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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;
}
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原文地址：http://www.cnblogs.com/csxy/p/5464050.html
