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Butterknife source analysis

本文分析的源码为8.4.0。

项目结构

butterknife包含ButterKnife核心的Api，如ButterKnife。butterknife-annotations包含了所有定义的注解。butterknife-compiler包含了生成模板代码的代码。这里我们重点研究ButterKnife是如何生成模板代码以及在我们的类中是如何调用的。

核心类

ViewBinding是一个接口，有两个直接的子类FieldViewBinding和MethodViewBinding。

FieldViewBinding封装了使用@BindView注解的Field的信息。 

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private final String name;//字段名
private final TypeName type;//类型
private final boolean required;//是否必须

MethodViewBinding封装了使用@OnClick等注解注解的方法的信息。

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private final String name;//方法名
private final List<Parameter> parameters;//方法参数
private final boolean required;//是否必须

注解@ListenerClass代表一个监听器的类，作用在如@OnClick等事件监听的注解上。 

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@Retention(RUNTIME) @Target(ANNOTATION_TYPE)
public @interface ListenerClass {
  String targetType();//目标类型
  /** Name of the setter method on the {@linkplain #targetType() target type} for the listener. */
  String setter();//将该类设置给目标类型的方法
  /**
   * Name of the method on the {@linkplain #targetType() target type} to remove the listener. If
   * empty {@link #setter()} will be used by default.
   */
  String remover() default "";
  /** Fully-qualified class name of the listener type. */
  String type();

  /** Enum which declares the listener callback methods. Mutually exclusive to {@link #method()}. */
  Class<? extends Enum<?>> callbacks() default NONE.class;

  /**
   * Method data for single-method listener callbacks. Mutually exclusive with {@link #callbacks()}
   * and an error to specify more than one value.
   */
  ListenerMethod[] method() default { };

  /** Default value for {@link #callbacks()}. */
  enum NONE { }
}
@ListenerMethod注解代表一个监听类的方法，因为类里面可能有多个方法，所以是一个数组。 
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@Retention(RUNTIME) @Target(FIELD)
public @interface ListenerMethod {
  /** Name of the listener method for which this annotation applies. */
  String name();

  /** List of method parameters. If the type is not a primitive it must be fully-qualified. */
  String[] parameters() default { };

  /** Primitive or fully-qualified return type of the listener method. May also be {@code void}. */
  String returnType() default "void";

  /** If {@link #returnType()} is not {@code void} this value is returned when no binding exists. */
  String defaultReturn() default "null";
}
@OnClick 
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@Target(METHOD)
@Retention(CLASS)
@ListenerClass(
    targetType = "android.view.View",
    setter = "setOnClickListener",
    type = "butterknife.internal.DebouncingOnClickListener",
    method = @ListenerMethod(
        name = "doClick",
        parameters = "android.view.View"
    )
)
public @interface OnClick {
  /** View IDs to which the method will be bound. */
  @IdRes int[] value() default { View.NO_ID };
}

ViewBindings封装了一个View的所有ViewBinding。例如在SimpleActivity中有如下代码 

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@BindView(R.id.hello) Button hello;//对应封装成一个FieldViewBinding
@OnClick(R.id.hello) void sayHello() {
    Toast.makeText(this, "Hello, views!", LENGTH_SHORT).show();
    ButterKnife.apply(headerViews, ALPHA_FADE);//对应封装成一个MethodViewBinding
}
@OnLongClick(R.id.hello) boolean sayGetOffMe() {
    Toast.makeText(this, "Let go of me!", LENGTH_SHORT).show();//对应封装成一个MethodViewBinding
    return true;
}
这里R.id.hello这个view对应的ViewBindings包含一个FieldViewBinding和两个MethodViewBinding。 
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final class ViewBindings {
  private final Id id; //View对应的id
  private final Map<ListenerClass, Map<ListenerMethod, Set<MethodViewBinding>>> methodBindings =
      new LinkedHashMap<>();//Map用于存储多个MethodViewBinding
  private FieldViewBinding fieldBinding;
  ...
}
这里使用 Map<ListenerMethod, Set来存储MethodViewBinding。因为同一个ListenerMethod可能对应多个MethodViewBinding。比如下面代码就是一个ListenerMethod就对应了两个MethodViewBinding。 
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  @OnClick({R.id.hello})void sayHello() {
    Toast.makeText(this, "Hello, views!", LENGTH_SHORT).show();
    ButterKnife.apply(headerViews, ALPHA_FADE);
  }
  @OnClick({R.id.hello})void sayHello2() {
    Toast.makeText(this, "Hello, views!", LENGTH_SHORT).show();
    ButterKnife.apply(headerViews, ALPHA_FADE);
  }
BindingSet可以看做是一个类里面所有所有Binding信息。同时还负责生成代码的逻辑。 
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final class BindingSet {
    private final TypeName targetTypeName;//
    private final ClassName bindingClassName;//生成辅助类的类名
    private final boolean isFinal;//是否是final类型
    private final List<ViewBindings> viewBindings;//用于存储ViewBinding
    private final List<FieldCollectionViewBinding> collectionBindings;
    private final List<ResourceBinding> resourceBindings;
    private final BindingSet parentBinding;//父parentBinding
}

