<h2><strong>Handler的运行机制</strong></h2>    <h3><strong>Handler的作用：</strong></h3>    <p>当我们需要在子线程处理耗时的操作（例如访问网络，数据库的操作），而当耗时的操作完成后，需要更新UI，这就需要使用Handler来处理，因为子线程不能做更新UI的操作。Handler能帮我们很容易的把任务（在子线程处理）切换回它所在的线程。简单理解，Handler就是解决线程和线程之间的通信的。</p>    <h3><strong>Handler的使用</strong></h3>    <p>使用的handler的两种形式：</p>    <p>1.在主线程使用handler；</p>    <p>2.在子线程使用handler。</p>    <p>在主线程使用handler的示例：</p>    <pre>  <code class="language-java">public class TestHandlerActivity extends AppCompatActivity {              private static final String TAG = "TestHandlerActivity";            private Handler mHandler = new Handler(){              @Override              public void handleMessage(Message msg) {                  super.handleMessage(msg);                  //获得刚才发送的Message对象，然后在这里进行UI操作                  Log.e(TAG,"------------> msg.what = " + msg.what);              }          };              @Override          protected void onCreate(Bundle savedInstanceState) {              super.onCreate(savedInstanceState);              setContentView(R.layout.activity_handler_test);              initData();          }            private void initData() {                //开启一个线程模拟处理耗时的操作              new Thread(new Runnable() {                  @Override                  public void run() {                        SystemClock.sleep(2000);                      //通过Handler发送一个消息切换回主线程（mHandler所在的线程）                      mHandler.sendEmptyMessage(0);                  }              }).start();            }</code></pre>    <p><img src="https://simg.open-open.com/show/b362997ea235b6f8210c395a91fae73d.png"></p>    <p style="text-align:center">这里写图片描述</p>    <p>在主线程使用handler很简单，只需在主线程创建一个handler对象，在子线程通过在主线程创建的handler对象发送Message，在handleMessage（）方法中接受这个Message对象进行处理。通过handler很容易的从子线程切换回主线程了。</p>    <p>那么来看看在子线程中使用是否也是如此。</p>    <pre>  <code class="language-java">public class TestHandlerActivity extends AppCompatActivity {              private static final String TAG = "TestHandlerActivity";          //主线程中的handler          private Handler mHandler = new Handler(){              @Override              public void handleMessage(Message msg) {                  super.handleMessage(msg);                  //获得刚才发送的Message对象，然后在这里进行UI操作                  Log.e(TAG,"------------> msg.what = " + msg.what);              }          };          //子线程中的handler          private Handler mHandlerThread = null;            @Override          protected void onCreate(Bundle savedInstanceState) {              super.onCreate(savedInstanceState);              setContentView(R.layout.activity_handler_test);              initData();          }            private void initData() {                //开启一个线程模拟处理耗时的操作              new Thread(new Runnable() {                  @Override                  public void run() {                        SystemClock.sleep(2000);                      //通过Handler发送一个消息切换回主线程（mHandler所在的线程）                      mHandler.sendEmptyMessage(0);                      //在子线程中创建Handler                      mHandlerThread = new Handler(){                          @Override                          public void handleMessage(Message msg) {                              super.handleMessage(msg);                              Log.e("sub thread","---------> msg.what = " + msg.what);                          }                      };                        mHandlerThread.sendEmptyMessage(1);                  }              }).start();            }</code></pre>    <p style="text-align:center"><img src="https://simg.open-open.com/show/0b47a0fb5683c64763aa3e5e8b014c23.png"></p>    <p style="text-align:center">这里写图片描述</p>    <p>程序崩溃了。报的错误是没有在子线程调用Looper.prepare()的方法。而为什么在主线程中使用不会报错？通过源码的分析可以解析这个问题。</p>    <p>在子线程中正确的使用Handler应该是这样的。</p>    <pre>  <code class="language-java">public class TestHandlerActivity extends AppCompatActivity {              private static final String TAG = "TestHandlerActivity";            //主线程的Handler          private Handler mHandler = new Handler(){              @Override              public void handleMessage(Message msg) {                  super.handleMessage(msg);                  //获得刚才发送的Message对象，然后在这里进行UI操作                  Log.e(TAG,"------------> msg.what = " + msg.what);              }          };          //子线程中的Handler          private Handler mHandlerThread = null;            @Override          protected void onCreate(Bundle savedInstanceState) {              super.onCreate(savedInstanceState);              setContentView(R.layout.activity_handler_test);              initData();          }            private void initData() {                //开启一个线程模拟处理耗时的操作              new Thread(new Runnable() {                  @Override                  public void run() {                        SystemClock.sleep(2000);                      //通过Handler发送一个消息切换回主线程（mHandler所在的线程）                      mHandler.sendEmptyMessage(0);                        //调用Looper.prepare（）方法                      Looper.prepare();                        mHandlerThread = new Handler(){                          @Override                          public void handleMessage(Message msg) {                              super.handleMessage(msg);                              Log.e("sub thread","---------> msg.what = " + msg.what);                          }                      };                        mHandlerThread.sendEmptyMessage(1);                        //调用Looper.loop（）方法                      Looper.loop();                  }              }).start();            }</code></pre>    <p><img src="https://simg.open-open.com/show/d1385198899b9183f45e905a328c8574.png"></p>    <p style="text-align:center">这里写图片描述</p>    <p>可以看到，通过调用Looper.prepare()运行正常，handleMessage方法中就可以接收到发送的Message。</p>    <p>至于为什么要调用这个方法呢？去看看源码。</p>    <h3><strong>Handler的源码分析</strong></h3>    <p>Handler的消息处理主要有五个部分组成，Message，Handler，Message Queue，Looper和ThreadLocal。首先简要的了解这些对象的概念</p>    <p>Message：Message是在线程之间传递的消息，它可以在内部携带少量的数据，用于线程之间交换数据。Message有四个常用的字段，what字段，arg1字段，arg2字段，obj字段。what，arg1，arg2可以携带整型数据，obj可以携带object对象。</p>    <p>Handler：它主要用于发送和处理消息的发送消息一般使用sendMessage（）方法,还有其他的一系列sendXXX的方法，但最终都是调用了sendMessageAtTime方法，除了sendMessageAtFrontOfQueue（）这个方法</p>    <p>而发出的消息经过一系列的辗转处理后，最终会传递到Handler的handleMessage方法中。</p>    <p>Message Queue：MessageQueue是消息队列的意思，它主要用于存放所有通过Handler发送的消息，这部分的消息会一直存在于消息队列中，等待被处理。每个线程中只会有一个MessageQueue对象。</p>    <p>Looper：每个线程通过Handler发送的消息都保存在，MessageQueue中，Looper通过调用loop（）的方法，就会进入到一个无限循环当中，然后每当发现Message Queue中存在一条消息，就会将它取出，并传递到Handler的handleMessage（）方法中。每个线程中只会有一个Looper对象。</p>    <p>ThreadLocal：MessageQueue对象，和Looper对象在每个线程中都只会有一个对象，怎么能保证它只有一个对象，就通过ThreadLocal来保存。Thread Local是一个线程内部的数据存储类，通过它可以在指定线程中存储数据，数据存储以后，只有在指定线程中可以获取到存储到数据，对于其他线程来说则无法获取到数据。</p>    <p>了解了这些基本概念后，我们深入源码来了解Handler的工作机制。</p>    <h3><strong>MessageQueue的工作原理</strong></h3>    <p>MessageQueue消息队列是通过一个单链表的数据结构来维护消息列表的。下面主要看enqueueMessage方法和next（）方法。如下：</p>    <pre>  <code class="language-java">boolean enqueueMessage(Message msg, long when) {              if (msg.target == null) {                  throw new IllegalArgumentException("Message must have a target.");              }              if (msg.isInUse()) {                  throw new IllegalStateException(msg + " This message is already in use.");              }                synchronized (this) {                  if (mQuitting) {                      IllegalStateException e = new IllegalStateException(                              msg.target + " sending message to a Handler on a dead thread");                      Log.w(TAG, e.getMessage(), e);                      msg.recycle();                      return false;                  }                    msg.markInUse();                  msg.when = when;                  Message p = mMessages;                  boolean needWake;                  if (p == null || when == 0 || when < p.when) {                      // New head, wake up the event queue if blocked.                      msg.next = p;                      mMessages = msg;                      needWake = mBlocked;                  } else {                      // Inserted within the middle of the queue.  Usually we don't have to wake                      // up the event queue unless there is a barrier at the head of the queue                      // and the message is the earliest asynchronous message in the queue.                      needWake = mBlocked && p.target == null && msg.isAsynchronous();                      Message prev;                      for (;;) {                          prev = p;                          p = p.next;                          if (p == null || when < p.when) {                              break;                          }                          if (needWake && p.isAsynchronous()) {                              needWake = false;                          }                      }                      msg.next = p; // invariant: p == prev.next                      prev.next = msg;                  }                    // We can assume mPtr != 0 because mQuitting is false.                  if (needWake) {                      nativeWake(mPtr);                  }              }              return true;          }</code></pre>    <p>可以看出，在这个方法里主要是根据时间的顺序向单链表中插入一条消息。</p>    <p>next（）方法。如下</p>    <pre>  <code class="language-java">Message next() {              // Return here if the message loop has already quit and been disposed.              // This can happen if the application tries to restart a looper after quit              // which is not supported.              final long ptr = mPtr;              if (ptr == 0) {                  return null;              }                int pendingIdleHandlerCount = -1; // -1 only during first iteration              int nextPollTimeoutMillis = 0;              for (;;) {                  if (nextPollTimeoutMillis != 0) {                      Binder.flushPendingCommands();                  }                    nativePollOnce(ptr, nextPollTimeoutMillis);                    synchronized (this) {                      // Try to retrieve the next message.  Return if found.                      final long now = SystemClock.uptimeMillis();                      Message prevMsg = null;                      Message msg = mMessages;                      if (msg != null && msg.target == null) {                          // Stalled by a barrier.  Find the next asynchronous message in the queue.                          do {                              prevMsg = msg;                              msg = msg.next;                          } while (msg != null && !msg.isAsynchronous());                      }                      if (msg != null) {                          if (now < msg.when) {                              // Next message is not ready.  Set a timeout to wake up when it is ready.                              nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);                          } else {                              // Got a message.                              mBlocked = false;                              if (prevMsg != null) {                                  prevMsg.next = msg.next;                              } else {                                  mMessages = msg.next;                              }                              msg.next = null;                              if (DEBUG) Log.v(TAG, "Returning message: " + msg);                              msg.markInUse();                              return msg;                          }                      } else {                          // No more messages.                          nextPollTimeoutMillis = -1;                      }                        // Process the quit message now that all pending messages have been handled.                      if (mQuitting) {                          dispose();                          return null;                      }                        // If first time idle, then get the number of idlers to run.                      // Idle handles only run if the queue is empty or if the first message                      // in the queue (possibly a barrier) is due to be handled in the future.                      if (pendingIdleHandlerCount < 0                              && (mMessages == null || now < mMessages.when)) {                          pendingIdleHandlerCount = mIdleHandlers.size();                      }                      if (pendingIdleHandlerCount <= 0) {                          // No idle handlers to run.  Loop and wait some more.                          mBlocked = true;                          continue;                      }                        if (mPendingIdleHandlers == null) {                          mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];                      }                      mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);                  }                    // Run the idle handlers.                  // We only ever reach this code block during the first iteration.                  for (int i = 0; i < pendingIdleHandlerCount; i++) {                      final IdleHandler idler = mPendingIdleHandlers[i];                      mPendingIdleHandlers[i] = null; // release the reference to the handler                        boolean keep = false;                      try {                          keep = idler.queueIdle();                      } catch (Throwable t) {                          Log.wtf(TAG, "IdleHandler threw exception", t);                      }                        if (!keep) {                          synchronized (this) {                              mIdleHandlers.remove(idler);                          }                      }                  }                    // Reset the idle handler count to 0 so we do not run them again.                  pendingIdleHandlerCount = 0;                    // While calling an idle handler, a new message could have been delivered                  // so go back and look again for a pending message without waiting.                  