[Blazor] Cascading RouteData in Layout causes OnInitializedAsync to be called twice when navigating to another Layout

[Charnock]

Describe the bug

When navigating between pages with two different layouts, if the page you're navigating from has a CascadingParameter for RouteData, OnInitializedAsync will be called twice on the page you're navigating to.

To Reproduce

I've included a minimal repro here. I've added logging to OnInitializedAsync of the Index, Counter, and Fetch Data pages.

• Start the project and navigate from the Index page to the Counter page.
• Then navigate from Counter to either the Index or Fetch Data
• Observe that OnInitializedAsync is called twice for Index or Fetch Data

This behavior does not occur when Navigating between Index and Fetch Data.

Additionally, I've commented out a second CascadingValue in App.razor which cascades a string. Uncommenting that CascadingValue appears to resolve the issue, even when that value isn't being used on any of the pages. Also, only using the cascading string doesn't appear to reproduce the issue.

Further technical details

.NET Core SDK (reflecting any global.json):
Version: 3.1.300-preview-015048
Commit: 13f19b4682

Runtime Environment:
OS Name: Windows
OS Version: 10.0.18362
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\3.1.300-preview-015048\

Host (useful for support):
Version: 3.1.3
Commit: 4a9f85e9f8

Microsoft Visual Studio Community 2019
Version 16.5.3

[captainsafia]

Thanks for posting the issue and providing the repro, @Charnock!

I was able to reproduce the same behavior you saw. There's some funkiness with how the cascading parameter after the processing of the components. The component is initialized before the layout and the re-initialized again after the layout is initialized.

| Switch page | --> | Initialize FetchData |--> | Initialize MainLayout |  --> | Initialize FetchData |

As opposed to

| Switch page | -->  | Initialize MainLayout |  --> | Initialize FetchData |

Marking this as a bug for now. I suspect that we will have to look into the logic in the CascadingValue component a little bit closer.

[dazinator]

Just a hit a similar problem, we were navigating from a page with a non default layout, to one that was missing the layout directive (and so using the default layout), and seeing OnInitAsync called twice, in a blazor wasm application.

We didn't use cascaded values on either page, but we do have a cascaded value surrounding the app.