ResourceBinding是一个接口，有两个实现类FieldResourceBinding和FieldDrawableBinding。 

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//例如  @BindColor(android.R.color.black) @ColorInt int blackColor;
//将会生成代码    target.blackColor = ContextCompat.getColor(context, android.R.color.black);
//ResourceMethod方法指的是生成代码的对象
final class FieldResourceBinding implements ResourceBinding {
    enum Type {
        BITMAP(new ResourceMethod(BindingSet.BITMAP_FACTORY, "decodeResource", true, 1)),
        BOOL("getBoolean"),
        COLOR(new ResourceMethod(BindingSet.CONTEXT_COMPAT, "getColor", false, 1),
                new ResourceMethod(null, "getColor", false, 23)),
        COLOR_STATE_LIST(new ResourceMethod(BindingSet.CONTEXT_COMPAT, "getColorStateList", false, 1),
                new ResourceMethod(null, "getColorStateList", false, 23)),
        DIMEN_AS_INT("getDimensionPixelSize"),
        DIMEN_AS_FLOAT("getDimension"),
        FLOAT(new ResourceMethod(BindingSet.UTILS, "getFloat", false, 1)),
        INT("getInteger"),
        INT_ARRAY("getIntArray"),
        STRING("getString"),
        STRING_ARRAY("getStringArray"),
        TEXT_ARRAY("getTextArray"),
        TYPED_ARRAY("obtainTypedArray");

        private final List<ResourceMethod> methods;

        Type(ResourceMethod... methods) {
            List<ResourceMethod> methodList = new ArrayList<>(methods.length);
            Collections.addAll(methodList, methods);
            Collections.sort(methodList);
            Collections.reverse(methodList);
            this.methods = unmodifiableList(methodList);
        }

        Type(String methodName) {
            methods = singletonList(new ResourceMethod(null, methodName, true, 1));
        }

        ResourceMethod methodForSdk(int sdk) {
            for (ResourceMethod method : methods) {
                if (method.sdk <= sdk) {
                    return method;
                }
            }
            throw new AssertionError();
        }
    }
    //资源方法
    static final class ResourceMethod implements Comparable<ResourceMethod> {
        final ClassName typeName;//类名 
        final String name;//方法名
        final boolean requiresResources;
        final int sdk;//sdk版本号 因为sdk版本号不一样，调用的方法可能不一样。

        ResourceMethod(ClassName typeName, String name, boolean requiresResources, int sdk) {
            this.typeName = typeName;
            this.name = name;
            this.requiresResources = requiresResources;
            this.sdk = sdk;
        }

        @Override
        public int compareTo(ResourceMethod other) {
            return Integer.compare(sdk, other.sdk);//按照sdk排序
        }
    }

    //构造函数
    private final Id id;
    private final String name;
    private final Type type;

    FieldResourceBinding(Id id, String name, Type type) {
        this.id = id;
        this.name = name;
        this.type = type;
    }

    @Override
    public Id id() {
        return id;
    }

    @Override
    public boolean requiresResources(int sdk) {
        return type.methodForSdk(sdk).requiresResources;
    }

    @Override
    public CodeBlock render(int sdk) {
        ResourceMethod method = type.methodForSdk(sdk);
        if (method.typeName == null) {
            if (method.requiresResources) {
                return CodeBlock.of("target.$L = res.$L($L)", name, method.name, id.code);
            }
            return CodeBlock.of("target.$L = context.$L($L)", name, method.name, id.code);
        }
        if (method.requiresResources) {
            return CodeBlock.of("target.$L = $T.$L(res, $L)", name, method.typeName, method.name,
                    id.code);
        }
        return CodeBlock.of("target.$L = $T.$L(context, $L)", name, method.typeName, method.name,
                id.code);
    }
}

