EventBus 开源库学习（三）-编程知识

源码细节阅读

上一节根据EventBus的使用流程把实现源码大体梳理了一遍，因为精力有限，所以看源码都是根据实现过程把基本流程看下，中间实现细节先忽略，否则越看越深不容易把握大体思路，这节把一些细节的部分再看看。

注解函数查找源码逻辑

#EventBus   public void register(Object subscriber) {Class<?> subscriberClass = subscriber.getClass();List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);synchronized (this) {for (SubscriberMethod subscriberMethod : subscriberMethods) {subscribe(subscriber, subscriberMethod);}}}

在进行注册的时候，我们使用subscriberMethodFinder对象的findSubscriberMethods方法来查找到所有该类中以@Subscribe注解的函数（以下简称为注解函数）。让我们继续看下查找部分的逻辑。

    List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass) {List<SubscriberMethod> subscriberMethods = METHOD_CACHE.get(subscriberClass);if (subscriberMethods != null) {return subscriberMethods;}if (ignoreGeneratedIndex) {subscriberMethods = findUsingReflection(subscriberClass);} else {subscriberMethods = findUsingInfo(subscriberClass);}if (subscriberMethods.isEmpty()) {throw new EventBusException("Subscriber " + subscriberClass+ " and its super classes have no public methods with the @Subscribe annotation");} else {METHOD_CACHE.put(subscriberClass, subscriberMethods);return subscriberMethods;}}

METHOD_CACHE是一个ConcurrentHashMap类型的常量，用来保存subscriberClass和对应SubscriberMethod的集合，以提高注册效率，防止重复查找。

如果METHOD_CACHE缓存中不存在，判断变量ignoreGeneratedIndex的值，该值是在EventBusBuilder中初始化的，表示是否忽略注解生成器，默认是false。然后会进入到findUsingInfo函数中进行查找（下面再看），最后将找到的subscriberMethods添加到METHOD_CACHE中。

    private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {FindState findState = prepareFindState();findState.initForSubscriber(subscriberClass);````return getMethodsAndRelease(findState);}

先看findUsingInfo前两行，出现一个FindState类，它是SubscriberMethodFinder的内部类，用来辅助查找订阅事件的方法。

    private FindState prepareFindState() {synchronized (FIND_STATE_POOL) {for (int i = 0; i < POOL_SIZE; i++) {FindState state = FIND_STATE_POOL[i];if (state != null) {FIND_STATE_POOL[i] = null;return state;}}}return new FindState();}static class FindState {```void initForSubscriber(Class<?> subscriberClass) {this.subscriberClass = clazz = subscriberClass;skipSuperClasses = false;subscriberInfo = null;}}

prepareFindState()方法主要是返回一个FindState对象，先从FIND_STATE_POOL这个缓存池中获取一个现成的，如果没有就新建一个对象。获取对象后调用初始化方法，将subscriberClass赋值给clazz变量，subscriberInfo对象赋值为null。

    private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {FindState findState = prepareFindState();findState.initForSubscriber(subscriberClass);while (findState.clazz != null) {findState.subscriberInfo = getSubscriberInfo(findState);if (findState.subscriberInfo != null) {```} else {findUsingReflectionInSingleClass(findState);}findState.moveToSuperclass();}return getMethodsAndRelease(findState);}

再回到findUsingInfo，因为findState.clazz刚赋值过，所以不为空。再来看下getSubscriberInfo方法。

    private SubscriberInfo getSubscriberInfo(FindState findState) {if (findState.subscriberInfo != null && findState.subscriberInfo.getSuperSubscriberInfo() != null) {SubscriberInfo superclassInfo = findState.subscriberInfo.getSuperSubscriberInfo();if (findState.clazz == superclassInfo.getSubscriberClass()) {return superclassInfo;}}if (subscriberInfoIndexes != null) {for (SubscriberInfoIndex index : subscriberInfoIndexes) {SubscriberInfo info = index.getSubscriberInfo(findState.clazz);if (info != null) {return info;}}}return null;}

