android消息处理机制学习（一）-Handler,Message,MessageQueue,Looper简介
android消息处理机制，是android当中最重要的话题之一，来看看官方文档的分析（这里都直接粘贴处国内的镜像网站，快捷方便）：
一 .Hanler简介：
源码地址：http://wear.techbrood.com/reference/android/os/Handler.html
A Handler allows you to send and process Message and Runnable objects associated with a thread's MessageQueue. Each Handler instance is associated with a single thread and that thread's message queue. When you create a new Handler, it is bound to the thread / message queue of the thread that is creating it -- from that point on, it will deliver messages and runnables to that message queue and execute them as they come out of the message queue.
一个Handler允许你发送和处理Message和Runable对象，这些对象和一个线程的MessageQueue相关联。每一个线程实例和一个单独的线程以及该线程的MessageQueue相关联。当你创建一个新的Handler时，它就和创建它的线程绑定在一起了。这里，线程我们也可以理解为线程的MessageQueue。从这一点上来看，Handler把Message和Runable对象传递给MessageQueue，而且在这些对象离开MessageQueue时，Handler负责执行他们。
There are two main uses for a Handler: (1) to schedule messages and runnables to be executed as some point in the future; and (2) to enqueue an action to be performed on a different thread than your own.
Handler有两个主要的用途：（1）确定在将来的某个时间点执行一个或者一些Message和Runnable对象。（2）在其他线程（不是Handler绑定线程）中排入一些要执行的动作。
Scheduling messages is accomplished with the post(Runnable), postAtTime(Runnable, long),postDelayed(Runnable, long), sendEmptyMessage(int), sendMessage(Message), sendMessageAtTime(Message, long), andsendMessageDelayed(Message, long) methods. The post versions allow you to enqueue Runnable objects to be called by the message queue when they are received; the sendMessage versions allow you to enqueue a Messageobject containing a bundle of data that will be processed by the Handler's handleMessage(Message) method (requiring that you implement a subclass of Handler).
Scheduling Message，即（1），可以通过以下方法完成：
post(Runnable):Runnable在handler绑定的线程上执行，也就是说不创建新线程。
postAtTime(Runnable,long):
postDelayed(Runnable,long):
sendEmptyMessage(int):
sendMessage(Message):
sendMessageAtTime(Message,long):
sendMessageDelayed(Message,long):
post这个动作让你把Runnable对象排入MessageQueue,MessageQueue受到这些消息的时候执行他们，当然以一定的排序。sendMessage这个动作允许你把Message对象排成队列，这些Message对象包含一些信息，Handler的hanlerMessage(Message)会处理这些Message.当然，handlerMessage(Message)必须由Handler的子类来重写。这是编程人员需要作的事。
When posting or sending to a Handler, you can either allow the item to be processed as soon as the message queue is ready to do so, or specify a delay before it gets processed or absolute time for it to be processed. The latter two allow you to implement timeouts, ticks, and other timing-based behavior.
当posting或者sending到一个Hanler时，你可以有三种行为：当MessageQueue准备好就处理，定义一个延迟时间，定义一个精确的时间去处理。后两者允许你实现timeout,tick,和基于时间的行为。
When a process is created for your application, its main thread is dedicated to running a message queue that takes care of managing the top-level application objects (activities, broadcast receivers, etc) and any windows they create. You can create your own threads, and communicate back with the main application thread through a Handler. This is done by calling the same post or sendMessage methods as before, but from your new thread. The given Runnable or Message will then be scheduled in the Handler's message queue and processed when appropriate.
当你的应用创建一个新的进程时，主线程（也就是UI线程）自带一个MessageQueue，这个MessageQueue管理顶层的应用对象（像activities,broadcast receivers等）和主线程创建的窗体。你可以创建自己的线程，并通过一个Handler和主线程进行通信。这和之前一样，通过post和sendmessage来完成，差别在于在哪一个线程中执行这么方法。在恰当的时候，给定的Runnable和Message将在Handler的MessageQueue中被Scheduled。
二 .Message简介：
源码地址：http://wear.techbrood.com/reference/android/os/Message.html
Defines a message containing a description and arbitrary data object that can be sent to a Handler. This object contains two extra int fields and an extra object field that allow you to not do allocations in many cases.
Message类就是定义了一个信息，这个信息中包含一个描述符和任意的数据对象，这个信息被用来传递给Handler.Message对象提供额外的两个int域和一个Object域，这可以让你在大多数情况下不用作分配的动作。
While the constructor of Message is public, the best way to get one of these is to call Message.obtain() or one of the Handler.obtainMessage() methods, which will pull them from a pool of recycled objects.
尽管Message的构造函数是public的，但是获取Message实例的最好方法是调用Message.obtain(),或者Handler.obtainMessage()方法，这些方法会从回收对象池中获取一个。
三 .MessageQueue简介：
源码地址：http://wear.techbrood.com/reference/android/os/MessageQueue.html
Low-level class holding the list of messages to be dispatched by a Looper. Messages are not added directly to a MessageQueue, but rather through MessageQueue.IdleHandler objects associated with the Looper.
这是一个包含message列表的底层类。Looper负责分发这些message。Messages并不是直接加到一个MessageQueue中，而是通过MessageQueue.IdleHandler关联到Looper。
You can retrieve the MessageQueue for the current thread with Looper.myQueue().
你可以通过Looper.myQueue()从当前线程中获取MessageQueue。
四 .Looper简介：
源码地址：http://wear.techbrood.com/reference/android/os/Looper.html
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 prepare() in the thread that is to run the loop, and then loop() to have it process messages until the loop is stopped.
大多数和message loop的交互是通过Handler。
Looper类被用来执行一个线程中的message循环。默认情况，没有一个消息循环关联到线程。在线程中调用prepare()创建一个Looper，然后用loop()来处理messages，直到循环终止。
Most interaction with a message loop is through the Handler class.
大多数和message loop的交互是通过Handler。
This is a typical example of the implementation of a Looper thread, using the separation of prepare() and loop()to create an initial Handler to communicate with the Looper.
下面是一个典型的带有Looper的线程实现， 添加了prepare和loop方法并创建和初始化一个Handler与当前looper 通信。class LooperThread extends Thread {
public Handler mHandler;
public void run() {
Looper.prepare();
mHandler = new Handler() {
public void handleMessage(Messagemsg) {
// process incoming messages here
}
};
Looper.loop();
}
}