解析注解

知道各个类的作用之后，下一步工作就是了解如何解析这些标注了注解的字段和方法的信息，并封装成类。 ButterKnife解析这些信息的类为ButterKnifeProcessor，解析的工作由findAndParseTargets方法完成。 

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private Map<TypeElement, BindingSet> findAndParseTargets(RoundEnvironment env) {   //使用Map存储BindingSet.Builder避免同一个类重复创建BindingSet.Builder
    Map<TypeElement, BindingSet.Builder> builderMap = new LinkedHashMap<>();//
    Set<TypeElement> erasedTargetNames = new LinkedHashSet<>();//
    scanForRClasses(env);
    // Process each @BindArray element.
    //获取注解@BindArray的Element
    for (Element element : env.getElementsAnnotatedWith(BindArray.class)) {
      if (!SuperficialValidation.validateElement(element)) continue;
      try {
        parseResourceArray(element, builderMap, erasedTargetNames);
      } catch (Exception e) {
        logParsingError(element, BindArray.class, e);
      }
    }
    ...
}
findAndParseTargets获取不同注解对应的Element，然后调用对应的parse方法来解析。

解析@BindView

parseBindView解析@BindView注解的Element。 

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private void parseBindView(Element element, Map<TypeElement, BindingSet.Builder> builderMap,
  Set<TypeElement> erasedTargetNames) {
    //验证注解是否使用正确
    boolean hasError = isInaccessibleViaGeneratedCode(BindView.class, "fields", element)
        || isBindingInWrongPackage(BindView.class, element);

    TypeMirror elementType = element.asType();
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();
    //例如当@BindView T view; 这时候就是一个TypeKind.TYPEVAR类型
    if (elementType.getKind() == TypeKind.TYPEVAR) {//泛型类型
      TypeVariable typeVariable = (TypeVariable) elementType;
      elementType = typeVariable.getUpperBound();//获取上边界
    }
    if (!isSubtypeOfType(elementType, VIEW_TYPE) && !isInterface(elementType)) {//判断field的类型
      if (elementType.getKind() == TypeKind.ERROR) {
        note(element, "@%s field with unresolved type (%s) "
                + "must elsewhere be generated as a View or interface. (%s.%s)",
            BindView.class.getSimpleName(), elementType, enclosingElement.getQualifiedName(),
            element.getSimpleName());
      } else {
        error(element, "@%s fields must extend from View or be an interface. (%s.%s)",
            BindView.class.getSimpleName(), enclosingElement.getQualifiedName(),
            element.getSimpleName());
        hasError = true;
      }
    }
    if (hasError) {
      return;
    }
    int id = element.getAnnotation(BindView.class).value();//获取id
    BindingSet.Builder builder = builderMap.get(enclosingElement);//创建  BindingSet.Builder
    if (builder != null) {
      ViewBindings viewBindings = builder.getViewBinding(getId(id));
      //当FieldBinding已经存在说明已经绑定过了。重复绑定将抛异常
      if (viewBindings != null && viewBindings.getFieldBinding() != null) {
        FieldViewBinding existingBinding = viewBindings.getFieldBinding();

        error(element, "Attempt to use @%s for an already bound ID %d on '%s'. (%s.%s)",
            BindView.class.getSimpleName(), id, existingBinding.getName(),
            enclosingElement.getQualifiedName(), element.getSimpleName());
        return;
      }
    } else {
      builder = getOrCreateBindingBuilder(builderMap, enclosingElement);
    }
    String name = element.getSimpleName().toString();//获取FieldName
    TypeName type = TypeName.get(elementType);//获取Field的Type
    boolean required = isFieldRequired(element);//是否必须
    builder.addField(getId(id), new FieldViewBinding(name, type, required));
    // Add the type-erased version to the valid binding targets set.
    erasedTargetNames.add(enclosingElement);
}
isInaccessibleViaGeneratedCode方法用于通过修饰符来判断字段是否能够被生成的代码访问。由于ButterKnife的原理是通过辅助类来完成findViewById的操作，所以必须引用原来的的字段，因此不能设置为private 
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target.title = Utils.findRequiredViewAsType(source, R.id.title, "field 'title'", TextView.class);
isInaccessibleViaGeneratedCode方法的源码 
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private boolean isInaccessibleViaGeneratedCode(Class<? extends Annotation> annotationClass,
  String targetThing, Element element) {
    boolean hasError = false;
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();
    // Verify method modifiers.
    Set<Modifier> modifiers = element.getModifiers();
    if (modifiers.contains(PRIVATE) || modifiers.contains(STATIC)) {
      error(element, "@%s %s must not be private or static. (%s.%s)",
          annotationClass.getSimpleName(), targetThing, enclosingElement.getQualifiedName(),
          element.getSimpleName());
      hasError = true;
    }
    // Verify containing type.
    if (enclosingElement.getKind() != CLASS) {
      error(enclosingElement, "@%s %s may only be contained in classes. (%s.%s)",
          annotationClass.getSimpleName(), targetThing, enclosingElement.getQualifiedName(),
          element.getSimpleName());
      hasError = true;
    }
    // Verify containing class visibility is not private.
    if (enclosingElement.getModifiers().contains(PRIVATE)) {
      error(enclosingElement, "@%s %s may not be contained in private classes. (%s.%s)",
          annotationClass.getSimpleName(), targetThing, enclosingElement.getQualifiedName(),
          element.getSimpleName());
      hasError = true;
    }
    return hasError;
}

