EvenBus for Android 之 訂閱者註冊

EventBus中的訂閱者其實就是具體的方法他的註冊過程分爲如下幾步：

一、在類中找到要註冊的方法

二、把找到的方法註冊到EventBus中

在類中找到要註冊的方法：

先上一段 SubscriberMethodFinder類中查找訂閱者方法的代碼

 List<SubscriberMethod> findSubscriberMethods(Class<?> subscriberClass, String eventMethodName) {
        String key = subscriberClass.getName() + '.' + eventMethodName;
        List<SubscriberMethod> subscriberMethods;
        synchronized (methodCache) {
            subscriberMethods = methodCache.get(key);
        }
        if (subscriberMethods != null) {
            return subscriberMethods;
        }
        subscriberMethods = new ArrayList<SubscriberMethod>();
        Class<?> clazz = subscriberClass;
        HashSet<String> eventTypesFound = new HashSet<String>();
        StringBuilder methodKeyBuilder = new StringBuilder();
        while (clazz != null) {
            String name = clazz.getName();
            if (name.startsWith("java.") || name.startsWith("javax.") || name.startsWith("android.")) {
                // Skip system classes, this just degrades performance
                break;
            }

            // Starting with EventBus 2.2 we enforced methods to be public (might change with annotations again)
            Method[] methods = clazz.getMethods();
            for (Method method : methods) {
                String methodName = method.getName();
                if (methodName.startsWith(eventMethodName)) {
                    int modifiers = method.getModifiers();
                    if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                        Class<?>[] parameterTypes = method.getParameterTypes();
                        if (parameterTypes.length == 1) {
                            String modifierString = methodName.substring(eventMethodName.length());
                            ThreadMode threadMode;
                            if (modifierString.length() == 0) {
                                threadMode = ThreadMode.PostThread;
                            } else if (modifierString.equals("MainThread")) {
                                threadMode = ThreadMode.MainThread;
                            } else if (modifierString.equals("BackgroundThread")) {
                                threadMode = ThreadMode.BackgroundThread;
                            } else if (modifierString.equals("Async")) {
                                threadMode = ThreadMode.Async;
                            } else {
                                if (skipMethodVerificationForClasses.containsKey(clazz)) {
                                    continue;
                                } else {
                                    throw new EventBusException("Illegal onEvent method, check for typos: " + method);
                                }
                            }
                            Class<?> eventType = parameterTypes[0];
                            methodKeyBuilder.setLength(0);
                            methodKeyBuilder.append(methodName);
                            methodKeyBuilder.append('>').append(eventType.getName());
                            String methodKey = methodKeyBuilder.toString();
                            if (eventTypesFound.add(methodKey)) {
                                // Only add if not already found in a sub class
                                subscriberMethods.add(new SubscriberMethod(method, threadMode, eventType));
                            }
                        }
                    } else if (!skipMethodVerificationForClasses.containsKey(clazz)) {
                        Log.d(EventBus.TAG, "Skipping method (not public, static or abstract): " + clazz + "."
                                + methodName);
                    }
                }
            }
            clazz = clazz.getSuperclass();
        }
        if (subscriberMethods.isEmpty()) {
            throw new EventBusException("Subscriber " + subscriberClass + " has no public methods called "
                    + eventMethodName);
        } else {
            synchronized (methodCache) {
                methodCache.put(key, subscriberMethods);
            }
            return subscriberMethods;
        }
    }

經過這個方法能夠看到，須要傳2個參數，Class subscriberClass和String eventMethodName

subscriberClass是包含訂閱者方法的類，能夠獲取到該類中的全部方法，

eventMethodName是約定做爲訂閱者的方法的前綴，經過這個來過濾出事訂閱者的方法

接下在看下具體實現

 String key = subscriberClass.getName() + '.' + eventMethodName;
        List<SubscriberMethod> subscriberMethods;
        synchronized (methodCache) {
            subscriberMethods = methodCache.get(key);
        }
        if (subscriberMethods != null) {
            return subscriberMethods;
        }

經過類名和eventMethodName組成一個key，在一個靜態的methodCache中查找是否已經存在要找的該類下的訂閱者，不存在就得進行下面的解析

 String name = clazz.getName();
            if (name.startsWith("java.") || name.startsWith("javax.") || name.startsWith("android.")) {
                // Skip system classes, this just degrades performance
                break;
            }

對要解析的類進行判斷，排除是系統的類

  if (methodName.startsWith(eventMethodName)) {
                    int modifiers = method.getModifiers();
                    if ((modifiers & Modifier.PUBLIC) != 0 && (modifiers & MODIFIERS_IGNORE) == 0) {
                        Class<?>[] parameterTypes = method.getParameterTypes();
                        if (parameterTypes.length == 1) {
                            String modifierString = methodName.substring(eventMethodName.length());
                            ThreadMode threadMode;
                            if (modifierString.length() == 0) {
                                threadMode = ThreadMode.PostThread;
                            } else if (modifierString.equals("MainThread")) {
                                threadMode = ThreadMode.MainThread;
                            } else if (modifierString.equals("BackgroundThread")) {
                                threadMode = ThreadMode.BackgroundThread;
                            } else if (modifierString.equals("Async")) {
                                threadMode = ThreadMode.Async;
                            } else {
                                if (skipMethodVerificationForClasses.containsKey(clazz)) {
                                    continue;
                                } else {
                                    throw new EventBusException("Illegal onEvent method, check for typos: " + method);
                                }
                            }
                            Class<?> eventType = parameterTypes[0];
                            methodKeyBuilder.setLength(0);
                            methodKeyBuilder.append(methodName);
                            methodKeyBuilder.append('>').append(eventType.getName());
                            String methodKey = methodKeyBuilder.toString();
                            if (eventTypesFound.add(methodKey)) {
                                // Only add if not already found in a sub class
                                subscriberMethods.add(new SubscriberMethod(method, threadMode, eventType));
                            }
                        }
                    } else if (!skipMethodVerificationForClasses.containsKey(clazz)) {
                        Log.d(EventBus.TAG, "Skipping method (not public, static or abstract): " + clazz + "."
                                + methodName);
                    }
                }
            }

遍歷該類下的方法，找出是eventMethodName前綴的方法，在經過方法名稱定義訂閱者的線程類型

PostThread，訂閱者將在發起事件的同一個線程中執行

MainThread，訂閱者將在主線程中執行

BackgroupThread，訂閱者將在一個後臺線程中執行，若是發送的線程不是主線程，則直接在發起時間的線程中執行，若是發送的線程是主線程，則會在惟一的一個後臺線程中按照順序來執行全部的後臺時間響應函數

AsyncThread，訂閱者將在另外一個異步線程中執行

 if (subscriberMethods.isEmpty()) {
            throw new EventBusException("Subscriber " + subscriberClass + " has no public methods called "
                    + eventMethodName);
        } else {
            synchronized (methodCache) {
                methodCache.put(key, subscriberMethods);
            }
            return subscriberMethods;
        }

把獲取去到的訂閱者保存到methodCache中。