Butterknife源码分析与原理简述

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/a820703048/article/details/81206337

一、背景

Butterknife 作为一款几乎每一个Android开发都会用到的常用开源组件，可以绑定android视图和事件回调到字段和方法，它通过使用注解处理并生成模版代码，为你绑定android视图中到字段和方法。如此有用到第三方开源组件，我们有必要去了解它到实现流程。

二、原理解析

这里我们来看常用的注解BindView

@Retention(Class)表明@BindView采用的是编译时注解

@Target(FIELD)则表明它应用于成员变量

接下来我们写一个很简单的例子，后面将会用到此代码

public class HelloActivity extends Activity {
    @BindView(R.id.tv_hello)
    TextView mHelloTv;
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_hello);
        ButterKnife.bind(this);
    }
}

butterknife的原理主要分为三个部分来介绍，主要为：注解生成模板代码分析、butterknife.bind()方法分析、生成的模板类代码分析。butterknife注册的注解器为ButterKnifeProcessor，源码在在butterknife-compiler工程下

@AutoService(Processor.class)
public final class ButterKnifeProcessor extends AbstractProcessor {
  ...
  @Override public boolean process(Set<? extends TypeElement> elements, RoundEnvironment env) {
    Map<TypeElement, BindingSet> bindingMap = findAndParseTargets(env);//1
    for (Map.Entry<TypeElement, BindingSet> entry : bindingMap.entrySet()) {
      TypeElement typeElement = entry.getKey();
      BindingSet binding = entry.getValue();//8
      JavaFile javaFile = binding.brewJava(sdk, debuggable);
      try {
        javaFile.writeTo(filer);
      } catch (IOException e) {
        error(typeElement, "Unable to write binding for type %s: %s", typeElement, e.getMessage());
      }
    }
    return false;
  }
  ...
}

先来看注释1处调用的findAndParseTargets方法，顾名思义此方法为查找并解析目标注解，源码如下：

private Map<TypeElement, BindingSet> findAndParseTargets(RoundEnvironment env) {
    Map<TypeElement, BindingSet.Builder> builderMap = new LinkedHashMap<>();
    Set<TypeElement> erasedTargetNames = new LinkedHashSet<>();
    scanForRClasses(env);
    ...
    //Process each @BindView element.
    for (Element element : env.getElementsAnnotatedWith(BindView.class)) {
      // we don't SuperficialValidation.validateElement(element)
      // so that an unresolved View type can be generated by later processing rounds
      try {
        parseBindView(element, builderMap, erasedTargetNames); //2
      } catch (Exception e) {
        logParsingError(element, BindView.class, e);
      }
    }
    ...
    return bindingMap;
}

接着查看注释2处parseBindView方法：

private void parseBindView(Element element, Map<TypeElement, BindingSet.Builder> builderMap,
      Set<TypeElement> erasedTargetNames) {
      TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();
      ... //非本文重点略掉。此处主要为一些限定性验证，（如元素修饰符不能为private,static、元素包含类型不能为非Class类型、包名不能为java. android.等）。
      // Assemble information on the field.
      String name = element.getSimpleName().toString();
      int[] ids = element.getAnnotation(BindViews.class).value();
      BindingSet.Builder builder = getOrCreateBindingBuilder(builderMap, enclosingElement);//3
      builder.addFieldCollection(new FieldCollectionViewBinding(name, type, kind, idVars,   required));
}

来看注释3处,如下：

private BindingSet.Builder getOrCreateBindingBuilder(
      Map<TypeElement, BindingSet.Builder> builderMap, TypeElement enclosingElement) {
    BindingSet.Builder builder = builderMap.get(enclosingElement);
    if (builder == null) {
      builder = BindingSet.newBuilder(enclosingElement);//4
      builderMap.put(enclosingElement, builder);
    }
    return builder;
  }

来看注释3处,如下：

private BindingSet.Builder getOrCreateBindingBuilder(
      Map<TypeElement, BindingSet.Builder> builderMap, TypeElement enclosingElement) {
    BindingSet.Builder builder = builderMap.get(enclosingElement);
    if (builder == null) {
      builder = BindingSet.newBuilder(enclosingElement);//4
      builderMap.put(enclosingElement, builder);
    }
    return builder;
  }

顾名思义获取或创建BindingBuilder,从builderMap中获取BindingSet.Builder如果有则return, 如果没有则创建并放入Map缓存中。那么BindingSet.Builder存储的是什么的？接下来我们看注释4处builder对象的创建，如下：

static Builder newBuilder(TypeElement enclosingElement) {
    TypeMirror typeMirror = enclosingElement.asType();

    boolean isView = isSubtypeOfType(typeMirror, VIEW_TYPE);
    boolean isActivity = isSubtypeOfType(typeMirror, ACTIVITY_TYPE);
    boolean isDialog = isSubtypeOfType(typeMirror, DIALOG_TYPE);

    TypeName targetType = TypeName.get(typeMirror);
    if (targetType instanceof ParameterizedTypeName) {
      targetType = ((ParameterizedTypeName) targetType).rawType;
    }

    String packageName = getPackage(enclosingElement).getQualifiedName().toString();
    String className = enclosingElement.getQualifiedName().toString().substring(
        packageName.length() + 1).replace('.', '$');
    ClassName bindingClassName = ClassName.get(packageName, className + "_ViewBinding");//5

    boolean isFinal = enclosingElement.getModifiers().contains(Modifier.FINAL);
    return new Builder(targetType, bindingClassName, isFinal, isView, isActivity, isDialog);//6
  }

里面可以看到实际上它是创建了一个BindingSet对象。而这个BindingSet对象里面存储着生成类的名称以及注解类名称等。

接下来findAndParseTargets会把此BindingSet对象返回来，到ButterKnifeProcessor类的process方法， 重新贴一下代码:

