How does javascript version (asp-append-version) work in ASP.NET Core MVC?

Question

It seems that there is no dynamic bundling supported in the new MVC (link), and it should be done using a gulp task. MVC supports some new attribute called asp-append-version, but I have not found any explanation on how it works. I suspect that it's calculating some hash of the file contents and even updates it after a file change. Is there any documentation on how it works?

I am also wondering how it detects the file changes or whether it just recalculates the hash each time the MVC parses razor markup.

Answer 1

You can check the LinkTagHelper source code, where you will see it is basically adding a version query string to the href value via a FileVersionProvider:

if (AppendVersion == true) {     EnsureFileVersionProvider();      if (Href != null)     {         output.Attributes[HrefAttributeName].Value = _fileVersionProvider.AddFileVersionToPath(Href);     } }  private void EnsureFileVersionProvider() {     if (_fileVersionProvider == null)     {         _fileVersionProvider = new FileVersionProvider(                 HostingEnvironment.WebRootFileProvider,                 Cache,                 ViewContext.HttpContext.Request.PathBase);     } } 

The FileVersionProvider will calculate the hash of the file contents using the SHA256 algorithm. It will then url encode it and add it to the query string as in:

path/to/file?v=B95ZXzHiOuQJzhBoHlSlNyN1_cOjJnz2DFsr-3ZyyJs 

The hash will be recalculated only when the file changes, as it is added to the cache but with an expiration trigger based on a file watcher:

if (!_cache.TryGetValue(path, out value)) {     value = QueryHelpers.AddQueryString(path, VersionKey, GetHashForFile(fileInfo));     var cacheEntryOptions = new MemoryCacheEntryOptions().AddExpirationToken(_fileProvider.Watch(resolvedPath));     _cache.Set(path, value, cacheEntryOptions); } 

This watcher is provided by HostingEnvironment.WebRootFileProvider, which implements IFileProvider:

// // Summary: //     Creates a change trigger with the specified filter. // // Parameters: //   filter: //     Filter string used to determine what files or folders to monitor. Example: **/*.cs, //     *.*, subFolder/**/*.cshtml. // // Returns: //     An Microsoft.Framework.Caching.IExpirationTrigger that is triggered when a file //     matching filter is added, modified or deleted. IExpirationTrigger Watch(string filter); 

Note: You can see the cached values yourself by inspecting the values in the IMemoryCache:

//give the link: <link rel="stylesheet" asp-append-version="true" href="~/css/site.css" />  //You can check the cached version this.Context.RequestServices.GetRequiredService<IMemoryCache>().Get("/css/site.css")  //Which will show a value like: /css/site.css?v=B95ZXzHiOuQJzhBoHlSlNyN1_cOjJnz2DFsr-3ZyyJs