前面执行initForSubscriber的时候，subscriberInfo赋值为null，因此第一个if不成立。第二个判断是对象subscriberInfoIndexes，该变量也是在EventBusBuilder中初始化的，默认是null，因此该方法返回值null。

    private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {FindState findState = prepareFindState();findState.initForSubscriber(subscriberClass);while (findState.clazz != null) {findState.subscriberInfo = getSubscriberInfo(findState);if (findState.subscriberInfo != null) {```} else {findUsingReflectionInSingleClass(findState);}findState.moveToSuperclass();}return getMethodsAndRelease(findState);}

再回到findUsingInfo，由于getSubscriberInfo返回为null，逻辑会走到findUsingReflectionInSingleClass方法中，使用反射来获取所有方法（下面再看）。

获取当前类所有的注解方法后，findState.moveToSuperclass()表示修改findState.clazz为subscriberClass的父类Class，继续遍历父类中的注解方法。

private static final int MODIFIERS_IGNORE = Modifier.ABSTRACT | Modifier.STATIC | BRIDGE | SYNTHETIC;private void findUsingReflectionInSingleClass(FindState findState) {Method[] methods;try {// This is faster than getMethods, especially when subscribers are fat classes like Activitiesmethods = findState.clazz.getDeclaredMethods();} catch (Throwable th) {// Workaround for java.lang.NoClassDefFoundError, see https://github.com/greenrobot/EventBus/issues/149```}for (Method method : methods) {int modifiers = method.getModifiers();if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {Class<?>[] parameterTypes = method.getParameterTypes();if (parameterTypes.length == 1) {Subscribe subscribeAnnotation = method.getAnnotation(Subscribe.class);if (subscribeAnnotation != null) {Class<?> eventType = parameterTypes[0];if (findState.checkAdd(method, eventType)) {ThreadMode threadMode = subscribeAnnotation.threadMode();findState.subscriberMethods.add(new SubscriberMethod(method, eventType, threadMode,subscribeAnnotation.priority(), subscribeAnnotation.sticky()));}}} else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {String methodName = method.getDeclaringClass().getName() + "." + method.getName();throw new EventBusException("@Subscribe method " + methodName +"must have exactly 1 parameter but has " + parameterTypes.length);}} else if (strictMethodVerification && method.isAnnotationPresent(Subscribe.class)) {String methodName = method.getDeclaringClass().getName() + "." + method.getName();throw new EventBusException(methodName +" is a illegal @Subscribe method: must be public, non-static, and non-abstract");}}}

上面代码流程如下：

通过方法getDeclaredMethods()获取该类中所有声明的方法列表；
遍历方法列表，获取方法的修饰符，如果修饰符是public并且不是abstract、static等进入下一步，MODIFIERS_IGNORE定义如第一行代码；
获取方法的参数列表，如果参数的个数是1，并且使用了Subscribe注解，获取唯一的入参进行下一步；
然后checkAdd()方法用来判断FindState的anyMethodByEventType map是否已经添加过以当前eventType为key的键值对，没添加过则返回true（就是看看当前类中有没有多个事件相同的注解函数）；
通过注解对象subscribeAnnotation获取threadMode，然后新建SubscriberMethod添加到findState.subscriberMethods集合中。

分析完findUsingReflectionInSingleClass这个方法后，我们回到findUsingInfo。

    private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {FindState findState = prepareFindState();findState.initForSubscriber(subscriberClass);while (findState.clazz != null) {findState.subscriberInfo = getSubscriberInfo(findState);if (findState.subscriberInfo != null) {```} else {findUsingReflectionInSingleClass(findState);}findState.moveToSuperclass();}return getMethodsAndRelease(findState);}

遍历当前类及其父类，通过findUsingReflectionInSingleClass方法找到所有注解方法后，通过getMethodsAndRelease返回所有相关方法。

    private List<SubscriberMethod> getMethodsAndRelease(FindState findState) {List<SubscriberMethod> subscriberMethods = new ArrayList<>(findState.subscriberMethods);findState.recycle();synchronized (FIND_STATE_POOL) {for (int i = 0; i < POOL_SIZE; i++) {if (FIND_STATE_POOL[i] == null) {FIND_STATE_POOL[i] = findState;break;}}}return subscriberMethods;}