nextPollTimeoutMillis = 0;              }          }</code></pre>    <p>在next方法是一个无限循环的方法，如果有消息返回这条消息并从链表中移除，而没有消息则一直阻塞在这里。</p>    <h3><strong>Looper的工作原理</strong></h3>    <p>每个程序都有一个入口，而Android程序是基于java的，java的程序入口是静态的main函数，因此Android程序的入口也应该为静态的main函数，在android程序中这个静态的main在ActivityThread类中。我们来看一下这个main方法，如下：</p>    <pre>  <code class="language-java">public static void main(String[] args) {              SamplingProfilerIntegration.start();                // CloseGuard defaults to true and can be quite spammy.  We              // disable it here, but selectively enable it later (via              // StrictMode) on debug builds, but using DropBox, not logs.              CloseGuard.setEnabled(false);                Environment.initForCurrentUser();                // Set the reporter for event logging in libcore              EventLogger.setReporter(new EventLoggingReporter());                Security.addProvider(new AndroidKeyStoreProvider());                // Make sure TrustedCertificateStore looks in the right place for CA certificates              final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());              TrustedCertificateStore.setDefaultUserDirectory(configDir);                Process.setArgV0("<pre-initialized>");              //######              Looper.prepareMainLooper();                ActivityThread thread = new ActivityThread();              thread.attach(false);                if (sMainThreadHandler == null) {                  sMainThreadHandler = thread.getHandler();              }                if (false) {                  Looper.myLooper().setMessageLogging(new                          LogPrinter(Log.DEBUG, "ActivityThread"));              }                Looper.loop();                throw new RuntimeException("Main thread loop unexpectedly exited");          }</code></pre>    <p>在main方法中系统调用了 Looper.prepareMainLooper();来创建主线程的Looper以及MessageQueue，并通过Looper.loop（）来开启主线程的消息循环。来看看Looper.prepareMainLooper()是怎么创建出这两个对象的。如下：</p>    <pre>  <code class="language-java">public static void prepareMainLooper() {              prepare(false);              synchronized (Looper.class) {                  if (sMainLooper != null) {                      throw new IllegalStateException("The main Looper has already been prepared.");                  }                  sMainLooper = myLooper();              }          }</code></pre>    <p>可以看到，在这个方法中调用了 prepare(false);方法和 myLooper();方法，我在进入这个两个方法中，如下：</p>    <pre>  <code class="language-java">private static void prepare(boolean quitAllowed) {              if (sThreadLocal.get() != null) {                  throw new RuntimeException("Only one Looper may be created per thread");              }              sThreadLocal.set(new Looper(quitAllowed));          }</code></pre>    <p>在这里可以看出，sThreadLocal对象保存了一个Looper对象，首先判断是否已经存在Looper对象了，以防止被调用两次。sThreadLocal对象是ThreadLocal类型，因此保证了每个线程中只有一个Looper对象。Looper对象是什么创建的，我们进入看看，如下：</p>    <pre>  <code class="language-java">private Looper(boolean quitAllowed) {              mQueue = new MessageQueue(quitAllowed);              mThread = Thread.currentThread();          }</code></pre>    <p>可以看出，这里在Looper构造函数中创建出了一个MessageQueue对象和保存了当前线程。从上面可以看出一个线程中只有一个Looper对象，而Message Queue对象是在Looper构造函数创建出来的，因此每一个线程也只会有一个MessageQueue对象。</p>    <p>对prepare方法还有一个重载的方法：如下</p>    <pre>  <code class="language-java">public static void prepare() {              prepare(true);          }</code></pre>    <p>prepare()仅仅是对prepare(boolean quitAllowed) 的封装而已，在这里就很好解释了在主线程为什么不用调用Looper.prepare（）方法了。因为在主线程启动的时候系统已经帮我们自动调用了Looper.prepare()方法。</p>    <p>在Looper.prepareMainLooper（）方法中还调用了一个方法myLooper()，我们进去看看，如下：</p>    <pre>  <code class="language-java">/**           * Return the Looper object associated with the current thread.  Returns           * null if the calling thread is not associated with a Looper.           */          public static Looper myLooper() {              return sThreadLocal.get();          }</code></pre>    <p>在调用prepare（）方法中在当前线程保存一个Looper对象sThreadLocal.set(new Looper(quitAllowed));my Looper（）方法就是取出当前线程的Looper对象，保存在sMainLooper引用中。</p>    <p>在main（）方法中还调用了Looper.loop()方法，如下：</p>    <pre>  <code class="language-java">public static void loop() {              final Looper me = myLooper();              if (me == null) {                  throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");              }              final MessageQueue queue = me.mQueue;                // Make sure the identity of this thread is that of the local process,              // and keep track of what that identity token actually is.              