Dev : Using preferences
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This article is about using Mozilla Preferences system. Information here applies to the Mozilla Suite, Firefox, Thunderbird, and possibly other Mozilla-based applications. Intended audience is Mozilla extension developers who wish to learn details about using preferences in Mozilla.
If you haven't yet, read other documents about Mozilla preferences on XUL Planet and on mozilla.org (links below in Resources section).
Note: This article doesn't cover all available methods for manipulating preferences, please refer to XUL Planet XPCOM reference pages listed in Resources section for the complete list of methods. The interfaces dealing with preferences are fairly well documented, so using the methods not mentioned here should be straightforward.
XPCOM interfaces for preferences system
Mozilla exposes its preferences system through a few XPCOM interfaces. Look in the Resources section below for the link to list of preferences-related interfaces.
Three three used interfaces are nsIPrefService
, nsIPrefBranch
and nsIPrefBranch2
. They are frozen and will not change.
It's worth noting that there also is an nsIPref
interface. Despite it being used in some places, it is deprecated and should not be used.
Preferences service is instantiated exactly as any other service. (See Creating XPCOM Components document at XUL Planet for details.) To get an nsIPrefBranch
, either QI the pref service (that will give you the root branch) or call nsIPrefService.getBranch()
to get a sub-branch.
Here are two examples:
// Get the root branch var prefs = Components.classes["@mozilla.org/preferences-service;1"]. getService(Components.interfaces.nsIPrefBranch);
// Get the "extensions.myext." branch var prefs = Components.classes["@mozilla.org/preferences-service;1"]. getService(Components.interfaces.nsIPrefService); prefs = prefs.getBranch("extensions.myext.");
Simple types
There are three types of preferences: string, integer and boolean. Each entry in preferences database (prefs.js) has one of those types. There are six methods of nsIPrefBranch
that read and write them: getBoolPref
, setBoolPref
, getCharPref
, setCharPref
, getIntPref
and setIntPref
. Using them is as easy as:
// prefs is an nsIPrefBranch. // Look in the above section for examples of getting one. var value = prefs.getBoolPref("accessibility.typeaheadfind"); // get a pref prefs.setBoolPref("accessibility.typeaheadfind", !value); // set a pref
Complex types
As noted in previous section, each entry in prefs database (prefs.js) must have a string, an integer or a boolean value. However, there is a concept of complex types, which makes it easier for developers to save and load nsILocalFile
and nsISupportsString
objects in preferences (as strings — note that from the preferences system's POV, complex values have a nsIPrefBranch.PREF_STRING
type.)
There are two nsIPrefBranch
methods implementing the concept — setComplexValue
and getComplexValue
. You can look up their implementation in nsPrefBranch.cpp. Here is the IDL definition:
void getComplexValue(in string aPrefName, in nsIIDRef aType, [iid_is(aType), retval] out nsQIResult aValue); void setComplexValue(in string aPrefName, in nsIIDRef aType, in nsISupports aValue);
As you can see, both of them take aType
parameter. It can have the following values (to be precise, you should pass Components.interfaces.nsIWhatever
instead of just nsIWhatever
, which is undefined):
nsISupportsString
— used to handle unicode strings in preferences. Use this when the preference value may contain non-ASCII characters (for example, user's name).nsIPrefLocalizedString
— almost same asnsISupportsString
, but it is handled differently ingetComplexValue
when there's no user value for given preference, see below for details.nsILocalFile
andnsIRelativeFilePref
— store paths in preferences.nsILocalFile
is used to store absolute paths, whilensIRelativeFilePref
is used to store paths relative to one of “special” directories, like the profile folder.
nsISupportsString
As noted above, this is used to handle unicode strings in preferences. Example:
// prefs is an nsIPrefBranch // Example 1: getting unicode value var value = prefs.getComplexValue("preference.with.non.ascii.value", Components.interfaces.nsISupportsString).data; // Example 2: setting unicode value var str = Components.classes["@mozilla.org/supports-string;1"] .createInstance(Components.interfaces.nsISupportsString); str.data = "some non-ascii text"; prefs.setComplexValue("preference.with.non.ascii.value", Components.interfaces.nsISupportsString, str);
nsIPrefLocalizedString
Another complex type supported by Mozilla is nsIPrefLocalizedString
. It is similar to nsISupportsString
, the only difference is that when there is no user value, getComplexValue
gets the default value from a locale file (thus making the default value localizable).
