@@ -2,6 +2,11 @@
  * 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/. */
 
+/**
+ * @import { ExperimentManager } from "./lib/ExperimentManager.sys.mjs"
+ * @import { RemoteSettingsExperimentLoader } from "./lib/RemoteSettingsExperimentLoader.sys.mjs"
+ */
+
 import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 import { AppConstants } from "resource://gre/modules/AppConstants.sys.mjs";
 
@@ -74,26 +79,6 @@ const experimentBranchAccessor = {
 
 const NIMBUS_PROFILE_ID_PREF = "nimbus.profileId";
 
-let cachedProfileId = null;
-
-/**
- * Ensure the Nimbus profile ID exists.
- *
- * @returns {string} The profile ID.
- */
-function ensureNimbusProfileId() {
-  if (!cachedProfileId) {
-    if (Services.prefs.prefHasUserValue(NIMBUS_PROFILE_ID_PREF)) {
-      cachedProfileId = Services.prefs.getStringPref(NIMBUS_PROFILE_ID_PREF);
-    } else {
-      cachedProfileId = Services.uuid.generateUUID().toString().slice(1, -1);
-      Services.prefs.setStringPref(NIMBUS_PROFILE_ID_PREF, cachedProfileId);
-    }
-  }
-
-  return cachedProfileId;
-}
-
 /**
  * Metadata about an enrollment.
  *
@@ -131,11 +116,96 @@ export const EnrollmentType = Object.freeze({
   ROLLOUT: "rollout",
 });
 
-let initialized = false;
-let experimentManager = null;
-let experimentLoader = null;
+export const ExperimentAPI = new (class {
+  /**
+   * Whether or not the ExperimentAPI has been initialized.
+   */
+  #initialized = false;
+
+  /**
+   * The current ExperimentManager.
+   *
+   * @type {ExperimentManager | null}
+   */
+  #experimentManager = null;
+
+  /**
+   * The current RemoteSettingsExperimentLoader.
+   *
+   * @type {RemoteSettingsExperimentLoader | null}
+   */
+  #experimentLoader = null;
+
+  /**
+   * The unique ID of this Profile.
+   *
+   * @type {string | null}
+   */
+  #cachedProfileId = null;
+
+  /**
+   * Cached pref values.
+   */
+  #prefValues = {
+    /**
+     * Whether or not opt-out studies are enabled.
+     *
+     * @see {@link STUDIES_OPT_OUT_PREF}
+     */
+    studiesEnabled: false,
+
+    /**
+     * Whether or not telemetry is enabled.
+     *
+     * @see {@link UPLOAD_ENABLED_PREF}
+     */
+    telemetryEnabled: false,
+  };
+
+  constructor() {
+    // Ensure that the profile ID is cached in a pref.
+    if (Services.prefs.prefHasUserValue(NIMBUS_PROFILE_ID_PREF)) {
+      this.#cachedProfileId = Services.prefs.getStringPref(
+        NIMBUS_PROFILE_ID_PREF
+      );
+    } else {
+      this.#cachedProfileId = Services.uuid
+        .generateUUID()
+        .toString()
+        .slice(1, -1);
+      Services.prefs.setStringPref(
+        NIMBUS_PROFILE_ID_PREF,
+        this.#cachedProfileId
+      );
+    }
+
+    const onStudiesEnabledChanged = this.#onStudiesEnabledChanged.bind(this);
+
+    XPCOMUtils.defineLazyPreferenceGetter(
+      this.#prefValues,
+      "studiesEnabled",
+      STUDIES_OPT_OUT_PREF,
+      false,
+      onStudiesEnabledChanged
+    );
+
+    XPCOMUtils.defineLazyPreferenceGetter(
+      this.#prefValues,
+      "telemetryEnabled",
+      UPLOAD_ENABLED_PREF,
+      false,
+      onStudiesEnabledChanged
+    );
+
+    this._annotateCrashReport = this._annotateCrashReport.bind(this);
+    this._removeCrashReportAnnotator =
+      this._removeCrashReportAnnotator.bind(this);
+
+    ChromeUtils.defineLazyGetter(this, "_remoteSettingsClient", function () {
+      return lazy.RemoteSettings(lazy.COLLECTION_ID);
+    });
+  }
 
-export const ExperimentAPI = {
   /**
    * The topic that is notified when either the studies enabled pref or the
    * telemetry enabled pref changes.
@@ -145,7 +215,7 @@ export const ExperimentAPI = {
    */
   get STUDIES_ENABLED_CHANGED() {
     return "nimbus:studies-enabled-changed";
-  },
+  }
 
   /**
    * Initialize the ExperimentAPI.
@@ -166,13 +236,11 @@ export const ExperimentAPI = {
    *          Whether or not the ExperimentAPI was initialized.
    */
   async init({ extraContext, forceSync = false } = {}) {
-    if (initialized) {
+    if (this.#initialized) {
       return false;
     }
 
-    ensureNimbusProfileId();
-
-    initialized = true;
+    this.#initialized = true;
 
     const studiesEnabled = this.studiesEnabled;
 
@@ -240,78 +308,75 @@ export const ExperimentAPI = {
       );
     }
 
-    Services.prefs.addObserver(
-      UPLOAD_ENABLED_PREF,
-      this._onStudiesEnabledChanged
-    );
-    Services.prefs.addObserver(
-      STUDIES_OPT_OUT_PREF,
-      this._onStudiesEnabledChanged
-    );
-
     // If Nimbus was disabled between the start of this function and registering
     // the pref observers we have not handled it yet.
     if (studiesEnabled !== this.studiesEnabled) {
-      await this._onStudiesEnabledChanged();
+      await this.#onStudiesEnabledChanged();
     }
 
     return true;
-  },
+  }
 
   /**
    * Return the global ExperimentManager.
    *
    * The ExperimentManager will be lazily created upon first access to this
    * property.
+   *
+   * @type {ExperimentManager}
    */
   get manager() {
-    if (experimentManager === null) {
-      experimentManager = new lazy.ExperimentManager();
+    if (this.#experimentManager === null) {
+      this.#experimentManager = new lazy.ExperimentManager();
     }
 