Binder.clearCallingIdentity();              final long ident = Binder.clearCallingIdentity();                for (;;) {                  Message msg = queue.next(); // might block                  if (msg == null) {                      // No message indicates that the message queue is quitting.                      return;                  }                    // This must be in a local variable, in case a UI event sets the logger                  Printer logging = me.mLogging;                  if (logging != null) {                      logging.println(">>>>> Dispatching to " + msg.target + " " +                              msg.callback + ": " + msg.what);                  }                    msg.target.dispatchMessage(msg);                    if (logging != null) {                      logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);                  }                    // Make sure that during the course of dispatching the                  // identity of the thread wasn't corrupted.                  final long newIdent = Binder.clearCallingIdentity();                  if (ident != newIdent) {                      Log.wtf(TAG, "Thread identity changed from 0x"                              + Long.toHexString(ident) + " to 0x"                              + Long.toHexString(newIdent) + " while dispatching to "                              + msg.target.getClass().getName() + " "                              + msg.callback + " what=" + msg.what);                  }                    msg.recycle();              }      }</code></pre>    <p>在这个方法里，进入一个无限循环，不断的从MessageQueue的next方法获取消息，而next方法是一个阻塞操作，当没有消息的时候一直在阻塞，当有消息通过 msg.target.dispatchMessage(msg);这里的msg.target其实就是发送给这条消息的Handler对象。</p>    <h3><strong>Handler的运行机制</strong></h3>    <p>看看Handler的构造方法。如下：</p>    <pre>  <code class="language-java">public Handler(Callback callback) {              this(callback, false);          }            public Handler(Looper looper) {              this(looper, null, false);          }            public Handler(Looper looper, Callback callback) {              this(looper, callback, false);          }</code></pre>    <p>我们去看看没有Looper 对象的构造方法：</p>    <pre>  <code class="language-java">public Handler(Callback callback, boolean async) {              if (FIND_POTENTIAL_LEAKS) {                  final Class<? extends Handler> klass = getClass();                  if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&                          (klass.getModifiers() & Modifier.STATIC) == 0) {                      Log.w(TAG, "The following Handler class should be static or leaks might occur: " +                          klass.getCanonicalName());                  }              }                mLooper = Looper.myLooper();              if (mLooper == null) {                  throw new RuntimeException(                      "Can't create handler inside thread that has not called Looper.prepare()");              }              mQueue = mLooper.mQueue;              mCallback = callback;              mAsynchronous = async;          }</code></pre>    <p>可以看到，到looper对象为null，抛出 "Can't create handler inside thread that has not called Looper.prepare()"异常由这里可以知道，当我们在子线程使用Handler的时候要手动调用Looper.prepare()创建一个Looper对象，之所以主线程不用，是系统启动的时候帮我们自动调用了Looper.prepare()方法。</p>    <p>handler的工作主要包含发送和接收过程。消息的发送主要通过post和send的一系列方法，而post的一系列方法是最终是通过send的一系列方法来实现的。而send的一系列方法最终是通过sendMessageAtTime方法来实现的，除了sendMessageAtFrontOfQueue（）这个方法。去看看这些一系列send的方法，如下：</p>    <pre>  <code class="language-java">public final boolean sendMessage(Message msg)          {              return sendMessageDelayed(msg, 0);          }            public final boolean sendEmptyMessage(int what)          {              return sendEmptyMessageDelayed(what, 0);          }              public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {              Message msg = Message.obtain();              msg.what = what;              return sendMessageAtTime(msg, uptimeMillis);          }            public final boolean sendMessageDelayed(Message msg, long delayMillis)          {              if (delayMillis < 0) {                  delayMillis = 0;              }              return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);          }            public boolean sendMessageAtTime(Message msg, long uptimeMillis) {              MessageQueue queue = mQueue;              if (queue == null) {                  RuntimeException e = new RuntimeException(                          this + " sendMessageAtTime() called with no mQueue");                  Log.w("Looper", e.getMessage(), e);                  return false;              }              return enqueueMessage(queue, msg, uptimeMillis);          }            public final boolean