解析方法注解

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for (Class<? extends Annotation> listener : LISTENERS) {
  findAndParseListener(env, listener, builderMap, erasedTargetNames);
}

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  private void findAndParseListener(RoundEnvironment env,
      Class<? extends Annotation> annotationClass,
      Map<TypeElement, BindingSet.Builder> builderMap, Set<TypeElement> erasedTargetNames) {
    for (Element element : env.getElementsAnnotatedWith(annotationClass)) {
      if (!SuperficialValidation.validateElement(element)) continue;
      try {
        parseListenerAnnotation(annotationClass, element, builderMap, erasedTargetNames);
      } catch (Exception e) {
        StringWriter stackTrace = new StringWriter();
        e.printStackTrace(new PrintWriter(stackTrace));

        error(element, "Unable to generate view binder for @%s.\n\n%s",
            annotationClass.getSimpleName(), stackTrace.toString());
      }
    }
  }

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  private void parseListenerAnnotation(Class<? extends Annotation> annotationClass, Element element,
      Map<TypeElement, BindingSet.Builder> builderMap, Set<TypeElement> erasedTargetNames)
      throws Exception {
    // This should be guarded by the annotation's @Target but it's worth a check for safe casting.
    if (!(element instanceof ExecutableElement) || element.getKind() != METHOD) {
      throw new IllegalStateException(
          String.format("@%s annotation must be on a method.", annotationClass.getSimpleName()));
    }

    ExecutableElement executableElement = (ExecutableElement) element;
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();
    // Assemble information on the method.
    Annotation annotation = element.getAnnotation(annotationClass);
    Method annotationValue = annotationClass.getDeclaredMethod("value");
    if (annotationValue.getReturnType() != int[].class) {
      throw new IllegalStateException(
          String.format("@%s annotation value() type not int[].", annotationClass));
    }
    int[] ids = (int[]) annotationValue.invoke(annotation);//通过反射获取值
    String name = executableElement.getSimpleName().toString();//方法名
    boolean required = isListenerRequired(executableElement);
    // Verify that the method and its containing class are accessible via generated code.
    boolean hasError = isInaccessibleViaGeneratedCode(annotationClass, "methods", element);
    hasError |= isBindingInWrongPackage(annotationClass, element);
    //发现重复的id
    Integer duplicateId = findDuplicate(ids);
    if (duplicateId != null) {
      error(element, "@%s annotation for method contains duplicate ID %d. (%s.%s)",
          annotationClass.getSimpleName(), duplicateId, enclosingElement.getQualifiedName(),
          element.getSimpleName());
      hasError = true;
    }

    ListenerClass listener = annotationClass.getAnnotation(ListenerClass.class);
    if (listener == null) {
      throw new IllegalStateException(
          String.format("No @%s defined on @%s.", ListenerClass.class.getSimpleName(),
              annotationClass.getSimpleName()));
    }
    for (int id : ids) {
      if (id == NO_ID.value) {
        if (ids.length == 1) {
          if (!required) {
            error(element, "ID-free binding must not be annotated with @Optional. (%s.%s)",
                enclosingElement.getQualifiedName(), element.getSimpleName());
            hasError = true;
          }
        } else {
          error(element, "@%s annotation contains invalid ID %d. (%s.%s)",
              annotationClass.getSimpleName(), id, enclosingElement.getQualifiedName(),
              element.getSimpleName());
          hasError = true;
        }
      }
    }

