Handler使用简答，功能强大，我们主要用他来做线程之间的通信，也可以用来更新UI界面等。
Handler是什么？
Handler是Android SDK中处理异步类消息的核心类，它的作用就是让子线程通过Handler机制与UI通信来更新UI界面
为什么会有Handler？
在线程中，主线程用于更新UI，而子线程又不能进行耗时操作，所有，就有了Handler，Handler可以在主线程和子线程之间传递信息，从而达到想要的效果
Handler怎么用？
步骤1
主线程创建一个Handler复写handlerMessage(Message msg)方法，接收子线程发过来的消息，然后做你想做的操作即可。
private Handler mHandler = new Handler() {
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
switch (msg.what){
case 100:
//do something
break;
}
}
};
步骤2
子线程创建发送的消息，携带数据
//创建Message 方式1 new 方式直接创建对象
Message message = new Message();
message.obj = "";
message.what = 100;
//方式2 通过Handler.obtainMessage方式创建Message,这种方式好一些，可以复用
//源码注释
/*Returns a new {@link android.os.Message Message}
from the global message pool. More efficient than
* creating and allocating new instances.*/
Message obtainMessage = mHandler.obtainMessage();
obtainMessage.obj = "";
obtainMessage.what = 101;
mHandler.sendMessage(obtainMessage);
Handler源码分析
首先介绍一下主要用到的几个类
Handler，looper，Message，MessageQueue。
Handler：消息操作类，handler起到了处理消息的作用（只处理自己发出的消息），主要功能就是把消息Message添加到MessageQueue里，处理Looper发送过来的Message。
Looper：Looper是每个线程中的MessageQueue管家，调用Looper的loop()方法后，就会进入到一个无线循环中，然后每当发现MessageQueue中存在一条消息，就会把它取出来，并传递给Handler的HandlerMessage()方法中。每个线程只有一个Loope对象。
Message：Message是在线程之间传递消息，它可以在内部携带少量的信息，用于在不同线程之间交换数据。
MessageQueue：MessageQueue是消息队列的意思，它主要用于存放所有Handler发发送的消息，这个部分消息会一直存在于消息队列中，等待被处理，每个线程中只有一个MessageQueue对象。
简单流程为：
当一个应用启动时，会初始化一个UI线程，UI线程中又初始化了Looper，创建Looper的时候又创建了MessageQueue。当Handler把Message发送到MessageQueue里，然后Looper循环的取出发给Handler。由Handler处理这个信息。
这里我是先贴源码再说意思，fear你不看源码
Handler构造方法
Handler.java
public Handler() {
this(null, false);
}
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;
}
上面代码比较简单，主要就是Handler的构造方法，你会发现刚进来是上面第一个构造方法，它调用的是下面这个构造方法，接下来把第二个构造方法分开详解
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());
}
}
这段是判断有没有泄漏，接下来
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
判断当前线程的Looper是否为空，如果为空就抛出异常。
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
完成了对Looper的检验后，Handler就获取了当前线程mLooper.mQueue的引用。同时包含一个可有可无的callback和是否同步的标志位。
sendMessage
Handler.java
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
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);
}
所有的sendMessage发送消息方法最终都会调用下面这个方法
enqueueMessage(queue, msg, uptimeMillis);
这个方法的三个参数分别mLooper.mQueue(当前线程的消息队列)、将要send的Message和sendMessage的事件戳。
接着看enqueueMessage里面肝了什么
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
在这里，Message的target会持有当前Handler对象。最终调用queue的enqueueMessage方法。enqueueMessage顾名思义，就是带着Message去排队，我们接下来看看Message是如何去排队。
MessageQueue：
MessageQueue.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;
}
上面代码有点长，我们一点一点的看
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.");
}
这个就是对target进行判空，判断代发Message是否已经在使用
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;
}
对当前消息队列加锁
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
判断消息队列是否弃用（通常因为线程已死）,将待发Message标记已用，获取消息时间戳,Message p 持有当前Message
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;
}
如果当前Message为空，或待发Message需要立即执行，或待发Message的时间戳已小于当前Message的时间戳，将当前Message放至待发Message后面（从这里我们也可以看到MessageQueue中的Message其实是链式储存的，方便插队）
if (needWake) {
nativeWake(mPtr);
}
如果待发Message还需要等待，则将待发Message放至队尾
到这里，我们就将需要发送的Message放入到对垒中，接下来我们看看它是怎么被发送的
Looper
Looper.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.recycleUnchecked();
}
}
还是有点多，我们还分开看
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
获取当前线程的Looper对象进行判空
Binder.clearCallingIdentity();
final long ident = Binder.clearCallingIdentity();
确定当前线程的身份是本地进程，同时获取当前线程的身份
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);
}
在循环中判断调用了msg.target.dispatchMessage(msg)
之后线程身份有没有发生改变
msg.recycleUnchecked();
回收Meesage对象
我们之前在Handler代码中有读到过
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
//空的，什么操作都没有，注释说必须继承实现 然后用来接收消息
//麻烦了，没办法往下看了，并没有看到是如何传递消息过来的
//别着急，看这个方法在哪里被调用了
//搜索一下，dispatchMessage这里被调用了
}
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {//分发消息
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
//调用了我们复写的方法，传递了msg过去了
继续查看dispatchMessage 是哪里调用的，，然后，就发现，找不到
handleMessage(msg);
}
}
这一段代码比较简单，根据初始化时是否有设置Callback决定谁来调用handleMessage(Message msg)。
到这里new Handler复写handleMessage方法就分析完了，知道是调用dispatchMessage(Message msg) 的时候分发的消息。但是，找不到这个方法是在哪里被调用的，，，那么就要涉及到Activity的启动流程相关知识了。
Activity启动流程之ActivityThread
我们都知道一个程序只有一个入口，Activity也是，它的入口就是ActivityThread 的main方法，来看看源码分析吧
ActivityThread
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
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());
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("");
Looper.prepareMainLooper();//Looper准备，程序开始运行，Activity通讯，消息基本上都是靠Looper去驱动的
ActivityThread thread = new ActivityThread();//创建一个主线程
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();//创建Activity的Handler
}
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();//Looper开始循环 取消息
throw new RuntimeException("Main thread loop unexpectedly exited");
}
到这里还是没看到调用 dispatchMessage 方法，别着急，马上就来。
ActivityThead
public static void main(String[] args) {
...
Looper.loop(); //开始循环，看看loop方法做了什么
throw new RuntimeException("Main thread loop unexpectedly exited");
}
Looper
/**
* 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();//用get方法获取存储在集合中的Looper对象
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
final MessageQueue queue = me.mQueue;//获取Looper的消息队列
// 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 死循环 不断的去MessageQueue里面获取消息
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 traceTag = me.mTraceTag;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
//取到消息就调用target的dispatchMessage 方法，而这个target正式Handler对象 下面接着分析源码
msg.target.dispatchMessage(msg);
} 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();//移除消息 具体看源码，就是判断消息是否为空，然后对消息进行回收，置为空
}
}
Message
/*package*/ Handler target;//Message里面的target就是Handler
/** Constructor (but the preferred way to get a Message is to call {@link #obtain() Message.obtain()}).
*/
//这里也强调了，创建消息的方法首选obtain方式
public Message() {
}
//接着看target是在哪里赋值的
Handler
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;//在这里给msg的target赋值的，赋的值是Handler
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
到这里，Handler的整个执行流程就基本分析完了，建议大家多去看看源码，多点进去看看，可以结合源码一起看，加深印象，这样理解起来更快。