It's easier to explain this on example. Let's say you want to make the default value for extensions.myext.welcomemessage
preference localizable. You should do the following:
- Add this line to some
.properties
file (for all of your locales), say tochrome://myext/locale/defaults.properties
:extensions.myext.welcomemessage=Localized default value
- Add the default value for
extensions.myext.welcomemessage
, pointing to that properties file, by adding the following line to your file with default preferences (see below).pref("extensions.myext.welcomemessage", "chrome://myext/locale/defaults.properties");
- Read the preference with
getComplexValue
, passingnsIPrefLocalizedString
asaType
:var prefs = Components.classes["@mozilla.org/preferences-service;1"]. getService(Components.interfaces.nsIPrefService); var branch = prefs.getBranch("extensions.myext."); var value = branch.getComplexValue("welcomemessage", Components.interfaces.nsIPrefLocalizedString).data;
The code in step 3 will read the default value from chrome://myext/locale/defaults.properties
when no user value is set, and will behave exactly the same as if nsISupportsString
was passed as aType
otherwise.
Setting nsIPrefLocalizedString
preferences is similar to setting nsISupportsString
:
var pls = Components.classes["@mozilla.org/pref-localizedstring;1"] .createInstance(Components.interfaces.nsIPrefLocalizedString); pls.data = val; prefs.setComplexValue("preference.with.non.ascii.value", Components.interfaces.nsIPrefLocalizedString, pls);
nsILocalFile and nsIRelativeFilePref
Please see the File IO article for details on nsILocalFile
and nsIRelativeFilePref
.
Default preferences
Each preference may have up to two values — current and default. That means there are two “pref trees” — current and default, — and each of them may or may not have a value for preference in question.
You can see the list of preferences in about:config (where available). Preferences that have user value are bold, those that don't have a user value are printed in normal font.
You can get both trees using nsIPrefService.getBranch()
and nsIPrefService.getDefaultBranch()
functions. See below for details.
What effect do default preferences have on various get methods
When one of get methods of nsIPrefBranch
(assuming it's a branch of the tree with current values) is called, it does the following:
- Checks whether the “current” tree has a value for the pref and whether the pref is locked.
- If there's a value of correct type (f.e.
getBoolValue
expects a value of typensIPrefBranch.PREF_BOOL
), and the preference is not locked, the method returns that value. - If there's a value of incorrect type and the pref is not locked, an exception is thrown (
NS_ERROR_UNEXPECTED
) - If the preference is locked or if there is no value for that preference in “current” tree, the get method checks the default tree.
- If there's a value of expected type in the “default” tree, it is returned (with the only exception,
getComplexValue
withaType
parameter =nsIPrefLocalizedString
, described above) - Otherwise an exception is thrown (
NS_ERROR_UNEXPECTED
).
If the branch is from the “default” tree, the get method doesn't check the tree with current values at all.
(This is not exactly how it's coded in libpref
, but it's equivalent)
Where are the default values read from
- All Mozilla-based applications read
(application directory)/defaults/pref/*.js
. - In addition to that, recent versions of Toolkit applications (Firefox 1.0, Thunderbird 1.0 and the like; not Mozilla Suite) read extension defaults—usually
(profile folder)/extensions/(ID)/defaults/preferences/
These files use simple JS-like syntax. To add a default value for a preference, you should add a line like this to your default preferences file:
pref("extensions.infolister.hide_menu_item", false);
How to install extension's defaults files
For Mozilla, copy them to (appdir)/defaults/pref
in your install script. (See this page for a profile directory based solution).
For Firefox/Thunderbird, just put them in myext.xpi/defaults/preferences/
. They will be copied and registered with the prefs system automatically.
More about preferences "branches"
Preferences names consist of a few strings separated with dots, and related prefs usually share the same prefix. For example, most accessibility preferences in Mozilla start with "accessibility."
