【Android】Handler原理解析

要分析Handler的原理，首先需要了解Message和Looper，所以我们先来分析一下Message和Looper的源码。

Message

首先根据注释来看一下Message的定义：

/**
 *
 * 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.
 *
 * <p class="note">While the constructor of Message is public, the best way to get
 * one of these is to call {@link #obtain Message.obtain()} or one of the
 * {@link Handler#obtainMessage Handler.obtainMessage()} methods, which will pull
 * them from a pool of recycled objects.</p>
 */

定义一个包含描述和任意数据对象的对象，将这个对象发送给Handler处理。这个对象包含两个额外的int字段和一个额外的object字段，但在很多情况下，这些字段并要求你必须给他们赋值。同时提醒我们，虽然Message的构造函数是public的，但是最好是调用Message.obtain()方法，这个方法会从回收池里获取Message对象。obtain方法是怎么回收利用Message的呢，从以下方法中分析：

/**
 * Return a new Message instance from the global pool. Allows us to  
 * avoid allocating new objects in many cases.
 */
public static Message obtain() {
    synchronized (sPoolSync) {
        if (sPool != null) {
            Message m = sPool;
            sPool = m.next;
            m.next = null;
            m.flags = 0; // clear in-use flag
            sPoolSize--;
            return m;
        }
    }
    return new Message();
}

1、这里的sPoolSync是一个Object实例，sPool是一个Message对象，如果sPool不为null的话，会将sPool赋值给一个新的Message对象，并将m.next赋值给sPool，m.next也是一个Message，同时，最终返回m。由于取出了一个Message，所以sPoolSize–，如果有另外线程调用Message.obtain，则会直接创建一个新的Message返回；

/**
 * Same as {@link #obtain()}, but copies the values of an existing
 * message (including its target) into the new one.
 * @param orig Original message to copy.
 * @return A Message object from the global pool.
 */
public static Message obtain(Message orig) {
    Message m = obtain();
    m.what = orig.what;
    m.arg1 = orig.arg1;
    m.arg2 = orig.arg2;
    m.obj = orig.obj;
    m.replyTo = orig.replyTo;
    m.sendingUid = orig.sendingUid;
    if (orig.data != null) {
        m.data = new Bundle(orig.data);
    }
    m.target = orig.target;
    m.callback = orig.callback;

    return m;
}

2、根据注释可以看出，该方法与obtain()方法一样，不同之处在于，这个方法的参数是一个已存在的Message，它首先调用botain()方法，获取一个已存在的Message，然后将参数Message的值赋值给m对应的属性，最终返回m。其他几个obtain方法都是多了一些传参，对m的某些值做了修改，就不多做赘述了，Message内部还有一些其他方法，但是一般在使用Handler时不会使用，所以就不予以描述了。

Looper

Looper对与Handler来说，是一个很重要的概念，不了解Looper就不能真正弄懂Handler的机制，所以，先来了解一下Looper，先看一下官方对与Looper的定义。

/**
  * Class used to run a message loop for a thread.  Threads by default do
  * not have a message loop associated with them; to create one, call
  * {@link #prepare} in the thread that is to run the loop, and then
  * {@link #loop} to have it process messages until the loop is stopped.
  *
  * <p>Most interaction with a message loop is through the
  * {@link Handler} class.
  *
  * <p>This is a typical example of the implementation of a Looper thread,
  * using the separation of {@link #prepare} and {@link #loop} to create an
  * initial Handler to communicate with the Looper.
  *
  * <pre>
  *  class LooperThread extends Thread {
  *      public Handler mHandler;
  *
  *      public void run() {
  *          Looper.prepare();
  *
  *          mHandler = new Handler() {
  *              public void handleMessage(Message msg) {
  *                  // process incoming messages here
  *              }
  *          };
  *
  *          Looper.loop();
  *      }
  *  }</pre>
  */

官方对与Looper的定义为，针对一个线程运行的消息循环。线程默认是没有消息循环相关联的，要创建一个消息循环，就要调用Looper的prepare方法，然后调用Looper的loop方法来循环处理消息，直到循环停止。下边官方也说明大多数与消息循环的交互是通过Handler来实现的。最后官方给出了一个创造包含消息循环的线程的例子。下边看一下Looper具体的代码：

 /** Initialize the current thread as a looper.
  * This gives you a chance to create handlers that then reference
  * this looper, before actually starting the loop. Be sure to call
  * {@link #loop()} after calling this method, and end it by calling
  * {@link #quit()}.
  */
 public static void prepare() {
    prepare(true);
 }