    ListenerMethod method;
    ListenerMethod[] methods = listener.method();
    if (methods.length > 1) {
      throw new IllegalStateException(String.format("Multiple listener methods specified on @%s.",
          annotationClass.getSimpleName()));
    } else if (methods.length == 1) {
      if (listener.callbacks() != ListenerClass.NONE.class) {
        throw new IllegalStateException(
            String.format("Both method() and callback() defined on @%s.",
                annotationClass.getSimpleName()));
      }
      method = methods[0];
    } else {
      Method annotationCallback = annotationClass.getDeclaredMethod("callback");
      Enum<?> callback = (Enum<?>) annotationCallback.invoke(annotation);
      Field callbackField = callback.getDeclaringClass().getField(callback.name());
      method = callbackField.getAnnotation(ListenerMethod.class);
      if (method == null) {
        throw new IllegalStateException(
            String.format("No @%s defined on @%s's %s.%s.", ListenerMethod.class.getSimpleName(),
                annotationClass.getSimpleName(), callback.getDeclaringClass().getSimpleName(),
                callback.name()));
      }
    }

    // Verify that the method has equal to or less than the number of parameters as the listener.
    List<? extends VariableElement> methodParameters = executableElement.getParameters();
    if (methodParameters.size() > method.parameters().length) {
      error(element, "@%s methods can have at most %s parameter(s). (%s.%s)",
          annotationClass.getSimpleName(), method.parameters().length,
          enclosingElement.getQualifiedName(), element.getSimpleName());
      hasError = true;
    }
    // Verify method return type matches the listener.
    TypeMirror returnType = executableElement.getReturnType();
    if (returnType instanceof TypeVariable) {
      TypeVariable typeVariable = (TypeVariable) returnType;
      returnType = typeVariable.getUpperBound();
    }
    if (!returnType.toString().equals(method.returnType())) {
      error(element, "@%s methods must have a '%s' return type. (%s.%s)",
          annotationClass.getSimpleName(), method.returnType(),
          enclosingElement.getQualifiedName(), element.getSimpleName());
      hasError = true;
    }
    if (hasError) {
      return;
    }
    Parameter[] parameters = Parameter.NONE;
    if (!methodParameters.isEmpty()) {
      parameters = new Parameter[methodParameters.size()];
      BitSet methodParameterUsed = new BitSet(methodParameters.size());
      String[] parameterTypes = method.parameters();
      for (int i = 0; i < methodParameters.size(); i++) {
        VariableElement methodParameter = methodParameters.get(i);
        TypeMirror methodParameterType = methodParameter.asType();
        if (methodParameterType instanceof TypeVariable) {
          TypeVariable typeVariable = (TypeVariable) methodParameterType;
          methodParameterType = typeVariable.getUpperBound();
        }

        for (int j = 0; j < parameterTypes.length; j++) {
          if (methodParameterUsed.get(j)) {
            continue;
          }
          if (isSubtypeOfType(methodParameterType, parameterTypes[j])
              || isInterface(methodParameterType)) {
            parameters[i] = new Parameter(j, TypeName.get(methodParameterType));
            methodParameterUsed.set(j);
            break;
          }
        }
        if (parameters[i] == null) {
          StringBuilder builder = new StringBuilder();
          builder.append("Unable to match @")
              .append(annotationClass.getSimpleName())
              .append(" method arguments. (")
              .append(enclosingElement.getQualifiedName())
              .append('.')
              .append(element.getSimpleName())
              .append(')');
          for (int j = 0; j < parameters.length; j++) {
            Parameter parameter = parameters[j];
            builder.append("\n\n  Parameter #")
                .append(j + 1)
                .append(": ")
                .append(methodParameters.get(j).asType().toString())
                .append("\n    ");
            if (parameter == null) {
              builder.append("did not match any listener parameters");
            } else {
              builder.append("matched listener parameter #")
                  .append(parameter.getListenerPosition() + 1)
                  .append(": ")
                  .append(parameter.getType());
            }
          }
          builder.append("\n\nMethods may have up to ")
              .append(method.parameters().length)
              .append(" parameter(s):\n");
          for (String parameterType : method.parameters()) {
            builder.append("\n  ").append(parameterType);
          }
          builder.append(
              "\n\nThese may be listed in any order but will be searched for from top to bottom.");
          error(executableElement, builder.toString());
          return;
        }
      }
    }

