1、ButterKnife简介

ButterKnife是Android大神JakeWharton出品的又一神器，它通过注解的方式来替代findViewById来绑定view和一系列匿名的view的事件。最常见的用法如下：

 @BindView(R.id.title) TextView title;
 @BindView(R.id.subtitle) TextView subtitle;
 @BindView(R.id.footer) TextView footer;
 
 @OnClick(R.id.submit)
 public void sayHi(Button button) {
     button.setText("Hello!");
}

但是ButterKnife除了绑定view和添加事件，还能绑定Resource,有@BindBool, @BindColor, @BindDimen, @BindDrawable, @BindInt, @BindString等标签。

2、ButterKnife总体架构

以截止到2017.6.29的最新版本8.6.0源码分析。它的各个模块的依赖关系如下：

未命名文件.png-11.1kB

1、butterknife是对外提供接口的模块;
2、butterknife-compile负责编译期对注解进行解析，然后生成绑定view的java文件，以xxx_ViewBinding命名，放在对应工程/build/generated/source目录下;
3、butterknife-gradle—plugin是8.2.0之后为了支持library工程而新增的模块;
4、butterknife-annotations是专门放置注解的包,我们常使用的Bindview、BindString、OnClick等注解都在这个包里；
5、butterknife是针对butterknife-gradle—plugin提供的静态代码检查工具。

3、注解（Annotation）

阅读ButterKnife的源码，必须要对java的注解有一定的了解，比如@Override,@Deprecated等，这种注解大家一定见过。
注解的目的是将一些本来重复性的工作，变成程序自动完成，简化和自动化该过程。比如用于生成Javadoc，比如编译时进行格式检查，比如自动生成代码等，用于提升软件的质量和提高软件的生产效率。
作为Android开发，日常碰到的注解主要有来自JDK里的，也有Android SDK里的，也有自定义的。推荐阅读

ButterKnife其实就是基于自定义注解的方式实现的。

3.1 andorid-apt(Annotation Processing Tool)

android-apt 是一个Gradle插件，协助Android Studio 处理annotation processors, 它有两个目的：

1、允许配置只在编译时作为注解处理器的依赖，而不添加到最后的APK或library
2、设置源路径，使注解处理器生成的代码能被Android Studio正确的引用

使用该插件，添加如下到你的构建脚本中：

//配置在Project下的build.gradle中 buildscript {
repositories {
mavenCentral()
}
dependencies {
//替换成最新的 gradle版本
classpath 'com.android.tools.build:gradle:1.3.0'
//替换成最新android-apt版本
classpath 'com.neenbedankt.gradle.plugins:android-apt:1.8'
} } //配置到Module下的build.gradle中 apply plugin: 'com.android.application' apply plugin: 'com.neenbedankt.android-apt'

伴随着 Android Gradle 插件 2.2 版本的发布，，Android Gradle 插件提供了名为 annotationProcessor 的功能来完全代替 android-apt。
修改前配置如下：

compile 'com.jakewharton:butterknife:8.0.1'
apt 'com.jakewharton:butterknife-compiler:8.0.1'

修改后配置如下：

compile 'com.jakewharton:butterknife:8.6.0'
annotationProcessor 'com.jakewharton:butterknife-compiler:8.6.0'

整个注解处理过程包括三部分：1、注解处理器（Processor）；2、注册注解处理器（AutoService）;2、代码生成器（JavaPoet）.

示例：

package com.example;

import com.google.auto.service.AutoService;
import com.squareup.javapoet.JavaFile;
import com.squareup.javapoet.MethodSpec;
import com.squareup.javapoet.TypeSpec;

import java.io.IOException;
import java.util.LinkedHashSet;
import java.util.Set;

import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.Filer;
import javax.annotation.processing.Messager;
import javax.annotation.processing.ProcessingEnvironment;
import javax.annotation.processing.Processor;
import javax.annotation.processing.RoundEnvironment;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.Element;
import javax.lang.model.element.ElementKind;
import javax.lang.model.element.Modifier;
import javax.lang.model.element.TypeElement;
import javax.lang.model.util.Elements;
import javax.lang.model.util.Types;

@AutoService(Processor.class)
public class MyProcessor extends AbstractProcessor {

    private Types typeUtils;
    private Elements elementUtils;
    private Filer filer;
    private Messager messager;

