IdleHandler是什么？
/**
* 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();
}
IdleHandler是定义在MessageQueue里面的一个Interface，它在当线程开始阻塞等待消息的时候会调用的一个接口；当消息队列中的消息已用完或着等待更多消息时调用。返回true以保持IdleHandler处于活动状态，返回false以将其删除。如果队列中仍有挂起的消息，则可以调用此命令，但这些消息都计划在当前时间之后发送。
IdleHandler使用方法
//我们在子线程开启一个Looper
Handler mHandler；
new Thread(){
@Override
public void run(){
//初始化Looper
Looper.prepare();
//创建Handler
mHandler = new Handler(){
@Override
public void handleMessage(@NonNull Message msg) {
super.handleMessage(msg);
Log.i("TAG","接收线程："+Thread.currentThread().getId()+"收到Message");
}
};
Looper.myQueue().addIdleHandler(new MessageQueue.IdleHandler() {
@Override
public boolean queueIdle() {
//当前线程如果Looper获取消息处于阻塞状态会调用该方法
Log.i("TAG","运行IdleHandler");
//返回true说明重复使用IdleHandler
//返回false则使用后删除该IdleHandler
return true;
}
});
//开始循环读取消息
Looper.loop();
}
}.start()
//然后我们开始发送1秒间隔Message
for (int i = 0 ; i < 5 ; i++){
//每个消息间隔1000ms
mHandler.sendMessageDelayed(Message.obtain(),1000*i);
}
//打印日志如下，间隔1000ms之间会执行IdleHandler的逻辑
I/TAG: 接收线程：4660收到Message
I/TAG: 运行IdleHandler
I/TAG: 接收线程：4660收到Message
I/TAG: 运行IdleHandler
I/TAG: 接收线程：4660收到Message
I/TAG: 运行IdleHandler
I/TAG: 接收线程：4660收到Message
I/TAG: 运行IdleHandler
I/TAG: 接收线程：4660收到Message
I/TAG: 运行IdleHandler
根据上面例子看出，发送的Message直接的间隔为1000ms，意味着在MessageQueue#next()获取每个Message之间，会阻塞1000ms，而在这1000ms就会调用IdleHandler，现在我们分析下源码，IdleHandler是怎么被调用的
//MessageQueue.java
//存放IdleHandler
private final ArrayList mIdleHandlers = new ArrayList();
//添加IdleHandler
public void addIdleHandler(@NonNull IdleHandler handler) {
if (handler == null) {
throw new NullPointerException("Can't add a null IdleHandler");
}
synchronized (this) {
mIdleHandlers.add(handler);
}
}
//移除IdleHandler
public void removeIdleHandler(@NonNull IdleHandler handler) {
synchronized (this) {
mIdleHandlers.remove(handler);
}
}
//调用IdleHandler
Message next() {
//...省略部分代码
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
//开始循环找出下个执行Message
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;
//下面代码作用是如果存在同步屏障则找出异步Message
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());
//一般为同步消息，isAsynchronous = false
}
if (msg != null) {
//Message对象不为空
if (now < msg.when) {
//当前时间小于Message的执行时间
// Next message is not ready. Set a timeout to wake up when it is ready.
//计算出需要睡眠的时间
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
//否则返回Message对象
// 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 {
//没有可执行的Message对象
// No more messages.
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
//只有当没有可执行Message或者Message对象执行时间需要等待时，才会走下面代码
// 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.
//第一次循环，记录IdleHadnler的数量
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
//如果不存在IdleHandler，则跳出本次循环
// 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.
//开始循环执行IdleHandler
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IdleHandler返回false，则移除IderHandler
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// 重制pendingIdleHandlerCount = 0，这个变量在循环读取Message队列之前是被初始化为-1，所以在第二次循环和之后，在上面逻辑会直接跳出本次循环
//if (pendingIdleHandlerCount <= 0) {
// //如果不存在IdleHandler，则跳出本次循环
// // No idle handlers to run. Loop and wait some more.
// mBlocked = true;
// continue;
//}
// 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;
}
}
总结
IdleHandler会在Looper.loop()的MessageQueue.next()获取消息的时候，如果消息队列为空，或者消息队列中下个可执行的Message需要等待，则会循环遍历且执行MessageQueue中的mIdleHandler集合，而且只会在next()方法中，遍历消息队列的第一次得到执行；