Looper源码细说

Looper作为引擎般的存在，主要作用是维持一个死循环，一旦有消息到来就通知handler分发事件。

Looper的创建

   //线程中初始化 获取Looper     public static void prepare() {        prepare(true);    }    private static void prepare(boolean quitAllowed) {//sThreadLocal 存放线程的局部变量   每个线程拿到一个原始变量的副本进行操作//在这里就是为每个线程存放一个Looper对象，如果已经存在  则报异常        if (sThreadLocal.get() != null) {            throw new RuntimeException("Only one Looper may be created per thread");        }//sThreadLocal 存放线程的局部变量   每个线程拿到一个原始变量的副本进行操作//多了一个quitAllowed  默认是true  主线程赋值false表示looper不可退出        sThreadLocal.set(new Looper(quitAllowed));    }//主线程初始化Looper的地方  在MainThread的main方法里面调用 这也是主线程里面不用调prepare的原因    public static void prepareMainLooper() {        prepare(false);//下面是主线程多出来的部分  就是单独的为主线程的Looper设置一个线程局部变量sMainLooper         synchronized (Looper.class) {            if (sMainLooper != null) {                throw new IllegalStateException("The main Looper has already been prepared.");            }//myLooper()  方法里就一句话 return sThreadLocal.get();  最终拿到线程sThreadLocal            sMainLooper = myLooper();        }    }....//上面 进过prepare 最终new一个Looper对象//同时拿到MessageQueue和创建宿主线程thread；    private Looper(boolean quitAllowed) {        mQueue = new MessageQueue(quitAllowed);        mThread = Thread.currentThread();    }

Looper的退出

    public void quit() {        mQueue.quit(false);    }    public void quitSafely() {        mQueue.quit(true);    }

上面简短的代码可以看出 其调用的是成员变量MessageQueue的退出方法 那我们就跟进看一看
MessageQueue中的代码

    void quit(boolean safe) {//mQuitAllowed  主线程调用prepare时传false  不允许在主线程中退出循环         if (!mQuitAllowed) {            throw new IllegalStateException("Main thread not allowed to quit.");        }        synchronized (this) {            if (mQuitting) {                return;            }            mQuitting = true;//safe  looper中传入判断是否安全退出//安全退出则只移除等待传递的消息否则清空所有            if (safe) {                removeAllFutureMessagesLocked();            } else {                removeAllMessagesLocked();            }            // We can assume mPtr != 0 because mQuitting was previously false.            nativeWake(mPtr);        }    }

Looper运作
核心部分

  public static void loop() {  //拿到Looper对象        final Looper me = myLooper();        if (me == null) {            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");        }//拿到消息队列        final MessageQueue queue = me.mQueue;//重置并获取当前线程跟踪令牌 确保线程身份不变        Binder.clearCallingIdentity();        final long ident = Binder.clearCallingIdentity();//looper维持的无限循环        for (;;) {// 消息出队  获取消息              Message msg = queue.next(); // might block            if (msg == null) {                // No message indicates that the message queue is quitting.                return;            }            // This must be in a local variable, in case a UI event sets the logger            final Printer logging = me.mLogging;            if (logging != null) {                logging.println(">>>>> Dispatching to " + msg.target + " " +                        msg.callback + ": " + msg.what);            }            final long traceTag = me.mTraceTag;            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));            }//拿到消息后进行消息分发            try {                msg.target.dispatchMessage(msg);            } finally {                if (traceTag != 0) {                    Trace.traceEnd(traceTag);                }            }            if (logging != null) {                logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);            }//事件分发后判断线程身分是否有变            final long newIdent = Binder.clearCallingIdentity();            if (ident != newIdent) {                Log.wtf(TAG, "Thread identity changed from 0x"                        + Long.toHexString(ident) + " to 0x"                        + Long.toHexString(newIdent) + " while dispatching to "                        + msg.target.getClass().getName() + " "                        + msg.callback + " what=" + msg.what);            }//消息处理完成后标记为已使用 扔回回收池中 并清空消息的属性变量等信息            msg.recycleUnchecked();        }    }

消息如何出队 下一篇会说下MessageQueue和message