在之前的文章里，我们讲过关于handler的一些使用和原理。
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。
Looper.myQueue().addIdleHandler(new IdleHandler() {
@Override
public 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。
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msg
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 {
// 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
// handler
boolean 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