 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));
 }

通过注释可以看到，prepare方法可以将当前线程初始化为looper。并且该方法要在Looper.loop()方法之前调用，最后要调用quit()方法退出循环。这两个方法
唯一的不同就是多了一个boolean类型的参数。无参的prepare方法调用了有参的prepare方法，并默认参数为true。从有参的prepare方法中可以看到，有个sThreadLocal对象，并调用了该对象的get方法，我们看下它的get方法最终会返回什么：

/**
 * Returns the value in the current thread's copy of this
 * thread-local variable.  If the variable has no value for the
 * current thread, it is first initialized to the value returned
 * by an invocation of the {@link #initialValue} method.
 *
 * @return the current thread's value of this thread-local
 */
public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    return setInitialValue();
}

首先，该方法会获取当前线程，并获取当前线程的ThreadLocalMap，ThreadLocal是一个定制的HashMap，是为了维护线程本地值而诞生的，如果当前线程的ThreadlocalMap不为null，则根据key，获取Entry，将Entry.value返回。如果map为null，则调用setInitialValue方法，再看这个方法是用来干什么的：

/**
 * Variant of set() to establish initialValue. Used instead
 * of set() in case user has overridden the set() method.
 *
 * @return the initial value
 */
private T setInitialValue() {
    T value = initialValue();
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
    return value;
}

protected T initialValue() {
    return null;
}

/**
 * Create the map associated with a ThreadLocal. Overridden in
 * InheritableThreadLocal.
 *
 * @param t the current thread
 * @param firstValue value for the initial entry of the map
 */
void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}

可以看到，setInitialValue方法是将null作为value传入了当前线程的ThreadLocalMap中，在get方法中，若map为null，最终返回的值为null。如果get方法返回不为null，则会抛出异常：每个线程只能创建一个Looper。若get方法返回为null，则ThreadLocal中会存入一个新创建的Looper对象。接下来，看一下Looper的构造函数：

private Looper(boolean quitAllowed) {
    mQueue = new MessageQueue(quitAllowed);
    mThread = Thread.currentThread();
}

在构造函数中，创建一个MessageQueue对象，同时将当前线程赋值给mThread。其中quitAllowed参数是MessageQueue调用quit()方法时会用到，若该参数值为false，则会抛出异常：Main thread not allowed to quit。所以创建一个包含Looper的子线程时，需要调用无参或者参数值为true的prepare方法。接下来就是Looper中最重要的方法,loop方法：

 /**
  * Run the message queue in this thread. Be sure to call
  * {@link #quit()} to end the loop.
  */
 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
         final Printer logging = me.mLogging;
         if (logging != null) {
             logging.println(">>>>> Dispatching to " + msg.target + " " +
                     msg.callback + ": " + msg.what);
         }

         final long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;

         final long traceTag = me.mTraceTag;
         if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
             Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
         }
         final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
         final long end;
         try {
             msg.target.dispatchMessage(msg);
             end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
         } finally {
             if (traceTag != 0) {
                 Trace.traceEnd(traceTag);
             }
         }
         if (slowDispatchThresholdMs > 0) {
             final long time = end - start;
             if (time > slowDispatchThresholdMs) {
                 Slog.w(TAG, "Dispatch took " + time + "ms on "
                         + Thread.currentThread().getName() + ", h=" +
                         msg.target + " cb=" + msg.callback + " msg=" + msg.what);
             }
         }

         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.recycleUnchecked();
     }
 }

首先调用Looper的myLooper方法，获取Looper对象，若为null，则说明没有调用Looper的prepare方法创建Looper对象，抛出异常。接着获取Looper对应的MessageQueue，后边会用到。然后调用两次Binder的native方法：clearCallingIdentity，一次用作标识，一次是获取具体的标识，就不多做详解了。接下来就是一个循环，这个循环就是用来遍历MessageQueue中的Message的，首先用MessageQueue的next方法获取Message，这里看一下MessageQueue的next方法：

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;
    }
}

首先是将成员变量mPtr赋值给局部变量ptr，这个mPtr是从何而来的呢？通过MessageQueue的构造函数可以看到，是通过native方法，nativeInit来初始化的一个值，看一下nativeInit的代码：

static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) {
    NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();
    if (!nativeMessageQueue) {
        jniThrowRuntimeException(env, "Unable to allocate native queue");
        return 0;
    }

    nativeMessageQueue->incStrong(env);
    return reinterpret_cast<jlong>(nativeMessageQueue);
}