    MethodViewBinding binding = new MethodViewBinding(name, Arrays.asList(parameters), required);
    BindingSet.Builder builder = getOrCreateBindingBuilder(builderMap, enclosingElement);
    for (int id : ids) {
      if (!builder.addMethod(getId(id), listener, method, binding)) {
        error(element, "Multiple listener methods with return value specified for ID %d. (%s.%s)",
            id, enclosingElement.getQualifiedName(), element.getSimpleName());
        return;
      }
    }
    // Add the type-erased version to the valid binding targets set.
    erasedTargetNames.add(enclosingElement);
  }

解析资源绑定

解析资源的代码基本都差不多。这里只给出parseResourceString的源码和注释。

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  private void parseResourceString(Element element,
      Map<TypeElement, BindingSet.Builder> builderMap, Set<TypeElement> erasedTargetNames) {
    boolean hasError = false;
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();

    // Verify that the target type is String.
    if (!STRING_TYPE.equals(element.asType().toString())) {
      error(element, "@%s field type must be 'String'. (%s.%s)",
          BindString.class.getSimpleName(), enclosingElement.getQualifiedName(),
          element.getSimpleName());
      hasError = true;
    }

    // Verify common generated code restrictions.
    hasError |= isInaccessibleViaGeneratedCode(BindString.class, "fields", element);
    hasError |= isBindingInWrongPackage(BindString.class, element);

    if (hasError) {
      return;
    }

    // Assemble information on the field.
    String name = element.getSimpleName().toString();
    int id = element.getAnnotation(BindString.class).value();

    BindingSet.Builder builder = getOrCreateBindingBuilder(builderMap, enclosingElement);
    builder.addResource(
        new FieldResourceBinding(getId(id), name, FieldResourceBinding.Type.STRING));

    erasedTargetNames.add(enclosingElement);
  }
设置parentBinding

在findAndParseTargets最后几行代码的作用是为解析出来的BindingSet.Builder设置parentBinding 

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 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);
      if (parentType == null) {//没有parentBinding，不是某各类的子类
        bindingMap.put(type, builder.build());
      } else {
        BindingSet parentBinding = bindingMap.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);
        }
      }
    }

生成代码

生成代码主要由BindingSet的brewJava方法完成。 

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   JavaFile brewJava(int sdk) {
        return JavaFile.builder(bindingClassName.packageName(), createType(sdk))
                .addFileComment("Generated code from Butter Knife. Do not modify!")//添加注释
                .build();
    }

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private TypeSpec createType(int sdk) {
        TypeSpec.Builder result = TypeSpec.classBuilder(bindingClassName.simpleName())//类名
                .addModifiers(PUBLIC);//修饰符
        if (isFinal) {
            result.addModifiers(FINAL);
        }

        if (parentBinding != null) {
            result.superclass(parentBinding.bindingClassName);//添加父类
        } else {
            result.addSuperinterface(UNBINDER);//添加接口UnBinder
        }

        if (hasTargetField()) {
            result.addField(targetTypeName, "target", PRIVATE);//添加字段target
        }
        //
        if (!constructorNeedsView()) {
            // Add a delegating constructor with a target type + view signature for reflective use.
            result.addMethod(createBindingViewDelegateConstructor(targetTypeName));
        }
        result.addMethod(createBindingConstructor(targetTypeName, sdk));

        if (hasViewBindings() || parentBinding == null) {
            result.addMethod(createBindingUnbindMethod(result, targetTypeName));
        }

        return result.build();
    }
createBindingConstructor创建构造函数 
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private MethodSpec createBindingConstructor(TypeName targetType, int sdk) {
    MethodSpec.Builder constructor = MethodSpec.constructorBuilder()
            .addAnnotation(UI_THREAD)//添加注解
            .addModifiers(PUBLIC);//添加修饰符
    if (hasMethodBindings()) {//如果没有MethodBinding就不用声明成final类型
        constructor.addParameter(targetType, "target", FINAL);
    } else {
        constructor.addParameter(targetType, "target");
    }
    if (constructorNeedsView()) {//当构造函数需要View
        constructor.addParameter(VIEW, "source");
    } else {
        constructor.addParameter(CONTEXT, "context");
    }

