概括

IdleHandler 可以用来优化性能，它会在消息队列空闲时被调用。
详细介绍参看注释：

/**
 * 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();
}

使用

使用非常简单，比如说在 activity 的 onResume() 方法中执行一些可以延迟的操作。

@Override
protected void onResume() {
    super.onResume();
    Looper.myQueue().addIdleHandler(new ResumeIdleHandler());
}

private static class ResumeIdleHandler implements MessageQueue.IdleHandler {

    @Override
    public boolean queueIdle() {
        ···
        一些延迟操作
        ···
        // 返回 true 调用后不删除，下次进入空闲时会再次调用。
        // 返回 false 只调用一次。
        return false;
    }
}

源码分析

在 MessageQueue 类中，添加和移除两个方法：

/**
 * 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);
    }
}

可以看到 IdleHandler 被一个mIdleHandlers管理，它是一个ArrayList：
private final ArrayList<IdleHandler> mIdleHandlers = new ArrayList<IdleHandler>();

接下来看 IdleHandler 何时被调用，解释在注释中：

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.
                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.
                    // 有 msg 则返回
                    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.
            // 线程退出则返回 null
            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.
            // pendingIdleHandlerCount 创建时被赋值为 -1，首次空闲时才会执行这条 if 语句并对其赋值
            if (pendingIdleHandlerCount < 0
                    && (mMessages == null || now < mMessages.when)) {
                pendingIdleHandlerCount = mIdleHandlers.size();
            }

            // 如果没有 IdleHandler 直接 continue
            if (pendingIdleHandlerCount <= 0) {
                // No idle handlers to run.  Loop and wait some more.
                mBlocked = true;
                continue;
            }

            // mPendingIdleHandlers 创建一个 IdleHandler 数组用于存放
            if (mPendingIdleHandlers == null) {
                // 这里数组的最小长度为 4 ，因为并没有源码的注释，猜想是为了性能考虑，减少重复创建数组的几率。
                // 因为下边的 toArray 方法内部会判断，如果 List 长度小于传参的数组长度，则会直接拷贝进这个数组，否则会创建一个新的数组。
                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.
        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 {
                // queueIdle 在这里得到了调用
                keep = idler.queueIdle();
            } catch (Throwable t) {
                Log.wtf(TAG, "IdleHandler threw exception", t);
            }

            // 如果 idler.queueIdle() 返回了 false 则移除
            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;
    }
}