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。