以上代码表示，若指针为空，抛出异常，返回0，否则，将指针强转为Long类型，返回。同时，在next方法中可以看到，若prt=0，直接返回null。接着定义了两个int类型的值，具体作用用到了再看。接下来是个for循环，用于循环获取Message。若nextPollTimeoutMillis不为0，则调用Binder的native方法来刷新进程间通信线程的状态，具体代码不作详解。下边又是一个MessageQueue的native方法，来看一下具体代码：

static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,
        jlong ptr, jint timeoutMillis) {
    NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);
    nativeMessageQueue->pollOnce(env, obj, timeoutMillis);
}

void NativeMessageQueue::pollOnce(JNIEnv* env, jobject pollObj, int timeoutMillis) {
    mPollEnv = env;
    mPollObj = pollObj;
    mLooper->pollOnce(timeoutMillis);
    mPollObj = NULL;
    mPollEnv = NULL;

    if (mExceptionObj) {
        env->Throw(mExceptionObj);
        env->DeleteLocalRef(mExceptionObj);
        mExceptionObj = NULL;
    }
}

可以看到内部是一些赋值操作，具体用到这些值的时候再做具体分析。继续往下分析next方法，可以看到一个同步代码块，首先获取当前时间，接着将null赋值给prevMsg，将mMessages赋值给msg。判断如果msg不为null，且msg.target为null，即msg没有对应的Hanlder时，说明当前msg不应该被处理，就将msg赋值给prevMsg，将msg.next赋值给msg，同时进入循环，若msg不为null，且msg是同步消息，则继续执行msg赋值给prevMsg，msg.next赋值给msg。代码继续执行，判断msg是否为null，若为null则将nextPollTimeoutMillis赋值为-1，代表没有Message，若不为null，则比较当前时间和msg.when，若当前时间小于msg.when，则说明下一条Message没有准备好，将msg.when-now和Integer.MAX_VALUE间的最小值赋值给nextPollTimeoutMillis,在其他方法中，会在该值到0后，调用nativeWake方法唤醒Looper，处理该条消息。如果now大于等于msg.when，说明获取到需要处理的Message，将false赋值给mBlocked，该值会在其他方法用到。如果prevMsg不为null的话，说明进入了上边do-while循环中，msg为下一条异步消息，但是不是头节点，则将msg.next赋值给prevMsg.next，若prevMsg为null，说明msg为头节点，将msg.next赋值给mMessages。将null赋值给msg.next，并将msg标识为正在使用中，最后返回msg。分析完MessageQueue的next方法，再继续分析Looper的loop方法。

通过next方法获取Message后，如果Message为null，说明消息队列里边没有消息了，则return结束轮询。后续大部分代码是打印log相关的，中间有一行代码： msg.target.dispatchMessage(msg);这里可以看到，调用了Hanlder的dispatchMessage方法来处理消息。最后调用了Message的recycleUnchecked方法来回收消息。接着再看一下MessageQueue的enqueueMessage方法，消息是如何进入消息队列的：

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;
}

首先是两个判断，如果target为null，说明没有Handler来处理消息，则抛出异常，再判断消息是否正在被使用，若是，则抛出异常。下来是一个同步代码块，首先判断mQuitting是否为true，该值在调用quit方法是会赋值为true，若为true，说明已经调用quit方法，则调用Message的recycle方法，并返回false。如果mQuitting为false，首先用markInUse将Message标识为使用中，并将参数中的when赋值给msg.when。接下来将mMessages赋值给p。然后定义一个boolean类型的值，用于标识是否需要唤醒Looper。**接下来判断，若p为null，说明消息队列中没有消息，若when为0，则说明该消息需要立即处理，若when小于p.when，说明插入消息要比当前链表头的处理时间要早，三种判断有一种为true，则将新插入的消息插入到链表头，反之，三种判断都为false的话，将mBlocked && p.target == null && msg.isAsynchronous()赋值给needwake，用于判断是否唤醒Looper,接着用for循环来对比消息队列中的Message的when和新插入消息的when，来找出新插入消息插入的位置。最后判断是否需要唤醒，若需要，则调用native方法nativeWake即可。**Message和Looper分析完了，接下来正式分析Handler。

Handler

查看Handler源码，该类首先定义了三个成员变量。

/*
 * Set this flag to true to detect anonymous, local or member classes
 * that extend this Handler class and that are not static. These kind
 * of classes can potentially create leaks.
 */
private static final boolean FIND_POTENTIAL_LEAKS = false;
private static final String TAG = "Handler";
private static Handler MAIN_THREAD_HANDLER = null;

其中TAG很容易理解，MAIN_THREAD_HANDLER根据名字就可以看出来是主线程的Handler，但是FIND_POTENTIAL_LEAKS是做什么用的呢？根据注释，如果将这个变量值设为true时，就可以检测继承自Handler的非静态匿名类、局部类和成员类可能造成的泄露。至于怎么检测，后边分析到了再说。

