Handle/Looper源码分析；

1. Handle中的属性：

    final Looper mLooper;
    final MessageQueue mQueue;
    final Callback mCallback;
    final boolean mAsynchronous;
    IMessenger mMessenger;

　　　mQueue消息队列，拥有消息队列的引用，handle可以对已发送的消息或任务再进行取消操作，如下源码:

　　public final void removeMessages(int what) {
    　　mQueue.removeMessages(this, what, null);
　　}

　　public final void removeMessages(int what, Object object) {
        mQueue.removeMessages(this, what, object);
    }

    /**
     * Remove any pending posts of callbacks and sent messages whose
     * <var>obj</var> is <var>token</var>.  If <var>token</var> is null,
     * all callbacks and messages will be removed.
     */
    public final void removeCallbacksAndMessages(Object token) {
        mQueue.removeCallbacksAndMessages(this, token);
    }

往消息队列中添加消息：

    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;         //Message中记录了来自哪个handle
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

2.Handle中的初始化：

　　通常我们使用的方式是 Handle handle=new Handle();这一过程执行的代码是：

　　

   public Handler(Callback callback, boolean async) {
        if (FIND_POTENTIAL_LEAKS) {
            final Class<? extends Handler> klass = getClass();
            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                    (klass.getModifiers() & Modifier.STATIC) == 0) {
                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                    klass.getCanonicalName());
            }
        }

        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

　　可以看出  ，这里在Looper中拿到looper和MessageQueue的引用赋值给Handler中的变量,MessageQueue和Looper的创建是在looper中完成的。

3.Looper:

　　1）主要的属性：

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper;  // guarded by Looper.class
final MessageQueue mQueue;
final Thread mThread;

　　2）构造函数:

　　对MessageQueue和mThread进行了初始化

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

　　3）prepare()方法：中对Looper进行了构建；并存放在ThreadLocal中。

    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));
    }

　　 ※static final ThreadLocal<Looper> 可知线程本地变量是静态final 类型的 整个进程中只有一份，里面存储 各个线程的Looper，存储方式类似于键(thread)值(looper)的方式。

　　4）looper():

　　　　

   /**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    public static void loop() {
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue;

        // Make sure the identity of this thread is that of the local process,
        // and keep track of what that identity token actually is.
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();

        for (;;) {
            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));
            }
            try {
                msg.target.dispatchMessage(msg);
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }

            if (logging != null) {
                logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
            }

            // Make sure that during the course of dispatching the
            // identity of the thread wasn't corrupted.
            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);
            }

            msg.recycleUnchecked();
        }
    }

　　　　这里有个循环 不断监控mQueue中的消息，有消息了 就取出 交给Handle处理；

　　5）。Thread TheadLocal  ThreadLocalMap;

　　　　ThreadLocalMap是TheadLocal的内部类，ThreadLocalMap是Thread的成员变量；Loop.prepare()里面，ThreadLocal负责将Looper对象存储到Thread的ThreadLocalMap中；

主线程的Looper初始化：

　　源码的位置是在ActivityThread类中的main函数。

　　

public static void main(String[] args) {  
        SamplingProfilerIntegration.start();  
        CloseGuard.setEnabled(false);  
        Environment.initForCurrentUser();  
        // Set the reporter for event logging in libcore  
        EventLogger.setReporter(new EventLoggingReporter());  
        Process.setArgV0("<pre-initialized>");  
        Looper.prepareMainLooper();//这实际就是一个实例化一个looper对象,大家也可以看看prepareMainlooper方法(下方)。  
        ActivityThread thread = new ActivityThread();  
        thread.attach(false);  
        if (sMainThreadHandler == null) {  
            sMainThreadHandler = thread.getHandler();  
        }   
    Looper.loop();//启动循环器   
｝