// FREE indicates that the buffer is not currently being used and
// will not be used in the future until it gets dequeued and
// subsequently queued by the client.
// will not be used in the future until it gets dequeued and
// subsequently queued by the client.
// aka "owned by BufferQueue, ready to be dequeued"
aka, also known as 也就是的意思 (换一个广为人知的名称)