深入理解 ButterKnife，让你的程序学会写代码

TextView textView = (TextView) findViewById(R.id.text_view);
Button button = (Button) findViewById(R.id.button);

bindingClass.brewJava().writeTo(filer);

 private MethodSpec createBindMethod() {    /*创建了一个叫做 bind 的方法，添加了 @Override 注解，方法可见性为 public
     以及一些参数类型 */
    MethodSpec.Builder result = MethodSpec.methodBuilder("bind")
        .addAnnotation(Override.class)
        .addModifiers(PUBLIC)
        .addParameter(FINDER, "finder", FINAL)
        .addParameter(TypeVariableName.get("T"), "target", FINAL)
        .addParameter(Object.class, "source");    if (hasResourceBindings()) {      // Aapt can change IDs out from underneath us, just suppress since all will work at runtime.
      result.addAnnotation(AnnotationSpec.builder(SuppressWarnings.class)
          .addMember("value", "$S", "ResourceType")
          .build());
    }    // Emit a call to the superclass binder, if any.
    if (parentViewBinder != null) {
      result.addStatement("super.bind(finder, target, source)");
    }    /* 关于 unbinder，我们一直都没有提到过，如果我们有下面的注入配置：
        @Unbinder
        ButterKnife.Unbinder unbinder;
    * 那么这时候就会在生成的代码中添加下面的代码，这实际上就是构造 unbinder
    */
    // If the caller requested an unbinder, we need to create an instance of it.
    if (hasUnbinder()) {
      result.addStatement("$T unbinder = new $T($N)", unbinderBinding.getUnbinderClassName(),
          unbinderBinding.getUnbinderClassName(), "target");
    }    /*
    * 这里就是注入 view了，addViewBindings 这个方法其实就生成功能上类似
        TextView textView = (TextView) findViewById(...) 的代码
    */
    if (!viewIdMap.isEmpty() || !collectionBindings.isEmpty()) {      // Local variable in which all views will be temporarily stored.
      result.addStatement("$T view", VIEW);      // Loop over each view bindings and emit it.
      for (ViewBindings bindings : viewIdMap.values()) {
        addViewBindings(result, bindings);
      }      // Loop over each collection binding and emit it.
      for (Map.Entry<FieldCollectionViewBinding, int[]> entry : collectionBindings.entrySet()) {
        emitCollectionBinding(result, entry.getKey(), entry.getValue());
      }
    }    /*
    * 注入 unbinder
    */ 
    // Bind unbinder if was requested.
    if (hasUnbinder()) {
      result.addStatement("target.$L = unbinder", unbinderBinding.getUnbinderFieldName());
    }    /* ButterKnife 其实不止支持注入 View， 还支持注入 字符串，主题，图片。。
    * 所有资源里面你能想象到的东西
    */
    if (hasResourceBindings()) {        //篇幅有限，我还是省略掉他们吧
        ...
    }    return result.build();
  }

  @Override
  public void bind(final Finder finder, final T target, Object source) {    //构造 unbinder
    Unbinder unbinder = new Unbinder(target);    //下面开始 注入 view
    View view;
    view = finder.findRequiredView(source, 2130968576, "field 'title'");
    target.title = finder.castView(view, 2130968576, "field 'title'");    //... 省略掉其他成员的注入 ...
    //注入 unbinder
    target.unbinder = unbinder;
  }

@BindLayout(R.layout.main)public class AnyActivity extends Activity{...}

public class AnyActivity extends Activity{    @Override
    protected void onCreate(Bundle savedInstances){        super.onCreate(savedInstances);
        setContentView(R.layout.main);
    }
}

//注入 layout@BindLayout(R.layout.simple_activity)public class SimpleActivity extends Activity {
    ...
}

public class SimpleActivity$$ViewBinder<T extends SimpleActivity> implements ViewBinder<T> {  @Override
  public void bind(final Finder finder, final T target, Object source) {    //生成了这句代码来注入 layout
    target.setContentView(2130837504);    //下面省略掉的代码我们已经见过啦，就是注入 unbinder，注入 view
    ...
  }

  ...
}

@Retention(CLASS) @Target(TYPE)public @interface BindLayout {    @LayoutRes int value();
}

