1.1 IdleHandler 基本情况
IdleHandler 可以用来提升性能，主要用在我们希望能够在当前线程 消息队列空闲时 做些事情（例如UI线程在显示完成后，如果线程空闲我们就可以提前准备其他内容）的情况下，不过最好不要做耗时操作。
IdleHandler 位于 MessageQueue 类中的一个静态接口，如下：
MessageQueue#IdleHandler
class MessageQueue{
...
/**
* Callback interface for discovering when a thread is going to block
* waiting for more messages.
*
// 可以理解为消息暂时处理完的适合回调的
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.
*/
//返回true就是单次回调后不删除，下次进入空闲时继续回调该方法，false 只回调单次执行完之后会移除
boolean queueIdle();
}
//判断当前队列是不是空闲的，辅助方法
public boolean isIdle() {
synchronized (this) {
final long now = SystemClock.uptimeMillis();
return mMessages == null || now < mMessages.when;
}
}
//添加一个IdleHandler 到空闲队列中，ArrayList 存储
public void addIdleHandler(@NonNull IdleHandler handler) {
if (handler == null) {
throw new NullPointerException("Can't add a null IdleHandler");
}
synchronized (this) {
mIdleHandlers.add(handler);
}
}
// 删除一个IdleHandler
public void removeIdleHandler(@NonNull IdleHandler handler) {
synchronized (this) {
mIdleHandlers.remove(handler);
}
}
//message 的获取下一条消息
Message next() {
...
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {//循环获取下一条消息
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();
}
//表示没有设置idle handler 去运行，就阻塞
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
//设置长度，最小长度是4
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
//将集合转化为数组
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
//循环遍历这个空闲队列数组
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 {
//回调取出返回值
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
//这里看到了吧，如果是返回false, 那就进去了，执行操作后就从集合中remove了
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
....
}
}
...
}
1.2 使用场景
Looper.myQueue().addIdleHandler(new MessageQueue.IdleHandler() {
@Override
public boolean queueIdle() {
//这里做一些操作
Log.i("xx","addIdleHandler invoked..."+iv.getWidth() + "::"+iv.getHeight());
return false;
}
});
我记得之前我们想要获取xml中某个控件的
width和height时，在onCreate方法中直接获取，会获取到是 0 ， 因为这个view还未绘制完成，所以获取不到，当时的解决方案我记得是使用Handler发送一个延时消息获取，现在有更好的方式实现了，那就是通过IdleHandler, 如上面代码所示。 当然还可以做一些其它预处理的简单操作。
IdleHandler 你明白了吗？欢迎大家留言讨论。
