Is my implementation of unloaders proper? - firefox-addon

Question

I was re-reading this post here: https://stackoverflow.com/a/24473888/1828637
And got concerned about if I did things correctly. This is how I do unloading:
So I set up some stuff per window. And unload them on shutdown. (i dont unload on window close, i havent found a need to yet, as when it closes, everything i added to it goes with with the close [such as my mutation observers]).
All code below is theoretical, the mutation stuff is example, so there might be typos or bugs in it. I was wondering if the idea behind it is appropriate:
var unloadersOnShutdown = [];
var unloadersOnClose = [];
function startup() {
let DOMWindows = Services.wm.getEnumerator(null);
while (DOMWindows.hasMoreElements()) {
let aDOMWindow = DOMWindows.getNext();
var worker = new winWorker(aDOMWindow);
unloadersOnShutdown.push({DOMWindow: aDOMWindow, fn: worker.destroy});
}
}
function shutdown() {
Array.forEach.call(unloadersOnShutdown, function(obj) {
//should probably test if obj.DOMWindow exists/is open, but just put it in try-ctach
try {
obj.fn();
} catch(ex) {
//window was probably closed
console.warn('on shutdown unlaoder:', ex);
}
});
}
function winWorker(aDOMWindow) {
this.DOMWindow = aDOMWindow;
this.init();
}
winWorker.prototype = {
init: function() {
this.gMutationObserver = new this.DOMWindow.MutationObserver(gMutationFunc.bind(this));
this.myElement = this.DOMWindow.querySelector('#myXulEl');
this.gMutationObserver.observe(this.myElement, gMutationConfig);
if (this.DOMWindow.gBrowser && this.DOMWindow.gBrowser.tabContainer) {
this.onTabSelectBinded = this.onTabSelect.bind(this);
this.gBrowser.tabContainer.addEventListener('TabSelect', this.onTabSelectBinded, false);
}
},
destroy: function() {
this.gMutationObserver.disconnect();
if (this.onTabSelectBinded) {
this.gBrowser.tabContainer.removeEventListener('TabSelect', this.onTabSelectBinded, false);
}
},
onTabSelect: function() {
console.log('tab selected = ', thisDOMWindow.gBrowser.selectedTab);
}
};
var windowListener = {
onOpenWindow: function (aXULWindow) {},
onCloseWindow: function (aXULWindow) {
var DOMWindow = aXULWindow.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowInternal || Ci.nsIDOMWindow);
for (var i=0; i<unloadersOnClose.length; i++) {
if (unloadersOnClose.DOMWindow == DOMWindow) {
try {
unloadersOnClose.fn();
} catch(ex) {
console.warn('on close unloader:', ex);
}
unloadersOnClose.splice(i, 1);
i--;
}
}
},
onWindowTitleChange: function (aXULWindow, aNewTitle) {},
}
I think one problem is me not using weak references with DOMWindows but I'm not sure.

Answer 1

The idea around unloaders in general seems to be OK, but very limited (to windows only).
The implementation is lacking. E.g. there is a big, fat bug:
unloadersOnShutdown.push({DOMWindow: aDOMWindow, fn: worker.destroy});
// and
obj.fn();
// or
unloadersOnClose.fn();
This will call winWorker.prototype.destroy with the wrong this.
The i++/i-- loop also looks, um... "interesting"?!
Also, keep in mind that there can be subtle leaks, so you should mind and test for Zombie compartments.
Not only can a window leak parts of your add-on (e.g. bootstrap.js) but it is also possible to leak closed windows by keeping references in your add-on. And of course, it's not just windows you need to care about, but also e.g. observers, other types of (XPCOM) listeners etc.