ButterKnife第三方库源码分析

ButterKnife原理其实也很简单

我的Github博客详解

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ButterKnife是大名鼎鼎JakeWharton热门开源项目的其中一个，让开发者不再重复的进行findViewById的操作。

配合android studio的插件，一键自动生成xml文件所有view的实例。

ButterKnife为什么会那么神奇，自动帮助开发者省去了繁琐的操作，他的实现的原理到底是怎么样的呢？

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下面我们从代码使用上，一步一步的分析ButterKnife的实现原理

分析ButterKnife版本：com.jakewharton:butterknife:7.0.1

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1. 使用ButterKnife快速初始化xml布局对象.

        public class MainActivity extends AppCompatActivity {          @Bind(R.id.tv01) TextView tv01;          @Bind(R.id.tv02) TextView tv02;          @Bind(R.id.tv03) TextView tv03;          @Bind(R.id.tv04) TextView tv04;          @Bind(R.id.activity_main) LinearLayout activityMain;          @OnClick(R.id.tv01)          public void test(View v) {          }          @Override protected void onCreate(Bundle savedInstanceState) {            super.onCreate(savedInstanceState);            setContentView(R.layout.activity_main);            ButterKnife.bind(this);          }        }

一段很常规使用ButterKnife快速初始化xml布局对象的代码。为什么当onCreate方法的ButterKnife.bind(this);

调用完毕，xml所有的布局对象都初始化好了呢？

我们进入到@Bind注解里看看究竟，看看是否找得到线索

        @Retention(CLASS) @Target(FIELD)        public @interface Bind {          /** View ID to which the field will be bound. */          int[] value();        }

可以看到@Retention(CLASS)，这句表示：保留时间 编译时，也就是工程编译时运行的注解.

常规获取View对象的方式是这样的：

        tv01 = (TextView) findViewById(R.id.tv01);        //现在变成这样        @Bind(R.id.tv01) TextView tv01;

所以说，ButterKnife这个库应该拿到了R.id.tv01 这个id值，也拿到了tv01成员变量，在通过findViewById给tv01赋值

但是，ButterKnife在哪里进行这样的操作呢？

从@Bind注解来看，应该是编译时拿到了id值

我们在build目录下找到了ButterKnife生成的新文件：

build\generated\source\apt\debug\com\butterknifedemo\MainActivity$$ViewBinder

        // Generated code from Butter Knife. Do not modify!        package com.butterknifedemo;        import android.view.View;        import butterknife.ButterKnife.Finder;        import butterknife.ButterKnife.ViewBinder;        public class MainActivity$$ViewBinder<T extends com.butterknifedemo.MainActivity> implements ViewBinder<T> {          @Override public void bind(final Finder finder, final T target, Object source) {            View view;            view = finder.findRequiredView(source, 2131427413, "field 'tv01' and method 'test'");            target.tv01 = finder.castView(view, 2131427413, "field 'tv01'");            view.setOnClickListener(              new butterknife.internal.DebouncingOnClickListener() {                @Override public void doClick(                  android.view.View p0                ) {                  target.test(p0);                }              });            view = finder.findRequiredView(source, 2131427414, "field 'tv02'");            target.tv02 = finder.castView(view, 2131427414, "field 'tv02'");            view = finder.findRequiredView(source, 2131427415, "field 'tv03'");            target.tv03 = finder.castView(view, 2131427415, "field 'tv03'");            view = finder.findRequiredView(source, 2131427416, "field 'tv04'");            target.tv04 = finder.castView(view, 2131427416, "field 'tv04'");            view = finder.findRequiredView(source, 2131427412, "field 'activityMain'");            target.activityMain = finder.castView(view, 2131427412, "field 'activityMain'");          }          @Override public void unbind(T target) {            target.tv01 = null;            target.tv02 = null;            target.tv03 = null;            target.tv04 = null;            target.activityMain = null;          }        }

