EventBus源码分析1

EventBus是一个基于观察者模式的一个开源框架，我这里从两个入口分析源码，一个是EventBus.getDefaut().register(),一个是EventBus.getDefaut().post(new Event())，分两篇文章分析。

注意：下面的代码EventBus是3.1.1版本的

compile 'org.greenrobot:eventbus:3.1.1'

我们可以在MainActivity的onCreate中执行EventBus.getDefault().regitser(this),在onDestroy中执行unregister(this),另外还需要一个注解@Subscribe下定义一个方法

代码如下：

  private Button secondAcitivity;

    private TextView showTv;

    @Override

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        EventBus.getDefault().register(this);

        secondAcitivity = (Button) findViewById(R.id.secondAcitivity);

        showTv = (TextView) findViewById(R.id.showTv);

        secondAcitivity.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View v) {

                Intent intent = new Intent(MainActivity.this,SecondActivity.class);

                startActivity(intent);

            }

        });

    }

    @Override

    protected void onDestroy() {

        EventBus.getDefault().unregister(this);

        super.onDestroy();

    }

    @Subscribe(threadMode = ThreadMode.MAIN)

    public void onClick(ClickEvent event){

            showTv.setText("clicked....");

    }

再创建一个SecondActivity，在页面中放一个按钮，点击按钮时执行post,代码如下

button.setOnClickListener(new View.OnClickListener() {

@Override

public void onClick(View v) {

EventBus.getDefault().post(new ClickEvent());

}

});

下面从入口EventBus.getDefault().register(this) 开始分析

先看getDefault()

    public static EventBus getDefault() {

        if (defaultInstance == null) {

            synchronized (EventBus.class) {

                if (defaultInstance == null) {

                    defaultInstance = new EventBus();

                }

            }

        }

        return defaultInstance;

    }

可以看到这里用了一个double-check-lock方式的单例模式

再看register方法

    public void register(Object subscriber) {

        Class<?> subscriberClass = subscriber.getClass();

        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);

        synchronized (this) {

            for (SubscriberMethod subscriberMethod : subscriberMethods) {

                subscribe(subscriber, subscriberMethod);

            }

        }

    }

register的就是建立订阅关系的过程，订阅者（观察者）就是MainActivity

通过反射拿到注解 Subscription的下方法Method

下面看建立订阅的方法subscribe

// Must be called in synchronized block

    private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {

        Class<?> eventType = subscriberMethod.eventType;

        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);

        CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);

        if (subscriptions == null) {

            subscriptions = new CopyOnWriteArrayList<>();

            subscriptionsByEventType.put(eventType, subscriptions);

        } else {

            if (subscriptions.contains(newSubscription)) {

                throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "

                        + eventType);

            }

        }

        int size = subscriptions.size();

        for (int i = 0; i <= size; i++) {

            if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {

                subscriptions.add(i, newSubscription);

                break;

            }

        }

        List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);

        if (subscribedEvents == null) {

            subscribedEvents = new ArrayList<>();

            typesBySubscriber.put(subscriber, subscribedEvents);

        }

        subscribedEvents.add(eventType);

        if (subscriberMethod.sticky) {

            if (eventInheritance) {

                // Existing sticky events of all subclasses of eventType have to be considered.

                // Note: Iterating over all events may be inefficient with lots of sticky events,

                // thus data structure should be changed to allow a more efficient lookup

                // (e.g. an additional map storing sub classes of super classes: Class -> List<Class>).

                Set<Map.Entry<Class<?>, Object>> entries = stickyEvents.entrySet();

                for (Map.Entry<Class<?>, Object> entry : entries) {

                    Class<?> candidateEventType = entry.getKey();

                    if (eventType.isAssignableFrom(candidateEventType)) {

                        Object stickyEvent = entry.getValue();

                        checkPostStickyEventToSubscription(newSubscription, stickyEvent);

                    }

                }

            } else {

                Object stickyEvent = stickyEvents.get(eventType);

                checkPostStickyEventToSubscription(newSubscription, stickyEvent);

            }

        }

    }

一个EvnetType可以有多个Subscriber(订阅者)

可参考学习的地方：用ConcurrentHashMap做缓存