主线程的Looper初始化是在类ActivityThread初始化的,main函数里,这也是AndroidApp里一个进程初始化就进来的地方。
Process.setArgV0("<pre-initialized>"); Looper.prepareMainLooper(); ActivityThread thread = new ActivityThread(); thread.attach(false); if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler(); }
AsyncTask.init(); if (false) {
Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); }
Looper.loop();
Handler涉及Looper,MessageQueue,Message类,为什么一开始在新的线程需要初始化Looper了,是因为:
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()); } } 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; }Handler在初始化的时候会判断当前线程有无Looper,若有则赋值,没有就抛异常。在这里还需要说下Looper
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)); }
private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }
sThreadLocal是ThreadLocal类,专门用于多线程开发,这样每个线程的ThreadLocal值都不同,属于一个复制类。
接下来Looper就开始从MessageQueue循环取消息,也就是loop函数。
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; }queue.next在取不到值时,会阻塞。
接下来说明下Handler发送消息时做了什么。
/** * Return a new Message instance from the global pool. Allows us to * avoid allocating new objects in many cases. */ 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(); }这是取得一个消息,建议用obtain来取,这里会循环利用生成的50个消息,如果当前没有可利用的消息,就生成一个,Message还是Parcelable,可用于进程间传递消息
msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis);在这里就设置msg的处理目标为当前Handler,并插入到MessageQueue里,在这里要注意是按照需要什么时候处理的时间排序的,因为可用postOnDelay()延时发送。
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); }这时候如果在阻塞,也就是没有取到消息处理,一直在循环,mBlocked就为thue,如果when为0,就插入到头部,唤醒next(),如果when不为0,就插入到比它大的前面。
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 (false) Log.v("MessageQueue", "Returning message: " + msg); 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("MessageQueue", "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; }这就是MessageQueue的next函数,一开始没消息,就会一直循环,当有消息时,就取出第一条消息,并与当前时间比较,如果当前时间小于消息的时间,就continue循环,这时会进入阻塞阶段,
nativePollOnce(ptr, nextPollTimeoutMillis);否则就返回消息,这时就交给looper处理
/** * 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(); 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(); } }说到Handler,就要说到Handler如果处理不当,容易导致Activity或者与它相关的类没有及时回收,这时会导致OOM。
这时延时发送了,因为handler引用了当前Activity,如果不取消息,Activity就一直有引用,导致虽然调用了onDestroy()但是无法回收。通过取消与Handler相关的消息,这样就可以把引用关系给消除。
void removeCallbacksAndMessages(Handler h, Object object) { if (h == null) { return; } synchronized (this) { Message p = mMessages; // Remove all messages at front. while (p != null && p.target == h && (object == null || p.obj == object)) { Message n = p.next; mMessages = n; p.recycleUnchecked(); p = n; } // Remove all messages after front. while (p != null) { Message n = p.next; if (n != null) { if (n.target == h && (object == null || n.obj == object)) { Message nn = n.next; n.recycleUnchecked(); p.next = nn; continue; } } p = n; } } }在这里有个更省事的例子,就是WeakHandler,外国人就是牛逼,
博客是:https://techblog.badoo.com/blog/2014/08/28/android-handler-memory-leaks
github是:https://github.com/badoo/android-weak-handler