通过上面的解析知道，所有的SubscriberMethod都添加到findState.subscriberMethods集合中，这个方法就是把findState.subscriberMethods集合中的内容复制出来，然后释放findState中的资源，并把findState放到FIND_STATE_POOL缓存中，POOL_SIZE常量是4，这样下次查找的时候可以重复利用无需每次都新建对象。

以上为注解方法查找的过程。在上面分析的过程中出现了两个变量subscriberInfoIndexes和ignoreGeneratedIndex看样子都是关于索引的，分析的时候都是使用的EventBusBuilder中的默认值。看下subscriberInfoIndexes赋值的地方，看注释意思是：将EventBus注解处理器生成索引添加添加进来。所以注解处理器需要再了解下。

    /** Adds an index generated by EventBus' annotation preprocessor. */public EventBusBuilder addIndex(SubscriberInfoIndex index) {if (subscriberInfoIndexes == null) {subscriberInfoIndexes = new ArrayList<>();}subscriberInfoIndexes.add(index);return this;}

Subscriber Index注解处理器

通过上面分析可知，EventBus注册事件时，主要是在项目运行时通过反射来查找注解方法信息，如果项目中有大量的注解方法，必然会对项目运行时的性能产生影响。

除了在项目运行时通过反射查找注解方法信息，EventBus 还提供了在项目编译时通过注解处理器查找注解方法信息的方式，生成一个辅助的索引类来保存这些信息，这个索引类就是Subscriber Index。

1、Subscriber Index使用方式
首先要在 app 的build.gradle中加入如下配置：

android {defaultConfig {javaCompileOptions {annotationProcessorOptions {// 指定辅助索引类的名称和包名，注意，这个类时自动生成的，不需要我们写arguments = [ eventBusIndex : 'com.jane.demo.MyEventBusIndex' ]}}}
}
dependencies {compile 'org.greenrobot:eventbus:3.3.1'// 引入注解处理器annotationProcessor 'org.greenrobot:eventbus-annotation-processor:3.3.1'
}

在项目的Application中添加如下配置，以生成一个默认的 EventBus 单例：

EventBus.builder().addIndex(new MyEventBusIndex()).installDefaultEventBus();

此时重新编译下项目，会生成一个MyEventBusIndex类（不同的gradle版本生成的位置可能不一样）。

生成的MyEventBusIndex代码如下：

public class MyEventBusIndex implements SubscriberInfoIndex {private static final Map<Class<?>, SubscriberInfo> SUBSCRIBER_INDEX;static {SUBSCRIBER_INDEX = new HashMap<Class<?>, SubscriberInfo>();putIndex(new SimpleSubscriberInfo(EventBusService.class, true, new SubscriberMethodInfo[] {new SubscriberMethodInfo("onMsgEventReceived", String.class),new SubscriberMethodInfo("onMsgEventReceived", MsgEvent.class),new SubscriberMethodInfo("onMsgEventReceived", org.greenrobot.eventbus.NoSubscriberEvent.class),}));putIndex(new SimpleSubscriberInfo(EventBusActivity.class, true, new SubscriberMethodInfo[] {new SubscriberMethodInfo("onMsgEventReceived", Event.class),}));}private static void putIndex(SubscriberInfo info) {SUBSCRIBER_INDEX.put(info.getSubscriberClass(), info);}@Overridepublic SubscriberInfo getSubscriberInfo(Class<?> subscriberClass) {SubscriberInfo info = SUBSCRIBER_INDEX.get(subscriberClass);if (info != null) {return info;} else {return null;}}
}

代码中SUBSCRIBER_INDEX是一个HashMap，保存了当前注册类的Class类型和其中注解方法的信息。

2、Subscriber Index源码

EventBus.builder().addIndex(new MyEventBusIndex()).installDefaultEventBus();

