This interface lets the application detect bidirectional writer users, and do some magic for them. A user is a bidirectional writer if they have keyboard layouts in both left-to-right and right-to-left directions (that is users who use Arabic, Iranian (Persian), or Israel (Hebrew) keyboard layout, beside an US (English) layout.)
Inherits from: nsISupports Last changed in Gecko 9.0 (Firefox 9.0 / Thunderbird 9.0 / SeaMonkey 2.6)

Method overview

boolean isLangRTL();
void setLangFromBidiLevel(in PRUint8 aLevel);


Attribute Type Description
haveBidiKeyboards boolean

Indicates whether or not the system has at least one keyboard for each direction (left-to-right and right-to-left) installed. Read only.

Note: This throws NS_ERROR_NOT_IMPLEMENTED if the widget layer doesn't provide this information.  Currently, this is only implemented on Windows.



Determines if the current keyboard language is right-to-left.

(supported on: win32, mac, gtk2)

Note: Prior to Gecko 1.9 this method used a parameter 'out PRBool aIsRTL' to return the value.

boolean isLangRTL();


Return value

true if the current keyboard is right-to-left, false if it is not.

Exceptions thrown
If no right-to-left keyboards are installed.


Sets the keyboard language to left-to-right or right-to-left.

(supported on: win32)

void setLangFromBidiLevel(
  in PRUint8 aLevel
If odd set the keyboard to right-to-left, if even set left-to-right.
Exceptions thrown
If no right-to-left keyboards are installed.




This implementation uses win32 API to get the language of the keyboard layout, and the direction of those languages.


This implementation depends on the Gecko and GTK+ version.

In the Gecko 1.8 branch, just isLangRTL() had been implemented, and uses GDK's API to check the direction of keyboard layout, which has a problem with Hebrew language. The problem is fixed on GTK+ version 2.10.1 and 2.12.

In the Gecko 1.9 branch, we are going to implement all the interface, with XKB functions for xlib and gtk2 backends. On the gtk2 backend, we will have GTK+ version checking, to use GTK+ implementations, if the problem doesn't exist. Some other part of the code is going to be on GTK+ later, and we will use GTK+ API instead.

Both GTK+ and Gecko versions, check keysym's on keyboard layout to detect the direction of the layout. For GTK+ (>=2.10.1, >=2.12.0) and all Gecko versions, we check only the first shift-level of the layout, to fix the problem with Hebrew layouts.

More info: bug 348724, bug 348724, and Gnome bug: 116626


Only isLangRTL is supported on this platform. bug 266551

See also

Document Tags and Contributors

 Last updated by: Sheppy,