 @Override public Set<String> getSupportedAnnotationTypes() {
    Set<String> types = new LinkedHashSet<>();
    ...
    types.add(BindLayout.class.getCanonicalName());
    ...    return types;
  }

private Map<TypeElement, BindingClass> findAndParseTargets(RoundEnvironment env) {
    Map<TypeElement, BindingClass> targetClassMap = new LinkedHashMap<>();
    Set<String> erasedTargetNames = new LinkedHashSet<>();    // Process each @Bind element.
    for (Element element : env.getElementsAnnotatedWith(BindLayout.class)) {      if (!SuperficialValidation.validateElement(element)) continue;      try {
          parseBindLayout(element, targetClassMap, erasedTargetNames);
      } catch (Exception e) {
          logParsingError(element, BindLayout.class, e);
      }
    }
    ...
}

private void parseBindLayout(Element element, Map<TypeElement, BindingClass> targetClassMap, Set<String> erasedTargetNames) {    /*与其他注解解析不同，BindLayout 标注的类型就是 TYPE，所以这里直接强转为 
     TypeElement，其实就是对应于 Activity 的类型*/
    TypeElement typeElement = (TypeElement) element;
    Set<Modifier> modifiers = element.getModifiers();    // 只有 private 不可以访问到，static 类型不影响，这也是与其他注解不同的地方
    if (modifiers.contains(PRIVATE)) {
        error(element, "@%s %s must not be private. (%s.%s)",
                BindLayout.class.getSimpleName(), "types", typeElement.getQualifiedName(),
                element.getSimpleName());        return;
    }    // 同样的，对于 android 开头的包内的类不予支持
    String qualifiedName = typeElement.getQualifiedName().toString();    if (qualifiedName.startsWith("android.")) {
        error(element, "@%s-annotated class incorrectly in Android framework package. (%s)",
                BindLayout.class.getSimpleName(), qualifiedName);        return;
    }    // 同样的，对于 java 开头的包内的类不予支持
    if (qualifiedName.startsWith("java.")) {
        error(element, "@%s-annotated class incorrectly in Java framework package. (%s)",
                BindLayout.class.getSimpleName(), qualifiedName);        return;
    }    /* 我们暂时只支持 Activity，如果你想支持 Fragment，需要区别对待哈，
    因为二者初始化 View 的代码不一样 */
    if(!isSubtypeOfType(typeElement.asType(), ACTIVITY_TYPE)){
        error(element, "@%s fields must extend from View or be an interface. (%s.%s)",
                BindLayout.class.getSimpleName(), typeElement.getQualifiedName(), element.getSimpleName());        return;
    }    // 拿到注解传入的值，比如 R.layout.main
    int layoutId = typeElement.getAnnotation(BindLayout.class).value();    if(layoutId == 0){
        error(element, "@%s for a Activity must specify one layout ID. Found: %s. (%s.%s)",
                BindLayout.class.getSimpleName(), layoutId, typeElement.getQualifiedName(),
                element.getSimpleName());        return;
    }

    BindingClass bindingClass = targetClassMap.get(typeElement);    if (bindingClass == null) {
        bindingClass = getOrCreateTargetClass(targetClassMap, typeElement);
    }    // 把这个布局的值塞给 bindingClass，这里我只是简单的存了下这个值
    bindingClass.setContentLayoutId(layoutId);
    log(element, "element:" + element + "; targetMap:" + targetClassMap + "; erasedNames: " + erasedTargetNames);
}

  private MethodSpec createBindMethod() {
    MethodSpec.Builder result = MethodSpec.methodBuilder("bind")
        .addAnnotation(Override.class)
        .addModifiers(PUBLIC)
        .addParameter(FINDER, "finder", FINAL)
        .addParameter(TypeVariableName.get("T"), "target", FINAL)
        .addParameter(Object.class, "source");    if (hasResourceBindings()) {
        ... 省略之 ...
    }    //如果 layoutId 不为 0 ，那说明有绑定，添加一句 setContentView 完事儿~~
    //要注意的是，这句要比 view 注入在前面。。。你懂的，不然自己去玩空指针
    if(layoutId != 0){
      result.addStatement("target.setContentView($L)", layoutId);
    }

    ...
}

    private void processThrowing(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) throws Exception {        if (nothingToDo(annotations, roundEnv)) {            return;
        }

        AnnotationElementsHolder extractedModel = extractAnnotations(annotations, roundEnv);
        AnnotationElementsHolder validatingHolder = extractedModel.validatingHolder();
        androidAnnotationsEnv.setValidatedElements(validatingHolder);        try {
            AndroidManifest androidManifest = extractAndroidManifest();
            LOGGER.info("AndroidManifest.xml found: {}", androidManifest);

            IRClass rClass = findRClasses(androidManifest);

            androidAnnotationsEnv.setAndroidEnvironment(rClass, androidManifest);

        } catch (Exception e) {            return;
        }

        AnnotationElements validatedModel = validateAnnotations(extractedModel, validatingHolder);

        ModelProcessor.ProcessResult processResult = processAnnotations(validatedModel);

        generateSources(processResult);
    }