虽然在iOS9以后ALAssetsLibrary已经弃用了，该用PhotoKit代替，但是对于一些老项目，来不及更改的，在选择图片并压缩的时候会出现照片模糊，解决方案如下将
thumbnail
使用
aspectRatioThumbnail
代替
#pragma mark - 完成选择后返回的图片Array(ALAsset*)
- (void)finishSelectImg{
//正方形缩略图
NSMutableArray *thumbnailImgArr = [NSMutableArray array];
for (ALAsset *set in _arrSelected) {
CGImageRef cgImg = [set aspectRatioThumbnail];
UIImage* image = [UIImage imageWithCGImage: cgImg];
[thumbnailImgArr addObject:image];
}
if (self.delegate && [self.delegate respondsToSelector:@selector(getSelectImageWithALAssetArray:thumbnailImageArray:)]) {
[self.delegate getSelectImageWithALAssetArray:_arrSelected thumbnailImageArray:thumbnailImgArr];
}
}