ButterKnife想必每一個Android開發(fā)者都或多或少使用過，它的功能強大之處就不用多說了。它的原理可以簡要概括為：編譯時注解（AbstractProcessor）+反射，網(wǎng)上已經(jīng)有很多ButterKnife源碼解析相關(guān)的文章了，閑暇之余將ButterKnife工程clone下來又翻了遍源碼，當(dāng)作學(xué)習(xí)筆記整理下。

ButterKnife使用了編譯時注解，入口就是ButterKnifeProcessor這個類，它繼承自AbstractProcessor，在跟進去ButterKnifeProcessor源碼前，我們先簡要概括下注解處理器的概念：

注解處理器（AbstractProcessor）是用來在編譯過程中掃描和處理注解的工具，我們在項目中可以為特定的注解注冊自己的注解處理器，生成.java 文件，但不能修改已經(jīng)存在的Java類（即不能向已有的類中添加方法）。而這些生成的Java文件，會同時與其他普通的手寫Java源代碼一起被javac編譯。若大家有對AbstractProcessor還不太了解的童鞋，請先移步至相關(guān)文章。

好了，我們跟進去ButterKnifeProcessor首先先看下它的getSupportedAnnotationTypes方法

@Override public Set<String> getSupportedAnnotationTypes() {
    Set<String> types = new LinkedHashSet<>();
    for (Class<? extends Annotation> annotation : getSupportedAnnotations()) {
      types.add(annotation.getCanonicalName());
    }
    return types;
  }

  //ButterKnife支持的注解類型
  private Set<Class<? extends Annotation>> getSupportedAnnotations() {
    Set<Class<? extends Annotation>> annotations = new LinkedHashSet<>();

    annotations.add(BindAnim.class);
    annotations.add(BindArray.class);
    annotations.add(BindBitmap.class);
    annotations.add(BindBool.class);
    annotations.add(BindColor.class);
    annotations.add(BindDimen.class);
    annotations.add(BindDrawable.class);
    annotations.add(BindFloat.class);
    annotations.add(BindFont.class);
    annotations.add(BindInt.class);
    annotations.add(BindString.class);
    annotations.add(BindView.class);
    annotations.add(BindViews.class);
    annotations.addAll(LISTENERS);

    return annotations;
  }

可以看到ButterKnife支持的注解很多，它不但支持我們常用的BindView、OnClick注解，還支持BindColor、BindDrawable等注解。接著我們跟進去ButterKnifeProcessor的process方法看下：

@Override public boolean process(Set<? extends TypeElement> elements, RoundEnvironment env) {
    //1、調(diào)用findAndParseTargets方法，處理所有的@BindXX注解
    Map<TypeElement, BindingSet> bindingMap = findAndParseTargets(env);
    //2、遍歷bindingMap，生成相應(yīng)的Java文件
    for (Map.Entry<TypeElement, BindingSet> entry : bindingMap.entrySet()) {
      TypeElement typeElement = entry.getKey();
      BindingSet binding = entry.getValue();

      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;
  }

關(guān)鍵部分我在上述代碼中已經(jīng)做了標注，我們接著跟進去findAndParseTargets方法，在findAndParseTargets方法中處理了所有支持的注解，由于該方法有點長，這里我們只關(guān)注下BindView相關(guān)的部分，其他注解原理類似：

private Map<TypeElement, BindingSet> findAndParseTargets(RoundEnvironment env) {
    Map<TypeElement, BindingSet.Builder> builderMap = new LinkedHashMap<>();
    Set<TypeElement> erasedTargetNames = new LinkedHashSet<>();

    ...

    // Process each @BindView element.   
    //1、處理每個被@BindView標注的元素
    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);
      } catch (Exception e) {
        logParsingError(element, BindView.class, e);
      }
    }
   
    ...

