「Android」IdleHandler 入门
based on Android API 28
IdleHandler，这是一种在只有 当音讯队列没有音讯时 或者是 队列中的音讯还没有到执行工夫时 才会执行的 IdleHandler，它寄存在 mPendingIdleHandlers 队列中。
/**
* Callback interface for discovering when a thread is going to block
* waiting for more messages.
*/
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.
* 返回 true，则放弃该回调；返回 false，则移除该回调
* 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();}依据正文可知：
IdleHandler 是一个回调接口，当线程中的音讯队列将要阻塞期待音讯的时候，就会回调该接口，也就是说音讯队列中的音讯都处理完毕了，没有新的音讯了，处于闲暇状态时就会回调该接口。
源码
android.os.Looper#loop：
/**
* 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();
// 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);
}
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;
try {msg.target.dispatchMessage(msg);
dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;} finally {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();}
}android.os.MessageQueue#next：
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;
}
}看 MessageQueue 的源码能够发现有两处对于 IdleHandler 的申明，别离是：
- 寄存 IdleHandler 的ArrayList：mIdleHandlers
- 寄存 IdleHandler 的 数组：mPendingIdleHandlers
前面的数组，它外面放的 IdleHandler 实例都是 长期 的，也就是每次应用完（调用了 queueIdle 办法）之后，都会置空mPendingIdleHandlers[i] = null。
大抵的流程是这样的：
- 如果本次循环 拿到的 Message 为空 ，或者 这个 Message 是一个延时的音讯而且还没到指定的触发工夫 ，那么，就认定以后的音讯队列为 闲暇状态
- 接着就会遍历 mPendingIdleHandlers 数组（这个数组外面的元素每次都会到 mIdleHandlers 中去拿）来 调用每一个 IdleHandler 实例的 queueIdle 办法
- 如果这个办法返回 false，那么这个实例就会 从 mIdleHandlers 中移除，也就是当下次队列闲暇的时候，不会持续回调它的 queueIdle 办法了
解决完 IdleHandler 后会将 nextPollTimeoutMillis 设置为 0，也就是不阻塞音讯队列，当然要留神这里执行的代码同样不能太耗时，因为它是同步执行的，如果太耗时必定会影响前面的 message 执行。
应用
在利用启动时，可能心愿把一些优先级没那么高的操作提早一点解决，个别会应用 Handler.postDelayed(Runnable r, long delayMillis) 来实现，然而又不晓得该提早多少工夫比拟适合，因为手机性能不同，有的性能较差可能须要提早较多，有的性能较好能够容许较少的延迟时间。所以在做我的项目 性能优化 的时候能够应用 IdleHandler，它 在主线程闲暇时执行工作 ，而 不影响其余工作的执行。
参考
https://zhuanlan.zhihu.com/p/…