-    return experimentManager;
-  },
+    return this.#experimentManager;
+  }
 
   /**
    * Return the global ExperimentManager.
    *
    * @deprecated Use ExperimentAPI.Manager instead of this property.
+   *
+   * @type {ExperimentManager}
    */
   get _manager() {
     return this.manager;
-  },
+  }
 
   /**
    * Return the global RemoteSettingsExperimentLoader.
+   *
+   * @type {RemoteSettingsExperimentLoader}
    */
   get _rsLoader() {
-    if (experimentLoader === null) {
-      experimentLoader = new lazy.RemoteSettingsExperimentLoader(this.manager);
+    if (this.#experimentLoader === null) {
+      this.#experimentLoader = new lazy.RemoteSettingsExperimentLoader(
+        this.manager
+      );
     }
 
-    return experimentLoader;
-  },
+    return this.#experimentLoader;
+  }
 
   _resetForTests() {
-    experimentLoader?.disable();
-    experimentLoader = null;
+    this.#experimentLoader?.disable();
+    this.#experimentLoader = null;
 
-    lazy.CleanupManager.removeCleanupHandler(
-      ExperimentAPI._removeCrashReportAnnotator
-    );
-    experimentManager?.store.off("update", this._annotateCrashReport);
-    experimentManager = null;
+    lazy.CleanupManager.removeCleanupHandler(this._removeCrashReportAnnotator);
+    this.#experimentManager?.store.off("update", this._annotateCrashReport);
+    this.#experimentManager = null;
 
-    initialized = false;
-  },
+    this.#initialized = false;
+  }
 
   get studiesEnabled() {
     return (
-      Services.prefs.getBoolPref(UPLOAD_ENABLED_PREF, false) &&
-      Services.prefs.getBoolPref(STUDIES_OPT_OUT_PREF, false) &&
+      this.#prefValues.studiesEnabled &&
+      this.#prefValues.telemetryEnabled &&
       Services.policies.isAllowed("Shield")
     );
-  },
+  }
 
   /**
    * Return the profile ID.
@@ -326,8 +391,8 @@ export const ExperimentAPI = {
    * @returns {string} The profile ID.
    */
   get profileId() {
-    return ensureNimbusProfileId();
-  },
+    return this.#cachedProfileId;
+  }
 
   /**
    * Wait for the ExperimentAPI to become ready.
@@ -342,7 +407,7 @@ export const ExperimentAPI = {
    */
   async ready() {
     return this.manager.store.ready();
-  },
+  }
 
   /**
    * Annotate the current crash report with current enrollments.
@@ -362,23 +427,27 @@ export const ExperimentAPI = {
       "NimbusEnrollments",
       activeEnrollments
     );
-  },
+  }
 
   _removeCrashReportAnnotator() {
-    if (initialized) {
-      experimentManager?.store.off("update", this._annotateCrashReport);
+    if (this.#initialized) {
+      this.#experimentManager?.store.off("update", this._annotateCrashReport);
+    }
+  }
+
+  async #onStudiesEnabledChanged() {
+    if (!this.#initialized) {
+      return;
     }
-  },
 
-  async _onStudiesEnabledChanged() {
     if (!this.studiesEnabled) {
       await this.manager._handleStudiesOptOut();
     }
 
     await this._rsLoader.onEnabledPrefChange();
 
     Services.obs.notifyObservers(null, this.STUDIES_ENABLED_CHANGED);
-  },
+  }
 
   /**
    * Returns the recipe for a given experiment slug
@@ -415,7 +484,7 @@ export const ExperimentAPI = {
     }
 
     return recipe;
-  },
+  }
 
   /**
    * Returns all the branches for a given experiment slug
@@ -437,7 +506,7 @@ export const ExperimentAPI = {
     return recipe?.branches.map(
       branch => new Proxy(branch, experimentBranchAccessor)
     );
-  },
+  }
 
   /**
    * Opt-in to the given experiment on the given branch.
@@ -465,8 +534,8 @@ export const ExperimentAPI = {
    */
   async optInToExperiment(options) {
     return this._rsLoader._optInToExperiment(options);
-  },
-};
+  }
+})();
 
 /**
  * Singleton that holds lazy references to _ExperimentFeature instances
@@ -933,21 +1002,6 @@ export class _ExperimentFeature {
   }
 }
 
-ExperimentAPI._annotateCrashReport =
-  ExperimentAPI._annotateCrashReport.bind(ExperimentAPI);
-ExperimentAPI._onStudiesEnabledChanged =
-  ExperimentAPI._onStudiesEnabledChanged.bind(ExperimentAPI);
-ExperimentAPI._removeCrashReportAnnotator =
-  ExperimentAPI._removeCrashReportAnnotator.bind(ExperimentAPI);
-
-ChromeUtils.defineLazyGetter(
-  ExperimentAPI,
-  "_remoteSettingsClient",
-  function () {
-    return lazy.RemoteSettings(lazy.COLLECTION_ID);
-  }
-);
-
 class ExperimentLocalizationError extends Error {
   constructor(reason, locale) {
     super(`Localized experiment error (${reason})`);