Looper & Handler的最简单定义:

Looper是一个将线程转换为管道线程的类，Handler为您提供了一种机制，可以将任务从任何其他线程推入该管道。

一般措词的详细说明:

因此，流水线线程是一个可以通过处理程序从其他线程接受更多任务的线程。

Looper之所以这样命名，是因为它实现了循环——接受下一个任务，执行它，然后再接受下一个任务，以此类推。Handler之所以被称为Handler，是因为它用于处理或接受每次来自任何其他线程的下一个任务，并传递给Looper(线程或管道线程)。

例子:

循环程序和处理程序或管道线程的非常完美的例子是下载多个图像或在单个线程中逐个上传到服务器(Http)，而不是在后台为每个网络调用启动一个新线程。

在这里阅读更多关于Looper和Handler以及Pipeline Thread的定义:

Android内脏:环形器和处理程序的介绍

什么是环形使者?

Looper是一个用于执行队列中的消息(可运行对象)的类。普通线程没有这样的队列，例如简单线程没有任何队列。它执行一次，方法执行完成后，线程将不再运行另一个Message(Runnable)。

我们可以在哪里使用Looper类?

如果有人想要执行多个消息(Runnables)，那么他应该使用负责在线程中创建队列的Looper类。 例如，在编写从internet下载文件的应用程序时，我们可以使用Looper类将要下载的文件放入队列中。

它是如何工作的?

有prepare()方法来准备Looper。然后，您可以使用loop()方法在当前线程中创建一个消息循环，现在您的Looper已经准备好在队列中执行请求，直到退出循环。

下面是您可以准备Looper的代码。

class LooperThread extends Thread {
      public Handler mHandler;

      @Override
      public void run() {
          Looper.prepare();

          mHandler = new Handler() {
              @Override
              public void handleMessage(Message msg) {
                  // process incoming messages here
              }
          };

          Looper.loop();
      }
  }

在Service中处理多个下拉或上传项是一个更好的例子。

Handler和AsnycTask通常用于在UI(线程)和工作线程之间传播事件/消息或延迟操作。所以它们与UI更相关。

循环程序在后台处理线程相关队列中的任务(Runnables, Futures) -即使没有用户交互或显示UI(应用程序在调用期间在后台下载文件)。

I will try to explain the purpose of looper class as simple as possible. With a normal thread of Java when the run method completes the execution we say the thread has done it's job and thread lives no longer after that. what if we want to execute more tasks throughout our program with that same thread which is not living anymore? Oh there is a problem now right? Yes because we want to execute more tasks but the thread in not alive anymore. It is where the Looper comes in to rescue us. Looper as the name suggests loops. Looper is nothing more than an infinite loop inside your thread. So, it keeps the thread alive for an infinite time until we explicitly calls quit() method. Calling quit() method on the infinitely alive thread will make the condition false in the infinite loop inside the thread thus, infinite loop will exit. so, the thread will die or will no longer be alive. And it's critical to call the quit() method on our Thread to which looper is attached otherwise they will be there in your system just like Zombies. So, for example if we want to create a background thread to do some multiple tasks over it. we will create a simple Java's thread and will use Looper class to prepare a looper and attach the prepared looper with that thread so that our thread can live as longer as we want them because we can always call quit() anytime whenever we want to terminate our thread. So our the looper will keep our thread alive thus we will be able to execute multiple tasks with the same thread and when we are done we will call quit() to terminate the thread. What if we want our Main thread or UI thread to display the results computed by the background thread or non-UI thread on some UI elements? for that purpose there comes in the concept of Handlers; via handlers we can do inter-process communication or say via handlers two threads can communicate with each other. So, the main thread will have an associated Handler and Background thread will communicate with Main Thread via that handler to get the task done of displaying the results computed by it on some UI elements on Main thread. I know I am explaining only theory here but try to understand the concept because understanding the concept in depth is very important. And I am posting a link below which will take you to a small video series about Looper, Handler and HandlerThread and I will highly recommend watching it and all these concepts will get cleared with examples there.

