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FeatureGate.jsm
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FeatureGate.jsm
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/* 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/. */
"use strict";
const { XPCOMUtils } = ChromeUtils.import(
"resource://gre/modules/XPCOMUtils.jsm"
);
ChromeUtils.defineModuleGetter(
this,
"AppConstants",
"resource://gre/modules/AppConstants.jsm"
);
ChromeUtils.defineModuleGetter(
this,
"FeatureGateImplementation",
"resource://featuregates/FeatureGateImplementation.jsm"
);
XPCOMUtils.defineLazyGlobalGetters(this, ["fetch"]);
var EXPORTED_SYMBOLS = ["FeatureGate"];
XPCOMUtils.defineLazyGetter(this, "gFeatureDefinitionsPromise", async () => {
const url = "resource://featuregates/feature_definitions.json";
return fetchFeatureDefinitions(url);
});
const kTargetFacts = new Map([
["release", AppConstants.MOZ_UPDATE_CHANNEL === "release"],
["beta", AppConstants.MOZ_UPDATE_CHANNEL === "beta"],
["dev-edition", AppConstants.MOZ_UPDATE_CHANNEL === "aurora"],
["nightly", AppConstants.MOZ_UPDATE_CHANNEL === "nightly"],
["win", AppConstants.platform === "win"],
["mac", AppConstants.platform === "macosx"],
["linux", AppConstants.platform === "linux"],
["android", AppConstants.platform === "android"],
]);
async function fetchFeatureDefinitions(url) {
const res = await fetch(url);
let definitionsJson = await res.json();
return new Map(Object.entries(definitionsJson));
}
/**
* Take a map of conditions to values, and return the value who's conditions
* match this browser, or the default value in the map.
*
* @example `evaluateTargetedValue({default: false, nightly: true})` would
* return true on Nightly, and false otherwise.
* @param {Object} targetedValue An object mapping string conditions to values. The
* conditions are comma separated values such as those specified
* in `kTargetFacts` above. A condition "default" is required, as
* the fallback valued.
* @param {Map} targetingFacts A map of target facts to use, such as `kTargetFacts`.
* @returns A value from `targetedValue`.
*/
function evaluateTargetedValue(targetedValue, targetingFacts) {
if (!Object.hasOwnProperty.call(targetedValue, "default")) {
throw new Error(
`Targeted value ${JSON.stringify(targetedValue)} has no default key`
);
}
for (const [key, value] of Object.entries(targetedValue)) {
if (key === "default") {
continue;
}
if (key.split(",").every(part => targetingFacts.get(part))) {
return value;
}
}
return targetedValue.default;
}
const kFeatureGateCache = new Map();
/** A high level control for turning features on and off. */
class FeatureGate {
/*
* This is structured as a class with static methods to that sphinx-js can
* easily document it. This constructor is required for sphinx-js to detect
* this class for documentation.
*/
constructor() {}
/**
* Constructs a feature gate object that is defined in ``Features.toml``.
* This is the primary way to create a ``FeatureGate``.
*
* @param {string} id The ID of the feature's definition in `Features.toml`.
* @param {string} testDefinitionsUrl A URL from which definitions can be fetched. Only use this in tests.
* @throws If the ``id`` passed is not defined in ``Features.toml``.
*/
static async fromId(id, testDefinitionsUrl = undefined) {
let featureDefinitions;
if (testDefinitionsUrl) {
featureDefinitions = await fetchFeatureDefinitions(testDefinitionsUrl);
} else {
featureDefinitions = await gFeatureDefinitionsPromise;
}
if (!featureDefinitions.has(id)) {
throw new Error(
`Unknown feature id ${id}. Features must be defined in toolkit/components/featuregates/Features.toml`
);
}
// Make a copy of the definition, since we are about to modify it
const definition = { ...featureDefinitions.get(id) };
const targetValueKeys = ["defaultValue", "isPublic"];
for (const key of targetValueKeys) {
definition[key] = evaluateTargetedValue(definition[key], kTargetFacts);
}
return new FeatureGateImplementation(definition);
}
/**
* Add an observer for a feature gate by ID. If the feature is of type
* boolean and currently enabled, `onEnable` will be called.
*
* The underlying feature gate instance will be shared with all other callers
* of this function, and share an observer.
*
* @param {string} id The ID of the feature's definition in `Features.toml`.
* @param {object} observer Functions to be called when the feature changes.
* All observer functions are optional.
* @param {Function()} [observer.onEnable] Called when the feature becomes enabled.
* @param {Function()} [observer.onDisable] Called when the feature becomes disabled.
* @param {Function(newValue)} [observer.onChange] Called when the
* feature's state changes to any value. The new value will be passed to the
* function.
* @param {string} testDefinitionsUrl A URL from which definitions can be fetched. Only use this in tests.
* @returns {Promise<boolean>} The current value of the feature.
*/
static async addObserver(id, observer, testDefinitionsUrl = undefined) {
if (!kFeatureGateCache.has(id)) {
kFeatureGateCache.set(
id,
await FeatureGate.fromId(id, testDefinitionsUrl)
);
}
const feature = kFeatureGateCache.get(id);
return feature.addObserver(observer);
}
/**
* Remove an observer of changes from this feature
* @param {string} id The ID of the feature's definition in `Features.toml`.
* @param observer Then observer that was passed to addObserver to remove.
*/
static async removeObserver(id, observer) {
let feature = kFeatureGateCache.get(id);
if (!feature) {
return;
}
feature.removeObserver(observer);
if (feature._observers.size === 0) {
kFeatureGateCache.delete(id);
}
}
/**
* Get the current value of this feature gate. Implementors should avoid
* storing the result to avoid missing changes to the feature's value.
* Consider using :func:`addObserver` if it is necessary to store the value
* of the feature.
*
* @async
* @param {string} id The ID of the feature's definition in `Features.toml`.
* @returns {Promise<boolean>} A promise for the value associated with this feature.
*/
static async getValue(id, testDefinitionsUrl = undefined) {
let feature = kFeatureGateCache.get(id);
if (!feature) {
feature = await FeatureGate.fromId(id, testDefinitionsUrl);
}
return feature.getValue();
}
/**
* An alias of `getValue` for boolean typed feature gates.
*
* @async
* @param {string} id The ID of the feature's definition in `Features.toml`.
* @returns {Promise<boolean>} A promise for the value associated with this feature.
* @throws {Error} If the feature is not a boolean.
*/
static async isEnabled(id, testDefinitionsUrl = undefined) {
let feature = kFeatureGateCache.get(id);
if (!feature) {
feature = await FeatureGate.fromId(id, testDefinitionsUrl);
}
return feature.isEnabled();
}
}