Handler MessageQueue Message Looper关系浅析

1、Handler

/** * A Handler allows you to send and process {@link Message} and Runnable * objects associated with a thread's {@link 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. */

从上面的官方文档看，Handler允许你通过一个线程类MessageQueue来发送和处理Message、Runnable对象，每一个Handler实例都与一个单线程以及该单线程的消息队列相关联。

/** * 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的两个作用就是：

1）安排消息和Runnable对象在未来某个时间点执行

2）在非当前线程上来执行一个排到的动作

注意第二个作用，当我们需要做一些耗时的动作时，不一定要用线程或异步来完成，使用Handler同样可以达到目的。

2、Message

/** * Defines a message containing a description and arbitrary data object that * can be sent to a {@link Handler}. This object contains two extra int * fields and an extra object field that allow you to not do allocations in * many cases. *  * While the constructor of Message is public,<span style="color:#ff0000;"> the best way to get one of * these is to call {@link #obtain Message.obtain()} or one of the * {@link Handler#obtainMessage Handler.obtainMessage()}</span> methods, which will * pull them from a pool of recycled objects. */

MessageQueue中存放的消息对象，官方推荐使用Message.obtain()或者Handler.obtainMessage()来从Message Pool中获取可回收的Message对象。

3、MessageQueue

1）用来存放Message的消息队列，是一种数据结构。

2）每一个线程最多只能拥有一个MessageQueue数据结构

3）主线程创建时，会创建一个默认的Looper对象，而Looper对象的创建，将自动创建一个Message Queue。其他非主线程，不会自动创建Looper，要需要的时候，通过调用prepare函数来实现。

4、Looper

1）MessageQueue的管理者，每一个Looper都与一个线程相关，创建一个Looper同时会创建一个MessageQueue结构

2）除了主线程有自己默认的Looper外，其他线程需要自定义一个Looper才能接受Message。

3）Looper从MessageQueue中取出Message然后，交由Handler的handleMessage进行处理。处理完成后，调用Message.recycle()将其放入Message Pool中。

整体结构如下图（来源于网络）：