This means that all existing preferences can be imagined as if they were in a tree, like this:
+ | +-- accessibility | | | +-- typeaheadfind | | | | | +-- autostart (accessibility.typeaheadfind.autostart) | | | | | +-- enablesound (accessibility.typeaheadfind.enablesound) | | | +-- usebrailledisplay (accessibility.usebrailledisplay) | +-- extensions | +-- lastAppVersion (extensions.lastAppVersion)
This is the metaphor behind nsIPrefBranch
. However, you should be aware of the fact that Mozilla preferences system doesn't treat dots in a special way. For example this code will also read the value of accessibility.typeaheadfind.enablesound
preference:
var prefs = Components.classes["@mozilla.org/preferences-service;1"]. getService(Components.interfaces.nsIPrefService); var branch = prefs.getBranch("acce"); var enablesound = branch.getBoolPref("ssibility.typeaheadfind.enablesound");
This is the reason why you should usually pass strings ending with a dot to getBranch()
, like prefs.getBranch("accessibility.")
.
Another caveat you should be aware of is that nsIPrefBranch.getChildList("")
returns an array of preferences names that start with that branch's root
, for example
var branch = prefs.getBranch("accessibility."); var children = branch.getChildList("", {});
will return these items (for the example tree above): "typeaheadfind.autostart", "typeaheadfind.enablesound", "usebrailledisplay"
, -- not just direct children ("typeaheadfind"
and "usebrailledisplay"
), as you might have expected.
Using preferences observers
You can use nsIPrefBranchInternal
interface to “listen” to changes to preferences in a certain branch.
- Note: During Gecko 1.8 development,
nsIPrefBranchInternal
was renamed tonsIPrefBranch2
[1] and was frozen.nsIPrefBranchInternal
name is still supported in Gecko 1.8, so this is what you should use in extensions that need to be compatible with Gecko 1.7 and Gecko 1.8 (Firefox 1.0/1.5). For newer extensions usensIPrefBranch2
.
Here's an example:
var myPrefObserver = { register: function() { var prefService = Components.classes["@mozilla.org/preferences-service;1"] .getService(Components.interfaces.nsIPrefService); this._branch = prefService.getBranch("extensions.myextension."); this._branch.QueryInterface(Components.interfaces.nsIPrefBranch2); this._branch.addObserver("", this, false); }, unregister: function() { if(!this._branch) return; this._branch.removeObserver("", this); }, observe: function(aSubject, aTopic, aData) { if(aTopic != "nsPref:changed") return; // aSubject is the nsIPrefBranch we're observing (after appropriate QI) // aData is the name of the pref that's been changed (relative to aSubject) switch (aData) { case "pref1": // extensions.myextension.pref1 was changed break; case "pref2": // extensions.myextension.pref2 was changed break; } } } myPrefObserver.register();
nsIPrefBranch2.idl has more documentation.
Using prefHasUserValue
nsIPrefBranch.prefHasUserValue(preference)
checks whether the preference has been changed from the default value. If so, it will return true
, otherwise false
. In particular, when no default value exists for a preference, prefHasUserValue()
indicates whether a preference exists.
Attempting to read an inexistent preference without a default value via one of get*Pref
methods will throw an exception. Using prefHasUserValue()
lets you check if the preference exists before attempting to read it. For example:
if(prefs.prefHasUserValue("mypref")) { alert(prefs.getCharPref("mypref"); }
Note that the getCharPref()
call may throw even if the preference exists, for example if it has a different type.
Using preferences in extensions
If you're writing your extension for one of Toolkit applications (Firefox, Thunderbird, Nvu), it's recommended to provide default values for your extension's preferences (see above for information on how to do it). It has the following benefits:
- You don't have to duplicate default values in various parts of your code.
- The code reading preferences is simplified, you don't need to worry about get* methods throw an exception, because under normal circumstances they won't do it.
JavaScript wrappers for preferences system
There are a few JavaScript wrappers to make your life easier: see
this, this, and the nsPreferences
wrapper included with Firefox and Thunderbird (chrome://global/content/nsUserSettings.js).
Resources
- Other documentation on preferences
- A Brief Guide to Mozilla Preferences — describes preferences system from user's / administrator's POV.
- XUL Planet's article on preferences — an explanation of the preferences system with simple examples. A must read.
- Mozilla XPCOM interfaces of the preferences system.
- Complete list
- Most used interfaces (these are frozen and will not change):
nsIPrefBranch
andnsIPrefService
nsIPrefBranchInternal
interface (in Gecko 1.8 it is callednsIPrefBranch2
and is frozen).
- LXR pages for libpref, the module implementing preferences system.
- Javascript Preferences wrappers: one, two.