原文地址：https://juejin.cn/post/7041576680648867877

知识点：IdleHandler 的执行时机是主线程无消息或者未到执行时机。并且IdleHandler的执行方式是一次性执行所有的IdleHandler任务，如果IdleHandler执行耗时，也会影响主队列中的任务执行。

//MessageQueue.java
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);

        ...
            // 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.
            //1
            if (pendingIdleHandlerCount < 0
                    && (mMessages == null || now < mMessages.when)) {
                pendingIdleHandlerCount = mIdleHandlers.size();
            }
            if (pendingIdleHandlerCount <= 0) {
                // No idle handlers to run.  Loop and wait some more.
                mBlocked = true;
                continue;
            }

            if (mPendingIdleHandlers == null) {
                mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
            }
            //2
            mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
        }

        // Run the idle handlers.
        // We only ever reach this code block during the first iteration.
        //3
        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);
            }
            //4
            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;
    }
}

//5
public void addIdleHandler(@NonNull IdleHandler handler) {
    if (handler == null) {
        throw new NullPointerException("Can't add a null IdleHandler");
    }
    synchronized (this) {
        mIdleHandlers.add(handler);
    }
}

正确使用方式：

private static List<Runnable> uiTasks = new ArrayList<>();

public static UIPoolManager addTask(Runnable runnable) {
    tasks.add(runnable);
    return this;
}

public static UIManager runUiTasks() {
    NullHelper.requireNonNull(uiTasks);
    IUiTask iUiTask = new IUiTask() {
        @Override
        public boolean queueIdle() {
            if (!uiTasks.isEmpty()) {
                Runnable task = uiTasks.get(0);
                task.run();
                uiTasks.remove(task);
            }
            //逐次取一个任务执行 避免占用主线程过久
            return !uiTasks.isEmpty();
        }
    };

    Looper.myQueue().addIdleHandler(iUiTask);
    return this;
}

uiTasks 是所有的IdleHandler 需要执行的任务，每次只执行一个任务，当uiTasks有任务，返回true，等待下一次空闲时机执行，否则返回false。