundle.LayoutFileBundle(originalFile, ...);
        // ViewBinding data == null，不会解析
        parseData(original, data, bundle);
        // 解析表达式
        parseExpressions(newTag, rootView, isMerge, bundle);

        return bundle;
    }

    private static String getViewName(XMLParser.ElementContext elm) {
        String viewName = elm.elmName.getText();
        if ("view".equals(viewName)) {
            String classNode = attributeMap(elm).get("class");
            if (Strings.isNullOrEmpty(classNode)) {
                L.e("No class attribute for 'view' node");
            }
            return classNode;
        }
        if ("include".equals(viewName) && !XmlEditor.hasExpressionAttributes(elm)) {
            return "android.view.View";
        }
        if ("fragment".equals(viewName)) {
            return "android.view.View";
        }
        return viewName;
    }
}

生成 xml 文件

public class LayoutXmlProcessor {
    public void writeLayoutInfoFiles(File xmlOutDir, JavaFileWriter writer) {
        for (ResourceBundle.LayoutFileBundle layout : mResourceBundle
                .getAllLayoutFileBundlesInSource()) {
            writeXmlFile(writer, xmlOutDir, layout);
        }
    }

    private void writeXmlFile(JavaFileWriter writer, File xmlOutDir,
            ResourceBundle.LayoutFileBundle layout) {
        // layout.getFileName() + '-' + layout.getDirectory() + ".xml
        // 如 activity_main.xml -> activity_main-layout.xml
        String filename = generateExportFileName(layout);
        writer.writeToFile(new File(xmlOutDir, filename), layout.toXML());
    }
}

生成的 xml 文件路径为 module/build/intermediates/data_binding_layout_info_type_merge/${buildTypes}/out/activity_main-layout.xml，描述了原始布局文件的相关信息。

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<Layout
    directory="layout"
    filePath="app\src\main\res\layout\activity_main.xml"
    isBindingData="false"
    isMerge="false"
    layout="activity_main"
    modulePackage="com.king.app.workhelper"
    rootNodeType="android.widget.LinearLayout">

    <Targets>

        <Target
            tag="layout/activity_main_0"
            view="LinearLayout">

            <Expressions />

            <location
                endLine="11"
                endOffset="14"
                startLine="1"
                startOffset="0" />
        </Target>

        <Target
            id="@+id/textView"
            view="TextView">

            <Expressions />

            <location
                endLine="9"
                endOffset="46"
                startLine="6"
                startOffset="4" />
        </Target>
    </Targets>
</Layout>

生成 Binding 类

class BaseDataBinder(val input : LayoutInfoInput) {
    fun generateAll(writer : JavaFileWriter) {
        input.invalidatedClasses.forEach {
            writer.deleteFile(it)
        }

        val useAndroidX = input.args.useAndroidX
        val libTypes = LibTypes(useAndroidX = useAndroidX)

        // 获取所有的 LayoutFileBundle，并根据文件名进行分组排序
        val layoutBindings = resourceBundle.allLayoutFileBundlesInSource
            .groupBy(LayoutFileBundle::getFileName)

        // 遍历 layoutBindings
        layoutBindings.forEach { layoutName, variations ->
            // 将 LayoutFileBundle 信息包装成 BaseLayoutModel
            val layoutModel = BaseLayoutModel(variations)

            val javaFile: JavaFile
            val classInfo: GenClassInfoLog.GenClass
            // 处理 DataBinding
            if (variations.first().isBindingData) {
                //...
            } else {
                // 处理 ViewBinding
                // 创建 ViewBinder 对象
                val viewBinder = layoutModel.toViewBinder()
                // 通过 ViewBinder 扩展方法，调用到 ViewBinderGenerateJava#create() 方法
                javaFile = viewBinder.toJavaFile(useLegacyAnnotations = !useAndroidX)
                classInfo = viewBinder.generatedClassInfo()
            }

            writer.writeToFile(javaFile)
        }
    }
}

// ViewBinderGenerateJava.kt

fun ViewBinder.toJavaFile(useLegacyAnnotations: Boolean = false) =
    JavaFileGenerator(this, useLegacyAnnotations).create()

private class JavaFileGenerator(
    private val binder: ViewBinder,
    private val useLegacyAnnotations: Boolean) {
    
    // 使用了 com.squareup.javapoet 库
    fun create() = javaFile(binder.generatedTypeName.packageName(), typeSpec()) {
        addFileComment("Generated by view binder compiler. Do not edit!")
    }

    private fun typeSpec() = classSpec(binder.generatedTypeName) {
        addModifiers(PUBLIC, FINAL)

        val viewBindingPackage = if (useLegacyAnnotations) "android" else "androidx"
        addSuperinterface(ClassName.get("$viewBindingPackage.viewbinding", "ViewBinding"))

        // 生成 rootView 字段
        addField(rootViewField())
        // 生成 View 字段
        addFields(bindingFields())

        // 生成构造方法
        addMethod(constructor())
        // 生成获取 rootView 的方法
        addMethod(rootViewGetter())

        if (binder.rootNode is RootNode.Merge) {
            addMethod(mergeInflate())
        } else {
            // 生成 inflate(LayoutInflater inflater) 方法
            addMethod(oneParamInflate())
            // 生成 inflate(LayoutInflater inflater, ViewGroup parent, boolean attachToParent) 方法
            addMethod(threeParamInflate())
        }

        addMethod(bind())
    }
}

1. 遍历收集的 Set<LayoutFileBundle>;
2. 根据 LayoutFileBundle 生成 ViewBinder 对象；
3. ViewBinder 传参给 JavaFileGenerator，通过 JavaPoet(com.squareup.javapoet) 库生成 Binding Class 文件。

参考

[1] ViewBinding - Developer
[2] 终于有人写：「新技术ViewBinding」 的本质了
[3] [译]深入研究ViewBinding 在 include, merge, adapter, fragment, activity 中使用
[4] hoc081098/ViewBindingDelegate