前言
最近复习Handler的源码的时候，看到MessageQueue中有一个addIdleHandler(IdleHandler handler)方法，参数需要传递一个接口IdleHandler并保存在mIdleHandlers中。
public void addIdleHandler(@NonNull IdleHandler handler) {
if (handler == null) {
throw new NullPointerException("Can't add a null IdleHandler");
}
synchronized (this) {
mIdleHandlers.add(handler);
}
}
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();
}
这个接口的作用是在MessageQueue中没有可以处理的消息的时候回调queueIdle，接口只有一个返回值，返回false的话会从mIdleHandlers集合中删除，反之保留。
源码
我们都知道Handler处理消息的时候是通过循环调用MessageQueue.next()方法，所以我直接就看next的源码：
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.
// 这里才是处理IdelHandler消息的位置
// 第一次进入，消息队列为空或者当前的时间小于将要处理消息的目标时间
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
//mIdleHandlers中为空，阻塞
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
//下面是将mIdleHandlers集合转化数组
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.
// 遍历数组回调queueIdle()
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);
}
//判断取出的返回值为false就idler从mIdleHandlers删除
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;
}
}
使用场景
以前我们在Activity中获取某个控件的宽高的时候总是得到的是0，那是因为view的测量还未完成。通常的做法是监听ViewTreeObserver，它是在ViewRootImpl测量完成之后调用ViewTreeObserver.dispatchOnGlobalLayout()方法，这时候在onGlobalLayout回调中获取的控件宽高都是正确的数据。
现在我们可以使用IdleHandler实现，在所有UI消息处理完成之后才处理IdleHandler中的消息，这样也可以正确的获取控件的宽高了，代码如下：
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
Looper.myQueue().addIdleHandler {
Loger.e("--Idle--${
mView.width}，${
mView.height}----")
false
}
}