Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/barback/src/transformer/transformer.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library barback.transformer.transformer;
+
+import 'dart:async';
+
+import '../asset/asset_id.dart';
+import '../utils.dart';
+import 'transform.dart';
+
+/// A [Transformer] represents a processor that takes in one or more input
+/// assets and uses them to generate one or more output assets.
+///
+/// Dart2js, a SASS->CSS processor, a CSS spriter, and a tool to concatenate
+/// files are all examples of transformers. To define your own transformation
+/// step, extend (or implement) this class.
+///
+/// If possible, transformers should implement [DeclaringTransformer] as well to
+/// help barback optimize the package graph.
+abstract class Transformer {
+  /// Override this to return a space-separated list of file extensions that are
+  /// allowed for the primary inputs to this transformer.
+  ///
+  /// Each extension must begin with a leading `.`.
+  ///
+  /// If you don't override [isPrimary] yourself, it defaults to allowing any
+  /// asset whose extension matches one of the ones returned by this. If you
+  /// don't override [isPrimary] *or* this, it allows all files.
+  String get allowedExtensions => null;
+
+  Transformer() {
+    if (allowedExtensions == null) return;
+
+    var invalidExtensions = allowedExtensions.split(" ")
+        .where((extension) => !extension.startsWith("."))
+        .map((extension) => '"$extension"');
+    if (invalidExtensions.isEmpty) return;
+
+    throw new FormatException('Each extension in $this.allowedExtensions '
+        'must begin with a ".", but ${toSentence(invalidExtensions)} '
+        '${pluralize("doesn't", invalidExtensions.length, plural: "don't")}.');
+  }
+
+  /// Returns `true` if [id] can be a primary input for this transformer.
+  ///
+  /// While a transformer can read from multiple input files, one must be the
+  /// "primary" input. This asset determines whether the transformation should
+  /// be run at all. If the primary input is removed, the transformer will no
+  /// longer be run.
+  ///
+  /// A concrete example is dart2js. When you run dart2js, it will traverse
+  /// all of the imports in your Dart source files and use the contents of all
+  /// of those to generate the final JS. However you still run dart2js "on" a
+  /// single file: the entrypoint Dart file that has your `main()` method.
+  /// This entrypoint file would be the primary input.
+  ///
+  /// If this is not overridden, defaults to allow any asset whose extension
+  /// matches one of the ones returned by [allowedExtensions]. If *that* is
+  /// not overridden, allows all assets.
+  ///
+  /// This may return a `Future<bool>` or, if it's entirely synchronous, a
+  /// `bool`.
+  isPrimary(AssetId id) {
+    // Allow all files if [primaryExtensions] is not overridden.
+    if (allowedExtensions == null) return true;
+
+    for (var extension in allowedExtensions.split(" ")) {
+      if (id.path.endsWith(extension)) return true;
+    }
+
+    return false;
+  }
+
+  /// Run this transformer on the primary input specified by [transform].
+  ///
+  /// The [transform] is used by the [Transformer] for two purposes (in
+  /// addition to accessing the primary input). It can call `getInput()` to
+  /// request additional input assets. It also calls `addOutput()` to provide
+  /// generated assets back to the system. Either can be called multiple times,
+  /// in any order.
+  ///
+  /// In other words, a Transformer's job is to find all inputs for a
+  /// transform, starting at the primary input, then generate all output assets
+  /// and yield them back to the transform.
+  ///
+  /// If this does asynchronous work, it should return a [Future] that completes
+  /// once it's finished.
+  apply(Transform transform);
+
+  String toString() => runtimeType.toString().replaceAll("Transformer", "");
+}