[Fix] ActiveEffect Mode OneTime Migration (#2078)

* Migrations for ActiveEffect Mode
* Start on migration handlers

---------

Co-authored-by: Carlos Fernandez <cfern1990@gmail.com>

module/systemRegistration/migration-handlers/base.mjs

@@ -0,0 +1,77 @@
+/** 
+ * @import DHItem from "../../documents/item.mjs";
+ */
+
+/** 
+ * The base class of an async migration. 
+ * These are generally run between versions for things that require compendiums or must be done in post.
+ * The migrate() functions calls the various updateXSource() functions.
+ * Generally a subclass will override the version and the updateXSource() functions.
+ */
+export class MigrationHandlerBase {
+    version = null;
+
+    /**
+     * Gets change data for an active effect's source, or null if no changes
+     * @param {object} effectSource 
+     * @param {DHItem} item 
+     * @returns {Promise<object>}
+     * @protected
+     */
+    async updateActiveEffectSource(effectSource, item) {
+        return null;
+    }
+
+    async migrate() {
+        // todo: handle more than just migrating effects. Right now this can only migrate effects
+        // NOTE: the preload is hardcoded, we should not hardcode it
+
+        const numActors = game.actors.size;
+        const numItems = game.items.size;
+        const finalUpdateProgress = 5;
+        const DhProgress = game.system.api.applications.ui.DhProgress;
+        const preRunProgress = game.packs.size;
+        
+        const progress = DhProgress.createMigrationProgress(
+            preRunProgress + numActors + numItems + finalUpdateProgress
+        );
+
+        // Preload. Avoid hardcoding in the future
+        for (const pack of game.packs) {
+            await pack.getDocuments();
+            progress.advance();
+        }
+
+        const batch = [];
+
+        const updateItem = async item => {
+            const itemUpdates = [];
+            for (const effect of item.effects) {
+                const changes = await this.updateActiveEffectSource(effect.toObject(), item);
+                if (changes) itemUpdates.push(changes);
+            }
+            if (itemUpdates.length) {
+                batch.push({
+                    action: 'update',
+                    documentName: 'ActiveEffect',
+                    updates: itemUpdates,
+                    parent: item
+                });
+            }
+        };
+
+        for (const actor of game.actors) {
+            for (const item of actor.items) {
+                await updateItem(item);
+            }
+            progress.advance();
+        }
+        for (const item of game.items) {
+            await updateItem(item);
+            progress.advance();
+        }
+
+        await foundry.documents.modifyBatch(batch);
+        progress.advance({ by: finalUpdateProgress });
+    }
+}