保存到本地的方式
1.UIImageWriteToSavedPhotosAlbum 函数
缺点: 只能保存图片,不能拿到图片的标示符引用到其他相册

    UIImageWriteToSavedPhotosAlbum(image, self, @selector(image:didFinishSavingWithError:contextInfo:), nil);
}
//无论图片保存成功或失败,都会来到这个方法
- (void)image:(UIImage *)image didFinishSavingWithError:(NSError *)error contextInfo:(void *)contextInfo
{
    
}

2.Photos.framework - 从iOS8开始可以使用，可以完全取AssetsLibrary.framework
Photos.framework使用注意:
1.PHAsset : 一个PHAsset对象代表相册中的一张图片或者一段视频
2.PHAssetCollection : 一个PHAssetCollection对象代表一个相册
3.PHAssetChangeRequest : 利用这个对象能添加,删除,修改PHAsset对象
4.PHAssetCollectionChangeRequest : 利用这个对象能添加、删除、修改PHAssetCollection对象
5.对相片\相册的任何改动操作，都必须放在以下方法的block中
1> -[PHPhotoLibrary performChanges:completionHandler:]
2> -[PHPhotoLibrary performChangesAndWait:error:]

#import <Photos/Photos.h>
- (void)saveImage
{
    NSError *error = nil;
    //先拿到图片的占位标示符
    __block NSString *assetId = nil;
    [[PHPhotoLibrary sharedPhotoLibrary] performChangesAndWait:^{
        assetId = [PHAssetChangeRequest creationRequestForAssetFromImage:self.imageView.image].placeholderForCreatedAsset.localIdentifier;
    } error:&error];
    //通过占位标示符拿到图片数组;后面再引用到相册
    PHFetchResult<PHAsset *> *assets = [PHAsset fetchAssetsWithLocalIdentifiers:@[assetId] options:nil];
    
    // 获得App的名称
    NSString *title = [NSBundle mainBundle].infoDictionary[(__bridge NSString *)kCFBundleNameKey];
    PHAssetCollection *createdCollection = nil;
    //先获取手机里面有没有该相册
    PHFetchResult<PHAssetCollection *> *collections = [PHAssetCollection fetchAssetCollectionsWithType:PHAssetCollectionTypeAlbum subtype:PHAssetCollectionSubtypeAlbumRegular options:nil];
    
    for (PHAssetCollection *collection in collections) {
        if ([collection.localizedTitle isEqualToString:title]) {
            createdCollection = collection;
            break;
        }
    }
    // 没有该相册,自己创建一个;同样也是先获取占位标示符
    if (createdCollection == nil) {
        // 创建【自定义相册】
        __block NSString *collectionId = nil;
        [[PHPhotoLibrary sharedPhotoLibrary] performChangesAndWait:^{
            collectionId = [PHAssetCollectionChangeRequest creationRequestForAssetCollectionWithTitle:title].placeholderForCreatedAssetCollection.localIdentifier;
        } error:&error];
        //通过占位标示符拿到对应相册
        createdCollection = [PHAssetCollection fetchAssetCollectionsWithLocalIdentifiers:@[collectionId] options:nil].firstObject;
    }
    
[[PHPhotoLibrary sharedPhotoLibrary] performChangesAndWait:^{
        //创建一个修改相册的请求
        PHAssetCollectionChangeRequest *request = [PHAssetCollectionChangeRequest changeRequestForAssetCollection:createdCollection];
        //添加图片到相册第一个位置
        [request insertAssets:assets atIndexes:[NSIndexSet indexSetWithIndex:0]];
    } error:&error];
    
    // 提示
    if (error) {
        [SVProgressHUD showErrorWithStatus:@"保存失败！"];
    } else {
        [SVProgressHUD showSuccessWithStatus:@"保存成功！"];
    }
}

3.AssetsLibrary.framework - 从iOS9开始废弃