Callback

接下来定义了一个接口Callback：

/**
 * 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);
}

根据注释可知，该接口是在实例化Handler时使用的，以避免必须实现自己的Handler子类。该接口与Handler的关系就跟Thread和Runnable的关系类似。以代码为例，通常我们使用Handler会写一个Hanlder的子类：

private static class MyHandler extends Handler {
    WeakReference<Activity> mWeakReference;

    public MyHandler(Activity activity) {
        mWeakReference = new WeakReference<>(activity);
    }

    @Override
    public void handleMessage(Message msg) {
        super.handleMessage(msg);
    }
}

但使用Callback的话，就不需要继承Hanlder了，代码如下：

class MyCallback implements Handler.Callback {
    @Override
    public boolean handleMessage(Message msg) {
        //处理消息
        return true;
    }
}

//使用时直接作为参数传进去即可
Handler handler = new Handler(new MyCallback());

可以看到Callback的handleMessage有个boolean类型的返回值，该值的作用就是判断是否处理Hanlder自己的handleMessage，如果为true，表示消息已经处理完，则不会调用Hanlder的handleMessage，反之，则会调用Hanlder的handleMessage。具体逻辑在dispatchMessage方法中，后续会讲该方法。
这种方法虽然编译器不会提示会发生内存泄露，但实际还是会发生内存泄露，一般在Activity中推荐使用继承Handler。

构造函数

Handler总共有7个构造函数，我们逐一分析：

/**
 * Default constructor associates this handler with the {@link Looper} for the
 * current thread.
 *
 * If this thread does not have a looper, this handler won't be able to receive messages
 * so an exception is thrown.
 */
public Handler() {
    this(null, false);
}

1、第一个构造函数，不需要传任何参数，它调用了另外一个两个参数的构造函数，两个参数分别为：Callback和一个boolean类型的值，根据其名称，可以看出它跟是否异步相关，这个值具体有什么用，后边用到后再进行了解，先记住有这么一个值，该构造函数传递的两个值分别为null和false，根据注释，使用该构造函数实例化Handler的话，Handler对象将于当前线程中的Looper关联，如果当前线程没有Looper，则会抛出异常。

/**
 * Constructor associates this handler with the {@link Looper} for the
 * current thread and takes a callback interface in which you can handle
 * messages.
 *
 * If this thread does not have a looper, this handler won't be able to receive messages
 * so an exception is thrown.
 *
 * @param callback The callback interface in which to handle messages, or null.
 */
public Handler(Callback callback) {
    this(callback, false);
}

2、第二个构造函数相较第一个构造函数,多了一个Callback参数，内部调用的跟第一个构造函数一样，不同之处第一个构造函数Callback为null，第二个构造函数Callback是作为参数传进来的。

/**
 * Use the provided {@link Looper} instead of the default one.
 *
 * @param looper The looper, must not be null.
 */
public Handler(Looper looper) {
    this(looper, null, false);
}

3、第三个构造函数跟第一个构造函数的区别在于，多了一个Looper参数，其内部调用的也是一个三参数的构造函数，无参的构造函数使用的是当前线程的Looper，这个构造函数使用的是参数传过来的Looper。

/**
 * Use the provided {@link Looper} instead of the default one and take a callback
 * interface in which to handle messages.
 *
 * @param looper The looper, must not be null.
 * @param callback The callback interface in which to handle messages, or null.
 */
public Handler(Looper looper, Callback callback) {
    this(looper, callback, false);
}

4、该构造函数有两个参数，分别为Looper和Callback，与只有Callback的构造函数不同点在于Looper。

/**
 * Use the {@link Looper} for the current thread
 * and set whether the handler should be asynchronous.
 *
 * Handlers are synchronous by default unless this constructor is used to make
 * one that is strictly asynchronous.
 *
 * Asynchronous messages represent interrupts or events that do not require global ordering
 * with respect to synchronous messages.  Asynchronous messages are not subject to
 * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
 *
 * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
 * each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
 *
 * @hide
 */
public Handler(boolean async) {
    this(null, async);
}

5、该构造函数有一个boolean类型的参数，这个参数是用来设置Handler是否异步的，根据注释可以看出，Handler默认是异步的，除非调用这个构造函数，可以修改Hanlder为同步的。

/**
 * Use the {@link Looper} for the current thread with the specified callback interface
 * and set whether the handler should be asynchronous.
 *
 * Handlers are synchronous by default unless this constructor is used to make
 * one that is strictly asynchronous.
 *
 * Asynchronous messages represent interrupts or events that do not require global ordering
 * with respect to synchronous messages.  Asynchronous messages are not subject to
 * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
 *
 * @param callback The callback interface in which to handle messages, or null.
 * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
 * each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
 *
 * @hide
 */
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;
}

