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.
*
* 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.
*/
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.
*
* Most interaction with a message loop is through the
* {@link Handler} class.
*
*
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.
*
*
* 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();
* }
* }
*/
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(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(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 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 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。