Values for all bound parameters are re-evaluated when a single parameter with two-way binding is changed.

[sam-wheat]

Steps to reproduce:

Clone this repo

Move the slider in the yellow div. Note the changed value of Config2.SomeValue is reported correctly by the control but not by the parent. This is expected as the binding is not two way.

Move the slider in the blue div. Note that ALL bound values for ALL controls are re-evaluated when the slider is moved. This behavior is not desirable and in fact appears to be incorrect since only one value is changing and only one value needs to be reported as changed. Note that bindings on People parameters on both controls are one-way and change notification is neither desired or expected. At the very least, the state of the control in the yellow div should never be affected by changes raised by the control in the blue div as the two controls are completely unrelated.

Expected / desired behavior: I would like like the parent to control (Home) to be notified of changes to Config.SomeValue without re-evaluating the People parameter and without making unnecessary calls to CreatePeople. How can I accomplish this?

https://learn.microsoft.com/en-us/aspnet/core/blazor/components/data-binding?view=aspnetcore-8.0#binding-with-component-parameters

You can't implement chained binds with @Bind syntax in the child component.

I did read the above help document - I am not trying to daisy-chain, only trying to report a (single) changed value to a (single) parent control.

[javiercn]

@sam-wheat thanks for contacting us.

This is by design. The reason is that the value (the config) is a reference type and the Blazor change detection can't know if the object has been mutated, so it considers it changed.

The reason for it is because it's ok to trigger additional renders (as that's always going to be correct even if it means "more work") as opposed to for example, checking that the object is the same reference.

What happens is that the Home component is being re-rendered when MyControl triggers the event (because of the @bind-Config) and as such, MyControl2 has to re-render because the component parameters in that case have changed. (It creates a new list every time).

Expected / desired behavior: I would like like the parent to control (Home) to be notified of changes to Config.SomeValue without re-evaluating the People parameter and without making unnecessary calls to CreatePeople. How can I accomplish this?

You'll have to define a classic .NET event on the Config object and manually subscribe to it within the Home component.

Note though, that this won't change things significantly. The moment you react in the event handler and call StateHasChanged that will trigger a re-render of the Home component and that will cause all the child components to re-render.