Android IdleHandler闲时加载

在之前的文章里，我们讲过关于handler的一些使用和原理。

Android Handler全使用

Android Handler原理源码全解析

今天讲一个系统预留的一个handler，IdleHandler，有了它，可以让我们在系统闲时进行一些预加载或者事务处理。

使用方式

Looper.myQueue().addIdleHandler(new IdleHandler() {@Overridepublic boolean queueIdle() {//具体的业务逻辑return false;}
}

需要注意的是，这里的返回值，可以为true也可以为false。

区别在于，return false代表该闲时任务仅执行一次。return true代表只要系统闲，就会反复执行多次。

原理

在MessageQueue.java中，有idleHandler的添加与移除

/*** 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.  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();
}/*** Add a new {@link IdleHandler} to this message queue.  This may be* removed automatically for you by returning false from* {@link IdleHandler#queueIdle IdleHandler.queueIdle()} when it is* invoked, or explicitly removing it with {@link #removeIdleHandler}.** <p>This method is safe to call from any thread.** @param handler The IdleHandler to be added.*/
public void addIdleHandler(@NonNull IdleHandler handler) {if (handler == null) {throw new NullPointerException("Can't add a null IdleHandler");}synchronized (this) {mIdleHandlers.add(handler);}
}/*** Remove an {@link IdleHandler} from the queue that was previously added* with {@link #addIdleHandler}.  If the given object is not currently* in the idle list, nothing is done.** <p>This method is safe to call from any thread.** @param handler The IdleHandler to be removed.*/
public void removeIdleHandler(@NonNull IdleHandler handler) {synchronized (this) {mIdleHandlers.remove(handler);}
}

在MessageQueue的next方法中

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.// 这里执行的操作是忽略所有的同步消息, 知道找出queue中的异步消息// 我理解是这个的同步消息会造成线程的阻塞, 所以忽略同步的消息do {prevMsg = msg;msg = msg.next;} while (msg != null && !msg.isAsynchronous());}// 走到这一步, 有两种可能,// 一种是遍历到队尾没有发现异步消息,// 另一种是找到queue中的第一个异步消息if (msg != null) {// 找到queue中的第一个异步消息if (now < msg.when) {// Next message is not ready. Set a timeout to wake up// when it is ready.// 没有到消息的执行时间nextPollTimeoutMillis = (int) Math.min(msg.when - now,Integer.MAX_VALUE);} else {// Got a message.// 当前消息到达可以执行的时间, 直接返回这个msgmBlocked = 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 {// 遍历到队尾, 没有发现异步消息或者没有消息了// No more messages.nextPollTimeoutMillis = -1;}// Process the quit message now that all pending messages have// been handled.// 检查当前的线程是否退出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.// 如果queue中没有msg, 或者msg没到可执行的时间,// 那么现在线程就处于空闲时间了, 可以执行IdleHandler了if (pendingIdleHandlerCount < 0&& (mMessages == null || now < mMessages.when)) {// pendingIdleHandlerCount在进入for循环之前是被初始化为-1的// 并且没有更多地消息要进行处理pendingIdleHandlerCount = mIdleHandlers.size();}if (pendingIdleHandlerCount <= 0) {// No idle handlers to run. Loop and wait some more.// 如果没有IdleHandler要进行处理, 则直接进入下次循环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.// 退出同步块, 接下来就可以执行IdleHandler的相关操作了for (int i = 0; i < pendingIdleHandlerCount; i++) {final IdleHandler idler = mPendingIdleHandlers[i];mPendingIdleHandlers[i] = null; // release the reference to the// handlerboolean keep = false;try {keep = idler.queueIdle();} catch (Throwable t) {Log.wtf(TAG, "IdleHandler threw exception", t);}if (!keep) {// 如果之前addIdleHandler中返回为false,// 就在执行完这个IdleHandler的callback之后, 将这个idler移除掉synchronized (this) {mIdleHandlers.remove(idler);}}}// Reset the idle handler count to 0 so we do not run them again.// 全部执行完, 重新设置这个值为0, 以便下次可以再次执行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;
}
}

从代码，可以看出，判断闲时的逻辑，即可以处理idleHandler的时机是：

如果queue中没有msg, 或者msg没到可执行的时间, 此时线程就处于空闲时间了,可以执行IdleHandler了