annotationProcessor

1.annotationProcessor 注解处理器，就是在编译阶段处理注解用的。可以通过它在编译的时候自动生成一些需要的代码，好像都是这么用的。

ButterKnife

1.bind 就是找到Unbinder 实例的构造方法，然后创建一个Unbinder 实例

  @NonNull @UiThread
  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 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);
    }
  }

2.Unbinder 实例如何工作的？

public class MainActivity_ViewBinding implements Unbinder
 @UiThread
  public MainActivity_ViewBinding(MainActivity target) {
    this(target, target.getWindow().getDecorView());
  }

  @UiThread
  public MainActivity_ViewBinding(final MainActivity target, View source) {
    this.target = target;

    View view;
    target.mSlMain = Utils.findRequiredViewAsType(source, R.id.sl_main, "field 'mSlMain'", VerticalDrawerLayout.class);
    view = Utils.findRequiredView(source, R.id.ll_bottom_bar, "field 'mLlBottomBar' and method 'onViewClicked'");
    target.mLlBottomBar = Utils.castView(view, R.id.ll_bottom_bar, "field 'mLlBottomBar'", LinearLayout.class);
    view2131296444 = view;
    view.setOnClickListener(new DebouncingOnClickListener() {
      @Override
      public void doClick(View p0) {
        target.onViewClicked(p0);
      }
    });
 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);
    throw new IllegalStateException("Required view '"
        + name
        + "' with ID "
        + id
        + " for "
        + who
        + " was not found. If this view is optional add '@Nullable' (fields) or '@Optional'"
        + " (methods) annotation.");
  }

可以看到构造方法里面直接为activity 的属性赋值（还是findviewbyid），这就可以理解为啥BindView 注解的成员变量不能是私有的了

3.Unbinder 实现类 是从哪里来的？

@AutoService(Processor.class)
@SuppressWarnings("NullAway") // TODO fix all these...
public final class ButterKnifeProcessor extends AbstractProcessor 

关键就是下面的这个方法中 

@Override public boolean process(Set elements, RoundEnvironment env) {
    Map bindingMap = findAndParseTargets(env);

    for (Map.Entry 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;
  }

package butterknife.compiler;

import butterknife.OnTouch;
import butterknife.internal.ListenerClass;
import butterknife.internal.ListenerMethod;
import com.google.common.collect.ImmutableList;
import com.squareup.javapoet.AnnotationSpec;
import com.squareup.javapoet.ClassName;
import com.squareup.javapoet.CodeBlock;
import com.squareup.javapoet.JavaFile;
import com.squareup.javapoet.MethodSpec;
import com.squareup.javapoet.ParameterizedTypeName;
import com.squareup.javapoet.TypeName;
import com.squareup.javapoet.TypeSpec;
import com.squareup.javapoet.WildcardTypeName;
import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;
import javax.annotation.Nullable;
import javax.lang.model.element.Modifier;
import javax.lang.model.element.TypeElement;
import javax.lang.model.type.TypeMirror;

import static butterknife.compiler.ButterKnifeProcessor.ACTIVITY_TYPE;
import static butterknife.compiler.ButterKnifeProcessor.DIALOG_TYPE;
import static butterknife.compiler.ButterKnifeProcessor.VIEW_TYPE;
import static butterknife.compiler.ButterKnifeProcessor.isSubtypeOfType;
import static com.google.auto.common.MoreElements.getPackage;
import static java.util.Collections.singletonList;
import static java.util.Objects.requireNonNull;
import static javax.lang.model.element.Modifier.FINAL;
import static javax.lang.model.element.Modifier.PRIVATE;
import static javax.lang.model.element.Modifier.PUBLIC;

/** A set of all the bindings requested by a single type. */
final class BindingSet {
  static final ClassName UTILS = ClassName.get("butterknife.internal", "Utils");
  private static final ClassName VIEW = ClassName.get("android.view", "View");
  private static final ClassName CONTEXT = ClassName.get("android.content", "Context");
  private static final ClassName RESOURCES = ClassName.get("android.content.res", "Resources");
  private static final ClassName UI_THREAD =
      ClassName.get("androidx.annotation", "UiThread");
  private static final ClassName CALL_SUPER =
      ClassName.get("androidx.annotation", "CallSuper");
  private static final ClassName SUPPRESS_LINT =
      ClassName.get("android.annotation", "SuppressLint");
  private static final ClassName UNBINDER = ClassName.get("butterknife", "Unbinder");
  static final ClassName BITMAP_FACTORY = ClassName.get("android.graphics", "BitmapFactory");
  static final ClassName CONTEXT_COMPAT =
      ClassName.get("androidx.core.content", "ContextCompat");
  static final ClassName ANIMATION_UTILS =
          ClassName.get("android.view.animation", "AnimationUtils");