6、该构造函数是使用当前线程的Looper的所有构造函数最终调用的函数。在这个函数中，看到了前面提到过的FIND_POTENTIAL_LEAKS，如果该值为true，首先通过getClass获取Handler的子类，接着判断该类是否是匿名类、成员类或者局部类并且是非静态的，就会输出log，提示Handler类应该为静态否则可能导致内存泄漏。接着根据Looper.myLooper()获取当前线程的Looper，如果Looper为null，则抛出异常,在异常里可以看到有个Looper.prepare()方法，该方法会在当前线程没有Looper时新建一个Looper实例。下边就是三个赋值操作，分别将Looper的MessageQueue、callback参数、async参数赋值给对应的成员变量。

/**
 * Use the provided {@link Looper} instead of the default one and take a callback
 * interface in which to handle messages.  Also set whether the handler
 * should be asynchronous.
 *
 * Handlers are synchronous by default unless this constructor is used to make
 * one that is strictly asynchronous.
 *
 * Asynchronous messages represent interrupts or events that do not require global ordering
 * with respect to synchronous messages.  Asynchronous messages are not subject to
 * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
 *
 * @param looper The looper, must not be null.
 * @param callback The callback interface in which to handle messages, or null.
 * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
 * each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
 *
 * @hide
 */
public Handler(Looper looper, Callback callback, boolean async) {
    mLooper = looper;
    mQueue = looper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

7、该构造函数与上个构造函数的不同在于Looper不是使用当前线程的Looper，而是使用参数中提供的Looper，该函数里边只有四个赋值操作，分别将looper参数、looper中的MessageQueue、callback参数、async参数赋值给对应的成员变量。

dispatchMessage与handleMessage

首先讲一下这两方法有啥区别，dispatchMessage是用于分发处理消息，通常我们不用实现该方法，而是用handleMessage来处理我们自己的方法。从代码看：

/**
 * Subclasses must implement this to receive messages.
 */
public void handleMessage(Message msg) {
}

/**
 * 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);
    }
}

可以看到，dispatchMessage内本身有一些逻辑，首先判断msg的callback是否为null，callback是一个Runnable接口，若不为bull，则调用handleCallback方法，该方法内部实现很简单，就是调用了callback的run方法，若为null，则判断Handler的Callback是否为null，若不为null，则调用Callback的handleMessage，若Callback的handleMessage返回true，说明不需要再进一步处理，则return，反之，调用handleMessage。Looper.loop中，循环取出消息后，就是调用的dispatchMessage方法来处理消息。

发送消息

首先整理一下Handler对外暴露的可以发送消息的方法，分别为：post(Runnable r)、postAtTime(Runnable r, long uptimeMillis)、postAtTime(Runnable r, Object token, long uptimeMillis)、postDelayed(Runnable r, long delayMillis)、sendEmptyMessage(int what)、sendEmptyMessageDelayed(int what, long delayMillis)、sendEmptyMessageAtTime(int what, long uptimeMillis)、sendMessageDelayed(Message msg, long delayMillis)、sendMessageAtTime(Message msg, long uptimeMillis)，可以看到，可以发送消息的方法很多，主要分为两种，一种是参数为Message的，一种是参数为Runnable的。下边我们分析一下两者有何区别：

public final boolean post(Runnable r)
{
   return  sendMessageDelayed(getPostMessage(r), 0);
}

可以看到，最终也是调用了sendMessageDelayed(Message msg, long delayMillis)这个方法，通过getPostMessage(r)方法获取到了Message，将r赋值给Message的callback，也就是说，最终处理消息时，不会执行handleMessage方法，而是执行Runnable的run方法。这就是两种参数发送消息之间的区别，上述方法最终调用的都是sendMessageAtTime(Message msg, long uptimeMillis)方法，来看一下这个方法：

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);
}

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    msg.target = this;
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
}

可以看到，其实最终调用的就是MessageQueue的enqueueMessage方法。这个方法对插入消息的处理我们上边也已经分析过了。Handler也就分析完了，可以看到其实Handler的源码本身是没有多少的，大部分内容是在Looper和MessageQueue中的。Handler的原理到此就算是分析完成了，中间若有不对的地方，希望各位大佬可以提出来。

代码中的彩蛋：算是一个彩蛋吧，是在Looper的源码中发现的，在打印log时，有这么一个方法，Log.wtf,看源码解释是What a Terrible Failure，但我想，应该也是包含what the fuck的意思吧，哈哈哈。