学习Looper,handle后感

从老罗那里学习了相关知识后，自己跟随6.0的源码学习一遍。此处只为记录自己的感想和见解，有错误之处望大家指正！！！谢谢各位看官！

总体分为以下3个步骤：

1、消息循环

2、消息发送

3、消息接收处理

1、消息循环

一、初始化

Process.setArgV0("<pre-initialized>");        .................        Looper.prepareMainLooper();        ActivityThread thread = new ActivityThread();        thread.attach(false);        if (sMainThreadHandler == null) {            sMainThreadHandler = thread.getHandler();        }        if (false) {            Looper.myLooper().setMessageLogging(new                    LogPrinter(Log.DEBUG, "ActivityThread"));        }        // End of event ActivityThreadMain.        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);       Looper.loop();

省略了不少无关代码。上面的代码初始化和looper ，得到了一个ActivityThread.下面具体看看代码。

frameworks/base/core/java/android/os/Looper.java

Looper.prepareMainLooper()，具体代码如下：

  /**     * Initialize the current thread as a looper, marking it as an     * application's main looper. The main looper for your application     * is created by the Android environment, so you should never need     * to call this function yourself.  See also: {@link #prepare()}     */    public static void prepareMainLooper() {        prepare(false);        synchronized (Looper.class) {            if (sMainLooper != null) {                throw new IllegalStateException("The main Looper has already been prepared.");            }            sMainLooper = myLooper();        }    }

/**     * Return the Looper object associated with the current thread.  Returns     * null if the calling thread is not associated with a Looper.     */    public static @Nullable Looper myLooper() {        return sThreadLocal.get();    }

private static void prepare(boolean quitAllowed) {        if (sThreadLocal.get() != null) {            throw new RuntimeException("Only one Looper may be created per thread");        }        sThreadLocal.set(new Looper(quitAllowed));    }

private Looper(boolean quitAllowed) {        mQueue = new MessageQueue(quitAllowed);        mThread = Thread.currentThread();    }

相关代码已经贴出来，6.0版本和2.3并没有什么区别。  这里将一个looper对象设置为唯一的主线程的looper用

sThreadLocal保存,是对象内的变量。这里quitAllowed=false。然后初始化了一个messageQueue.下面看下这个消息队列：处于同一目录

MessageQueue(boolean quitAllowed) {        mQuitAllowed = quitAllowed;        mPtr = nativeInit();    }

这里调用了JNI，

static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) {    NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();    if (!nativeMessageQueue) {        jniThrowRuntimeException(env, "Unable to allocate native queue");        return 0;    }    nativeMessageQueue->incStrong(env);    return reinterpret_cast<jlong>(nativeMessageQueue);}

这里将生成的nativeMessageQueue进行了加工，返回给java层，用ptr 保存了这个对应关系。下面贴下如何new NativeMessageQueue()

NativeMessageQueue::NativeMessageQueue() :        mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) {    mLooper = Looper::getForThread();    if (mLooper == NULL) {        mLooper = new Looper(false);        Looper::setForThread(mLooper);    }}

Looper目录在：system/core/libutils/Looper.cpp         注意大小写区分

Looper::Looper(bool allowNonCallbacks) :        mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),        mPolling(false), mEpollFd(-1), mEpollRebuildRequired(false),        mNextRequestSeq(0), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {    mWakeEventFd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);    LOG_ALWAYS_FATAL_IF(mWakeEventFd < 0, "Could not make wake event fd: %s",                        strerror(errno));    AutoMutex _l(mLock);    rebuildEpollLocked();}

void Looper::rebuildEpollLocked() {    // Close old epoll instance if we have one.    if (mEpollFd >= 0) {#if DEBUG_CALLBACKS        ALOGD("%p ~ rebuildEpollLocked - rebuilding epoll set", this);#endif        close(mEpollFd);    }    // Allocate the new epoll instance and register the wake pipe.    mEpollFd = epoll_create(EPOLL_SIZE_HINT);    LOG_ALWAYS_FATAL_IF(mEpollFd < 0, "Could not create epoll instance: %s", strerror(errno));    struct epoll_event eventItem;    memset(& eventItem, 0, sizeof(epoll_event)); // zero out unused members of data field union    eventItem.events = EPOLLIN;    eventItem.data.fd = mWakeEventFd;    int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem);    LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake event fd to epoll instance: %s",                        strerror(errno));    for (size_t i = 0; i < mRequests.size(); i++) {        const Request& request = mRequests.valueAt(i);        struct epoll_event eventItem;        request.initEventItem(&eventItem);        int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, request.fd, & eventItem);        if (epollResult < 0) {            ALOGE("Error adding epoll events for fd %d while rebuilding epoll set: %s",                  request.fd, strerror(errno));        }    }}

2、消息发送

3、消息接收处理