我们发现ButterKnife在build目录下生成了一个类，这个类竟然帮助我们完成了findVieweById的操作

那这个类是怎么制作出来的呢？

现在，我们直接去看ButterKnife源码：

        //先进入里面看看        ButterKnife.bind(this);      //显然Activity对象作为target往下传递了      //Finder.ACTIVITY 是什么呢      public static void bind(Activity target) {        bind(target, target, Finder.ACTIVITY);      }        //Finder.ACTIVITY 原来是 ButterKnife 内部枚举        //return ((Activity) source).findViewById(id); 注意看句代码        public final class ButterKnife {          private ButterKnife() {            throw new AssertionError("No instances.");          }          /** DO NOT USE: Exposed for generated code. */          @SuppressWarnings("UnusedDeclaration") // Used by generated code.          public enum Finder {            VIEW {              @Override protected View findView(Object source, int id) {                return ((View) source).findViewById(id);              }              @Override public Context getContext(Object source) {                return ((View) source).getContext();              }            },            ACTIVITY {              @Override protected View findView(Object source, int id) {                return ((Activity) source).findViewById(id);              }              @Override public Context getContext(Object source) {                return (Activity) source;              }            },            DIALOG {              @Override protected View findView(Object source, int id) {                return ((Dialog) source).findViewById(id);              }              @Override public Context getContext(Object source) {                return ((Dialog) source).getContext();              }            };      //findViewBinderForClass这个方法通过Activity对象去查找返回了一个ViewBinder类，      //然后viewBinder.bind(finder, target, source);      static void bind(Object target, Object source, Finder finder) {        Class<?> targetClass = target.getClass();        try {          if (debug) Log.d(TAG, "Looking up view binder for " + targetClass.getName());          ViewBinder<Object> viewBinder = findViewBinderForClass(targetClass);          if (viewBinder != null) {            viewBinder.bind(finder, target, source);          }        } catch (Exception e) {          throw new RuntimeException("Unable to bind views for " + targetClass.getName(), e);        }      }

这里有两个问题：
1. findViewBinderForClass通过Activity字节码如何找到的viewBinder？
2. viewBinder是什么？

首先，看viewBinder是什么：

        /** DO NOT USE: Exposed for generated code. */        public interface ViewBinder<T> {        void bind(Finder finder, T target, Object source);        void unbind(T target);        }

原来是个接口，注释说自动生成代码用的，看看刚刚在build找到的类：

        public class MainActivity$$ViewBinder        <T extends com.butterknifedemo.MainActivity> implements ViewBinder<T> {          @Override public void bind(final Finder finder, final T target, Object source) {

原来viewBinder.bind(finder, target, source);这行代码调用了MainActivity$$ViewBinder类里的bind方法，

帮助我们完成findViewById工作

现在我们知道了，平时我们调用ButterKnife.bind(this);

最终都会调用对应生成的$$ViewBinder类里的bind方法帮助我们完成繁琐的操作

问题2：怎么通过Activity字节码找到viewBinder对象的？

      private static ViewBinder<Object> findViewBinderForClass(Class<?> cls)          throws IllegalAccessException, InstantiationException {        ViewBinder<Object> viewBinder = BINDERS.get(cls);        if (viewBinder != null) {          if (debug) Log.d(TAG, "HIT: Cached in view binder map.");          return viewBinder;        }        String clsName = cls.getName();        if (clsName.startsWith(ANDROID_PREFIX) || clsName.startsWith(JAVA_PREFIX)) {          if (debug) Log.d(TAG, "MISS: Reached framework class. Abandoning search.");          return NOP_VIEW_BINDER;        }        try {          Class<?> viewBindingClass =                 Class.forName(clsName + ButterKnifeProcessor.SUFFIX);//SUFFIX = "$$ViewBinder";          //noinspection unchecked          viewBinder = (ViewBinder<Object>) viewBindingClass.newInstance();          if (debug) Log.d(TAG, "HIT: Loaded view binder class.");        } catch (ClassNotFoundException e) {          if (debug) Log.d(TAG, "Not found. Trying superclass " + cls.getSuperclass().getName());          viewBinder = findViewBinderForClass(cls.getSuperclass());        }        BINDERS.put(cls, viewBinder);        return viewBinder;      }

原来先从BINDERS.get(cls);里面取，空的话在通过Class.forName(clsName + ButterKnifeProcessor.SUFFIX);

创建出一个新对象出来，前提是这个$$ViewBinder已经生成好了

什么时候生成这个类，怎么生成的？通过注解@Bind我们应该猜到，是工程编译时就生成好了的

在ButterKnife源码中我们发现了这一个类：

        public final class ButterKnifeProcessor extends AbstractProcessor {          public static final String SUFFIX = "$$ViewBinder";          public static final String ANDROID_PREFIX = "android.";          public static final String JAVA_PREFIX = "java.";          static final String VIEW_TYPE = "android.view.View";          private static final String COLOR_STATE_LIST_TYPE = "android.content.res.ColorStateList";          private static final String DRAWABLE_TYPE = "android.graphics.drawable.Drawable";          private static final String NULLABLE_ANNOTATION_NAME = "Nullable";          private static final String ITERABLE_TYPE = "java.lang.Iterable<?>";          private static final String LIST_TYPE = List.class.getCanonicalName();          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 //          );      @Override public Set<String> getSupportedAnnotationTypes() {        Set<String> types = new LinkedHashSet<String>();        types.add(Bind.class.getCanonicalName());        for (Class<? extends Annotation> listener : LISTENERS) {          types.add(listener.getCanonicalName());        }        types.add(BindBool.class.getCanonicalName());        types.add(BindColor.class.getCanonicalName());        types.add(BindDimen.class.getCanonicalName());        types.add(BindDrawable.class.getCanonicalName());        types.add(BindInt.class.getCanonicalName());        types.add(BindString.class.getCanonicalName());        return types;      }