    //2、調(diào)用BindingSet.Builder的build方法，生成相應(yīng)的BindingSet對象，并put到bindingMap中
    Deque<Map.Entry<TypeElement, BindingSet.Builder>> entries =
        new ArrayDeque<>(builderMap.entrySet());
    Map<TypeElement, BindingSet> bindingMap = new LinkedHashMap<>();
    while (!entries.isEmpty()) {
      Map.Entry<TypeElement, BindingSet.Builder> entry = entries.removeFirst();

      TypeElement type = entry.getKey();
      BindingSet.Builder builder = entry.getValue();

      TypeElement parentType = findParentType(type, erasedTargetNames, classpathBindings.keySet());
      if (parentType == null) {
        bindingMap.put(type, builder.build());
      } else {
        BindingInformationProvider parentBinding = bindingMap.get(parentType);
        if (parentBinding == null) {
          parentBinding = classpathBindings.get(parentType);
        }
        if (parentBinding != null) {
          builder.setParent(parentBinding);
          bindingMap.put(type, builder.build());
        } else {
          // Has a superclass binding but we haven't built it yet. Re-enqueue for later.
          entries.addLast(entry);
        }
      }
    }
    
    //3、最后將bindingMap  return掉
    return bindingMap;
  }

我們接著跟進去1處的parseBindView方法：

private void parseBindView(Element element, Map<TypeElement, BindingSet.Builder> builderMap,
      Set<TypeElement> erasedTargetNames) {

   一系列校驗工作
   ···
   if (hasError) {
      return;
    }

    //1、通過getAnnotation.value()方法，獲取到相應(yīng)控件id，類似于R.id.btn_test
    int id = element.getAnnotation(BindView.class).value();
    BindingSet.Builder builder = builderMap.get(enclosingElement);
    //2、將int類型的id包裝成Id對象
    Id resourceId = elementToId(element, BindView.class, id);
    if (builder != null) {
      String existingBindingName = builder.findExistingBindingName(resourceId);
      if (existingBindingName != null) {
        error(element, "Attempt to use @%s for an already bound ID %d on '%s'. (%s.%s)",
            BindView.class.getSimpleName(), id, existingBindingName,
            enclosingElement.getQualifiedName(), element.getSimpleName());
        return;
      }
    } else {
      //3、創(chuàng)建TypeElement相對應(yīng)的BindingSet.Builder實例，并put到builderMap中
      builder = getOrCreateBindingBuilder(builderMap, enclosingElement);
    }

    String name = simpleName.toString();
    TypeName type = TypeName.get(elementType);
    boolean required = isFieldRequired(element);

    builder.addField(resourceId, new FieldViewBinding(name, type, required));

    // Add the type-erased version to the valid binding targets set.
    erasedTargetNames.add(enclosingElement);
  }

這里我們關(guān)注下3處，在調(diào)用getOrCreateBindingBuilder方法創(chuàng)建BindingSet.Builder對象的過程中會聲明編譯生成的.java文件名，相見getBindingClassName方法:

static ClassName getBindingClassName(TypeElement typeElement) {
    String packageName = getPackage(typeElement).getQualifiedName().toString();
    String className = typeElement.getQualifiedName().toString().substring(
            packageName.length() + 1).replace('.', '$');
    return ClassName.get(packageName, className + "_ViewBinding");
  }

上述代碼就印證了通過ButterKnife生成的Java文件名為className + "_ViewBinding"的形式。

好了，我們回到最初的ButterKnifeProcessor，接著看下process方法的2處，遍歷bindingMap，生成相應(yīng)的Java文件，這里生成Java文件用到了開源庫javapoet。我們跟進去brewJava方法看下：

JavaFile brewJava(int sdk, boolean debuggable) {
    //1、createType方法
    TypeSpec bindingConfiguration = createType(sdk, debuggable);
    return JavaFile.builder(bindingClassName.packageName(), bindingConfiguration)
        .addFileComment("Generated code from Butter Knife. Do not modify!")
        .build();
  }

跟進去createType方法:

private TypeSpec createType(int sdk, boolean debuggable) {
    //1、聲明Java文件的類名、修飾符、是否final等，其實就是開源庫javapoet的API操作
    TypeSpec.Builder result = TypeSpec.classBuilder(bindingClassName.simpleName())
        .addModifiers(PUBLIC)
        .addOriginatingElement(enclosingElement);
    if (isFinal) {
      result.addModifiers(FINAL);
    }
  
