Handler 源码从零学习笔记 （一）

Handler 是什么？

这个问题其实在 Handler 源码最前面的注释里其实已经介绍的很清楚了，下面都是我自己的翻译，英语好的可以自己理解：

• 我们先来看第一段话：

/**
 * 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.

这句话介绍了什么是 Hanlder 大概意思是说（这种格式看起来比较容易）：

 * 一个 Hanlder 允许你去发送和处理与 Thread（线程） 的 MessageQueue（消息队列） 相关联的
 * Message（消息） 和 Runnable 对象。每个 Hanlder 实例关联一个 Thread 实例和该 Thread 的
 * MessageQueue。当你创建一个新的 Hanlder 时，它（Handler）会和当前 Thread/当前 Thread 的
 * MessageQueue 联系起来 -- 此时，它（Handler）会将 Messages 和 Runnables 传递到该 MessageQueue
 * 并在他们从消息队列中出现时执行它们。

• 第二段话：

 * 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.

这句话介绍了 Handler 的两个主要用途：

1. 传入 Message 和 Runnable 用来执行。
2. 在另一个线程上执行一个操作。

• 第三段话：

 * cheduling messages is accomplished with the
 * {@link #post}, {@link #postAtTime(Runnable, long)},
 * {@link #postDelayed}, {@link #sendEmptyMessage},
 * {@link #sendMessage}, {@link #sendMessageAtTime}, and
 * {@link #sendMessageDelayed} methods.  The <em>post</em> versions allow
 * you to enqueue Runnable objects to be called by the message queue when
 * they are received; the <em>sendMessage</em> versions allow you to enqueue
 * a {@link Message} object containing a bundle of data that will be
 * processed by the Handler's {@link #handleMessage} method (requiring that
 * you implement a subclass of Handler).

这句话首先介绍了 Handler 调度 Message 的几种方法：

1. post
2. postAtTime(Runnable,long)
3. postDelayed
4. sendEmptyMessage
5. sendMessage
6. sendMessageAtTime
7. sendMessageDelayed
   post 允许你在接收到 MessageQueue 时调用 Runnable 对象。
   sendMessage 允许你对包含一些数据的 Message 对象进行排列，这些数据将由 handlerMessage 方法处理。

• 第四段话：

 * <p>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.

大概意思是：

当发布或者发送到 Handler 时，只要 MessageQueue 准备就绪就可以立即处理该任务。或者在处理 MessageQueue 之前指定一个延迟或者一个固定的时间。后两者允许你实现延时和一些其他的时间设置。

• 最后一段：

 * 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 <em>post</em> or <em>sendMessage</em> 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.

当一个进程在应用中被创建之后，它的主线程致力于运行一个负责管理顶层应用对象（Activity,Broadcast Receiver,etc）以及它创建一些 Window 的 MesssageQueue。你可以创建一些你自己的线程（子线程）来通过 Handler 和主线程通信。和以前一样，它需要调用 post 和 sendMessage 方法来完成，但是要在你新创建的子线程中。传递过来的 Message 或者 Runnable 将会在 Handler 的 MessageQueue 中调度，并在合适的时候处理。