document the pull-on-demand convention for reading simulation state in the UI

UI widgets hold the Simulation and read authoritative values via getters on
demand; state-change events are refresh signals, not carriers of truth. This
codifies the pattern used by BuildButtonGrid and avoids stale-cache bugs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7N59FsLA5e2kuVdqe4Uhc
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2026-07-19 20:25:01 +02:00
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@@ -97,6 +97,12 @@ All UI interactions — building selection, builder/blueprint mode transitions,
Bidirectional interactions use separate request/notification event types to avoid infinite recursion (e.g., `ExitBuilderModeRequestedEvent` from `BuildButtonGrid``GameWorldView`, vs. `BuilderModeExitedEvent` from `GameWorldView``BuildButtonGrid`).
### Reading Simulation State
The simulation is the single source of truth for every game value (building block stock, expansion cost, threat level, tick, etc.). A UI widget that needs such a value holds the `Simulation*` it was constructed with and **pulls the value on demand** via the corresponding getter (e.g., `m_sim->getBuildingBlocksStock()`), rather than caching its own copy.
State-change events (e.g., `BuildingBlocksChangedEvent`) are treated as *refresh signals*, not as carriers of truth: a widget subscribes to the event to learn *when* the value changed and then re-reads it from the simulation to learn *what* it now is. The value carried in the event payload is not authoritative and should not be stored. This keeps a single copy of each value and avoids stale-cache bugs (a widget acting on a value that has since moved on because nothing refreshed its local copy).
## Tick Order
Within a single simulation tick, subsystems run in this fixed order. The order is load-bearing for determinism and for avoiding one-tick-delay artifacts (e.g., items landing on a belt but not advancing in the same tick).