    //重點：createBindingConstructor 創(chuàng)建構(gòu)造方法
    result.addMethod(createBindingConstructor(sdk, debuggable));
    
    ...

    return result.build();
  }

createBindingConstructor方法中又調(diào)用到了addViewBinding方法，我們跟進去看下：

private void addViewBinding(MethodSpec.Builder result, ViewBinding binding, boolean debuggable) {
    if (binding.isSingleFieldBinding()) {
      // Optimize the common case where there's a single binding directly to a field.
      FieldViewBinding fieldBinding = requireNonNull(binding.getFieldBinding());
      CodeBlock.Builder builder = CodeBlock.builder()
          .add("target.$L = ", fieldBinding.getName());
      //標記是否需要強轉(zhuǎn)
      boolean requiresCast = requiresCast(fieldBinding.getType());
      if (!debuggable || (!requiresCast && !fieldBinding.isRequired())) {
        if (requiresCast) {
          builder.add("($T) ", fieldBinding.getType());
        }
        //直接拼接findViewById($L)  ，$L為占位符，findViewById括號中正好為相應(yīng)控件id，類似于R.id.btn_test
        builder.add("source.findViewById($L)", binding.getId().code);
      } else {
        //通過Utils類包裝了一層，內(nèi)部也是調(diào)用findViewById操作
        builder.add("$T.find", UTILS);
        builder.add(fieldBinding.isRequired() ? "RequiredView" : "OptionalView");
        if (requiresCast) {
          builder.add("AsType");
        }
        builder.add("(source, $L", binding.getId().code);
        if (fieldBinding.isRequired() || requiresCast) {
          builder.add(", $S", asHumanDescription(singletonList(fieldBinding)));
        }
        if (requiresCast) {
          builder.add(", $T.class", fieldBinding.getRawType());
        }
        builder.add(")");
      }
      result.addStatement("$L", builder.build());
      return;
    }

    ...
  }

方法addViewBinding中簡單明了，就是用來拼接我們在Activity或者Fragment中常寫的findViewById那行代碼。

到這里AbstractProcessor相關(guān)的部分我們就分析完畢了，簡單講就是在代碼編譯期間掃描每個Java文件中的特定的注解，通過開源庫javapoet來“拼湊”成ViewBinding文件，該Java文件的命名為className + "_ViewBinding"。那么那么中間文件又是在什么時候被調(diào)用的呢？答案就是ButterKnife.bind(this);

我們跟進去ButterKnife.bind方法看下：

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

接著跟進去bind方法：

@NonNull @UiThread
  public static Unbinder bind(@NonNull Object target, @NonNull View source) {
    //1、獲取目標class
    Class<?> targetClass = target.getClass();
    if (debug) Log.d(TAG, "Looking up binding for " + targetClass.getName());
   //2、通過反射獲取對應(yīng)_viewBinding類的構(gòu)造方法
    Constructor<? extends Unbinder> constructor = findBindingConstructorForClass(targetClass);

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

    //noinspection TryWithIdenticalCatches Resolves to API 19+ only type.
    try {
      //3、創(chuàng)建構(gòu)造對象，執(zhí)行findViewById操作（在編譯時，我們在構(gòu)造方法中“拼接”了findViewById代碼）
      return constructor.newInstance(target, source);
    } catch (IllegalAccessException e) {
      throw new RuntimeException("Unable to invoke " + constructor, e);
    } catch (InstantiationException e) {
      throw new RuntimeException("Unable to invoke " + constructor, e);
    } catch (InvocationTargetException e) {
      Throwable cause = e.getCause();
      if (cause instanceof RuntimeException) {
        throw (RuntimeException) cause;
      }
      if (cause instanceof Error) {
        throw (Error) cause;
      }
      throw new RuntimeException("Unable to create binding instance.", cause);
    }
  }

好了，ButterKnife源碼分析到這里就結(jié)束了，下一小節(jié)我們手動擼一個簡易的ButterKnife框架。