    @Override
    public synchronized void init(ProcessingEnvironment processingEnv) {
        super.init(processingEnv);
        //提供处理注解和生成代码的工具类
        typeUtils = processingEnv.getTypeUtils();
        elementUtils = processingEnv.getElementUtils();
        filer = processingEnv.getFiler();
        messager = processingEnv.getMessager();
    }

    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        //1、处理注解；
        //2、生成代码。
    }

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

    @Override
    public SourceVersion getSupportedSourceVersion() {
        return SourceVersion.latestSupported();
    }
}

定义完注解处理器后，还需要告诉编译器该注解处理器的信息，需在 src/main/resource/META-INF/service 目录下增加 javax.annotation.processing.Processor 文件，并将注解处理器的类名配置在该文件中。
![image.png-33kB][2]

![image.png-34.9kB][3]

javapoet也是square开源的生成.java文件的API。

4、ButterKnife整体流程

image.png-37.5kB

4.1 ButterKnifeProcessor

ButterKnifeProcessor主要做了两件事：

1、解析所有包含了ButterKnife注解的类；
2、根据解析结果，使用JavaPoet生成对应的类。

@Override public boolean process(Set<? extends TypeElement> elements, RoundEnvironment env) {
    //解析所有包含了ButterKnife注解的类
    Map<TypeElement, BindingSet> bindingMap = findAndParseTargets(env);

    for (Map.Entry<TypeElement, BindingSet> entry : bindingMap.entrySet()) {
      TypeElement typeElement = entry.getKey();
      BindingSet binding = entry.getValue();
        //根据解析结果，使用JavaPoet生成对应的类
      JavaFile javaFile = binding.brewJava(sdk);
      try {
        javaFile.writeTo(filer);
      } catch (IOException e) {
        error(typeElement, "Unable to write binding for type %s: %s", typeElement, e.getMessage());
      }
    }

    return false;
  }

ButterKnife支持的所有注解在getSupportedAnnotations方法里面定义。

private Set<Class<? extends Annotation>> getSupportedAnnotations() {
    Set<Class<? extends Annotation>> annotations = new LinkedHashSet<>();

    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(BindInt.class);
    annotations.add(BindString.class);
    annotations.add(BindView.class);
    annotations.add(BindViews.class);
    annotations.addAll(LISTENERS);

    return annotations;
  }
  private static final List<Class<? extends Annotation>> LISTENERS = Arrays.asList(//
      OnCheckedChanged.class, //
      OnClick.class, //
      OnEditorAction.class, //
      OnFocusChange.class, //
      OnItemClick.class, //
      OnItemLongClick.class, //
      OnItemSelected.class, //
      OnLongClick.class, //
      OnPageChange.class, //
      OnTextChanged.class, //
      OnTouch.class //
  );

4.2 @BindView、@BindString、@OnClick

下面以@BindView、@BindString、@OnClick三个比较常用的注解为例，分析下具体的绑定过程。
在findAndParseTargets方法中解析所有注解，每个类对应一个BindingSet，解析过程中会把每个注解的解析结果放到BindingSet中，比如@BindString对应FieldResourceBinding类型，@BindView对应FieldViewBinding类型，解析完成后调用BindingSet的brewJava生成对应的JavaFile，即可通过JavaPoet生成.java文件。

4.2.1@BindView：

 private void parseBindView(Element element, Map<TypeElement, BindingSet.Builder> builderMap,
      Set<TypeElement> erasedTargetNames) {
    
    //1、TypeElement就是该注解Element所在的类
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();
    
    // Start by verifying common generated code restrictions.
    //2、验证生成代码时是否可见和是否在错误的包里面
    boolean hasError = isInaccessibleViaGeneratedCode(BindView.class, "fields", element)
        || isBindingInWrongPackage(BindView.class, element);

    // Verify that the target type extends from View.
    //3、 验证目标类型是否继承自View
    TypeMirror elementType = element.asType();
    if (elementType.getKind() == TypeKind.TYPEVAR) {
      TypeVariable typeVariable = (TypeVariable) elementType;
      elementType = typeVariable.getUpperBound();
    }
    Name qualifiedName = enclosingElement.getQualifiedName();
    Name simpleName = element.getSimpleName();
    if (!isSubtypeOfType(elementType, VIEW_TYPE) && !isInterface(elementType)) {
      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, qualifiedName, simpleName);
      } else {
        error(element, "@%s fields must extend from View or be an interface. (%s.%s)",
            BindView.class.getSimpleName(), qualifiedName, simpleName);
        hasError = true;
      }
    }

