1 files changed, 133 insertions, 0 deletions
diff --git a/browser/components/newtab/lib/LinksCache.sys.mjs b/browser/components/newtab/lib/LinksCache.sys.mjs
new file mode 100644
index 0000000000..0dfb89e74e
--- /dev/null
+++ b/browser/components/newtab/lib/LinksCache.sys.mjs
@@ -0,0 +1,133 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// This should be slightly less than SYSTEM_TICK_INTERVAL as timer
+// comparisons are too exact while the async/await functionality will make the
+// last recorded time a little bit later. This causes the comparasion to skip
+// updates.
+// It should be 10% less than SYSTEM_TICK to update at least once every 5 mins.
+// https://github.com/mozilla/activity-stream/pull/3695#discussion_r144678214
+const EXPIRATION_TIME = 4.5 * 60 * 1000; // 4.5 minutes
+
+/**
+ * Cache link results from a provided object property and refresh after some
+ * amount of time has passed. Allows for migrating data from previously cached
+ * links to the new links with the same url.
+ */
+export class LinksCache {
+  /**
+   * Create a links cache for a given object property.
+   *
+   * @param {object} linkObject Object containing the link property
+   * @param {string} linkProperty Name of property on object to access
+   * @param {array} properties Optional properties list to migrate to new links.
+   * @param {function} shouldRefresh Optional callback receiving the old and new
+   *                                 options to refresh even when not expired.
+   */
+  constructor(
+    linkObject,
+    linkProperty,
+    properties = [],
+    shouldRefresh = () => {}
+  ) {
+    this.clear();
+
+    // Allow getting links from both methods and array properties
+    this.linkGetter = options => {
+      const ret = linkObject[linkProperty];
+      return typeof ret === "function" ? ret.call(linkObject, options) : ret;
+    };
+
+    // Always migrate the shared cache data in addition to any custom properties
+    this.migrateProperties = ["__sharedCache", ...properties];
+    this.shouldRefresh = shouldRefresh;
+  }
+
+  /**
+   * Clear the cached data.
+   */
+  clear() {
+    this.cache = Promise.resolve([]);
+    this.lastOptions = {};
+    this.expire();
+  }
+
+  /**
+   * Force the next request to update the cache.
+   */
+  expire() {
+    delete this.lastUpdate;
+  }
+
+  /**
+   * Request data and update the cache if necessary.
+   *
+   * @param {object} options Optional data to pass to the underlying method.
+   * @returns {promise(array)} Links array with objects that can be modified.
+   */
+  async request(options = {}) {
+    // Update the cache if the data has been expired
+    const now = Date.now();
+    if (
+      this.lastUpdate === undefined ||
+      now > this.lastUpdate + EXPIRATION_TIME ||
+      // Allow custom rules around refreshing based on options
+      this.shouldRefresh(this.lastOptions, options)
+    ) {
+      // Update request state early so concurrent requests can refer to it
+      this.lastOptions = options;
+      this.lastUpdate = now;
+
+      // Save a promise before awaits, so other requests wait for correct data
+      // eslint-disable-next-line no-async-promise-executor
+      this.cache = new Promise(async (resolve, reject) => {
+        try {
+          // Allow fast lookup of old links by url that might need to migrate
+          const toMigrate = new Map();
+          for (const oldLink of await this.cache) {
+            if (oldLink) {
+              toMigrate.set(oldLink.url, oldLink);
+            }
+          }
+
+          // Update the cache with migrated links without modifying source objects
+          resolve(
+            (await this.linkGetter(options)).map(link => {
+              // Keep original array hole positions
+              if (!link) {
+                return link;
+              }
+
+              // Migrate data to the new link copy if we have an old link
+              const newLink = Object.assign({}, link);
+              const oldLink = toMigrate.get(newLink.url);
+              if (oldLink) {
+                for (const property of this.migrateProperties) {
+                  const oldValue = oldLink[property];
+                  if (oldValue !== undefined) {
+                    newLink[property] = oldValue;
+                  }
+                }
+              } else {
+                // Share data among link copies and new links from future requests
+                newLink.__sharedCache = {};
+              }
+              // Provide a helper to update the cached link
+              newLink.__sharedCache.updateLink = (property, value) => {
+                newLink[property] = value;
+              };
+
+              return newLink;
+            })
+          );
+        } catch (error) {
+          reject(error);
+        }
+      });
+    }
+
+    // Provide a shallow copy of the cached link objects for callers to modify
+    return (await this.cache).map(link => link && Object.assign({}, link));
+  }
+}