Add the possibility to reference actor properties in the values of effects

Additionally, this also adds the possibility to do math inside the values of
effects. All math functions that are available for rolls are also available
here.

src/module/active-effect.ts

@@ -0,0 +1,24 @@
+// SPDX-FileCopyrightText: 2021 Johannes Loher
+//
+// SPDX-License-Identifier: MIT
+
+import { DS4Actor } from "./actor/actor";
+
+declare global {
+    interface DocumentClassConfig {
+        ActiveEffect: typeof DS4ActiveEffect;
+    }
+}
+
+export class DS4ActiveEffect extends ActiveEffect {
+    /** @override */
+    apply(actor: DS4Actor, change: foundry.data.ActiveEffectData["changes"][number]): unknown {
+        change.value = Roll.replaceFormulaData(change.value, actor.data);
+        try {
+            change.value = Roll.safeEval(change.value).toString();
+        } catch (e) {
+            // this is a valid case, e.g., if the effect change simply is a string
+        }
+        return super.apply(actor, change);
+    }
+}

src/module/hooks/init.ts

@@ -4,6 +4,7 @@
 //
 // SPDX-License-Identifier: MIT
 
+import { DS4ActiveEffect } from "../active-effect";
 import { DS4Actor } from "../actor/actor";
 import { DS4CharacterActorSheet } from "../actor/sheets/character-sheet";
 import { DS4CreatureActorSheet } from "../actor/sheets/creature-sheet";
@@ -43,6 +44,7 @@ async function init() {
 
     CONFIG.Actor.documentClass = DS4Actor;
     CONFIG.Item.documentClass = DS4Item;
+    CONFIG.ActiveEffect.documentClass = DS4ActiveEffect;
 
     CONFIG.Actor.typeLabels = DS4.i18n.actorTypes;
     CONFIG.Item.typeLabels = DS4.i18n.itemTypes;