refactoring and tests

website/src/lib/engine/observer.ts

@@ -0,0 +1,50 @@
+import type { Readable } from 'svelte/store';
+import { engine } from './engine';
+import type { FileState, FileStatesCallback } from './types';
+
+/**
+ * Tells when file stores appear and disappear, so that their subscribers know when to
+ * subscribe and unsubscribe. `onFilesAdded` is called with the stores of the new files only
+ * (including the ones already present when the observer is created).
+ */
+export class FileStateCollectionObserver {
+    private _fileIds = new Set<string>();
+    private _onFilesAdded: FileStatesCallback;
+    private _onFileRemoved: (fileId: string) => void;
+    private _onDestroy: () => void;
+    private _unsubscribe: () => void;
+
+    constructor(
+        onFilesAdded: FileStatesCallback,
+        onFileRemoved: (fileId: string) => void,
+        onDestroy: () => void
+    ) {
+        this._onFilesAdded = onFilesAdded;
+        this._onFileRemoved = onFileRemoved;
+        this._onDestroy = onDestroy;
+
+        this._unsubscribe = engine.files.subscribe((files) => {
+            this._fileIds.forEach((fileId) => {
+                if (!files.has(fileId)) {
+                    this._onFileRemoved(fileId);
+                    this._fileIds.delete(fileId);
+                }
+            });
+            const newFiles = new Map<string, Readable<FileState>>();
+            files.forEach((store, fileId) => {
+                if (!this._fileIds.has(fileId)) {
+                    newFiles.set(fileId, store);
+                    this._fileIds.add(fileId);
+                }
+            });
+            if (newFiles.size > 0) {
+                this._onFilesAdded(newFiles);
+            }
+        });
+    }
+
+    destroy() {
+        this._onDestroy();
+        this._unsubscribe();
+    }
+}