Add API which add listeners to GM value changes

An API for listening to GM value changes is useful for implement a polyfill for old sync GM_getValue, GM_setValue, ect. And TM already provided a similar one. If such API may be added, then it is possible to create GM_*value polyfills. Thats seems the fastest (maybe only possible) way to port some large userscripts written for old APIs work.

Something polyfill like this: (not tested)

if (!GM_getValue) {
  GM.listValues().then(function (keys) {
    var values = new Map();
    Promise.all(keys.map(function () {
      return GM.getValue(key).then(function (value) {
        if (value !== void 0) values.set(key, value);
        GM.onValueChange(key, function (newValue) {
          values.set(key, newValue);
        });
      });
    })).then(function () {
      doTheMainStuff();
    });
    GM_getValue = function (key, defaultValue) {
      if (values.has(key)) return values.get(key);
      return defaultValue;
    };
    GM_setValue = function (key, value) {
      values.set(key, value);
      return GM.setValue(key, value);
    };
    GM_deleteValue = function (key) {
      values.delete(key);
      return GM.deleteValue;
    };
  });

}

greasemonkey / greasemonkey

Add API which add listeners to GM value changes #2646