1.监听系统相册的变化，
[[PHPhotoLibrary sharedPhotoLibrary] registerChangeObserver:self];
//系统方法回调
- (void)photoLibraryDidChange:(PHChange *)changeInstance{
PHFetchResultChangeDetails *collectionChanges = [changeInstance changeDetailsForFetchResult:fetchRet];
if (collectionChanges) {
//相册内的元素变化，如相片的增减
}else{
//相册的增删，如用户自定义的相册
}
}
