backend-common: add UrlReaders for constructing a reader from config

packages/backend-common/src/index.ts

@@ -20,6 +20,7 @@ export * from './discovery';
 export * from './errors';
 export * from './logging';
 export * from './middleware';
+export * from './reading';
 export * from './service';
 export * from './paths';
 export * from './hot';

packages/backend-common/src/reading/UrlReaders.ts

@@ -0,0 +1,89 @@
+/*
+ * Copyright 2020 Spotify AB
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { Config } from '@backstage/config';
+import { GithubUrlReader } from './GithubUrlReader';
+import { UrlReader } from './types';
+import { UrlReaderHostMux } from './UrlReaderHostMux';
+
+export type ReaderFactoryOptions = {
+  config: Config;
+  logger: Logger;
+};
+export type ReaderFactory = (options: ReaderFactoryOptions) => UrlReader;
+
+/**
+ * UrlReaders provide various utilities related to the UrlReader interface.
+ */
+export class UrlReaders {
+  /**
+   * Creates a new UrlReaders instance without any known types.
+   */
+  static empty({ logger }: { logger: Logger }) {
+    return new UrlReaders(new Map(), logger);
+  }
+
+  /**
+   * Creates a new UrlReaders instance that includes factories for all types
+   * implemented in this package.
+   */
+  static default({ logger }: { logger: Logger }) {
+    return new UrlReaders(
+      new Map([['github', GithubUrlReader.factory]]),
+      logger,
+    );
+  }
+
+  private constructor(
+    private readonly factories: Map<string, ReaderFactory>,
+    private readonly logger: Logger,
+  ) {}
+
+  /**
+   * Constructs a new UrlReader using the provided configuration. Any encountered
+   * reader type needs to have a registered factory, or an error will be thrown.
+   */
+  createWithConfig(config: Config): UrlReader {
+    const readerItems = config.getOptionalConfigArray('readers.byHost') ?? [];
+    const mux = new UrlReaderHostMux();
+
+    for (const readerItem of readerItems) {
+      const type = readerItem.getString('type');
+      const host = readerItem.getString('host');
+      const readerConfig = readerItem.getConfig('config');
+
+      const factory = this.factories.get(type);
+      if (!factory) {
+        throw new Error(`Failed to create reader for unknown type '${type}'`);
+      }
+      const reader = factory({
+        config: readerConfig,
+        logger: this.logger,
+      });
+
+      mux.register(host, reader);
+    }
+
+    return mux;
+  }
+
+  /**
+   * Register a UrlReader factory for a given type
+   */
+  register(type: string, factory: ReaderFactory) {
+    this.factories.set(type, factory);
+  }
+}

packages/backend-common/src/reading/index.ts

@@ -15,3 +15,4 @@
  */
 
 export type { UrlReader } from './types';
+export { UrlReaders } from './UrlReaders';