extends AbstractProcessor，继承这一个类，表示它可以在工程编译是运行里面的process方法，

ButterKnife就是通过编译时，apt会自动查找集成AbstractProcessor的类，调用process方法

在process方法中找到存在ButterKnife的注解信息，获取在注解对应下的数据，例如id值

上面的代码我们还可以看到，Bind，OnClick等等注解已经存储好了，就等着遍历配对处理获取数据

看process方法：

        @Override public boolean process(Set<? extends TypeElement> elements, RoundEnvironment env) {        Map<TypeElement, BindingClass> targetClassMap = findAndParseTargets(env);        for (Map.Entry<TypeElement, BindingClass> entry : targetClassMap.entrySet()) {          TypeElement typeElement = entry.getKey();          BindingClass bindingClass = entry.getValue();          try {            JavaFileObject jfo = filer.createSourceFile(bindingClass.getFqcn(), typeElement);            Writer writer = jfo.openWriter();            writer.write(bindingClass.brewJava());            writer.flush();            writer.close();          } catch (IOException e) {            error(typeElement, "Unable to write view binder for type %s: %s", typeElement,                e.getMessage());          }        }        return true;        }

从JavaFileObject，Writer这个类就可以知道，ButterKnife把一些东西写到文件中去了，应该猜到

那些自动生成的java文件就从这里出来的

代码生成java文件，代码加载java文件去运行，有点意思

先看看findAndParseTargets方法做了什么：

        private Map<TypeElement, BindingClass> findAndParseTargets(RoundEnvironment env) {        Map<TypeElement, BindingClass> targetClassMap = new LinkedHashMap<TypeElement, BindingClass>();        Set<String> erasedTargetNames = new LinkedHashSet<String>();        // Process each @Bind element.        for (Element element : env.getElementsAnnotatedWith(Bind.class)) {          try {            parseBind(element, targetClassMap, erasedTargetNames);          } catch (Exception e) {            logParsingError(element, Bind.class, e);          }        }        // Process each annotation that corresponds to a listener.        for (Class<? extends Annotation> listener : LISTENERS) {          findAndParseListener(env, listener, targetClassMap, erasedTargetNames);        }        // Process each @BindBool element.        for (Element element : env.getElementsAnnotatedWith(BindBool.class)) {          try {            parseResourceBool(element, targetClassMap, erasedTargetNames);          } catch (Exception e) {            logParsingError(element, BindBool.class, e);          }        }        ....    

可以看到，正在查找Bind之类的注解，猜都猜到通过定位注解获取注解下面的值了吧

现在，我们回过头看看把什么东西写到文件中去了，看着行代码： writer.write(bindingClass.brewJava());

        String brewJava() {        StringBuilder builder = new StringBuilder();        builder.append("// Generated code from Butter Knife. Do not modify!\n");        builder.append("package ").append(classPackage).append(";\n\n");        if (!resourceBindings.isEmpty()) {          builder.append("import android.content.res.Resources;\n");        }        if (!viewIdMap.isEmpty() || !collectionBindings.isEmpty()) {          builder.append("import android.view.View;\n");        }        builder.append("import butterknife.ButterKnife.Finder;\n");        if (parentViewBinder == null) {          builder.append("import butterknife.ButterKnife.ViewBinder;\n");        }        builder.append('\n');        builder.append("public class ").append(className);        builder.append("<T extends ").append(targetClass).append(">");        if (parentViewBinder != null) {          builder.append(" extends ").append(parentViewBinder).append("<T>");        } else {          builder.append(" implements ViewBinder<T>");        }        builder.append(" {\n");        emitBindMethod(builder);        builder.append('\n');        emitUnbindMethod(builder);        builder.append("}\n");        return builder.toString();        }

