refactor: Establish BaseImporter.

- Establish BaseImporter as an abstract class that other Importers can
extend from. This will be where we add common utilities that Importers
may need for working with CSS Blocks files.
- Update NodeJsImporter to extend from BaseImporter.

Author: Timothy Lindvall

packages/@css-blocks/core/src/importing/BaseImporter.ts

@@ -0,0 +1,39 @@
+import { Syntax } from '../BlockParser';
+import { ResolvedConfiguration } from '../configuration';
+import { Importer, FileIdentifier, ImportedFile } from './Importer';
+
+/**
+ * The BaseImporter is an abstract class that Importer implementations may extend from.
+ * This follows the Importer interface that must be used for interacting with the BlockFactory.
+ * We also include additional utility methods that are useful for handling CSS Blocks,
+ * Compiled CSS, and Definition Files.
+ */
+export abstract class BaseImporter implements Importer {
+  /**
+   * Compute a unique identifier for a given import path. If `fromIdentifier` is provided,
+   * the importPath can be relative to the file that is identified by it.
+   */
+  abstract identifier(fromIdentifier: FileIdentifier | null, importPath: string, config: ResolvedConfiguration): FileIdentifier;
+  /**
+   * Import the file with the given metadata and return a string and meta data for it.
+   */
+  abstract import(identifier: FileIdentifier, config: ResolvedConfiguration): Promise<ImportedFile>;
+  /**
+   * The default name of the block used unless the block specifies one itself.
+   */
+  abstract defaultName(identifier: FileIdentifier, configuration: ResolvedConfiguration): string;
+  /**
+   * If a file identifier has an on-disk representation, return an absolute path to it.
+   */
+  abstract filesystemPath(identifier: FileIdentifier, config: ResolvedConfiguration): string | null;
+  /**
+   * Returns a string meant for human consumption that identifies the file.
+   * As is used in debug statements and error reporting. Unlike filesystemPath,
+   * this needn't resolve to an actual file or be an absolute path.
+   */
+  abstract debugIdentifier(identifier: FileIdentifier, config: ResolvedConfiguration): string;
+  /**
+   * Returns the syntax the contents are written in.
+   */
+  abstract syntax(identifier: FileIdentifier, config: ResolvedConfiguration): Syntax;
+}

packages/@css-blocks/core/src/importing/NodeJsImporter.ts

@@ -6,7 +6,8 @@ import * as path from "path";
 import { Syntax } from "../BlockParser";
 import { ResolvedConfiguration } from "../configuration";
 
-import { FileIdentifier, ImportedFile, Importer } from "./Importer";
+import { FileIdentifier, ImportedFile } from "./Importer";
+import { BaseImporter } from "./BaseImporter";
 
 const debug = debugGenerator("css-blocks:importer");
 
@@ -27,9 +28,11 @@ export interface Alias {
   path: string;
 }
 
-export class NodeJsImporter implements Importer {
+export class NodeJsImporter extends BaseImporter {
   aliases: Alias[];
   constructor(aliases: Alias[] | ObjectDictionary<string> = []) {
+    super();
+
     // Normalize aliases input.
     this.aliases = Array.isArray(aliases)
       ? aliases.slice()