开发步骤 :

1. 给UIWebView添加长按手势
2. 监听手势动作，拿到坐标点(x,y)
3. 弹出对话框，是否保存到相册
4. UIWebView注入js:Document.elementFromPoint(x,y).src拿到img标签的src
5. 拿到图片的url，生成UIImage
6. 图片保存到相册

//  WebView长按图片保存到相册
#import "ViewController.h"

@interface ViewController ()<
UIGestureRecognizerDelegate,
NSURLSessionDelegate>

@property (weak, nonatomic) IBOutlet UIWebView *webView;
@end


@implementation ViewController

#pragma mark -- life cycle
- (void)viewDidLoad{
    [super viewDidLoad];

    NSString *htmlURL = [[NSBundle mainBundle] pathForResource:@"saveImage" ofType:@"html"];
    [self.webView loadRequest:[NSURLRequest requestWithURL:[NSURL fileURLWithPath:htmlURL]]];
   
    // 给UIWebView添加手势
    UILongPressGestureRecognizer* longPressed = [[UILongPressGestureRecognizer alloc] initWithTarget:self action:@selector(longPressed:)];
    longPressed.delegate = self;
    [self.webView addGestureRecognizer:longPressed];
}

#pragma mark -- UIGestureRecognizerDelegate
- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldRecognizeSimultaneouslyWithGestureRecognizer:(UIGestureRecognizer *)otherGestureRecognizer{
    UIActivityTypeAddToReadingList
    return YES;
}

- (void)longPressed:(UILongPressGestureRecognizer*)recognizer{
    if (recognizer.state != UIGestureRecognizerStateBegan) {
        return;
    }
    CGPoint touchPoint = [recognizer locationInView:self.webView];
    NSString *imgURL = [NSString stringWithFormat:@"document.elementFromPoint(%f, %f).src", touchPoint.x, touchPoint.y];
    NSString *urlToSave = [self.webView stringByEvaluatingJavaScriptFromString:imgURL];
    if (urlToSave.length == 0) {
        return;
    }

    UIAlertController *alertVC =  [UIAlertController alertControllerWithTitle:@"提示" message:@"你真的要保存图片到相册吗？" preferredStyle:UIAlertControllerStyleAlert];
    UIAlertAction *okAction = [UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
        [self saveImageToDiskWithUrl:urlToSave];
    }];
    UIAlertAction *cancelAction = [UIAlertAction actionWithTitle:@"取消" style:UIAlertActionStyleDefault handler:nil];
    [alertVC addAction:okAction];
    [alertVC addAction:cancelAction];
    [self presentViewController:alertVC animated:YES completion:nil];
}

#pragma mark - private method
- (void)saveImageToDiskWithUrl:(NSString *)imageUrl{
    
    NSURL *url = [NSURL URLWithString:imageUrl]
    NSURLSessionConfiguration * configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
    NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:[NSOperationQueue new]];

    NSURLRequest *imgRequest = [NSURLRequest requestWithURL:url cachePolicy:NSURLRequestReturnCacheDataElseLoad timeoutInterval:30.0];

    NSURLSessionDownloadTask  *task = [session downloadTaskWithRequest:imgRequest completionHandler:^(NSURL * _Nullable location, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        if (error) {
            return ;
        }
        NSData * imageData = [NSData dataWithContentsOfURL:location];
        dispatch_async(dispatch_get_main_queue(), ^{

            UIImage * image = [UIImage imageWithData:imageData];
            UIImageWriteToSavedPhotosAlbum(image, self, @selector(imageSavedToPhotosAlbum:didFinishSavingWithError:contextInfo:), NULL);
        });
    }];
    [task resume];
}

#pragma mark 保存图片后的回调
- (void)imageSavedToPhotosAlbum:(UIImage*)image didFinishSavingWithError:  (NSError*)error contextInfo:(id)contextInfo{
    if(!error) {
        UIAlertController *alertControl = [UIAlertController alertControllerWithTitle:@"提示" message:@"成功保存到相册" preferredStyle:UIAlertControllerStyleAlert];

        UIAlertAction *action = [UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDestructive handler:nil];
        [alertControl addAction:action];
        [self presentViewController:alertControl animated:YES completion:nil];
    }else{
        UIAlertController *alertControl = [UIAlertController alertControllerWithTitle:@"提示" message:[error description] preferredStyle:UIAlertControllerStyleAlert];
        UIAlertAction *action = [UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleCancel handler:nil];
        [alertControl addAction:action];
        [self presentViewController:alertControl animated:YES completion:nil];
    }
}

@end