  private final TypeName targetTypeName;
  private final ClassName bindingClassName;
  private final boolean isFinal;
  private final boolean isView;
  private final boolean isActivity;
  private final boolean isDialog;
  private final ImmutableList viewBindings;
  private final ImmutableList collectionBindings;
  private final ImmutableList resourceBindings;
  private final @Nullable BindingSet parentBinding;

  private BindingSet(TypeName targetTypeName, ClassName bindingClassName, boolean isFinal,
      boolean isView, boolean isActivity, boolean isDialog, ImmutableList viewBindings,
      ImmutableList collectionBindings,
      ImmutableList resourceBindings, @Nullable BindingSet parentBinding) {
    this.isFinal = isFinal;
    this.targetTypeName = targetTypeName;
    this.bindingClassName = bindingClassName;
    this.isView = isView;
    this.isActivity = isActivity;
    this.isDialog = isDialog;
    this.viewBindings = viewBindings;
    this.collectionBindings = collectionBindings;
    this.resourceBindings = resourceBindings;
    this.parentBinding = parentBinding;
  }

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

  private TypeSpec createType(int sdk, boolean debuggable) {
    TypeSpec.Builder result = TypeSpec.classBuilder(bindingClassName.simpleName())
        .addModifiers(PUBLIC);
    if (isFinal) {
      result.addModifiers(FINAL);
    }

    if (parentBinding != null) {
      result.superclass(parentBinding.bindingClassName);
    } else {
      result.addSuperinterface(UNBINDER);
    }

    if (hasTargetField()) {
      result.addField(targetTypeName, "target", PRIVATE);
    }

    if (isView) {
      result.addMethod(createBindingConstructorForView());
    } else if (isActivity) {
      result.addMethod(createBindingConstructorForActivity());
    } else if (isDialog) {
      result.addMethod(createBindingConstructorForDialog());
    }
    if (!constructorNeedsView()) {
      // Add a delegating constructor with a target type + view signature for reflective use.
      result.addMethod(createBindingViewDelegateConstructor());
    }
    result.addMethod(createBindingConstructor(sdk, debuggable));

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

    return result.build();
  }

  private MethodSpec createBindingViewDelegateConstructor() {
    return MethodSpec.constructorBuilder()
        .addJavadoc("@deprecated Use {@link #$T($T, $T)} for direct creation.\n    "
                + "Only present for runtime invocation through {@code ButterKnife.bind()}.\n",
            bindingClassName, targetTypeName, CONTEXT)
        .addAnnotation(Deprecated.class)
        .addAnnotation(UI_THREAD)
        .addModifiers(PUBLIC)
        .addParameter(targetTypeName, "target")
        .addParameter(VIEW, "source")
        .addStatement(("this(target, source.getContext())"))
        .build();
  }

  private MethodSpec createBindingConstructorForView() {
    MethodSpec.Builder builder = MethodSpec.constructorBuilder()
        .addAnnotation(UI_THREAD)
        .addModifiers(PUBLIC)
        .addParameter(targetTypeName, "target");
    if (constructorNeedsView()) {
      builder.addStatement("this(target, target)");
    } else {
      builder.addStatement("this(target, target.getContext())");
    }
    return builder.build();
  }

  private MethodSpec createBindingConstructorForActivity() {
    MethodSpec.Builder builder = MethodSpec.constructorBuilder()
        .addAnnotation(UI_THREAD)
        .addModifiers(PUBLIC)
        .addParameter(targetTypeName, "target");
    if (constructorNeedsView()) {
      builder.addStatement("this(target, target.getWindow().getDecorView())");
    } else {
      builder.addStatement("this(target, target)");
    }
    return builder.build();
  }

  private MethodSpec createBindingConstructorForDialog() {
    MethodSpec.Builder builder = MethodSpec.constructorBuilder()
        .addAnnotation(UI_THREAD)
        .addModifiers(PUBLIC)
        .addParameter(targetTypeName, "target");
    if (constructorNeedsView()) {
      builder.addStatement("this(target, target.getWindow().getDecorView())");
    } else {
      builder.addStatement("this(target, target.getContext())");
    }
    return builder.build();
  }

  private MethodSpec createBindingConstructor(int sdk, boolean debuggable) {
    MethodSpec.Builder constructor = MethodSpec.constructorBuilder()
        .addAnnotation(UI_THREAD)
        .addModifiers(PUBLIC);

    if (hasMethodBindings()) {
      constructor.addParameter(targetTypeName, "target", FINAL);
    } else {
      constructor.addParameter(targetTypeName, "target");
    }

    if (constructorNeedsView()) {
      constructor.addParameter(VIEW, "source");
    } else {
      constructor.addParameter(CONTEXT, "context");
    }

