nsIPrefBranch for Firefox for android extension returns NS_ERROR_UNEXPECTED

Question

I want to store a value in the preference of my extension (a firefox for android ext). So, in the prefs.js file (under defaults/preferences/) I write:

pref("extensions.trackdetect.idUser","nothing");

Then, in boostrap.js I get the preference branch:

var prefs = Components.classes["@mozilla.org/preferences-service;1"]
                .getService(Components.interfaces.nsIPrefService).getBranch("extensions.trackdetect.");

But, when I try to get idUser value like this:

var idPref = prefs.getCharPref("idUser");

I get this error:`

Exception... "Component returned failure code: 0x8000ffff (NS_ERROR_UNEXPECTED) [nsIPrefBranch.getCharPref]"  nsresult: "0x8000ffff (NS_ERROR_UNEXPECTED)"

This post explains the error origins, but my preference as I showed bellow is set, so I don't understand the problem.

Answer 1

Bootstrapped (restartless) add-ons do not support auto-prefs via defaults/preferences. It is the responsibility of your bootstrap code to set up default preferences accordingly.

Since your auto-prefs were never loaded, the getCharPref() call has to fail.

You will need to get the default branch and seed your preferences. An easy way to do so would be:

const PREF_BRANCH = "extensions.myaddon.";
const PREFS = {
  someIntPref: 1,
  someStringPref: "some text value"
};

function setDefaultPrefs() {
  let branch = Services.prefs.getDefaultBranch(PREF_BRANCH);
  for (let [key, val] in Iterator(PREFS)) {
    switch (typeof val) {
      case "boolean":
        branch.setBoolPref(key, val);
        break;
      case "number":
        branch.setIntPref(key, val);
        break;
      case "string":
        branch.setCharPref(key, val);
        break;
    }
  }
}

function startup(aData, aReason) {
  // Always set the default prefs as they disappear on restart
  setDefaultPrefs();
  ...
}

There are alternatives, like using mozISubscriptLoader or Sandboxes, but I don't really have stand-alone code that can be easily copied for that.