    if (hasUnqualifiedResourceBindings()) {//添加SuppressWarnings注解
        // Aapt can change IDs out from underneath us, just suppress since all will work at runtime.
        constructor.addAnnotation(AnnotationSpec.builder(SuppressWarnings.class)
                .addMember("value", "$S", "ResourceType")
                .build());
    }
    if (parentBinding != null) {//添加父类构造方法的调用
        if (parentBinding.constructorNeedsView()) {
            constructor.addStatement("super(target, source)");
        } else if (constructorNeedsView()) {
            constructor.addStatement("super(target, source.getContext())");
        } else {
            constructor.addStatement("super(target, context)");
        }
        constructor.addCode("\n");
    }
    if (hasTargetField()) {//
        constructor.addStatement("this.target = target");
        constructor.addCode("\n");
    }

    if (hasViewBindings()) {
        if (hasViewLocal()) {
            // Local variable in which all views will be temporarily stored.
            constructor.addStatement("$T view", VIEW);
        }
        for (ViewBindings bindings : viewBindings) {
            addViewBindings(constructor, bindings);
        }
        for (FieldCollectionViewBinding binding : collectionBindings) {
            constructor.addStatement("$L", binding.render());
        }
        if (!resourceBindings.isEmpty()) {
            constructor.addCode("\n");
        }
    }
    //
    if (!resourceBindings.isEmpty()) {
        if (constructorNeedsView()) {
            constructor.addStatement("$T context = source.getContext()", CONTEXT);
        }
        if (hasResourceBindingsNeedingResource(sdk)) {
            constructor.addStatement("$T res = context.getResources()", RESOURCES);
        }
        for (ResourceBinding binding : resourceBindings) {
            constructor.addStatement("$L", binding.render(sdk));
        }
    }
    return constructor.build();
}
addViewBindings用于添加ViewBindings对象所对应的表达式。当只有FieldViewBinding时直接生成代码 target.title = Utils.findRequiredViewAsType(source, R.id.title, "field 'title'", TextView.class);。 

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private void addViewBindings(MethodSpec.Builder result, ViewBindings bindings) {
    if (bindings.isSingleFieldBinding()) {//如果只有FieldBinding
        // Optimize the common case where there's a single binding directly to a field.
        FieldViewBinding fieldBinding = bindings.getFieldBinding();
        CodeBlock.Builder builder = CodeBlock.builder()
                .add("target.$L = ", fieldBinding.getName());
    //当type是View类型不需要强制转换
        boolean requiresCast = requiresCast(fieldBinding.getType());
        if (!requiresCast && !fieldBinding.isRequired()) {
            builder.add("source.findViewById($L)", bindings.getId().code);
        } else {
            builder.add("$T.find", UTILS);//Utils.findRequiredViewAsType(source,12345,
            builder.add(fieldBinding.isRequired() ? "RequiredView" : "OptionalView");
            if (requiresCast) {
                builder.add("AsType");
            }
            builder.add("(source, $L", bindings.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;
    }
  //如同时存在FieldViewBinding和MethodViewBinding
  //先生成    view = Utils.findRequiredView(source, R.id.hello, "field 'hello', method 'sayHello', method 'sayHello2', and method 'sayGetOffMe'");
//调用addFieldBindings方法生成    target.hello = Utils.castView(view, R.id.hello, "field 'hello'", Button.class);

    List<ViewBinding> requiredViewBindings = bindings.getRequiredBindings();
    if (requiredViewBindings.isEmpty()) {
        result.addStatement("view = source.findViewById($L)", bindings.getId().code);
    } else if (!bindings.isBoundToRoot()) {
        result.addStatement("view = $T.findRequiredView(source, $L, $S)", UTILS,
                bindings.getId().code, asHumanDescription(requiredViewBindings));
    }
    addFieldBindings(result, bindings);
    addMethodBindings(result, bindings);
}

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 private void addFieldBindings(MethodSpec.Builder result, ViewBindings bindings) {
    FieldViewBinding fieldBinding = bindings.getFieldBinding();
    if (fieldBinding != null) {
        if (requiresCast(fieldBinding.getType())) {
            result.addStatement("target.$L = $T.castView(view, $L, $S, $T.class)",
                    fieldBinding.getName(), UTILS, bindings.getId().code,
                    asHumanDescription(singletonList(fieldBinding)), fieldBinding.getRawType());
        } else {
            result.addStatement("target.$L = view", fieldBinding.getName());
        }
    }
}

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 private void addMethodBindings(MethodSpec.Builder result, ViewBindings bindings) {
    Map<ListenerClass, Map<ListenerMethod, Set<MethodViewBinding>>> classMethodBindings =
            bindings.getMethodBindings();
    if (classMethodBindings.isEmpty()) {
        return;
    }