    //4、上述两步只要有错误，直接返回
    if (hasError) {
      return;
    }

    // Assemble information on the field.
    //5、解析注解的value
    int id = element.getAnnotation(BindView.class).value();

    //6、查找所在类对应的BindingSet构造类。如果有还要判断这个view的id是否已经绑定过，如果没有则新建。
    BindingSet.Builder builder = builderMap.get(enclosingElement);
    QualifiedId qualifiedId = elementToQualifiedId(element, id);
    if (builder != null) {
      String existingBindingName = builder.findExistingBindingName(getId(qualifiedId));
      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 {
      builder = getOrCreateBindingBuilder(builderMap, enclosingElement);
    }

    //7、解析注解的view。比如@BindView(R.id.tv_title) TextView tvTitle，那么name就是tvTitle,type就是TextView，required只要没有@Nullable的注解就是true。
    String name = simpleName.toString();
    TypeName type = TypeName.get(elementType);
    boolean required = isFieldRequired(element);

    //8、将上述信息封装成FieldViewBinding，加到builder中。
    builder.addField(getId(qualifiedId), new FieldViewBinding(name, type, required));

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

4.2.2@BindString:

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

    erasedTargetNames.add(enclosingElement);
  }

看代码，跟绑定View的过程基本一致，唯一的区别是它最后封装成了
FieldResourceBinding类，然后通过addResource加到builder中。

4.2.3@OnClick：

所有Listener的解析都在parseListenerAnnotation方法中，代码总共200多行，这里不一一分析了，下面截取关键代码做解析。
以如下注解代码为例：

package test;
import butterknife.OnClick;
    public class Test {
      @OnClick(1) void doStuff() {}
   }

 ExecutableElement executableElement = (ExecutableElement) element;//（doStuff()）
    TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();//(test.Test)

    Annotation annotation = element.getAnnotation(annotationClass);//(interface butterknife.OnClick)
    Method annotationValue = annotationClass.getDeclaredMethod("value");//(public abstract int[] butterknife.OnClick.value())
    
int[] ids = (int[])annotationValue.invoke(annotation);//(1)
String name = executableElement.getSimpleName().toString();//(doStuff)
    boolean required = isListenerRequired(executableElement);//(true)

    //

上面是注解及其节点的一些基本属性。后面的值是调试过程中看到的值。

下面看下OnClick注解的定义:

@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 };
}

它有一个ListenerClass的注解，用来定义目标类型、设置方法、listener类型、method(listener里面只有一个回调时使用)、callbacks（listener里面有多个回调时使用）。

比如OnTextChanged里面的ListenerClass注解是这样的，它只实现OnTextChanged方法的回调：

@ListenerClass(
    targetType = "android.widget.TextView",
    setter = "addTextChangedListener",
    remover = "removeTextChangedListener",
    type = "android.text.TextWatcher",
    callbacks = OnTextChanged.Callback.class
)

继续看parseListenerAnnotation方法里面的代码。

ListenerClass listener = annotationClass.getAnnotation(ListenerClass.class);

ListenerMethod method;
ListenerMethod[] methods = listener.method();

通过解析ListenerClass，获取一个method实例。然后再根据methodParameters及其其他一些逻辑生成一个参数列表。

MethodViewBinding binding = new MethodViewBinding(name, Arrays.asList(parameters), required);

builder.addMethod(getId(qualifiedId), listener, method, binding)

最后把注解listener、method和binding加入到builder中，用户后续生成代码。

4.2.4 上面大致分析了三种不同注解的解析方式，下面看下到底是如何通过BuildSet生成java类的。

4.3 生成.java文件

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

ButterKnifeProcessor中process方法中上面的代码触发了生成代码的逻辑，最后写到文件中。

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

brewJava调用createType构造具体的类。其中最重要的代码是：
result.addMethod(createBindingConstructor(sdk));
它负责在构造方法中绑定view，生成的代码类似（源码中SimpleActivity）如下：

@UiThread
  public SimpleActivity_ViewBinding(final SimpleActivity target, View source) {
    this.target = target;