可以一目了然的看到使用了StringBuilder手动拼接字符串的方式，生成了java文件，挺不容易的。

问题又来了，这些java文件静态不变化的部分可以写死，那些动态灵活的部分呢？例如View的id，对象名称

继续深入看源码：

        //bind方法代码拼接        private void emitBindMethod(StringBuilder builder) {        builder.append("  @Override ")            .append("public void bind(final Finder finder, final T target, Object source) {\n");        // Emit a call to the superclass binder, if any.        if (parentViewBinder != null) {          builder.append("    super.bind(finder, target, source);\n\n");        }        if (!viewIdMap.isEmpty() || !collectionBindings.isEmpty()) {          // Local variable in which all views will be temporarily stored.          builder.append("    View view;\n");          // Loop over each view bindings and emit it.          for (ViewBindings bindings : viewIdMap.values()) {            //进入里面看看            emitViewBindings(builder, bindings);          }          private void emitViewBindings(StringBuilder builder, ViewBindings bindings) {            builder.append("    view = ");            List<ViewBinding> requiredViewBindings = bindings.getRequiredBindings();            if (requiredViewBindings.isEmpty()) {              builder.append("finder.findOptionalView(source, ")                  .append(bindings.getId())//这个就是View的id了                  .append(", null);\n");            } else {              if (bindings.getId() == View.NO_ID) {                builder.append("target;\n");              } else {                builder.append("finder.findRequiredView(source, ")                    .append(bindings.getId())                    .append(", \"");                emitHumanDescription(builder, requiredViewBindings);                builder.append("\");\n");              }            }            //字段看这里，进去            emitFieldBindings(builder, bindings);            emitMethodBindings(builder, bindings);          }        static void emitHumanDescription(StringBuilder builder,              Collection<? extends ViewBinding> bindings) {            Iterator<? extends ViewBinding> iterator = bindings.iterator();            switch (bindings.size()) {              case 1:                builder.append(iterator.next().getDescription());//View变量名称                break;              case 2:                builder.append(iterator.next().getDescription())        @Override public String getDescription() {            return "field '" + name + "'";          }

从上面的代码可以知道，动态的部分通过BindingClass这个类来获取的，那这是类怎么来的，看之前的代码：

          @Override public boolean process(Set<? extends TypeElement> elements, RoundEnvironment env) {            Map<TypeElement, BindingClass> targetClassMap = findAndParseTargets(env);

原来这个类是从findAndParseTargets里来的，刚刚我们知道，里面做了定位Bind，OnClick注解的操作，

定位的同时也把注解的值，例如id值，变量名称存在到BindingClass对象中了，很符合面向对象的思想

        private void parseBindOne(Element element, Map<TypeElement, BindingClass> targetClassMap,          Set<String> erasedTargetNames) {        boolean hasError = false;        TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();            ...            BindingClass bindingClass = targetClassMap.get(enclosingElement);        if (bindingClass != null) {          ViewBindings viewBindings = bindingClass.getViewBinding(id);          if (viewBindings != null) {            Iterator<FieldViewBinding> iterator = viewBindings.getFieldBindings().iterator();            if (iterator.hasNext()) {              FieldViewBinding existingBinding = iterator.next();              error(element, "Attempt to use @%s for an already bound ID %d on '%s'. (%s.%s)",                  Bind.class.getSimpleName(), id, existingBinding.getName(),                  enclosingElement.getQualifiedName(), element.getSimpleName());              return;            }          }        } else {          bindingClass = getOrCreateTargetClass(targetClassMap, enclosingElement);        }        String name = element.getSimpleName().toString();        String type = elementType.toString();        boolean required = isRequiredBinding(element);        FieldViewBinding binding = new FieldViewBinding(name, type, required);        bindingClass.addField(id, binding);        // Add the type-erased version to the valid binding targets set.        erasedTargetNames.add(enclosingElement.toString());        }      private BindingClass getOrCreateTargetClass(Map<TypeElement, BindingClass> targetClassMap,          TypeElement enclosingElement) {        BindingClass bindingClass = targetClassMap.get(enclosingElement);        if (bindingClass == null) {          String targetType = enclosingElement.getQualifiedName().toString();          String classPackage = getPackageName(enclosingElement);          String className = getClassName(enclosingElement, classPackage) + SUFFIX;          bindingClass = new BindingClass(classPackage, className, targetType);          targetClassMap.put(enclosingElement, bindingClass);        }        return bindingClass;      }

最后，还有一点的是：ButterKnife通过注解获取id值并没有使用到反射，获取到变量也是通过Activity.view的形式

不同与一些反射获取注解的框架，使用反射会增加IO操作，增加了时间操作，多了会变得卡顿

反射的方式成员变量可使用private，而ButterKnife不可以，必须public或者protected

因为ButterKnife没有使用反射，需要Activity.view这样去获取一些对象赋值

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具体可以去看源码，总的来说：

1, ButterKnife将View的id值放到@Bind注解中
2, ButterKnife通过extends AbstractProcessor编译时自动调用process方法来定位和存在注解与注解上的id值
3, 找到所有带注解与值的对象，存储在集合中，一个for循环一顿狂写，把java文件写到build目录下
4, 当调用ButterKnife.bind(this)的时候，最终会调用生成的viewBinder类里的bind方法
5, viewBinder里的bind方法，找已动态生成好了finfViewById的过程，通过Activity.view的形式初始化所有view

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分析就到这里了

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11/9/2016 11:33:29 PM