Handler详解：消息处理与多线程操作的关键

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Handler是Android系统中一个重要的组件，它允许开发者在不同的线程之间传递消息并进行任务调度。Handler的核心作用是建立了一个线程间的通信桥梁，使得主线程（UI线程）可以与其它工作线程进行交互，这对于实现异步操作、避免UI阻塞至关重要。 Handler的工作原理是这样的：每个Handler对象都关联一个特定的Thread及其MessageQueue。当你创建一个新的Handler时，它会绑定到创建它的线程的MessageQueue。这意味着所有的消息和Runnable对象都会被发送到这个特定的队列，然后按照队列中的顺序执行。这就确保了消息的顺序和线程安全。 Handler的主要用途有两个： 1. **消息调度**：通过Handler提供的方法如`post(Runnable)`, `postAtTime(Runnable, long)`, `postDelayed(Runnable, long)`等，开发者可以在未来某个时间点将Runnable对象放入消息队列，让它们在指定时间后被执行。这些方法允许你在非UI线程执行耗时操作，然后在UI线程回调，避免阻塞用户界面。 2. **跨线程操作**：除了延迟执行，还可以通过`sendMessage(Message)`、`sendMessageAtTime(Message, long)`和`sendMessageDelayed(Message, long)`来发送包含数据的Message对象。这些消息会被传递给Handler，由Handler内部的handleMessage()方法进行处理，从而实现在不同线程上执行特定的任务。在实际应用中，我们经常使用Handler来实现线程间的通信，比如在后台线程执行网络请求或计算密集型任务，然后通过Handler将结果传递回主线程更新UI。同时，也常用于处理Activity/Fragment的生命周期事件，确保在适当的时间执行相关的操作，如销毁前保存数据或清理资源。总结来说，Handler是Android中不可或缺的工具，它简化了线程间通信的复杂性，促进了应用程序的异步处理和多线程编程。理解并熟练运用Handler，是开发高质量Android应用的关键技能之一。

Handler 的官方解释如下：

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.

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.

Scheduling messages is accomplished with the post(Runnable), postAtTime(Runnable, long),

postDelayed(Runnable, long), sendEmptyMessage(int), sendMessage(Message),

sendMessageAtTime(Message, long), and sendMessageDelayed(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 Message object containing a bundle of data that will be

processed by the Handler's handleMessage(Message) method (requiring that you implement a subclass of

Handler).

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.

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 than be scheduled in the Handler's message queue and processed when

appropriate.

大致意思是：Handler 是一种能够发送和处理与消息队列关联的 Message 和 Runnable 的一种对象。

它隶属于调用 Handler 对象的线程，自它创建伊始，它就在当前线程发送消息到当前线程的消息队列，

并且负责处理线程消息队列的消息。

Handler 的用法主要有两种：其一，发送消息，其二，在另外一个线程处理消息。

对其一：

post(Runnable), postAtTime(Runnable, long), postDelayed(Runnable, long), sendEmptyMessage(int),

sendMessage(Message), sendMessageAtTime(Message, long), and sendMessageDelayed(Message, long) 这些

方法主要用来发送消息，其中，post version 主要是发送 Runnable，而 sendMessage version 则是用来发

送消息，然后在 handler 的 HandleMessage 中处理。

如何处理的呢？最后一段非常关键：当我们创建了一个应用程序进程，它内部就开启了一个处理消息

的 Looper.

在 new 一个 Handler 的时候，

mLooper = Looper.myLooper();

public static final Looper myLooper() {

return (Looper)sThreadLocal.get();

}

mQueue = mLooper.mQueue;

mCallback = callback;

获取当前进程的 Looper，然后获取它的消息队列，这样就将一个消息加入了消息队列，这个 Looper 开

启类似下面，先运行 Run 方法，在 Run 方法中 Looper.prepare,之后进入一个无限循环，Looper.loop(),在

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