Coordinate Components

Coordinate Components

Step 1 — Read AGENTS.md

Read AGENTS.md at the workspace root to learn the project's conventions before making changes.

Step 2 — Decide the scope

For parent→child one level: use [Parameter] / EventCallback (see author-component skill). For persisting state across prerender→interactive: see support-prerendering skill.

Workflow (quick reference)

1. Choose the mechanism from the table in Step 2
2. If crossing render mode boundaries → use CascadingValueSource<T> (Step 4)
3. Register in Program.cs with AddCascadingValue(...) and isFixed: false
4. Consume via [CascadingParameter] in child components
5. Update via NotifyChangedAsync(newValue) — never page reload
6. For additional mutable state within a circuit → add scoped service (Step 5)
7. Wrap any StateHasChanged from background threads in InvokeAsync
8. Implement IDisposable — dispose timers, cancel tokens, unsubscribe events

Step 3 — CascadingValue for subtree state

Wrap a subtree with <CascadingValue> to flow data to all descendants without passing it through every intermediate component.

@* In a layout or parent component *@
<CascadingValue Value="theme">
    @Body
</CascadingValue>

@code {
    private ThemeInfo theme = new() { ButtonClass = "btn-primary" };
}

Consume in any descendant:

[CascadingParameter]
private ThemeInfo? Theme { get; set; }

Rules:

• Matched by type, not name. To cascade multiple values of the same type, add Name:

  <CascadingValue Value="primary" Name="PrimaryTheme">...</CascadingValue>
  

  [CascadingParameter(Name = "PrimaryTheme")]
  private ThemeInfo? Primary { get; set; }
  

• Set IsFixed="true" when the value never changes — avoids subscription overhead.
• Does NOT cross render mode boundaries. A <CascadingValue> in a static SSR parent is invisible to interactive children. See Step 6.

Step 4 — CascadingValueSource<T> for app-wide state

Register a CascadingValueSource<T> in DI when the value must be available to all components regardless of render mode.

// Program.cs
builder.Services.AddCascadingValue(sp =>
{
    var theme = new ThemeInfo { ButtonClass = "btn-primary" };
    return new CascadingValueSource<ThemeInfo>(theme, isFixed: false);
});

Consume identically to Step 3:

[CascadingParameter]
private ThemeInfo? Theme { get; set; }

To update and notify subscribers, either mutate the existing object or replace it:

@* Component that changes the theme *@
@inject CascadingValueSource<ThemeInfo> ThemeSource

<button @onclick="ToggleDarkMode">Toggle theme</button>

@code {
    private bool isDark;

    private async Task ToggleDarkMode()
    {
        isDark = !isDark;
        // Replace the value entirely:
        var newTheme = new ThemeInfo { ButtonClass = isDark ? "btn-dark" : "btn-primary" };
        await ThemeSource.NotifyChangedAsync(newTheme);
    }
}

NotifyChangedAsync() (no argument) also works — mutate the object and then call it. NotifyChangedAsync(newValue) replaces the value and notifies in one step.

Update protocol: Whenever shared state changes, the component that changes it MUST inject CascadingValueSource<T> and call NotifyChangedAsync(). This is the only mechanism that triggers re-rendering in all [CascadingParameter] subscribers. Without this call, no subscribers update. Do not use NavigationManager.Refresh() or page reloads as a substitute.

Rules:

• isFixed: false enables change notifications. isFixed: true is better for truly static values (feature flags).
• Crosses render mode boundaries — works for per-page interactivity, global interactivity, and WebAssembly. Key advantage over <CascadingValue>.
• Keep cascaded types granular. Every NotifyChangedAsync re-renders ALL subscribers regardless of which property changed. Don't put all app state into one cascaded type.
• For Auto/WebAssembly apps, register in both server and .Client Program.cs. The type must be in a shared assembly.

Step 5 — Scoped state service with change events

For mutable shared state that multiple components read and write (shopping cart, notification count, filters), use a scoped service with an event for change notification.

Define the service:

public class CartState
{
    private readonly List<CartItem> _items = [];

    public IReadOnlyList<CartItem> Items => _items;
    public int Count => _items.Count;

    public event Action? OnChange;

    public void Add(CartItem item)
    {
        _items.Add(item);
        OnChange?.Invoke();
    }

    public void Remove(CartItem item)
    {
        _items.Remove(item);
        OnChange?.Invoke();
    }
}

Register as scoped:

builder.Services.AddScoped<CartState>();

Subscribe in components:

@inject CartState Cart
@implements IDisposable

<span class="badge">@Cart.Count</span>

@code {
    protected override void OnInitialized()
    {
        Cart.OnChange += StateHasChanged;
    }

    public void Dispose()
    {
        Cart.OnChange -= StateHasChanged;
    }
}

The simple Action OnChange pattern works when the event fires from the Blazor sync context (button click → Cart.Add(…)). If the event fires from outside the sync context (timer, background task, SignalR hub), wrap in InvokeAsync:

private Action? _handler;

protected override void OnInitialized()
{
    _handler = () => InvokeAsync(StateHasChanged);
    Cart.OnChange += _handler;
}

public void Dispose() => Cart.OnChange -= _handler;

Store the delegate in a field so you can unsubscribe the exact same instance.

Step 6 — Render mode and service lifetime rules

Cascading values don't cross render mode boundaries

A <CascadingValue> placed in a static SSR layout (MainLayout.razor when the layout renders statically) will not reach interactive children. The interactive component sees null for the cascading parameter.

Fix: Use CascadingValueSource<T> registered in DI (Step 4) or a scoped service (Step 5). Both cross boundaries because DI services are resolved per-circuit, not from the component tree.

Service lifetime on Server vs WebAssembly

On Server, never store user-specific state in a singleton — every user's circuit shares the same singleton. One user's cart leaks into another's. Use AddScoped<T>().

On WebAssembly, singletons are per-tab and safe. But code meant for both Server and WebAssembly (Auto mode) must use scoped.

Auto/WebAssembly with prerendering

State services must be defined in the .Client project or a shared assembly — they cannot reference server-only types. Register the service in both Program.cs files. State created during prerender does not survive the switch to the interactive runtime. Use the support-prerendering skill's [PersistentState] pattern to carry state across.

Don'ts

• Don't use a singleton for per-user state on Server — all circuits share it, leaking state between users.
• Don't put all app state into one cascaded object — NotifyChangedAsync re-renders ALL subscribers on every change. Separate concerns into distinct types (ThemeState, CartState, UserPreferences).
• Don't forget to unsubscribe — omitting Dispose on event subscriptions causes memory leaks that grow per-circuit.
• Don't use <CascadingValue> in a static layout expecting it to reach interactive children — it won't cross render mode boundaries. Use DI-registered CascadingValueSource<T> or scoped services.
• Don't use NavigationManager.Refresh(forceReload: true) to propagate cascading value changes — this destroys the circuit and forces a full page reload. Instead, inject CascadingValueSource<T> and call NotifyChangedAsync(newValue) to push updates to all [CascadingParameter] subscribers without a page reload.
• Don't call StateHasChanged from a non-Blazor thread — wrap in InvokeAsync. The framework throws InvalidOperationException: The current thread is not associated with the Dispatcher.