    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");
    }
    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("$T view", VIEW);
      }
      for (ViewBinding binding : viewBindings) {
        addViewBinding(constructor, binding, debuggable);
      }
      for (FieldCollectionViewBinding binding : collectionBindings) {
        constructor.addStatement("$L", binding.render(debuggable));
      }

      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();
  }

  private MethodSpec createBindingUnbindMethod(TypeSpec.Builder bindingClass) {
    MethodSpec.Builder result = MethodSpec.methodBuilder("unbind")
        .addAnnotation(Override.class)
        .addModifiers(PUBLIC);
    if (!isFinal && parentBinding == null) {
      result.addAnnotation(CALL_SUPER);
    }

    if (hasTargetField()) {
      if (hasFieldBindings()) {
        result.addStatement("$T target = this.target", targetTypeName);
      }
      result.addStatement("if (target == null) throw new $T($S)", IllegalStateException.class,
          "Bindings already cleared.");
      result.addStatement("$N = null", hasFieldBindings() ? "this.target" : "target");
      result.addCode("\n");
      for (ViewBinding binding : viewBindings) {
        if (binding.getFieldBinding() != null) {
          result.addStatement("target.$L = null", binding.getFieldBinding().getName());
        }
      }
      for (FieldCollectionViewBinding binding : collectionBindings) {
        result.addStatement("target.$L = null", binding.name);
      }
    }

    if (hasMethodBindings()) {
      result.addCode("\n");
      for (ViewBinding binding : viewBindings) {
        addFieldAndUnbindStatement(bindingClass, result, binding);
      }
    }

    if (parentBinding != null) {
      result.addCode("\n");
      result.addStatement("super.unbind()");
    }
    return result.build();
  }

  private void addFieldAndUnbindStatement(TypeSpec.Builder result, MethodSpec.Builder unbindMethod,
      ViewBinding bindings) {
    // Only add fields to the binding if there are method bindings.
    Map>> classMethodBindings =
        bindings.getMethodBindings();
    if (classMethodBindings.isEmpty()) {
      return;
    }

    String fieldName =
        bindings.isBoundToRoot()
            ? "viewSource"
            : "view" + Integer.toHexString(bindings.getId().value);
    result.addField(VIEW, fieldName, PRIVATE);

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

    for (ListenerClass listenerClass : classMethodBindings.keySet()) {
      // We need to keep a reference to the listener
      // in case we need to unbind it via a remove method.
      boolean requiresRemoval = !"".equals(listenerClass.remover());
      String listenerField = "null";
      if (requiresRemoval) {
        TypeName listenerClassName = bestGuess(listenerClass.type());
        listenerField = fieldName + ((ClassName) listenerClassName).simpleName();
        result.addField(listenerClassName, listenerField, PRIVATE);
      }

      String targetType = listenerClass.targetType();
      if (!VIEW_TYPE.equals(targetType)) {
        unbindMethod.addStatement("(($T) $N).$N($N)", bestGuess(targetType),
            fieldName, removerOrSetter(listenerClass, requiresRemoval), listenerField);
      } else {
        unbindMethod.addStatement("$N.$N($N)", fieldName,
            removerOrSetter(listenerClass, requiresRemoval), listenerField);
      }

      if (requiresRemoval) {
        unbindMethod.addStatement("$N = null", listenerField);
      }
    }

    unbindMethod.addStatement("$N = null", fieldName);

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

  private String removerOrSetter(ListenerClass listenerClass, boolean requiresRemoval) {
    return requiresRemoval
        ? listenerClass.remover()
        : listenerClass.setter();
  }

  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());

      boolean requiresCast = requiresCast(fieldBinding.getType());
      if (!debuggable || (!requiresCast && !fieldBinding.isRequired())) {
        if (requiresCast) {
          builder.add("($T) ", fieldBinding.getType());
        }
        builder.add("source.findViewById($L)", binding.getId().code);
      } else {
        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;
    }

    List requiredBindings = binding.getRequiredBindings();
    if (!debuggable || requiredBindings.isEmpty()) {
      result.addStatement("view = source.findViewById($L)", binding.getId().code);
    } else if (!binding.isBoundToRoot()) {
      result.addStatement("view = $T.findRequiredView(source, $L, $S)", UTILS,
          binding.getId().code, asHumanDescription(requiredBindings));
    }

    addFieldBinding(result, binding, debuggable);
    addMethodBindings(result, binding, debuggable);
  }

