记录一次H5生成blob文件url，iOS app内部WKWebView不能正常下载问题。
因为我发现在系统浏览器Safari里能下载，按理说在WKWebView里也能，后来查了下资料解决了这个问题。
1.首先要拦截跳转把以blob开头的跳转链接转成WKNavigationActionPolicyDownload 代码如下，注意这个只能在iOS14.5以后才能使用

- (void)webView:(WKWebView *)webView decidePolicyForNavigationAction:(WKNavigationAction *)navigationAction decisionHandler:(void (^)(WKNavigationActionPolicy))decisionHandler{
  if([navigationAction.request.URL.absoluteString hasPrefix:@"blob"]){
        if (@available(iOS 14.5, *)) {
            decisionHandler(WKNavigationActionPolicyDownload);
        } else {
            // Fallback on earlier versions
            decisionHandler(WKNavigationActionPolicyCancel);
        }
    }
    else{
        decisionHandler(WKNavigationActionPolicyAllow);
    }
}

2.然后实现代理方法

- (void)webView:(WKWebView *)webView navigationAction:(WKNavigationAction *)navigationAction didBecomeDownload:(WKDownload *)download API_AVAILABLE(ios(14.5)){
    download.delegate = self;// 遵守WKDownloadDelegate
}

3.最后实现WKDownloadDelegate代理

- (void)download:(WKDownload *)download decideDestinationUsingResponse:(NSURLResponse *)response suggestedFilename:(NSString *)suggestedFilename completionHandler:(void (^)(NSURL * _Nullable))completionHandler API_AVAILABLE(ios(14.5)){
    NSURL *temporaryDirectory = [NSFileManager.defaultManager temporaryDirectory];
    NSURL *url = [temporaryDirectory URLByAppendingPathComponent:suggestedFilename];
    completionHandler(url);
    //这里是提示框
    [ShowAlertTool showSheetwithTitles:@[@"分享文件"] confirmBlock:^(NSInteger number) {
        SHOW_HUD;
        XMainAfter(1, ^{
            DISMISS_HUD;
        });
      //这里是分享文件 封的一个方法，内部用的UIActivityViewController ，下面顺便会把分享代码也贴出来
        [SFShare showSystemShare:@[url] completion:nil];
    }];
}

4.分享方法的代码

+(void)showSystemShare:(NSArray *)items completion:(ShareCallBackBlock)shareBlock{
    UIActivityViewController *activityView = [[UIActivityViewController alloc] initWithActivityItems:items applicationActivities:nil];
    activityView.excludedActivityTypes = @[UIActivityTypePostToFacebook,
                                           UIActivityTypeMessage,
                                           UIActivityTypePostToTwitter,
                                           UIActivityTypePostToWeibo,
                                           UIActivityTypeMail,
                                           UIActivityTypePrint,
                                           UIActivityTypeCopyToPasteboard,
                                           UIActivityTypeAssignToContact,
//                                           UIActivityTypeSaveToCameraRoll,
                                           UIActivityTypeAddToReadingList,
                                           UIActivityTypePostToFlickr,
                                           UIActivityTypePostToVimeo,
                                           UIActivityTypePostToTencentWeibo,
                                           UIActivityTypeAirDrop,
                                           UIActivityTypeOpenInIBooks,
                                           UIActivityTypeMarkupAsPDF,
                                        ];
    activityView.completionWithItemsHandler = ^(UIActivityType __nullable activityType, BOOL completed, NSArray * __nullable returnedItems, NSError * __nullable activityError){
        if (activityType == UIActivityTypeSaveToCameraRoll) {//保存到相册
            NSString *msg = nil ;
             if(!completed){
                 msg = @"保存图片失败" ;
             }else{
                 msg = @"保存图片成功" ;
             }
             [[UIApplication sharedApplication].keyWindow makeToast:(msg) duration:1.5 position:CSToastPositionCenter];
        }
        if(shareBlock){
            shareBlock(completed);
        }
    };
    
    [[UIApplication sharedApplication].keyWindow.rootViewController presentViewController:activityView animated:YES completion:nil];
}

总结：这个功能只能在iOS14.5以上才能使用，因为网上关于iOS处理的资料有点少，就特地分享出来。希望能帮到iOS开发小伙伴们。