nsIPrefBranchInternal.addObserver requires the entire pref name




Preferences: Backend
14 years ago
13 years ago


(Reporter: Heath Stewart, Unassigned)



Firefox Tracking Flags

(Not tracked)



(1 attachment)



14 years ago
User-Agent:       Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.5) Gecko/20041107 Firefox/1.0
Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.5) Gecko/20041107 Firefox/1.0

When getting or setting preferences you can request a certain branch using
nsIPrefService.getBranch and request a certain "tree" by which all access to
preferences are scoped from that "tree". When using
nsIPrefBranchInternal.addObserver with such a scope (having passed something
other than nsnull or an empty string to getBranch) the observer is not notified
(i.e., observe is not called).

Reproducible: Always
Steps to Reproduce:
1. Request a particular branch of preferences:
const prefService = Components.classes["@mozilla.org/preferences-service;1"]

2. QI for the nsIPrefBranchInternal interface.
const branch = prefService.QueryInterface(

3. Add an observer for a preference under the "browser." tree.
branch.addObserver("active_color", prefObserver, false);

4. Define a prefObserver object with the observe method to handle the
notification and simply alert the user.
var prefObserver = 
  observe: function(subject, topic, data)
    alert(topic + ": " + data);
Actual Results:  
Nothing happened. The alert window is never displayed.

Expected Results:  
The alert window should be displayed with the correct topic and data.

The workaround is to not request a particular branch, passing either null or an
empty string. Call addObserver with the entire preference name as the first
parameter and the observer is notified.

Comment 1

14 years ago
Created attachment 167140 [details]
Example XUL file with documented problem

Install this into chrome. I personally keep an "example" directory with its
"content" sub-directory set as the base path for an "example" package
registered with "chrome.rdf" in the "chrome" directory of my user profile.

This displays two labels, one watching for a change as documented by this bug,
the other watching for a chance to an entire preference name.
The prefbranch has a root string set. This makes it not a simple copy of the
prefservice, but a new thing. If there are no pointers to this new thing, it
goes away. And if it is away, there is nothing that has the observers registered.
So the other workaround is to keep a global pointer to the prefbranch.
The summary is incorrect, since the prefbranch does resolve the pref name
correctly; see

So the only issue is that there is that the prefbranch goes away here, so can't
really notify any observers.
This is an automated message, with ID "auto-resolve01".

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Last Resolved: 13 years ago
Resolution: --- → EXPIRED
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