Trigger: conditional actions when expressions transition

[jlowin]

Exploration of a conditional trigger primitive that fires actions when a boolean expression transitions between true and false, rather than on every state change.

Trigger(
    "{{ cart_total > 100 }}",
    on_true=ServerAction("/api/apply-discount"),
    on_false=ServerAction("/api/remove-discount"),
)

This would be distinct from Computed (which derives values) and from component on_change handlers (which react to specific user interactions). A trigger would react to state crossing a threshold, regardless of what caused the change.

Composes naturally with Computed: the expression can reference computed keys, and computed keys can encode the conditions. The trigger just detects the transition.

This came out of the discussion that led to removing Watch (#283). Watch was designed to fire actions on any state change, but its primary use cases (derived state, normalization) were better served by Computed. The one gap neither Computed nor on_change fills is threshold/condition-based reactions: "when this expression becomes true, do something." That's what this would address.

Note: This doesn't have a strong concrete use case yet. Filing to capture the design direction for when one emerges.