Handler机制原理
Handler、Looper、Message是Android线程间通信的重要概念，我们在项目中会经常用到，最常用的写法，创建一个Handler对象，在线程中通过Handler发送消息来更新UI，这是很常用的写法，那么有时候我们把Handler的创建写在线程里面，运行就会报错，错误信息大都是“Can't create handler inside thread that has notcalled ，Looper.prepare().”，意思是说我们没有调用Loope.prepare方法，那么Looper的prepare方法到底都干了什么，它和Handler又有什么关系呢，它们的实现原理又是怎样的呢，下面我们详细分析一下。
1. Looper.perpare();
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));
}
ThreadLocal，用来存储以当前线程为键的一对键值对（可以理解为HashMap），它的键默认是当前的线程对象，通过set()方法设置对应的值，可以是任意对象，在这里是指一个Looper对象。get方法就是获取以当前线程为键对应的值，这里也是指一个Looper对象。
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
Looper的构造方法，创建了一个MessageQueue对象赋给了Looper的MessageQueue成员变量mQueue，把当前线程赋给了Looper的Thread成员变量mThread。
2.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 (;;) {
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();
}
}
首先调用
myLooper方法返回
prepare方法中
sThreadLocal保存的
Looper对象
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
接着进入消息循环，这个循环的作用就是不停的从消息队列queue中取出消息，并交给handler来处理，下面我们仔细来分析。
Message msg = queue.next(); // might block
通过queue的next方法从消息队列中取出消息，queue.next()这个方法是阻塞式的，如果消息队列中没有消息对象就一直等待，直到有消息为止，类似于Java Socket编程中ServerSocket.accept()方法，也是一直等待Socket去连接，如果没有连接就一直等待。如果取到的消息为空就直接return。我们看一下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;
}
}
首先进入循环，这是个死循环，也即是说如果没有消息它会一直循环等待，直到有消息，
Message msg = mMessages，这个mMessage应该是在添加消息时把一个Message对象赋给了它的，然后对它判空，如果不为空就再遍历消息链表，直到找到一个异步消息，接着做了一些变量更新操作，用于下一次取出消息。最后返回这条消息。
继续到msg.target.dispatchMessage(msg);
通过查看Message源码我们发现Message的target属性是一个Handler对象，为了了解清楚这个target是怎么被初始化的，我们从获取Message对象开始分析。handler.obtainMessage();
public final Message obtainMessage(){
return Message.obtain(this);
}
走到Message.obtain(this);
public static Message obtain(Handler h) {
Message m = obtain();
m.target = h;
return m;
}
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();
}
回到上面msg.target.dispatchMessage();
msg.target其实就是初始化的handler，然后调用Handler的dispatchMessage();
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
首先判断msg.的callback是否为空，接着再判断handler的mCallBack是否为空，这些在对象初始化的时候都为空，所以执行handleMessage()，这就到了我们重写的方法了。
说了这么多，可能大家不太明白，我们在代码中没有调用过Looper.prepare()和Looper.loop()，那这些代码是在什么时候调用的呢，其实在Activity启动的时候，系统就已经在主线程中调用Looper.prepare()和Looper.loop()了，不过在进入到消息循环后，没有Message对象，所以处于阻塞状态，直到Handler发送消息到MessageQueue，这时才开始处理消息。下面我们分析一下Handler的初始化以及发送消息的具体过程。
首先看下初始化过程
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()) &&`11
(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;
}
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
接着把looper对象的消息队列赋给Handler的MessageQueue成员变量mQueue。
callback和async默认分别是null和false。到这Handler的初始化就结束了。
3.handler.sendMessage();
发送消息其实有很多个方法，sendEmptyMessage()，sendMessageDelay()等，这些方法大体上都差不多，只是msg一些属性的值不同，或者有一些延时，最终都会到sendMessageAtTime
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);
}
这里把this对象赋给了msg.target，接着把消息保存到消息队列中。在前面初始化Message的时候，我们建议用handle.obtainMessage()方法获取Message对象，这时会把handler对象复制给msg.target属性，但是如果我们没有这样获取msg对象，那这个属性就为空了吗?，当然不，这里就有了合理的解释，在发送消息后，enqueueMessage方法中会将Handler赋给msg的target属性，也就是说每个消息都有对应一个Handler，一个Handler对应多个Message，现在这些看起来是那么的顺理成章。
继续来看
enqueueMessage(Message msg, long when)
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;
}
第一次
第二次
第三次
这就是添加消息的大概过程，总结起来就是使用单链表结构存储，进行头插操作。
4.handler.post()
在讨论这个问题之前，我们先看一下下面这段代码
class TestThread extends Thread{
@Override
public void run(){
Runnable r = new Runnable(){
@Override
public void run() {
String currentThreadName = Thread.currentThread().getName();
System.out.println("currentThreadName:" + currentThreadName);
}
};
handler.post(r);
}
}
开启这个线程后，我们想一下它的输出结果，可能我们会觉得输出应该是”Thread-xxx”，但是实际结果却是main，明明在线程里面执行的怎么能是main呢，为了解决这个疑惑，我们就要求助于handler.post()的源码了。
public final boolean post(Runnable r){
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
private static void handleCallback(Message message) {
message.callback.run();
}
用这张图来总结一下
追求优雅的代码
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