@AutoService(Processor.class)
public final class ButterKnifeProcessor extends AbstractProcessor {
  ...
  @Override public boolean process(Set<? extends TypeElement> elements, RoundEnvironment env) {
    Map<TypeElement, BindingSet> bindingMap = findAndParseTargets(env);//1
    for (Map.Entry<TypeElement, BindingSet> entry : bindingMap.entrySet()) {
      TypeElement typeElement = entry.getKey();
      BindingSet binding = entry.getValue();//8
      JavaFile javaFile = binding.brewJava(sdk, debuggable);
      try {
        javaFile.writeTo(filer);
      } catch (IOException e) {
        error(typeElement, "Unable to write binding for type %s: %s", typeElement, e.getMessage());
      }
    }
    return false;
  }
  ...
}

注释8获取到了上面生成的BindingSet对象。

JavaFile javaFile = binding.brewJava(sdk, debuggable);
javaFile.writeTo(filer);

这两行代码为javapoet的范畴，其功能根据返回的binding对象配置信息生成我们需要用到的模板类代码，到此第一部分注解生成模板代码的源码就分析完了。

butterknife.bind()

来看butterknife工程下butterknife包下的ButterKnife.java类bind方法。

public static Unbinder bind(@NonNull Activity target) {
    View sourceView = target.getWindow().getDecorView();
    return createBinding(target, sourceView);
  }

此方法有很多重载的方法， 这里我们只看绑定activity场景的重载方法。获取到activity中的decorview,将activity和decorview传入createBinding()方法。

private static Unbinder createBinding(@NonNull Object target, @NonNull View source) {
    Class<?> targetClass = target.getClass();
    if (debug) Log.d(TAG, "Looking up binding for " + targetClass.getName());
    Constructor<? extends Unbinder> constructor = findBindingConstructorForClass(targetClass);//1

    if (constructor == null) {
      return Unbinder.EMPTY;
    }

    //noinspection TryWithIdenticalCatches Resolves to API 19+ only type.
    try {
      return constructor.newInstance(target, source);//5
    ...
 }

注释1 进入findBindingConstructorForClass并传入了activity为参数，方法如下：

private static Constructor<? extends Unbinder> findBindingConstructorForClass(Class<?> cls) {
    Constructor<? extends Unbinder> bindingCtor = BINDINGS.get(cls);//4
    if (bindingCtor != null) {
      if (debug) Log.d(TAG, "HIT: Cached in binding map.");
      return bindingCtor;
    }
    String clsName = cls.getName();
    if (clsName.startsWith("android.") || clsName.startsWith("java.")) {
      if (debug) Log.d(TAG, "MISS: Reached framework class. Abandoning search.");
      return null;
    }
    try {
      Class<?> bindingClass = cls.getClassLoader().loadClass(clsName + "_ViewBinding");//2
      //noinspection unchecked
      bindingCtor = (Constructor<? extends Unbinder>) bindingClass.getConstructor(cls, View.class);
      if (debug) Log.d(TAG, "HIT: Loaded binding class and constructor.");
    } catch (ClassNotFoundException e) {
      if (debug) Log.d(TAG, "Not found. Trying superclass " + cls.getSuperclass().getName());
      bindingCtor = findBindingConstructorForClass(cls.getSuperclass());
    } catch (NoSuchMethodException e) {
      throw new RuntimeException("Unable to find binding constructor for " + clsName, e);
    }
    BINDINGS.put(cls, bindingCtor); //3
    return bindingCtor;
  }

先来看注释2处通过类加载器加载模板类,然后获取到它的构造方法，此处用到了反射会对性能有一定影响，为了优化性能看注解3会把构造方法加入到缓存map中，而注释4也就是方法开始的地方会对缓存做判断，如果有数据的话就直接返回了。createBinding ()方法 注释5处根据构造器创建xx_ViewBinding模板类对象，我们例子里面的模板类ming成为“HelloActivity_ViewBinding”。

板类代码分析

接下来看HelloActivity_ViewBinding类，代码如下：

public class HelloActivity_ViewBinding implements Unbinder {
  private HelloActivity target;

  @UiThread
  public HelloActivity_ViewBinding(HelloActivity target) {
    this(target, target.getWindow().getDecorView());
  }

  @UiThread
  public HelloActivity_ViewBinding(HelloActivity target, View source) {
    this.target = target;

    target.mHelloTv = Utils.findRequiredViewAsType(source, R.id.tv_hello, "field 'mHelloTv'", TextView.class);//1
  }

  @Override
  @CallSuper
  public void unbind() {
    HelloActivity target = this.target;
    if (target == null) throw new IllegalStateException("Bindings already cleared.");
    this.target = null;

    target.mHelloTv = null;
  }
}

接下来进入注释1 findRequiredViewAsType方法

public static <T> T findRequiredViewAsType(View source, @IdRes int id, String who,
      Class<T> cls) {
    View view = findRequiredView(source, id, who);//2
    return castView(view, id, who, cls); //3
  }

继续看注释2

public static View findRequiredView(View source, @IdRes int id, String who) {
    View view = source.findViewById(id);
    if (view != null) {
      return view;
    }
    String name = getResourceEntryName(source, id);
    ...
}

此处看到了我们熟悉的View view = source.findViewById(id);。

注释3return castView(view, id, who, cls); 此处将view强制转型为cls类型。cls类型也就是下面的TextView.class。

target.mHelloTv = Utils.findRequiredViewAsType(source, R.id.tv_hello, “field ‘mHelloTv’”, TextView.class);此处的TextView.class。

将mHelloTv赋值给，target（也就是HelloActivity）。

至此我们的原理简单的分析完了。

哈哈，断断续续几个小时的时间又重新温习了一下butterknife原理。