    // We only need to emit the null check if there are zero required bindings.
    boolean needsNullChecked = bindings.getRequiredBindings().isEmpty();
    if (needsNullChecked) {
        result.beginControlFlow("if (view != null)");
    }

    // Add the view reference to the binding.
    String fieldName = "viewSource";
    String bindName = "source";
    if (!bindings.isBoundToRoot()) {
        fieldName = "view" + bindings.getId().value;
        bindName = "view";
    }
    result.addStatement("$L = $N", fieldName, bindName);

    for (Map.Entry<ListenerClass, Map<ListenerMethod, Set<MethodViewBinding>>> e
            : classMethodBindings.entrySet()) {
        ListenerClass listener = e.getKey();
        Map<ListenerMethod, Set<MethodViewBinding>> methodBindings = e.getValue();

        TypeSpec.Builder callback = TypeSpec.anonymousClassBuilder("")
                .superclass(ClassName.bestGuess(listener.type()));

        for (ListenerMethod method : getListenerMethods(listener)) {
            MethodSpec.Builder callbackMethod = MethodSpec.methodBuilder(method.name())
                    .addAnnotation(Override.class)
                    .addModifiers(PUBLIC)
                    .returns(bestGuess(method.returnType()));
            String[] parameterTypes = method.parameters();
            for (int i = 0, count = parameterTypes.length; i < count; i++) {
                callbackMethod.addParameter(bestGuess(parameterTypes[i]), "p" + i);
            }

            boolean hasReturnType = !"void".equals(method.returnType());
            CodeBlock.Builder builder = CodeBlock.builder();
            if (hasReturnType) {
                builder.add("return ");
            }

            if (methodBindings.containsKey(method)) {
                for (MethodViewBinding binding : methodBindings.get(method)) {
                    builder.add("target.$L(", binding.getName());
                    List<Parameter> parameters = binding.getParameters();
                    String[] listenerParameters = method.parameters();
                    for (int i = 0, count = parameters.size(); i < count; i++) {
                        if (i > 0) {
                            builder.add(", ");
                        }

                        Parameter parameter = parameters.get(i);
                        int listenerPosition = parameter.getListenerPosition();

                        if (parameter.requiresCast(listenerParameters[listenerPosition])) {
                            builder.add("$T.<$T>castParam(p$L, $S, $L, $S, $L)", UTILS, parameter.getType(),
                                    listenerPosition, method.name(), listenerPosition, binding.getName(), i);
                        } else {
                            builder.add("p$L", listenerPosition);
                        }
                    }
                    builder.add(");\n");
                }
            } else if (hasReturnType) {
                builder.add("$L;\n", method.defaultReturn());
            }
            callbackMethod.addCode(builder.build());
            callback.addMethod(callbackMethod.build());
        }

        boolean requiresRemoval = listener.remover().length() != 0;
        String listenerField = null;
        if (requiresRemoval) {
            TypeName listenerClassName = bestGuess(listener.type());
            listenerField = fieldName + ((ClassName) listenerClassName).simpleName();
            result.addStatement("$L = $L", listenerField, callback.build());
        }

        if (!VIEW_TYPE.equals(listener.targetType())) {
            result.addStatement("(($T) $N).$L($L)", bestGuess(listener.targetType()), bindName,
                    listener.setter(), requiresRemoval ? listenerField : callback.build());
        } else {
            result.addStatement("$N.$L($L)", bindName, listener.setter(),
                    requiresRemoval ? listenerField : callback.build());
        }
    }

    if (needsNullChecked) {
        result.endControlFlow();
    }
}

调用辅助方法

我们在类中通过调用ButterKnife的bind方法来调用辅助类的。

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@NonNull @UiThread
public static Unbinder bind(@NonNull Object target, @NonNull View source) {
    return createBinding(target, source);
}
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);
    if (constructor == null) {
      return Unbinder.EMPTY;
    }
//noinspection TryWithIdenticalCatches Resolves to API 19+ only type.
    try {
      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);
    }
}
 @Nullable @CheckResult @UiThread
  private static Constructor<? extends Unbinder> findBindingConstructorForClass(Class<?> cls) {
    Constructor<? extends Unbinder> bindingCtor = BINDINGS.get(cls);
    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 = Class.forName(clsName + "_ViewBinding");
      //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);
    return bindingCtor;
  }