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

  private void addMethodBindings(MethodSpec.Builder result, ViewBinding binding,
      boolean debuggable) {
    Map>> classMethodBindings =
        binding.getMethodBindings();
    if (classMethodBindings.isEmpty()) {
      return;
    }

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

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

    for (Map.Entry>> e
        : classMethodBindings.entrySet()) {
      ListenerClass listener = e.getKey();
      Map> 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 hasReturnValue = false;
        CodeBlock.Builder builder = CodeBlock.builder();
        Set methodViewBindings = methodBindings.get(method);
        if (methodViewBindings != null) {
          for (MethodViewBinding methodBinding : methodViewBindings) {
            if (methodBinding.hasReturnValue()) {
              hasReturnValue = true;
              builder.add("return "); // TODO what about multiple methods?
            }
            builder.add("target.$L(", methodBinding.getName());
            List parameters = methodBinding.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])) {
                if (debuggable) {
                  builder.add("$T.castParam(p$L, $S, $L, $S, $L, $T.class)", UTILS,
                      listenerPosition, method.name(), listenerPosition, methodBinding.getName(), i,
                      parameter.getType());
                } else {
                  builder.add("($T) p$L", parameter.getType(), listenerPosition);
                }
              } else {
                builder.add("p$L", listenerPosition);
              }
            }
            builder.add(");\n");
          }
        }

        if (!"void".equals(method.returnType()) && !hasReturnValue) {
          builder.add("return $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());
      }

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

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

  private static List getListenerMethods(ListenerClass listener) {
    if (listener.method().length == 1) {
      return Arrays.asList(listener.method());
    }

    try {
      List methods = new ArrayList<>();
      Class> callbacks = listener.callbacks();
      for (Enum callbackMethod : callbacks.getEnumConstants()) {
        Field callbackField = callbacks.getField(callbackMethod.name());
        ListenerMethod method = callbackField.getAnnotation(ListenerMethod.class);
        if (method == null) {
          throw new IllegalStateException(String.format("@%s's %s.%s missing @%s annotation.",
              callbacks.getEnclosingClass().getSimpleName(), callbacks.getSimpleName(),
              callbackMethod.name(), ListenerMethod.class.getSimpleName()));
        }
        methods.add(method);
      }
      return methods;
    } catch (NoSuchFieldException e) {
      throw new AssertionError(e);
    }
  }

  static String asHumanDescription(Collection bindings) {
    Iterator iterator = bindings.iterator();
    switch (bindings.size()) {
      case 1:
        return iterator.next().getDescription();
      case 2:
        return iterator.next().getDescription() + " and " + iterator.next().getDescription();
      default:
        StringBuilder builder = new StringBuilder();
        for (int i = 0, count = bindings.size(); i < count; i++) {
          if (i != 0) {
            builder.append(", ");
          }
          if (i == count - 1) {
            builder.append("and ");
          }
          builder.append(iterator.next().getDescription());
        }
        return builder.toString();
    }
  }

  private static TypeName bestGuess(String type) {
    switch (type) {
      case "void": return TypeName.VOID;
      case "boolean": return TypeName.BOOLEAN;
      case "byte": return TypeName.BYTE;
      case "char": return TypeName.CHAR;
      case "double": return TypeName.DOUBLE;
      case "float": return TypeName.FLOAT;
      case "int": return TypeName.INT;
      case "long": return TypeName.LONG;
      case "short": return TypeName.SHORT;
      default:
        int left = type.indexOf('<');
        if (left != -1) {
          ClassName typeClassName = ClassName.bestGuess(type.substring(0, left));
          List typeArguments = new ArrayList<>();
          do {
            typeArguments.add(WildcardTypeName.subtypeOf(Object.class));
            left = type.indexOf('<', left + 1);
          } while (left != -1);
          return ParameterizedTypeName.get(typeClassName,
              typeArguments.toArray(new TypeName[typeArguments.size()]));
        }
        return ClassName.bestGuess(type);
    }
  }

  /** True when this type's bindings require a view hierarchy. */
  private boolean hasViewBindings() {
    return !viewBindings.isEmpty() || !collectionBindings.isEmpty();
  }

  /** True when this type's bindings use raw integer values instead of {@code R} references. */
  private boolean hasUnqualifiedResourceBindings() {
    for (ResourceBinding binding : resourceBindings) {
      if (!binding.id().qualifed) {
        return true;
      }
    }
    return false;
  }

