Finding the tab associated with a DOM window

问题:

I'm adding some new functionality in a Firefox extension, TryAgain, that traps HTTP error codes (e.g. 500) and automatically retries loading the page after some interval.

Trapping the codes works excellently, and I'm trying to tally the total number of retries and store this in the tab using Session Store. Unfortunately, right now I'm getting a reference to a DOM window (through interfaceRequestor.getInterface(Components.interfaces.nsIDOMWindow)), but I need a reference to a tab, which is a nsIDOMNode as per nsISessionStore docs on setTabValue().

What I have so far (I've snipped the actual retrying out of this example):

// This function implements the nsIObserverService interface and observes
// the status of all HTTP channels
observe : function(aSubject, aTopic, aData) {
    var httpChannel = aSubject
            .QueryInterface(Components.interfaces.nsIHttpChannel);
    if (httpChannel.responseStatus == 500) {
        var domWindow;
        try {
            var notificationCallbacks;
            if (httpChannel.notificationCallbacks) {
                notificationCallbacks = httpChannel.notificationCallbacks;
            } else {
                notificationCallbacks = aSubject.loadGroup
                        .notificationCallbacks;
            }
            var interfaceRequestor = notificationCallbacks
                    .QueryInterface(Components.interfaces
                        .nsIInterfaceRequestor);
            domWindow = interfaceRequestor
                    .getInterface(Components.interfaces.nsIDOMWindow);
        } catch (e) {
            // No window associated with this channel
            return;
        }
        var ss = Components.classes["@mozilla.org/browser/sessionstore;1"]
                .getService(Components.interfaces.nsISessionStore);
        ss.setTabValue(domWindow, "foo", "bar");
    }
},

This of course fails on setTabValue with an invalid parameter. How can I get the tab associated with the DOM window?

As an alternative solution, can I store variables associated with a DOM window in some way such that I don't have to clean up the memory myself?

回答1:

domWindow is a content window, first you need to get the chrome window containing it. That's done by this rather ugly code:

var chromeWindow = window.QueryInterface(Ci.nsIInterfaceRequestor)
                         .getInterface(Ci.nsIWebNavigation)
                         .QueryInterface(Ci.nsIDocShellTreeItem)
                         .rootTreeItem
                         .QueryInterface(Ci.nsIInterfaceRequestor)
                         .getInterface(Ci.nsIDOMWindow);

Then you want to ask the <tabbrowser> element about the tab (note that you should pass in domWindow.top because domWindow might not be the top frame):

var browser = chromeWindow.gBrowser.getBrowserForDocument(domWindow.top.document);

Note that this is the <browser> element, not the associated <tab> (for the latter you would need getBrowserIndexForDocument and then look at gBrowser.tabs[index]). But I think you simply want to store a property for this tab, not have this property persist across sessions? Then you can use an expando property:

if (!("_myExtensionErrorCount" in browser))
  browser._myExtensionErrorCount = 0;
browser._myExtensionErrorCount++;

Here _myExtensionErrorCount should be a name that is unique enough to avoid conflicts with other extensions that might want to use expando properties as well.

回答2:

The nsISessionStore APIs referencing nsIDOMNode aTab require you to get hold of the tab node storing the data. If you you are working in a browser.xul overlay and want the currently selected tab, it's as simple as gBrowser.selectedTab. If you just hold a reference to some content document in the overlay's chrome window, here is how you find its corresponding tab:

function tabFromDoc(doc) {
    var no = gBrowser.getBrowserIndexForDocument(doc);
    return gBrowser.tabContainer.childNodes[no];
}

// example use:
Cc['@mozilla.org/browser/sessionstore;1']
    .getService(Ci.nsISessionStore)
    .setTabValue(tabFromDoc(myContentDoc), 'myKey', 'myValue');