Computed state

The convention

Mark a parameterless method named ComputeXxx() or GetXxx() with [BlexComputed] and the generator emits a memoized public property Xxx. The value is calculated on first read, cached, and thrown away the moment any [BlexState] field on that store changes.

[BlexStore(Name = "cart")]
public partial class CartStore
{
    [BlexState] private IReadOnlyList<CartLine> _lines = [];
    [BlexState] private decimal _discountRate;

    [BlexComputed] private decimal ComputeSubtotal() => Lines.Sum(l => l.Total);

    // Computed values may build on other computed values.
    [BlexComputed] private decimal ComputeTotal() => Math.Round(Subtotal * (1 - DiscountRate), 2);

    [BlexComputed] private int ComputeItemCount() => Lines.Sum(l => l.Quantity);
}

Which becomes, in the generated partial:

private bool __SubtotalValid;
private decimal __SubtotalValue = default!;

public decimal Subtotal
{
    get
    {
        if (!__SubtotalValid)
        {
            __SubtotalValue = ComputeSubtotal();
            __SubtotalValid = true;
        }
        return __SubtotalValue;
    }
}

// Called by BlexStoreBase after every state change.
protected override void InvalidateComputed()
{
    __SubtotalValid = false;
    __TotalValid = false;
    __ItemCountValid = false;
}

Memoization in practice

Reading a computed property ten times evaluates it once. Only a state change makes it stale. The counters below are incremented inside the compute methods, so they show exactly how often each one actually ran.

Composing computed values

A computed method may read other computed properties. ComputeTotal() above reads Subtotal, so the first read of Total after a change evaluates both, in dependency order, with no wiring on your part.

Rules

• The method must be parameterless - a computed value backs a property, so there is nowhere to pass arguments (BLEX004).
• The name must start with Compute or Get and have something after it (BLEX003).
• It must be an instance method, not static (BLEX008).
• The generated property name must not collide with anything else on the store (BLEX005).
• Any return type is allowed, including collections and records.

Computed values are not serialized

Only [BlexState] fields go into SerializeState(). Computed values are derived, so they are recalculated after a restore rather than stored - which means a time-travel jump or a rehydrate can never leave a derived value out of sync with the state it came from.