使用的时候，获取EventBusBuilder对象，然后调用addIndex方法，把生成的MyEventBusIndex加入进去。

    /** Adds an index generated by EventBus' annotation preprocessor. */public EventBusBuilder addIndex(SubscriberInfoIndex index) {if (subscriberInfoIndexes == null) {subscriberInfoIndexes = new ArrayList<>();}subscriberInfoIndexes.add(index);return this;}

上面的方法是不是比较眼熟，就是上面代码分析的时候出现的变量subscriberInfoIndexes赋值函数。这样，运行的时候就把之前编译好的索引类的实例保存在subscriberInfoIndexes集合中。然后调用installDefaultEventBus方法创建EventBus实例。

    /*** Installs the default EventBus returned by {@link EventBus#getDefault()} using this builders' values. Must be* done only once before the first usage of the default EventBus.** @throws EventBusException if there's already a default EventBus instance in place*/public EventBus installDefaultEventBus() {synchronized (EventBus.class) {if (EventBus.defaultInstance != null) {throw new EventBusException("Default instance already exists." +" It may be only set once before it's used the first time to ensure consistent behavior.");}EventBus.defaultInstance = build();return EventBus.defaultInstance;}}/** Builds an EventBus based on the current configuration. */public EventBus build() {return new EventBus(this);}

上面通过build方法，以当前EventBusBuilder对象作为参数生成EventBus单例对象，并赋值给defaultInstance，这样就把subscriberInfoIndexes信息传递给了EventBus。以后调用EventBus 的getDefault()返回的就是这里生成的单例对象，里面包含了subscriberInfoIndexes信息。

而且在Application中生成了 EventBus 的默认单例，这样就保证了在项目其它地方执行EventBus.getDefault()就能得到就是上面包含subscriberInfoIndexes信息的单例。

在上面分析查找注解函数的时候，分析过一个函数：

    private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {FindState findState = prepareFindState();findState.initForSubscriber(subscriberClass);while (findState.clazz != null) {findState.subscriberInfo = getSubscriberInfo(findState);if (findState.subscriberInfo != null) {····} else {findUsingReflectionInSingleClass(findState);}findState.moveToSuperclass();}return getMethodsAndRelease(findState);}

如果没有使用注解处理器的话getSubscriberInfo这个方法返回值是null，因此会走到findUsingReflectionInSingleClass，通过反射区获取注解方法。现在使用使用注解处理器的话在重新看下这个函数。

    private SubscriberInfo getSubscriberInfo(FindState findState) {if (findState.subscriberInfo != null && findState.subscriberInfo.getSuperSubscriberInfo() != null) {SubscriberInfo superclassInfo = findState.subscriberInfo.getSuperSubscriberInfo();if (findState.clazz == superclassInfo.getSubscriberClass()) {return superclassInfo;}}//不为空if (subscriberInfoIndexes != null) {for (SubscriberInfoIndex index : subscriberInfoIndexes) {SubscriberInfo info = index.getSubscriberInfo(findState.clazz);if (info != null) {return info;}}}return null;}@Overridepublic SubscriberInfo getSubscriberInfo(Class<?> subscriberClass) {SubscriberInfo info = SUBSCRIBER_INDEX.get(subscriberClass);if (info != null) {return info;} else {return null;}}

如果使用了注解处理器，因为subscriberInfoIndexes这个集合不为空，遍历获取当前类对应的注解方法列表， index.getSubscriberInfo这个方法实际调用的就是系统生成的MyEventBusIndex中的getSubscriberInfo方法，这样就获取到了之前编译生成的注解方法。

    private List<SubscriberMethod> findUsingInfo(Class<?> subscriberClass) {FindState findState = prepareFindState();findState.initForSubscriber(subscriberClass);while (findState.clazz != null) {findState.subscriberInfo = getSubscriberInfo(findState);if (findState.subscriberInfo != null) {SubscriberMethod[] array = findState.subscriberInfo.getSubscriberMethods();for (SubscriberMethod subscriberMethod : array) {if (findState.checkAdd(subscriberMethod.method, subscriberMethod.eventType)) {findState.subscriberMethods.add(subscriberMethod);}}} else {findUsingReflectionInSingleClass(findState);}findState.moveToSuperclass();}return getMethodsAndRelease(findState);}