Can you subscribe / observe an object has changed in a Blazor component

Question

Blazor seems aware of object changes within components without any external user intervention, for example if I have:

<p>@Data.SomeProperty</p>
<input type="text" @bind="Data.SomeProperty" />

@code {
   [Parameter]SomeModel Data {get;set;} // How do I know when this/its internals have changed?
}

Whenever the input changes it seems to understand that it needs to refresh the UI (not sure if its whole UI or just the specific parts changed). So is there a way for a developer to hook into this and be notified when something has changed?

My use case here is I want to be able to have a title field for a component (ComponentA) but the title is computed within a child component (ComponentB) so I want to be able to have ComponentB raise an event or notify in some way to indicate the computed property has changed then ComponentA can take that change and update its title.

So is there a way to be able to notify externally that the models changed WITHOUT altering the models internals? (Happy for it to be a custom EventCallback I somehow trigger as long as logic for it lives inside the component not the model)

Answer 1

You already have [Parameter]SomeModel Data {get;set;} which is oneway communication from parent (ComponentA) to the child (ComponentB).

For the reverse direction you can use an [Parameter]EventCallback<string> OnChange { get set;}. Or use a StateManager object as a cascading value.

Your main questions seems to be

but how do I KNOW in ComponentB that data has changed?

That is easy, just use

override onParametersSet()
{
   // ... compute title 
   OnChange.InvokeAsync(title);
}

But you could easily get into a loop here, depending on how you handle this in the Parent.