Handler机制的作用：
可以实现线程见得通讯，将数据封装到消息中，并将消息发送给UI线程，并取出消息中的数据刷新UI。
涉及到Handler机制的4个重要的类：
Handler：发送消息到MessageQueue，Looper循环拿出消息，交给Handler的handlermessage处理
Message：消息；包含消息描述和数据
Looper：循环消息，当消息队列有消息的时候，Looper不断循环的处理消息，当消息队列没有消息了，Looper就阻塞在原地
MessageQueue：消息队列，用于储存消息
Message：消息；包含消息描述和数据
Looper：循环消息，当消息队列有消息的时候，Looper不断循环的处理消息，当消息队列没有消息了，Looper就阻塞在原地
MessageQueue：消息队列，用于储存消息
Handler类：
public Handler(Looper looper) {
this(looper, null, false);
}
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());
}
}
// 这里获取到一个Looper对象
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;
}
从sThreadLocal取出looper，如果sThreadLocal中有就返回looper没有就返回空
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}是从哪里给sThreadLocal设置looper的？Looper.prepare()
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));
}从上面可以看出，首先判断sThreadLocal是否存在Looper，如果没有就创建一个新的Looper设置进去。所以我们要先调用Looper.prepare()，才能创建Handler对象（否则会抛出异常"Can't create handler inside thread that has not called Looper.prepare()"），每个线程也只会创建一个Looper对象。
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("<pre-initialized>");
// 这里调用了prepareMainLooper()
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"));
}
// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}prepareMainLooper()：
public static void prepareMainLooper() {
// 在这里调用prepare方法创建Looper，并设置到sThreadLocal中
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
// 从sThreadLocal取出上面创建的Looper对象赋值给sMainLooper
sMainLooper = myLooper();
}
}发送消息：handler.sendMessage(msg);
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);
}
/** * msg：发送的消息 * uptimeMillis：发送消息的时间，如果不是调用的sendMessageDelayed()，时间都会为0 */ 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); }sendMessageAtFrontOfQueue()：将消息添加到消息队列的头部位置
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);
}MessageQueue的enqueueMessage方法：
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;
// 只是使用一个message对象来标识当前的处理的消息
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;
}所有的消息入队列其实就是将所有的消息按时间来进行排序，这个时间就是我们传入的uptimeMillis。具体操作就是根据时间顺序调用msg.next，从而为每个消息指定它下一个消息是什么。如果是通过sendMessageAtFrontOfQueue()来发送消息，它会调用enqueueMessage()来让消息入队列，时间为0，会把mMessage赋值为新入队的这条消息，然后将这条消息的next指定为刚才的mMessages，这样就完成了消息队列的头部添加。
从队列里取出消息：Looper.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 (;;) {
// 通过一个死循环不断地调用MessageQueue的next()方法来取出消息
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 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 {
// 每当有一个消息出队列，就会调用Handler的dispatchMessage()方法
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();
}
}
Handler的dispatchMessage()方法：转发处理消息
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
// 如果mCallback，就调用mCallback.handleMessage(msg)，否则直接调用Handler的handleMessage(msg)，
// 我们就可以在handleMessage()中获取到之前发送的消息了
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
总结：
Handler机制的流程：
1、将子线程中获取到的数据封装在Message对象中
2、通过handler.sendMessage()或者handler.post()将消息发送到消息队列中，最终都是走的sendMessageAtTime(),在sendMessageAtTime中调用了MessageQueue的enqueueMessage()，完成消息插入到队列的操作
3、每次我们创建Hanlder实例时，都会初始化Looper对象，是调用Looper.prepare()创建Looper对象，将Looper对象通过
ThreadLocal存入到线程中
4、Looper调用Looper.loop() --> MessageQueue.next()从消息队列中不断取出消息
5、取出来的消息通过handler.dispatchMessage()转发处理
1、将子线程中获取到的数据封装在Message对象中
2、通过handler.sendMessage()或者handler.post()将消息发送到消息队列中，最终都是走的sendMessageAtTime(),在sendMessageAtTime中调用了MessageQueue的enqueueMessage()，完成消息插入到队列的操作
3、每次我们创建Hanlder实例时，都会初始化Looper对象，是调用Looper.prepare()创建Looper对象，将Looper对象通过
ThreadLocal存入到线程中
4、Looper调用Looper.loop() --> MessageQueue.next()从消息队列中不断取出消息
5、取出来的消息通过handler.dispatchMessage()转发处理
还有以下几种方式可以在子线程中操作UI：
1、handler.post()：
public final boolean post(Runnable r)
{
// 还是调用了sendMessageDelayed去发送消息
return sendMessageDelayed(getPostMessage(r), 0);
}
/**
* 将Runnable对象转化成Message对象
*/
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
在handler的dispatchMessage中有个判断，msg.callback如果不为null就会进入以下方法：
private static void handleCallback(Message message) {
// 直接就调用了Runnable对象的run()
message.callback.run();
}
2、view.post(): 就是调用的Handler的post()
public boolean post(Runnable action) {
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
return attachInfo.mHandler.post(action);
}
// Postpone the runnable until we know on which thread it needs to run.
// Assume that the runnable will be successfully placed after attach.
getRunQueue().post(action);
return true;
}
3、Activity的runOnUiThread():
@Override
public final void runOnUiThread(Runnable action) {
// 如果不是主线程就调用Handler的post()，否则就直接调用Runnable的run()
if (Thread.currentThread() != mUiThread) {
mHandler.post(action);
} else {
action.run();
}
}