SilverBullet pivot to become an offline-first PWA (#403)

common/spaces/fallback_space_primitives.ts

@@ -0,0 +1,45 @@
+import { FileMeta } from "../types.ts";
+import type { SpacePrimitives } from "./space_primitives.ts";
+
+/**
+ * FallbackSpacePrimitives is a SpacePrimitives implementation that will try to fall back to another SpacePrimitives implementation for two
+ * operations:
+ *  - readFile
+ *  - getFileMeta
+ * The use case is primarily sync: when sync hasn't completed yet, we can fall back to HttpSpacePrimitives to fetch the file from the server.
+ */
+export class FallbackSpacePrimitives implements SpacePrimitives {
+  constructor(
+    private primary: SpacePrimitives,
+    private fallback: SpacePrimitives,
+  ) {
+  }
+  fetchFileList(): Promise<FileMeta[]> {
+    return this.primary.fetchFileList();
+  }
+  async readFile(name: string): Promise<{ data: Uint8Array; meta: FileMeta }> {
+    try {
+      return await this.primary.readFile(name);
+    } catch {
+      return this.fallback.readFile(name);
+    }
+  }
+  async getFileMeta(name: string): Promise<FileMeta> {
+    try {
+      return await this.primary.getFileMeta(name);
+    } catch {
+      return this.fallback.getFileMeta(name);
+    }
+  }
+  writeFile(
+    name: string,
+    data: Uint8Array,
+    selfUpdate?: boolean | undefined,
+    lastModified?: number | undefined,
+  ): Promise<FileMeta> {
+    return this.primary.writeFile(name, data, selfUpdate, lastModified);
+  }
+  deleteFile(name: string): Promise<void> {
+    return this.primary.deleteFile(name);
+  }
+}