Lazy loading might encounter issues if host page rerenders during loading

[SteveSandersonMS]

Each time the host page rerenders (e.g., for reasons unrelated to navigation), it will call the router's SetParametersAsync, which in turn calls Refresh which then calls _renderHandle.Render(Found(routeData));. It does this even if a lazy-load is currently in progress, which means it might throw if the route relies on assemblies not yet loaded.

At minimum, we could address this by making the router a bit more stateful. It could keep track of whether a load is in progress, and if so, render the Navigating fragment if there is one, or skip rendering if not.

A more sophisticated solution might involve changing the OnNavigateAsync callback to be an EventCallback<T> and letting the event system be responsible for rerendering during navigation. This would have some benefits like auto-rendering the host page on navigation just like any other event handler. It's a bit tricky to wrap one eventcallback inside another but I think it's technically possible because we're inside the M.A.Components project and hence can freely read the internal properties of EventCallback<T>. Update: Filed #24215 to track this separate aspect of it.

[captainsafia]

@SteveSandersonMS and I just had a call about this. Dumping the notes from our conversation here.

There's two dimensions to this problem:

1. How we react when the parent of the router is re-rendered by the user
2. Exposing OnNavigateAsync as an EventCallback so that Blazor users don't have to learn the specific re-rendering semantics of the OnNavigateAsync function

The two dimensions are related because they both require us to be a little bit more clever about when we call the Refresh method in our Router. The Refresh method is what renders the component associated with the route. It's called in the SetParametersAsync of the Router (which gets called whenever the router is re-rendered) and on location changed.

Enter the problem, if the Router component is re-rendered:

• By the parent component
• By the StateHasChanged invocation in the event processing

Then Refresh will be called, causing the route component to be rendered, potentially before it's OnNavigateAsync task has completed.

So, we need some sort of way to ensure that the route component is only rendered after OnNavigateAsync has finished rendering. We already do this kind of check with the shouldRefresh logic in our RunOnNavigateAsyncWithRefresh method but we need to account for re-renders that are caused by the parent/don't happen during navigation.

To resolve this issue, we'll probably need to:

• Add a firstRender check to ensure that Refresh only gets called on first render in the SetParametersAsync call
• Add a check inside the Refresh method that ensures the rendering of the route's component only happens once the previousNavigate task is completed