sendMessageAtFrontOfQueue(Message msg) {              MessageQueue queue = mQueue;              if (queue == null) {                  RuntimeException e = new RuntimeException(                      this + " sendMessageAtTime() called with no mQueue");                  Log.w("Looper", e.getMessage(), e);                  return false;              }              return enqueueMessage(queue, msg, 0);          }            private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {              msg.target = this;              if (mAsynchronous) {                  msg.setAsynchronous(true);              }              return queue.enqueueMessage(msg, uptimeMillis);          }</code></pre>    <p>可以看出，handler发送一条消息其实就是在消息队列插入一条消息。在Looper的loop方法中，从Message Queue中取出消息调msg.target.dispatchMessage(msg);这里其实就是调用了Handler的dispatchMessage(msg)方法，进去看看，如下：</p>    <pre>  <code class="language-java">/**           * Handle system messages here.           */          public void dispatchMessage(Message msg) {              if (msg.callback != null) {                  handleCallback(msg);              } else {                  if (mCallback != null) {                      if (mCallback.handleMessage(msg)) {                          return;                      }                  }                  handleMessage(msg);              }          }</code></pre>    <p>判断msg.callback是否为空，不为空调用 handleCallback(msg);来处理消息。其实callback是一个Runnable对象，就是Handler发送post消息传过来的对象。</p>    <pre>  <code class="language-java">public final boolean post(Runnable r)          {             return  sendMessageDelayed(getPostMessage(r), 0);          }             public final boolean postAtTime(Runnable r, long uptimeMillis)          {              return sendMessageAtTime(getPostMessage(r), uptimeMillis);          }            public final boolean postAtTime(Runnable r, Object token, long uptimeMillis)          {              return sendMessageAtTime(getPostMessage(r, token), uptimeMillis);          }              public final boolean postDelayed(Runnable r, long delayMillis)          {              return sendMessageDelayed(getPostMessage(r), delayMillis);          }              public final boolean postAtFrontOfQueue(Runnable r)          {              return sendMessageAtFrontOfQueue(getPostMessage(r));          }            private static Message getPostMessage(Runnable r) {              Message m = Message.obtain();              m.callback = r;              return m;          }</code></pre>    <p>进去handleCallback方法看看怎么处理消息的，如下：</p>    <pre>  <code class="language-java">private static void handleCallback(Message message) {              message.callback.run();          }</code></pre>    <p>可以看出，其实就是回调Runnable对象的run方法。Activity的runOnUiThread，View的postDelayed方法也是同样的原理，我们先看看runOnUiThread方法，如下：</p>    <pre>  <code class="language-java">public final void runOnUiThread(Runnable action) {              if (Thread.currentThread() != mUiThread) {                  mHandler.post(action);              } else {                  action.run();              }          }</code></pre>    <p>View的postDelayed方法。如下：</p>    <pre>  <code class="language-java">public boolean postDelayed(Runnable action, long delayMillis) {              final AttachInfo attachInfo = mAttachInfo;              if (attachInfo != null) {                  return attachInfo.mHandler.postDelayed(action, delayMillis);              }              // Assume that post will succeed later              ViewRootImpl.getRunQueue().postDelayed(action, delayMillis);              return true;          }</code></pre>    <p>实质上都是在UI线程中执行了Runnable的run方法。</p>    <p>如果msg.callback是否为null，判断mCallback是否为null？mCallback是一个接口，如下：</p>    <pre>  <code class="language-java">/**           * Callback interface you can use when instantiating a Handler to avoid           * having to implement your own subclass of Handler.           *           * @param msg A {@link android.os.Message Message} object           * @return True if no further handling is desired           */          public interface Callback {              public boolean handleMessage(Message msg);          }</code></pre>    <p>CallBack其实提供了另一种使用Handler的方式，可以派生子类重写handleMessage（）方法，也可以通过设置CallBack来实现。</p>    <p>我们梳理一下我们在主线程使用Handler的过程。</p>    <p>首先在主线程创建一个Handler对象 ，并重写handleMessage（）方法。然后当在子线程中需要进行更新UI的操作，我们就创建一个Message对象，并通过handler发送这条消息出去。之后这条消息被加入到MessageQueue队列中等待被处理，通过Looper对象会一直尝试从Message Queue中取出待处理的消息，最后分发会Handler的handler Message（）方法中。</p>    <p><img src="https://simg.open-open.com/show/23272d8d219670def992782ad3480f7b.png"></p>    <p style="text-align:center">这里写图片描述</p>    <p> </p>    <p> </p>    <p>来自：http://www.jianshu.com/p/78b44756df2d</p>    <p> </p>