Handler的使用场景：有时候在子线程中进行耗时的 I/O 操作，在操作完成会需要对UI进行改变，由于Android开发机制限制，我们并不能在子线程修改UI，否则会抛出异常。这个时候可以将更新UI操作通过Handler切换到主线程完成。
以上涉及到的几个概念：
1>为什么不能在主线程进行耗时操作？
2>为什么不能在子线程访问UI？
1）Android主线程中进行UI绘制，当进行耗时操作时，UI会停止绘制，界面阻塞程序看起来卡死，这对用户来说是极其糟糕的体验。而且当Android系统发现有在UI线程内进行网络请求时会抛出异常
RuntimeException: Unable to start activity ComponentInfo{com.example/com.example.ExampleActivity}: android.os.NetworkOnMainThreadException
2）最直观体现在更改UI时，ViewRootImpl中通过cheakThread()对UI进行校验。当在子线程中更改UI时会抛出异常。
void checkThread() {
if (mThread != Thread.currentThread()) {
throw new CalledFromWrongThreadException(
"Only the original thread that created a view hierarchy can touch its views.");
}
}
我们考虑下为什么系统不允许在子线程中访问UI呢？这是因为Android的UI控件不是线程安全的，如果在多线程中并发访问可能会造成UI控件处于不可预期的状态。好为什么系统不对UI控件访问加上锁机制呢？缺点有两个：首先加上锁机制会让UI访问的逻辑变的复杂；其次锁机制会降低UI访问的效率。源自----《Android开发艺术探索》
Tips：其实在Activity的onCreate中进行不太耗时的操作是可以在子线程中更新UI的。因为checkThread方法是在界面初始化完成绘制（onWindowFocusChanged）后进行调用判断。而在onCreate方法中界面UI还未完成渲染，不用调用checkThread方法，所以此时在对于不太耗时的操作在子线程中可以更新UI。
消息传递原理
好的，现在我们正式了解Handler机制，完成整个消息传递过程需要Handler、MeaasgeQueue、Looper、Meaasge四个对象。
大致和流程为：Handler通过sendMessage发送消息（Message），发送的消息存放在消息队列（MessageQueue）中；主线程中默认创建Looper，而Looper的looper()方法会不停去消息队列(messageQueue)中获取消息(msg)，当有发现有新消息(msg != null)时则传递给Handler的dispatchMessage()进行处理，内部即调用handleMessage()完成消息传递。
1首先通过handler.sendMessage(Message msg)发送消息，我们去源码看看
//调用延迟发送发送
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
其实就是调用sendMessageAtTime方法
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
在sendMessageAtTime中创建MessageQueue对象用来存放消息（单链表数据结构来维护消息列表），把MeaasgeQueue、Message、延时时间传递到enqueueMessage方法中。
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);
}
接着我们来看enqueueMessage方法,其中msg.target = this 是此handler将自己作为一个标记传入到了Message中。(此target后文会用到)
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
MessageQueue的enqueueMessage()主要是实现单链表的插入操作；next()方法是一个无限循环的方法，如果消息队列中没有消息，那next方法会一直阻塞在这里，当有新消息到来，next方法会条消息并将其从单链表中删除。
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;
}
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;
}
}
Looper工作原理
一个线程中必须也只能有一个Looper，UI线程中默认创建Looper对象，子线程中可以通过 Looper.prepare()创建Looper，并调用Looper.looper()方法实现去MessageQueue中轮循Message.(可调用Looper的quit方法终止消息循环)
Looper.prepare()内会调用以下方法创建MessageQueue消息队列
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
下面再来看looper方法,可以看到looper方法是一个死循环，唯一跳出循环的方式是MeaasgeQueue的next方法(msg)返回了null。当返回不为空时，走msg.target.dispatchMessage(msg);其中此时target即为前面Message中标记的Handler对象，所以就是调用了handler.dispatchMessage()方法。
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 (;;) {
//关键代码（如下）-------------------------------------
Message msg = queue.next(); // might block
if (msg == null) {
// No message indicates that the message queue is quitting.
return;
}
//关键代码（往上）-------------------------------------
// 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 traceTag = me.mTraceTag;
if (traceTag != 0) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
//关键代码（如下）-------------------------------------
msg.target.dispatchMessage(msg);
//关键代码（往上）-------------------------------------
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
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();
}
}
再看Handler的dispatchMessage方法，在日常开发中创建Handler并重写handleMessage方法时，callback（就是Runnable）为null，看到最后调用了handleMessage方法也就是回到我们重写的handleMessage方法。最终完成了的消息传递过程。
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
补充：以上可知Handler中创建了MessageQueue，但是这个消息队列如何跟Looper中消息队列的关联起来呢？其实在Handler的构造函数中获取到Looper对象，将Looper中的queue赋值给Handler中queue，所以Handler跟Looper中的MessageQueue对象是一样的。
public Handler(Callback callback, boolean async) {
......
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
......
}
有关Handler消息传递机制的大致到这里，如有问题，欢迎指正~
参考：《Android开发艺术探索》