ConsumerBase 实现了 ConsumerListener ，内部定义了 FrameAvailableListener 结构体，把 frame 的状态变化转发给了 FrameAvailableListener。

class ConsumerBase : public virtual RefBase,
        protected ConsumerListener {
...
    struct FrameAvailableListener : public virtual RefBase {
        // See IConsumerListener::onFrame{Available,Replaced}
        virtual void onFrameAvailable(const BufferItem& item) = 0;
        virtual void onFrameReplaced(const BufferItem& /* item */) {}
        virtual void onFrameDequeued(const uint64_t){};
        virtual void onFrameCancelled(const uint64_t){};
        virtual void onFrameDetached(const uint64_t){};
    };
    ...
    // Implementation of the IConsumerListener interface.  These
    // calls are used to notify the ConsumerBase of asynchronous events in the
    // BufferQueue.  The onFrameAvailable, onFrameReplaced, and
    // onBuffersReleased methods should not need to be overridden by derived
    // classes, but if they are overridden the ConsumerBase implementation must
    // be called from the derived class. The ConsumerBase version of
    // onSidebandStreamChanged does nothing and can be overriden by derived
    // classes if they want the notification.
    virtual void onFrameAvailable(const BufferItem& item) override;
    virtual void onFrameReplaced(const BufferItem& item) override;
    virtual void onFrameDequeued(const uint64_t bufferId) override;
    virtual void onFrameCancelled(const uint64_t bufferId) override;
    virtual void onFrameDetached(const uint64_t bufferId) override;
    virtual void onBuffersReleased() override;
    virtual void onSidebandStreamChanged() override;
    ...
    wp<FrameAvailableListener> mFrameAvailableListener;
    ...
...

void ConsumerBase::onFrameAvailable(const BufferItem& item) {
    CB_LOGV("onFrameAvailable");

    sp<FrameAvailableListener> listener;
    { // scope for the lock
        Mutex::Autolock lock(mFrameAvailableMutex);
        listener = mFrameAvailableListener.promote();
    }

    if (listener != nullptr) {
        CB_LOGV("actually calling onFrameAvailable");
        listener->onFrameAvailable(item);
    }
}