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build/node_modules/hydrolysis/lib/loader/file-loader.js

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+/**
+ * @license
+ * Copyright (c) 2015 The Polymer Project Authors. All rights reserved.
+ * This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
+ * The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
+ * The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
+ * Code distributed by Google as part of the polymer project is also
+ * subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
+ */
+
+// jshint node:true
+'use strict';
+
+// jshint -W079
+// Promise polyfill
+var Promise = global.Promise || require('es6-promise').Promise;
+// jshint +W079
+
+function Deferred() {
+  var self = this;
+  this.promise = new Promise(function(resolve, reject) {
+    self.resolve = resolve;
+    self.reject = reject;
+  });
+}
+
+/**
+ * An object that knows how to resolve resources.
+ * @typedef {Object} Resolver
+ * @memberof hydrolysis
+ * @property {function(string, Deferred): boolean} accept Attempt to resolve
+ *     `deferred` with the contents the specified URL. Returns false if the
+ *     Resolver is unable to resolve the URL.
+ */
+
+
+/**
+ * A FileLoader lets you resolve URLs with a set of potential resolvers.
+ * @constructor
+ * @memberof hydrolysis
+ */
+function FileLoader() {
+  this.resolvers = [];
+  // map url -> Deferred
+  this.requests = {};
+}
+FileLoader.prototype = {
+
+  /**
+   * Add an instance of a Resolver class to the list of url resolvers
+   *
+   * Ordering of resolvers is most to least recently added
+   * The first resolver to "accept" the url wins.
+   * @param {Resolver} resolver The resolver to add.
+   */
+  addResolver: function(resolver) {
+    this.resolvers.push(resolver);
+  },
+
+  /**
+   * Return a promise for an absolute url
+   *
+   * Url requests are deduplicated by the loader, returning the same Promise for
+   * identical urls
+   *
+   * @param {string} url        The absolute url to request.
+   * @return {Promise.<string>} A promise that resolves to the contents of the URL.
+   */
+  request: function(uri) {
+    var promise;
+
+    if (!(uri in this.requests)) {
+      var handled = false;
+      var deferred = new Deferred();
+      this.requests[uri] = deferred;
+
+      // loop backwards through resolvers until one "accepts" the request
+      for (var i = this.resolvers.length - 1, r; i >= 0; i--) {
+        r = this.resolvers[i];
+        if (r.accept(uri, deferred)) {
+          handled = true;
+          break;
+        }
+      }
+
+      if (!handled) {
+        deferred.reject(new Error('no resolver found for ' + uri));
+      }
+
+      promise = deferred.promise;
+    } else {
+      promise = this.requests[uri].promise;
+    }
+
+    return promise;
+  }
+};
+
+module.exports = FileLoader;