Android消息机制 空闲消息处理器
之前我们分析过Android的消息机制，但是有一个空闲消息处理器我们没有分析过，这篇博客我们来看下。
一、MessageQueue空闲处理器
之前在分析MessageQueue的next函数，这个函数就是获取队列的下个Message，我们再来看下。
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);//native函数，底层就是epoll机制
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);//下个Message的时间
} 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;//返回message
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;//message为空，底层永久阻塞
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
//当消息为空，或者消息时间没到 （这个时候代表线程空闲）
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);//把mIdleHandler的数据放入mPendingIdelHandlers
}
// 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();//调用IdleHandler的queueIdle函数
} 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;
}
}上面这个函数是一个无线循环直到找到一个Message，先调用nativePollOnce，这个函数之前分析过是在c层调用epoll机制。然后继续分析，当有消息时，消息的时间没有，我们就计算下个消息的时间，下次传入nativePollOnce。当有消息时间到了，我们就返回该消息。如果没有消息naxtPollTimeoutMillis就为-1（永久阻塞）。
下面我们再来看看关于空闲消息处理器，首先只有第一次才会赋值给pendingIdleHandlerCount。其次一定要Message为空，或者Message还没有到时间（这样代表线程空闲）。这样才会赋值给pendingIdleHandlerCount， 然后把mIdleHandlers 给mPendingIdleHandlers。
最后遍历mPendingIdleHandlers，调用每一个IdleHandler 的queueIdle方法，调用完之后就去除这个IdleHandler。
下面我们再来看下IdleHandler 这个接口。是MessageQueue的内部接口。
public static interface IdleHandler {
/**
* Called when the message queue has run out of messages and will now
* wait for more. Return true to keep your idle handler active, false
* to have it removed. This may be called if there are still messages
* pending in the queue, but they are all scheduled to be dispatched
* after the current time.
*/
boolean queueIdle();
}二、举例（ActivityThread的Idler内部类）
下面我们通过一个例子，我们先来看ActivityThread的Idler类，这个类就是在Activity的线程空闲时，调用AMS的activityIdle函数。
private class Idler implements MessageQueue.IdleHandler {
@Override
public final boolean queueIdle() {
ActivityClientRecord a = mNewActivities;
boolean stopProfiling = false;
if (mBoundApplication != null && mProfiler.profileFd != null
&& mProfiler.autoStopProfiler) {
stopProfiling = true;
}
if (a != null) {
mNewActivities = null;
IActivityManager am = ActivityManagerNative.getDefault();
ActivityClientRecord prev;
do {
if (localLOGV) Slog.v(
TAG, "Reporting idle of " + a +
" finished=" +
(a.activity != null && a.activity.mFinished));
if (a.activity != null && !a.activity.mFinished) {
try {
am.activityIdle(a.token, a.createdConfig, stopProfiling);
a.createdConfig = null;
} catch (RemoteException ex) {
// Ignore
}
}
prev = a;
a = a.nextIdle;
prev.nextIdle = null;
} while (a != null);
}
if (stopProfiling) {
mProfiler.stopProfiling();
}
ensureJitEnabled();
return false;
}
}......
if (!r.onlyLocalRequest) {
r.nextIdle = mNewActivities;
mNewActivities = r;
if (localLOGV) Slog.v(
TAG, "Scheduling idle handler for " + r);
Looper.myQueue().addIdleHandler(new Idler());
}
......addIdleHandler就是往mIdleHandlers成员变量加入给IdleHandler。
public void addIdleHandler(@NonNull IdleHandler handler) {
if (handler == null) {
throw new NullPointerException("Can't add a null IdleHandler");
}
synchronized (this) {
mIdleHandlers.add(handler);
}
}