Firefox Addon Development : Detecting non-compatible addons?

Question

Few addons are not compatible with mine, so how to detect their presence and inform the user.

Thanks

Answer 1

OK got it here is how this is done :

  function isExtEnabled(){

    if(!Application.extensions.has('EXTENSION_ID_HERE')) {

      return false;
    }
    return true;
  }

Answer 2

Here is an example from the evil noscript-1.9.2.xpi which modified adblock plugin settings

function MRD(ns) {
  this.enabled = ns.getPref("mrd", true);
  if (!this.enabled) return;

  var c = CC[this.id];
  if (c) {
    this.ns = ns;
    this.c = c.createInstance().wrappedJSObject;
    this._w._mrd = this;
    var eh = this.c["elemhide"];
    eh.watch("url", this._w);
    eh.apply();
    ns.mrd = this;
    ns.initContentPolicy();
  } else this.enabled = false;
}  
MRD.prototype = {
  id: "@mozilla.org/adblockplus;1",
  _nobind: "{-moz-binding: none !important}",
  _ms: null,
  _w: function(p, o, n) {
    if (!n) return n;
    var mrd = arguments.callee._mrd;
    var u = decodeURIComponent(n.spec);

    var mm = u.match(/@-moz-document\s+domain[^\)]*?(?:(?:noscript|flashgot|hackademix)\.net|informaction\.com|googlesyndication\.com)[^\}]*\}/g);
    if (mm) {
      var ns = mrd.ns;
      mrd._ms = mm.join('').replace(/(\{[^\{\}]*)\{[^\}]*/g, '$1' + mrd._nobind);
    }

    /*
    var uu = n.spec.split(',');
    uu[1] = encodeURIComponent(decodeURIComponent(uu[1]).replace(/@-moz-document\s+domain[^\)]*?(?:(?:noscript|flashgot|hackademix)\.net|informaction\.com|googlesyndication\.com)[^\}]*\}/g, ''));
    n.spec = uu.join(',');
    */
    mrd.ns.delayExec(function() { mrd.apply(); }, 0);
    return n;
  },
  _dd: function(a, s) {
    return "@-moz-document domain(" + a.join("),domain(") + "){" + s + "} ";
  },

  get _def() {
    delete this.__proto__._def;
    return this.__proto__._def = this.ns.prefService.getDefaultBranch(this.ns.prefs.root).getCharPref("default");
  },
  get _wl() {
    delete this.__proto__._wl;
    return this.__proto__._wl = this._def.match(/\w+[^r].\.n\w+|in\w+on\.c\w+/g).concat(this.ns.getPref("xblHack", "").split(/\s+/));
  },
  get _wlrx() {
    delete this.__proto__._wlrx;
    return this.__proto__._wlrx = new RegExp("^(?:[\\w\\-\\.]*\\.)?(?:" + this._wl.join("|").replace(/\./g, "\\.").concat(")$"));
  },
  get _es() {
    delete this.__proto__._es;
    try {
      var ss = [], lastS = '';
      for(var j = 0; j < 5; j++) {
        ss.push(lastS += " #k" + j);
      }
      es = this._dd(this._wl, ss.join(' *,') + ' *' + this._nobind) +
           this._dd(this._def.match(/\w+[^r].\.n\w+|\w+on\.c\w+/g), "#a\u0064s, #\u0061ds .ad" + this._nobind);
    } catch (e) {
      if (this.ns.consoleDump) this.ns.dump("MRD ES Error: " + e);
    }
    return this.__proto__._es = es;
  },

  apply: function() {
    var ns = this.ns; 
    for each(var s in [this._es, this._ms]){
      if (s) {
        ns.updateStyleSheet(s, false);
        ns.updateStyleSheet(s, true);
      }
    }
  },

  attach: function() {
    if (!this.enabled) return false;
    try {
      var p = this.c.policy;
      var ns = this.ns;
      var wlrx = this._wlrx;
      if (!wlrx) return false;
      ns._mrd_shouldLoad = ns.shouldLoad;
      ns.shouldLoad = function(ct, cl, ro, ctx, mm, internal) {
        if (!internal) try {
          var w = ctx && (ctx.defaultView || ctx.ownerDocument && ctx.ownerDocument.defaultView || ctx);
          if (w) {
            l = w.top.location;
            if (!(/^https?/.test(l.protocol) && wlrx.test(l.hostname))) {
              var res = p.shouldLoad(ct, cl, ro, ctx, mm, internal);
              if (res != CP_OK) return res;
            }
          }
        } catch(e) {
          if (ns.consoleDump) ns.dump(e);
        }
        return ns._mrd_shouldLoad(ct, cl, ro, ctx, mm, internal);
      };
    } catch(e) {
      if (this.ns.consoleDump) this.ns.dump("MRD Attach Error: " + e);
      return false;
    }
    return true;
  }
}