  /** True when this type's bindings use Resource directly instead of Context. */
  private boolean hasResourceBindingsNeedingResource(int sdk) {
    for (ResourceBinding binding : resourceBindings) {
      if (binding.requiresResources(sdk)) {
        return true;
      }
    }
    return false;
  }

  private boolean hasMethodBindings() {
    for (ViewBinding bindings : viewBindings) {
      if (!bindings.getMethodBindings().isEmpty()) {
        return true;
      }
    }
    return false;
  }

  private boolean hasOnTouchMethodBindings() {
    for (ViewBinding bindings : viewBindings) {
      if (bindings.getMethodBindings()
          .containsKey(OnTouch.class.getAnnotation(ListenerClass.class))) {
        return true;
      }
    }
    return false;
  }

  private boolean hasFieldBindings() {
    for (ViewBinding bindings : viewBindings) {
      if (bindings.getFieldBinding() != null) {
        return true;
      }
    }
    return !collectionBindings.isEmpty();
  }

  private boolean hasTargetField() {
    return hasFieldBindings() || hasMethodBindings();
  }

  private boolean hasViewLocal() {
    for (ViewBinding bindings : viewBindings) {
      if (bindings.requiresLocal()) {
        return true;
      }
    }
    return false;
  }

  /** True if this binding requires a view. Otherwise only a context is needed. */
  private boolean constructorNeedsView() {
    return hasViewBindings() //
        || (parentBinding != null && parentBinding.constructorNeedsView());
  }

  static boolean requiresCast(TypeName type) {
    return !VIEW_TYPE.equals(type.toString());
  }

  @Override public String toString() {
    return bindingClassName.toString();
  }

  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");

    boolean isFinal = enclosingElement.getModifiers().contains(Modifier.FINAL);
    return new Builder(targetType, bindingClassName, isFinal, isView, isActivity, isDialog);
  }

  static final class Builder {
    private final TypeName targetTypeName;
    private final ClassName bindingClassName;
    private final boolean isFinal;
    private final boolean isView;
    private final boolean isActivity;
    private final boolean isDialog;

    private @Nullable BindingSet parentBinding;

    private final Map viewIdMap = new LinkedHashMap<>();
    private final ImmutableList.Builder collectionBindings =
        ImmutableList.builder();
    private final ImmutableList.Builder resourceBindings = ImmutableList.builder();

    private Builder(TypeName targetTypeName, ClassName bindingClassName, boolean isFinal,
        boolean isView, boolean isActivity, boolean isDialog) {
      this.targetTypeName = targetTypeName;
      this.bindingClassName = bindingClassName;
      this.isFinal = isFinal;
      this.isView = isView;
      this.isActivity = isActivity;
      this.isDialog = isDialog;
    }

    void addField(Id id, FieldViewBinding binding) {
      getOrCreateViewBindings(id).setFieldBinding(binding);
    }

    void addFieldCollection(FieldCollectionViewBinding binding) {
      collectionBindings.add(binding);
    }

    boolean addMethod(
        Id id,
        ListenerClass listener,
        ListenerMethod method,
        MethodViewBinding binding) {
      ViewBinding.Builder viewBinding = getOrCreateViewBindings(id);
      if (viewBinding.hasMethodBinding(listener, method) && !"void".equals(method.returnType())) {
        return false;
      }
      viewBinding.addMethodBinding(listener, method, binding);
      return true;
    }

    void addResource(ResourceBinding binding) {
      resourceBindings.add(binding);
    }

    void setParent(BindingSet parent) {
      this.parentBinding = parent;
    }

    @Nullable String findExistingBindingName(Id id) {
      ViewBinding.Builder builder = viewIdMap.get(id);
      if (builder == null) {
        return null;
      }
      FieldViewBinding fieldBinding = builder.fieldBinding;
      if (fieldBinding == null) {
        return null;
      }
      return fieldBinding.getName();
    }

    private ViewBinding.Builder getOrCreateViewBindings(Id id) {
      ViewBinding.Builder viewId = viewIdMap.get(id);
      if (viewId == null) {
        viewId = new ViewBinding.Builder(id);
        viewIdMap.put(id, viewId);
      }
      return viewId;
    }

    BindingSet build() {
      ImmutableList.Builder 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);
    }
  }
}

利用javapoet 生成对应的Unbinder class，原理很简单。

总结：

1.编译的时候遇到@BindView 或者其他在 processor 里面添加的其他注解，就会去生成对应的Unbinder 实现类 文件

2.调用ButterKinfe.bind 方法会去查询 Unbinder 的构造方法并实例化一个Unbinder，在Unbinder 的构造方法里面去初始化那些注解绑定的view id->findviewById

3.难点： javapoet 生成 实现类