https://www.youtube.com/watch?v=rfLMwbOKLRk&list=PL6nth5sRD25hVezlyqlBO9dafKMc5fAU2&index=1

java线程的生命周期在run()方法完成后结束。同一线程不能再次启动。

Looper将普通线程转换为消息循环。Looper的主要方法有:

void prepare ()

将当前线程初始化为循环程序。这使您有机会创建处理程序，然后在实际开始循环之前引用此循环程序。请确保在调用此方法后调用loop()，并通过调用quit()结束它。

void loop ()

在此线程中运行消息队列。确保调用quit()来结束循环。

void quit()

退出循环器。 导致loop()方法终止，而不处理消息队列中的任何其他消息。

Janishar的这篇mindorks文章很好地解释了核心概念。

循环程序与线程相关联。如果你在UI线程上需要Looper, loop . getmainlooper()将返回相关的线程。

您需要将Looper与Handler相关联。

Looper, Handler和HandlerThread是Android解决异步编程问题的方法。

有了Handler之后，就可以调用下面的api了。

post (Runnable r)

导致Runnable r被添加到消息队列。可运行对象将在附加此处理程序的线程上运行。

boolean sendMessage (Message msg)

在当前时间之前的所有挂起消息之后，将消息压入消息队列的末尾。它将在附加到此处理程序的线程中的handleMessage(Message)中接收。

HandlerThread是一个方便的类，用于启动具有循环程序的新线程。循环程序可以用来创建处理程序类

在某些情况下，你不能在UI线程上运行可运行任务。 例如，网络操作:在套接字上发送消息，打开URL并通过读取InputStream获取内容

在这些情况下，HandlerThread很有用。你可以从HandlerThread获取Looper对象，并在HandlerThread上创建Handler而不是主线程。

HandlerThread代码是这样的:

@Override
public void run() {
    mTid = Process.myTid();
    Looper.prepare();
    synchronized (this) {
        mLooper = Looper.myLooper();
        notifyAll();
    }
    Process.setThreadPriority(mPriority);
    onLooperPrepared();
    Looper.loop();
    mTid = -1;
}

参考下面的示例代码:

Android:在线程中吐司

什么是环形使者?

从文档

电影

循环程序类，用于为线程运行消息循环。默认情况下，线程没有与之关联的消息循环;要创建一个循环，在要运行循环的线程中调用prepare()，然后调用loop()让它处理消息，直到循环停止。

A Looper is a message handling loop: An important character of Looper is that it's associated with the thread within which the Looper is created The Looper class maintains a MessageQueue, which contains a list messages. An important character of Looper is that it's associated with the thread within which the Looper is created. The Looper is named so because it implements the loop – takes the next task, executes it, then takes the next one and so on. The Handler is called a handler because someone could not invent a better name Android Looper is a Java class within the Android user interface that together with the Handler class to process UI events such as button clicks, screen redraws and orientation switches.

它是如何工作的?

创造电影

线程通过在运行后调用loop .prepare()来获得Looper和MessageQueue。loop .prepare()标识调用线程，创建一个Looper和MessageQueue对象并关联线程

示例代码

class MyLooperThread extends Thread {

      public Handler mHandler; 

      public void run() { 

          // preparing a looper on current thread  
          Looper.prepare();

          mHandler = new Handler() { 
              public void handleMessage(Message msg) { 
                 // process incoming messages here
                 // this will run in non-ui/background thread
              } 
          }; 

          Looper.loop();
      } 
  }

欲了解更多信息，请查看下面的帖子

在Android中，Looper, Handler和MessageQueue之间是什么关系? Android内脏:环形器和处理程序的介绍 理解Android核心:Looper, Handler和HandlerThread Android中的Handler 什么是Android Looper? Android: Looper, Handler, HandlerThread。我一部分。 Android中的MessageQueue和Looper