juce AsyncUpdaterMessage 分析

这个类同样是基于 CallbackMessage, 主要目的是为了在主线程中进行回调，只不过在收到消息的时候进行检测，检测消息发送对象是否已经删除，如果消息发送对象已经没了。消息回调最终调用了调用者的 handleAsyncUpdate， 只是这样的话应当还是存在问题，主线程在处理，后台线程在删除，这样问题就大了，看来作者是没把这个问题给解决掉。

    void messageCallback() override
    {
        if (shouldDeliver.compareAndSetBool (0, 1))
            owner.handleAsyncUpdate();
    }

源码码如下：

class AsyncUpdater::AsyncUpdaterMessage  : public CallbackMessage
{
public:
    AsyncUpdaterMessage (AsyncUpdater& au)  : owner (au) {}

    void messageCallback() override
    {
        if (shouldDeliver.compareAndSetBool (0, 1))
            owner.handleAsyncUpdate();
    }

    AsyncUpdater& owner;
    Atomic<int> shouldDeliver; 

    JUCE_DECLARE_NON_COPYABLE (AsyncUpdaterMessage)
};

//==============================================================================
AsyncUpdater::AsyncUpdater()
{
    activeMessage = new AsyncUpdaterMessage (*this);
}

AsyncUpdater::~AsyncUpdater()
{
    // You‘re deleting this object with a background thread while there‘s an update
    // pending on the main event thread - that‘s pretty dodgy threading, as the callback could
    // happen after this destructor has finished. You should either use a MessageManagerLock while
    // deleting this object, or find some other way to avoid such a race condition.
    jassert ((! isUpdatePending())
              || MessageManager::getInstanceWithoutCreating() == nullptr
              || MessageManager::getInstanceWithoutCreating()->currentThreadHasLockedMessageManager());

    activeMessage->shouldDeliver.set (0);
}

void AsyncUpdater::triggerAsyncUpdate()
{
    // If you‘re calling this before (or after) the MessageManager is
    // running, then you‘re not going to get any callbacks!
    jassert (MessageManager::getInstanceWithoutCreating() != nullptr);

    if (activeMessage->shouldDeliver.compareAndSetBool (1, 0))
        if (! activeMessage->post())
            cancelPendingUpdate(); // if the message queue fails, this avoids getting
                                   // trapped waiting for the message to arrive
}

void AsyncUpdater::cancelPendingUpdate() noexcept
{
	activeMessage->shouldDeliver.set(0);
}

void AsyncUpdater::handleUpdateNowIfNeeded()
{
    // This can only be called by the event thread.
    jassert (MessageManager::getInstance()->currentThreadHasLockedMessageManager());

    if (activeMessage->shouldDeliver.exchange (0) != 0)
        handleAsyncUpdate();
}

bool AsyncUpdater::isUpdatePending() const noexcept
{
    return activeMessage->shouldDeliver.value != 0;
}