nsIPrefBranch

This interface is used to manipulate the preferences data. This object may be obtained from the preferences service (nsIPrefService) and used to get and set default and/or user preferences across the application.
Inherits from: nsISupports Last changed in Gecko 13 (Firefox 13 / Thunderbird 13 / SeaMonkey 2.10)

This object is created with a "root" value which describes the base point in the preferences "tree" from which this "branch" stems. Preferences are accessed off of this root by using just the final portion of the preference. For example, if this object is created with the root "browser.startup.", the preferences "browser.startup.page", "browser.startup.homepage", and "browser.startup.homepage_override" can be accessed by simply passing "page", "homepage", or "homepage_override" to the various Get/Set methods.

Method overview

void addObserver(in string aDomain, in nsIObserver aObserver, in boolean aHoldWeak);
void clearUserPref(in string aPrefName);
void deleteBranch(in string aStartingAt);
boolean getBoolPref(in string aPrefName);
string getCharPref(in string aPrefName);
void getChildList(in string aStartingAt, [optional] out unsigned long aCount, [array, size_is(aCount), retval] out string aChildArray);
void getComplexValue(in string aPrefName, in nsIIDRef aType, [iid_is(aType), retval] out nsQIResult aValue);
long getIntPref(in string aPrefName);
long getPrefType(in string aPrefName);
void lockPref(in string aPrefName);
boolean prefHasUserValue(in string aPrefName);
boolean prefIsLocked(in string aPrefName);
void removeObserver(in string aDomain, in nsIObserver aObserver);
void resetBranch(in string aStartingAt);
void setBoolPref(in string aPrefName, in long aValue);
void setCharPref(in string aPrefName, in string aValue);
void setComplexValue(in string aPrefName, in nsIIDRef aType, in nsISupports aValue);
void setIntPref(in string aPrefName, in long aValue);
void unlockPref(in string aPrefName);

Attributes

Attribute Type Description
root string Called to get the root on which this branch is based, such as "browser.startup." Read only.

Constants

Constant Value Description
PREF_INVALID 0 long
PREF_STRING 32 long data type.
PREF_INT 64 long data type.
PREF_BOOL 128 long data type.

Methods

Requires Gecko 13 (Firefox 13 / Thunderbird 13 / SeaMonkey 2.10)

addObserver()

Adds a preference change observer. On preference changes, the following arguments will be passed to nsIObserver.observe():

aSubject - The nsIPrefBranch object (this).

aTopic - The string defined by NS_PREFBRANCH_PREFCHANGE_TOPIC_ID

aData - The name of the preference which has changed, relative to the "root" of the aSubject branch.

aSubject.get*Pref(aData) will get the new value of the modified preference. For example, if your observer is registered with addObserver("bar.", ...) on a branch with root "foo.", modifying the preference "foo.bar.baz" will trigger the observer, and aData parameter will be "bar.baz".

void addObserver(
  in string aDomain, 
  in nsIObserver aObserver, 
  in boolean aHoldWeak
);
Parameters
aDomain
The preference on which to listen for changes. This can be the name of an entire branch to observe. For example holding the "root" prefbranch and calling addObserver("foo.bar.", ...) will observe changes to foo.bar.baz and foo.bar.bzip.
aObserver
The object to be notified if the preference changes.
aHoldWeak
true holds a weak reference to aObserver. The object must implement the nsISupportsWeakReference interface or this will fail. false holds a strong reference to aObserver.

clearUserPref()

Called to clear a user set value from a specific preference. This will, in effect, reset the value to the default value. If no default value exists the preference will cease to exist.

Note: This method does nothing if the prefbranch it is called on is a default branch.

void clearUserPref(
  in string aPrefName
);
Parameters
aPrefName
The preference to be cleared.
Remarks

Note: Prior to Gecko 6.0, this method would throw an exception if there was no user value set for the specified preference. Now, this method never throws. Instead, it simply does nothing.

deleteBranch()

Called to remove all of the preferences referenced by this branch.

Note: This method can be called on either a default or user branch but, in effect, always operates on both.

void deleteBranch(
  in string aStartingAt
);
Parameters
aStartingAt
The point on the branch at which to start the deleting preferences. Pass in "" to remove all preferences referenced by this branch.

getBoolPref()

Called to get the state of an individual boolean preference.

boolean getBoolPref(
  in string aPrefName
);
Parameters
aPrefName
The boolean preference to get the state of.
Return value

The value of the requested boolean preference.

getCharPref()

Called to get the state of an individual string preference.

string getCharPref(
  in string aPrefName
);
Parameters
aPrefName
The string preference to retrieve.
Return value

Returns string - The value of the requested string preference.

getChildList()

Returns an array of strings representing the child preferences of the root of this branch.

Note: This method can be called on either a default or user branch but, in effect, always operates on both.

(To call from javascript use children = nsIPrefBranch.getChildList("",obj), which will fill in obj.value with the count and return an array of keys! (It is not void in javascript)

void getChildList(
  in string aStartingAt,
  out unsigned long aCount,
  [array, size_is(aCount), retval] out string aChildArray
);
Parameters
aStartingAt
The point on the branch at which to start enumerating the child preferences. Pass in "" to enumerate all preferences referenced by this branch.
aCount Optional from Gecko 2.0
Receives the number of elements in the array.
aChildArray
Receives the array of child preferences.

getComplexValue()

Called to get the state of an individual complex preference. A complex preference is a preference which represents an XPCOM object that can not be easily represented using a standard boolean, integer or string value.

void getComplexValue(
  in string aPrefName,
  in nsIIDRef aType,
  [iid_is(aType), retval] out nsQIResult aValue
);
Parameters
aPrefName
The complex preference to get the value of.
aType
The XPCOM interface that this complex preference represents. Interfaces currently supported are:
aValue
The XPCOM object into which to the complex preference value should be retrieved.

getIntPref()

Called to get the state of an individual integer preference.

long getIntPref(
  in string aPrefName
);
Parameters
aPrefName
The integer preference to get the value of.
Return value

Returns long - The value of the requested integer preference.

getPrefType()

Called to determine the type of a specific preference.

long getPrefType(
  in string aPrefName
);
Parameters
aPrefName
The preference to get the type of.
Return value

Returns long - A value representing the type of the preference. This value will be PREF_STRING, PREF_INT, PREF_BOOL, or PREF_INVALID.

lockPref()

Called to lock a specific preference. Locking a preference will cause the preference service to always return the default value regardless of whether there is a user set value or not.