    View view;
    target.title = Utils.findRequiredViewAsType(source, R.id.title, "field 'title'", TextView.class);
    target.subtitle = Utils.findRequiredViewAsType(source, R.id.subtitle, "field 'subtitle'", TextView.class);
    view = Utils.findRequiredView(source, R.id.hello, "field 'hello', method 'sayHello', and method 'sayGetOffMe'");
    target.hello = Utils.castView(view, R.id.hello, "field 'hello'", Button.class);
    view2130968578 = view;
    view.setOnClickListener(new DebouncingOnClickListener() {
      @Override
      public void doClick(View p0) {
        target.sayHello();
      }
    });
    view.setOnLongClickListener(new View.OnLongClickListener() {
      @Override
      public boolean onLongClick(View p0) {
        return target.sayGetOffMe();
      }
    });
    view = Utils.findRequiredView(source, R.id.list_of_things, "field 'listOfThings' and method 'onItemClick'");
    target.listOfThings = Utils.castView(view, R.id.list_of_things, "field 'listOfThings'", ListView.class);
    view2130968579 = view;
    ((AdapterView<?>) view).setOnItemClickListener(new AdapterView.OnItemClickListener() {
      @Override
      public void onItemClick(AdapterView<?> p0, View p1, int p2, long p3) {
        target.onItemClick(p2);
      }
    });
    target.footer = Utils.findRequiredViewAsType(source, R.id.footer, "field 'footer'", TextView.class);
    target.headerViews = Utils.listOf(
        Utils.findRequiredView(source, R.id.title, "field 'headerViews'"), 
        Utils.findRequiredView(source, R.id.subtitle, "field 'headerViews'"), 
        Utils.findRequiredView(source, R.id.hello, "field 'headerViews'"));
  }

下面结合createBindingConstructor方法做具体分析,直接在代码中进行注释说明。

private MethodSpec createBindingConstructor(int sdk) {
========
    MethodSpec.Builder constructor = MethodSpec.constructorBuilder()
        .addAnnotation(UI_THREAD)
        .addModifiers(PUBLIC);
========1、上面代码在方法SimpleActivity_ViewBinding增加UI_THREAD注解和public修饰符。
    if (hasMethodBindings()) {
      constructor.addParameter(targetTypeName, "target", FINAL);
    } else {
      constructor.addParameter(targetTypeName, "target");
    }
========2、上面代码增加参数final SimpleActivity target
    if (constructorNeedsView()) {
      constructor.addParameter(VIEW, "source");
    } else {
      constructor.addParameter(CONTEXT, "context");
    }
========3、上面代码增加参数View source

    if (hasUnqualifiedResourceBindings()) {
      // 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 (hasOnTouchMethodBindings()) {
      constructor.addAnnotation(AnnotationSpec.builder(SUPPRESS_LINT)
          .addMember("value", "$S", "ClickableViewAccessibility")
          .build());
    }

===========
    if (parentBinding != null) {
      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");
    }
    ========上面代码是判断有继承的情况
    
    if (hasTargetField()) {
      constructor.addStatement("this.target = target");
      constructor.addCode("\n");
    }
========上面代码增加this.target = target;
    if (hasViewBindings()) {
      if (hasViewLocal()) {
        // Local variable in which all views will be temporarily stored.
        constructor.addStatement("$T view", VIEW);
      }
      for (ViewBinding binding : viewBindings) {
        addViewBinding(constructor, binding);
      }
      for (FieldCollectionViewBinding binding : collectionBindings) {
        constructor.addStatement("$L", binding.render());
      }

      if (!resourceBindings.isEmpty()) {
        constructor.addCode("\n");
      }
    }
========上面代码实现view的绑定,具体代码在addViewBinding，它会根据不同的情况生成不同的代码;
    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));
      }
    }
========上面代码实现resource的绑定;
    return constructor.build();
  }

addViewBinding方法里面的大致步骤是先findView，然后调用addFieldBinding(result, binding)和addMethodBindings(result, binding)实现view的绑定和listener的设置。

5 运行期间

运行期间,ButterKnife.bind(this)通过反射去获取对应的xxx__ViewBinding类的实例，在该类的构造方法中，完成view或者resource的查找和绑定，以及listener的设置。

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

 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);
    }
  }

参考链接：
1、https://github.com/ShowJoy-com/showjoy-blog/issues/30?hmsr=toutiao.io&utm_medium=toutiao.io&utm_source=toutiao.io
2、http://www.jianshu.com/p/b8b59fb80554
3、http://blog.csdn.net/crazy1235/article/details/51876192
4、http://blog.csdn.net/asce1885/article/details/52878076