简介

butterknife来自于著名的大神JakeWharton，github 上是这么描述它的功能和原理的。

功能

Bind Android views and callbacks to fields and methods.

绑定android视图和事件回调到字段和方法。

原理

Field and method binding for Android views which uses annotation processing to generate boilerplate code for you.

通过使用注解处理并生成模板代码，为你绑定android视图中的字段和方法。

源码解读

工欲善其事，必先利其器。我们把butterknife 导入到insight.io中。
上面已经简述了butterknife的原理，先来看它定义的所有注解如下图：

注解.png

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。

注解生成模板代码分析

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;
  }

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
  }

看注释5ClassName bindingClassName = ClassName.get(packageName, className + "_ViewBinding");此bindingClassName就是要即将生成的模板类名称。
继续看注释6此处new 了 Builder类，根据名字我们可以看出这是一个创建者模式，来看看Builder类的build方法，如下：

 BindingSet build() {
      ImmutableList.Builder<ViewBinding> viewBindings = ImmutableList.builder();
      for (ViewBinding.Builder builder : viewIdMap.values()) {
        viewBindings.add(builder.build());
      }
      return new BindingSet(targetTypeName, bindingClassName, isFinal, isView, isActivity, isDialog,
          viewBindings.build(), collectionBindings.build(), resourceBindings.build(),
          parentBinding);//7
    }

注释7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”。

模板类代码分析
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。

butterknife源码简单分析&原理简述

butterknife源码简单分析&原理简述

简介

源码解读

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