Note: This method can be called on either a default or user branch but, in effect, always operates on the default branch.

void lockPref(
  in string aPrefName
);
Parameters
aPrefName
The preference to be locked.

prefHasUserValue()

Called to check if a specific preference has a user value associated to it.

Note: This method can be called on either a default or user branch but, in effect, always operates on the user branch.

Note: If a preference was manually set to a value that equals the default value, then the preference no longer has a user set value, i.e. it is considered reset to its default value. In particular, this method will return false for such a preference and the preference will not be saved to a file by nsIPrefService.savePrefFile().

boolean prefHasUserValue(
  in string aPrefName
);
Parameters
aPrefName
The preference to be tested.
Return value

Returns boolean - true The preference has a user set value. false The preference only has a default value.

prefIsLocked()

Called to check if a specific preference is locked. If a preference is locked calling its Get method will always return the default value.

Note: This method can be called on either a default or user branch but, in effect, always operates on the default branch.

boolean prefIsLocked(
  in string aPrefName
);
Parameters
aPrefName
The preference to be tested.
Return value

Returns boolean - true The preference is locked. false The preference is not locked.

Requires Gecko 13 (Firefox 13 / Thunderbird 13 / SeaMonkey 2.10)

removeObserver()

Remove a preference change observer.

Note: You must call removeObserver method on the same nsIPrefBranch instance on which you called addObserver method in order to remove aObserver; otherwise, the observer will not be removed.

  void removeObserver(
     in string aDomain, 
     in nsIObserver aObserver
  );
Parameters
aDomain
The preference which is being observed for changes.
aObserver
An observer previously registered with addObserver.

resetBranch()

Called to reset all of the preferences referenced by this branch to their default values.

Note: This method can be called on either a default or user branch but, in effect, always operates on the user branch.

Note: As of Firefox 3.0, this function has not yet been implemented.

void resetBranch(
  in string aStartingAt
);
Parameters
aStartingAt
The point on the branch at which to start the resetting preferences to their default values. Pass in "" to reset all preferences referenced by this branch.

setBoolPref()

Called to set the state of an individual boolean preference.

void setBoolPref(
  in string aPrefName,
  in long aValue
);
Parameters
aPrefName
The boolean preference to set the state of.
aValue
The boolean value to set the preference to.

setCharPref()

Called to set the state of an individual string preference.

Note: The preferences system is not designed to store large amounts of data: all preferences are stored in a single file, which is read at the application startup. If you find yourself wondering what is the maximum amount of data you can store in a string preference, consider storing data separately, for example in a flat file or an sqlite database.
void setCharPref(
  in string aPrefName,
  in string aValue
);
Parameters
aPrefName
The string preference to set.
aValue
The string value to set the preference to.

setComplexValue()

Called to set the state of an individual complex preference. A complex preference is a preference which represents an XPCOM object that can not be easily represented using a standard boolean, integer or string value.

void setComplexValue(
  in string aPrefName,
  in nsIIDRef aType,
  in nsISupports aValue
);
Parameters
aPrefName
The complex preference to set the value of.
aType
The XPCOM interface that this complex preference represents. Interfaces currently supported are:
aValue
The XPCOM object from which to set the complex preference value.

setIntPref()

Called to set the state of an individual integer preference.

void setIntPref(
  in string aPrefName,
  in long aValue
);
Parameters
aPrefName
The integer preference to set the value of.
aValue
The integer value to set the preference to.

unlockPref()

Called to unlock a specific preference. Unlocking a previously locked preference allows the preference service to once again return the user set value of the preference.

Note: This method can be called on either a default or user branch but, in effect, always operates on the default branch.

void unlockPref(
  in string aPrefName
);
Parameters
aPrefName
The preference to be unlocked.

Remarks

Registering as a preference observer can open an object to potential cyclical references which will cause memory leaks. These cycles generally occur because an object both registers itself as an observer (causing the branch to hold a reference to the observer) and holds a reference to the branch object for the purpose of getting/setting preference values. There are 3 approaches which have been implemented in an attempt to avoid these situations:

  1. The nsPrefBranch object supports nsISupportsWeakReference. Any consumer may hold a weak reference to it instead of a strong one.
  2. The nsPrefBranch object listens for xpcom-shutdown and frees all of the objects currently in its observer list. This ensures that long lived objects (services for example) will be freed correctly.
  3. The observer can request to be held as a weak reference when it is registered. This insures that shorter lived objects (say one tied to an open window) will not fall into the cyclical reference trap.

The list of registered observers may be changed during the dispatch of nsPref:changed notification. However, the observers are not guaranteed to be notified in any particular order, so you can't be sure whether the added/removed observer will be called during the notification when it is added/removed. 

It is possible to change preferences during the notification.

It is not safe to change observers during this callback in releases before Gecko 1.9. If you want a safe way to remove a preference observer, please use an nsITimer.

See also

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Last updated by: Sheppy,