[_playerItem addObserver:self forKeyPath:@"status" options:NSKeyValueObservingOptionNew context:&ItemStatusContext];
之后
-- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
。。。（此方法没有进入）
｝
self.playerItem.status 一直是AVPlayerItemStatusUnknown 求解