private boolean isPollingLocked() {
// If the loop is quitting then it must not be idling.
// We can assume mPtr != 0 when mQuitting is false.
return !mQuitting && nativeIsPolling(mPtr);
}
/\*\*
\* Returns whether this looper's thread is currently polling for more work to do.
\* This is a good signal that the loop is still alive rather than being stuck
\* handling a callback. Note that this method is intrinsically racy, since the
\* state of the loop can change before you get the result back.
\*/
bool isPolling() const;
未到处理时间，如果都是排过序的，期望 空转保持阻塞 or 返回静默并设置唤醒？按照前面的讨论，是期望 保持空转
不存在消息，阻塞 or 返回null？-- 如果返回null，则在外部需要需要 保持空转 或者 唤醒机制，以支持正常运作。从封装角度出发，应当 保持空转，自己解决问题
存在消息
class MessageQueue {
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.
// 1. 如果 native消息队列指针映射已经为0，即虚引用，说明消息队列已经退出，没有消息了。
// 则返回 null
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
// 2. 死循环，当为获取到需要 \`分发处理\` 的消息时，保持空转
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
// 3. 调用native层方法，poll message，注意，消息还存在于native层
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;
//4. 如果发现 barrier ，即同步屏障，则寻找队列中的下一个可能存在的异步消息
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) {
// 5. 发现了消息，
// 如果是还没有到约定时间的消息，则设置一个 \`下次唤醒\` 的最大时间差
// 否则 \`维护单链表信息\` 并返回消息
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 {
// 寻找到了 \`到处理时间\` 的消息。 \`维护单链表信息\` 并返回消息
// 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;
}
// 处理 是否需要 停止消息队列
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// 维护 接下来需要处理的 IDLEHandler 信息，
// 如果没有 IDLEHandler，则直接进入下一轮消息获取环节
// 否则处理 IDLEHandler
// 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);
}
// 处理 IDLEHandler
// 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;
}
}
}
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
synchronized (this) {
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
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;
}
class Looper {
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));
}
}
class Looper {
public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
if (me.mInLoop) {
Slog.w(TAG, "Loop again would have the queued messages be executed"
+ " before this one completed.");
}
me.mInLoop = true;
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();
// Allow overriding a threshold with a system prop. e.g.
// adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
final int thresholdOverride =
SystemProperties.getInt("log.looper."
+ Process.myUid() + "."
+ Thread.currentThread().getName()
+ ".slow", 0);
boolean slowDeliveryDetected = false;
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
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
// Make sure the observer won't change while processing a transaction.
final Observer observer = sObserver;
final long traceTag = me.mTraceTag;
long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
if (thresholdOverride > 0) {
slowDispatchThresholdMs = thresholdOverride;
slowDeliveryThresholdMs = thresholdOverride;
}
final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);
final boolean needStartTime = logSlowDelivery || logSlowDispatch;
final boolean needEndTime = logSlowDispatch;
if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
final long dispatchEnd;
Object token = null;
if (observer != null) {
token = observer.messageDispatchStarting();
}
long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
try {
//注意这里
msg.target.dispatchMessage(msg);
if (observer != null) {
observer.messageDispatched(token, msg);
}
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
} catch (Exception exception) {
if (observer != null) {
observer.dispatchingThrewException(token, msg, exception);
}
throw exception;
} finally {
ThreadLocalWorkSource.restore(origWorkSource);
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
if (logSlowDelivery) {
if (slowDeliveryDetected) {
if ((dispatchStart - msg.when) <= 10) {
Slog.w(TAG, "Drained");
slowDeliveryDetected = false;
}
} else {
if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
msg)) {
// Once we write a slow delivery log, suppress until the queue drains.
slowDeliveryDetected = true;
}
}
}
if (logSlowDispatch) {
showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", 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();
}
}
}
下班 quit/quitSafely
这是比较粗暴的行为，MQ离开了Looper就没法正常工作了，即下班即意味着辞职
class Looper {
public void quit() {
mQueue.quit(false);
}
public void quitSafely() {
mQueue.quit(true);
}
}
/ Handler /
这里就比较清晰了。API基本分为以下几类：
面向使用者：
创建Message，通过Message的 享元模式
发送消息，注意postRunnable也是一个消息
移除消息，
退出等
面向消息处理：
class Handler {
/\*\*
\* Subclasses must implement this to receive messages.
\*/
public void handleMessage(@NonNull Message msg) {
}
/\*\*
\* Handle system messages here.
\* Looper分发时调用的API
\*/
public void dispatchMessage(@NonNull Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
}
如果有 Handler callback，则交给callback处理，否则自己处理，如果没覆写 handleMessage ，消息相当于被 drop 了。
消息发送部分可以结合下图梳理：
class Message {
public int what;
public int arg1;
public int arg2;
public Object obj;
public Messenger replyTo;
/\*package\*/ int flags;
public long when;
/\*package\*/ Bundle data;
/\*package\*/ Handler target;
/\*package\*/ Runnable callback;
}
Looper::Looper(bool allowNonCallbacks) :
mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),
mResponseIndex(0), mNextMessageUptime(LLONG\_MAX) {
int wakeFds\[2\];
int result = pipe(wakeFds);
LOG\_ALWAYS\_FATAL\_IF(result != 0, "Could not create wake pipe. errno=%d", errno);
mWakeReadPipeFd = wakeFds\[0\];
mWakeWritePipeFd = wakeFds\[1\];
result = fcntl(mWakeReadPipeFd, F\_SETFL, O\_NONBLOCK);
LOG\_ALWAYS\_FATAL\_IF(result != 0, "Could not make wake read pipe non-blocking. errno=%d",
errno);
result = fcntl(mWakeWritePipeFd, F\_SETFL, O\_NONBLOCK);
LOG\_ALWAYS\_FATAL\_IF(result != 0, "Could not make wake write pipe non-blocking. errno=%d",
errno);
// Allocate the epoll instance and register the wake pipe.
mEpollFd = epoll\_create(EPOLL\_SIZE\_HINT);
LOG\_ALWAYS\_FATAL\_IF(mEpollFd < 0, "Could not create epoll instance. errno=%d", errno);
struct epoll\_event eventItem;
memset(& eventItem, 0, sizeof(epoll\_event)); // zero out unused members of data field union
eventItem.events = EPOLLIN;
eventItem.data.fd = mWakeReadPipeFd;
result = epoll\_ctl(mEpollFd, EPOLL\_CTL\_ADD, mWakeReadPipeFd, & eventItem);
LOG\_ALWAYS\_FATAL\_IF(result != 0, "Could not add wake read pipe to epoll instance. errno=%d",
errno);
}
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发布于：2022-04-11 16:08
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