Android FrameWork源码浅析之(一)－－handler，looper,messagequeue

我们经常会用handler接收异步线程的数据，然后进行ui刷新，或是其他逻辑操作。

希望通过了解相关的操作，作为framework源码入门的阶梯。

handler主要涉及framework的三个类：

Handler,Looper,MessageQueue

1.handler的创建和初始流程。

Handler的主要创建如下：

</pre>
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;
    }

    public interface Callback {
        public boolean handleMessage(Message msg);
    }

 public Handler(Callback callback) {
        this(callback, false);
    }
一般来说，创建handler时，使用new handler(this)，然后复写handleMessage方法，从源码的角度就可以很清楚的看到handler创建和复写方法的流程。
在创建过程中，看一下下面这段代码。
mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
我们再来看一下Looper是什么概念。

* Class used to run a message loop for a thread.  Threads by default do
* not have a message loop associated with them; to create one, call
* {@link #prepare} in the thread that is to run the loop, and then
* {@link #loop} to have it process messages until the loop is stopped.
这时候可能会有疑问，在activity使用的时候，并没有prepare的操作啊。
这里请先看一下looper的两个全局变量。
</pre><pre name="code" class="java"><span style="font-size:14px;"> static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
    private static Looper sMainLooper;  // guarded by Looper.class</span>

这里要看一下ThreadLocal这个类。下面是百度百科的结果，具体的用法可以自行搜索，
JDK 1.2的版本中就提供java.lang.ThreadLocal，ThreadLocal为解决多线程程序的并发问题提供了一种新的思路。
使用这个工具类可以很简洁地编写出优美的多线程程序，ThreadLocal并不是一个Thread，而是Thread的局部变量。
通常情况下，在activity中使用handler是不需要创建looper的，因为在activity的创建过程中，
ActivityThread.java会使用 Looper.prepareMainLooper();创建一个主线程的looper,一般handler都是
在主线程中处理数据，就不需要初始化了。

在looper创建的时候，会初始化一个messagequeue.
   private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }

MessageQueue.java
* Low-level class holding the list of messages to be dispatched by a
* {@link Looper}.  Messages are not added directly to a MessageQueue,
* but rather through {@link Handler} objects associated with the Looper.
* 
* <p>You can retrieve the MessageQueue for the current thread with
* {@link Looper#myQueue() Looper.myQueue()}.
 MessageQueue(boolean quitAllowed) {
        mQuitAllowed = quitAllowed;
        mPtr = nativeInit();
    }


nativeInit()的源码在：/frameworks/base/core/jni/android_os_MessageQueue.cpp，偶对C++不熟，就不搬门弄斧了。
关于messagequeue的处理可以参考下面这篇博文：http://blog.youkuaiyun.com/kesalin/article/details/37765707

2.handler的发送消息和接收消息。
 

  
    public final Message obtainMessage(int what, Object obj)
    {
        return Message.obtain(this, what, obj);
    }
<pre name="code" class="java">    public static Message obtain(Handler h, int what, Object obj) {
        Message m = obtain();
        m.target = h;
        m.what = what;
        m.obj = obj;

        return m;
    }

 /**
     * Return a new Message instance from the global pool. Allows us to
     * avoid allocating new objects in many cases.
     */
    public static Message obtain() {
        synchronized (sPoolSync) {
            if (sPool != null) {
                Message m = sPool;
                sPool = m.next;
                m.next = null;
                m.flags = 0; // clear in-use flag
                sPoolSize--;
                return m;
            }
        }
        return new Message();
    }
从上面的源码可以看到message发送消息的时候，实际上是在handler下创建一个message顺序链表。
 

handler又是如何取数据的呢？

在Looper loop会创建一个队列，依次从queue中取出数据。

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
            Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            msg.target.dispatchMessage(msg);

            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();
        }
    }

其他代码的函数，如handler中的sendMessageAtTime等就请自行阅读吧。～