Lazy loaded `BlazorCustomElements.js` file does not adhere to the `loadBootResource` configuration

[martinlingstuyl]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

Hi guys,

Since .NET 7 is out, I've been test driving Blazor WASM to see how to use it in combination with other frameworks.

I've configured the following to be able to point the loading of resources to a specific server.
https://learn.microsoft.com/en-us/aspnet/core/blazor/fundamentals/startup?view=aspnetcore-7.0#load-boot-resources

Which works nicely.

I'm now using the Microsoft.AspNetCore.Components.CustomElements nuget package to get CustomElements to work.
Initially you had to reference the script manually:

<script src="_content/Microsoft.AspNetCore.Components.CustomElements/BlazorCustomElements.js"></script>

But since .NET 7 has been released it's lazy loaded by ASP.NET. Which is awesome. The only problem is: it uses the URL of the server you are on, to lazy load the file, while it should be respecting the loadBootResource configuration.

Which is a problem for me, as I'm depending on the loadBootResource function to get the WASM files from another server, for example a CDN.

Is that something you guys can fix?

cc: @guardrex dotnet/AspNetCore.Docs#27638

Expected Behavior

Loading the JS for CustomElements should respect the loadBootResource configuration.

Steps To Reproduce

1. Create a Blazor WASM client app
2. Create a simple webserver that loads the Blazor script with autostart="false" and add the following bootstrapping lines:

Blazor.start({
        loadBootResource: function (type, name, defaultUri, integrity) {
          return 'https://localhost:8088/' + defaultUri`;
        }
      });

3. Install the CustomElements nuget and implement a basic customelement.
4. Try loading the basic webserver with the customelement and blazor script.

Exceptions (if any)

No response

.NET Version

7

Anything else?

No response

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[SteveSandersonMS]

I understand why it might be confusing, but JavaScript files loaded via dynamic import (i.e., invoking import in a JS interop call) are not Blazor boot resources, so their loading process will not be affected by loadBootResource customization. That's not a bug, it's just that loadBootResource doesn't get to control anything other than boot resources whose loading is triggered by Blazor itself.

We could theoretically add an extensibility point that overrides what a JS interop call to invokeAsync('import', 'some/url') does, but there isn't any such extensibility point today. If that sounds like what you'd want, please consider filing an issue to request that as a more general feature.

Note that you would have the same issue if you were using any JavaScript library that did dynamic imports - you wouldn't have a way to change the URL it imports from.

[martinlingstuyl]

Hi @SteveSandersonMS,

Ok, so you're saying it's not literally a boot resource and that this just is how dynamic imports work.

It is a dependency that blazor needs though. And a quite fundamental one as well. You probably see my point, that it is a bit odd how it currently works.
It's not just odd, it's blocking the use of the feature. CustomElements are (how I see it) specifically useful in integration scenarios with other applications.

If dynamic imports will not fix this, I'd say: let me be in control and allow me to disable loading the scriptfile dynamically. I'm perfectly fine with loading it from the proper place myself, that would be an acceptable workaround for me as well.

This issue has been resolved and has not had any activity for 1 day. It will be closed for housekeeping purposes.

See our Issue Management Policies for more information.

[martinlingstuyl]

Hi @SteveSandersonMS,

The bot closed the issue. But it's a bit unclear to me now as to what the follow-up action is.

I got a message that it's on the backlog, but you also added the label Resolution:answered, which suggests that you think nothing needs to be fixed.

If it's on the backlog, should the issue not remain open till it's fixed?

[SteveSandersonMS]

I've filed an issue describing this as a general feature request: #45148

A more practical solution for you in the short term would be to implement a service worker that will let you override how all static resources are fetched.