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先上handler机制的图片，然后来逐步分析，查看他的源码
Handler源码分析
创建Handler
Handler提供了多种构造方法的重载，主要分为两类，一类是不指定Looper对象，也就是直接使用当前线程的Looper，另一类是制定Looper，先看看他的构造方法
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;
}
从这个构造函数可以看出，如果不是静态的内部类的话，会打印出内存泄漏的警告，而且可以知道在创建线程之前当前线程必须有looper对象，主线程的looper在源码中已经默认实现好了
在看下另外个构造方法
public Handler(Looper looper, Callback callback, boolean async) {
mLooper = looper;
mQueue = looper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
就是简单对传入的looper进行赋值
发送消息
使用handler最多的就是sentMessage（message）先看看他的源码
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
他其实就是调用sendMessageDelayed(Message,long)我们看下这个方法内部
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
就是为当前时间加上个目标时间，然后穿进去作为参数
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);
}
可以看到这个方法调用了enqueueMessage(MessageQueue,Message,long)方法，就是让消息去排队
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
从这里可以看出两个事情
1.Message对象被加入MessageQueue的队列中
2.Messagetarget是用来持有Handler的强应用，而Handler如果是非静态内部类，是默认持有外部Activity强引用的，这样就会造成所谓的内存泄漏
除了sentMessage外，还有一种就是post(Runnable)
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}
其实就是调用了sentMessageDelayed(Message,long),只不过就是把Runable封装进入Message
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
最后被送入消息队列中
处理消息
当消息被丢进消息对垒中，就要被处理，我们经常用Handler就是重写handleMessage（Message）方法来处理Message，其实还有另外一种方式就是实现Handler.Callback接口，这个接口要求必须实现带boolean值得handleMessage（Message）方法，这两种实现方法如下
private static class MyHandler extends Handler{
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
//处理消息，
}
}
private static class MyHanderCallback implements Handler.Callback{
@Override
public boolean handleMessage(Message msg) {
return false;
}
}
private Handler myHandler = new Handler(new MyHanderCallback());
这两种处理方式有什么区别呢？就在于，第二种实现handler.Callback的处理方法是有boolean值得，Handler会优先处理Handler.Callback实现，然后根据返回值决定是否调用Handler的handlerMassage（Message）方法，接下来看下dispatchMessage（Message）方法
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);//处理Runable类型消息
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {//当Callback.handleMessage返回true时候直接return，就不再执行handleMessage
return;
}
}
handleMessage(msg);
}
}
当一个消息从队列中被取出时候，会通过dispatchMessage（Message）方法分配给Handler。在处理消息的时候优先处理Runable类型的消息，然后再根据Handle.Callback.handleMessage(Message)最后才是Handler.handleMessage（Message）
Message源码分析
Message相当于一个消息媒介，我们通过对这个message对象进行设置，然后最后将他发送出去
创建Message
创建消息时我们不是直接newMessage，而是通过Message.obtain()方法，从Message内部维护的一个消息池里面获取一个Message对象，这样的话就可以复用，而不会因为频繁发送消息而造成频繁gc，内存抖动那些，先看看Message源码
public final class Message Implements Parcelable{
private static final int MAX_POOL_SIZE = 50；//消息池最大数量为50
private static Message sPool;//消息池
Message next;//消息池实际上是一个链表结构
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();
}
}
他是通过维护一个链表来构成消息池子的，只要链表中有一个消息对象，就从这里取出
回收Message
为了实现复用，Message被处理完后就会回到消息池中，回收的过程就是清空所有数据，全部设置被默认值
public void recycle() {
if (isInUse()) {
if (gCheckRecycle) {
throw new IllegalStateException("This message cannot be recycled because it "
+ "is still in use.");
}
return;
}
recycleUnchecked();
}
void recycleUnchecked() {
// Mark the message as in use while it remains in the recycled object pool.
// Clear out all other details.
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = -1;
when = 0;
target = null;
callback = null;
data = null;
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
MessageQueue源码分析
MessageQueue就是消息队列，我们通过Handler发送消息都会进入到这个Mess个Queue中，之后Looper会提取消息进行处理，看看MessageQUeue源码
消息入列
先从他的enqueueMessage（Message，long）源码看下
boolean enqueueMessage(Message msg, long when) {
synchronized (this) {
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
//根据时间节点来插入Message
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;
}
}
}
可以看出他是根据when这个时间节点进行排队的，根据时间的排序找到一个合适的Message对象插入队列中
消息出列
接下来看看消息出列的代码，也就是MessageQueue.next()方法，Looper实际上通过这个方法获取下一个要执行的Message对象，
Message next() {
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
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)
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
//还没有到达时间的情况，就设置一个定时器让他唤醒
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
//有barrier情况
prevMsg.next = msg.next;
} else {
//无barrier的情况
mMessages = msg.next;
}
msg.next = null;
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;
}
}
上面逻辑大概就是：判断消息队列中第一个消息是否到达执行时间点，如果满足条件就移除并返回第一个Message对象，否则逻辑往下走执行idlerHandler，所谓的idlerHandler就是在空闲的时候会被执行一次，如果queueIdle返回false的话就会执行之后移除空闲队列
关闭队列###
void quit(boolean safe) {
if (!mQuitAllowed) {
throw new IllegalStateException("Main thread not allowed to quit.");
}
synchronized (this) {
if (mQuitting) {
return;
}
mQuitting = true;
if (safe) {
//安全的方式关闭队列
//就是对没有到达时间点的消息进行移除，把没有到达时间点的执行完先
removeAllFutureMessagesLocked();
} else {
//直接暴力的关闭队列
removeAllMessagesLocked();
}
// We can assume mPtr != 0 because mQuitting was previously false.
nativeWake(mPtr);
}
}