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A Requirements document for an Input Method Subsystem in Qt4

  1. A Requirements document for an Input Method Subsystem in Qt4

Intro

This document outlines a set of functional requirements for an IM system that we would like to see supported in Qt4. Where relevant, this document also points out current design and implementation issues, and suggests some solutions.

User Requirements

  • Input method modules should be configurable and switchable at runtime.
  • A user should be able to quickly find an input method for a particular language
  • A simple configuration should not require any 'magic' environment variables. Things should 'just work', and yet still be configurable if needed.
  • User should be able to perform configuration through a simple GUI
    • Automated configuration through script/config file should also be possible
  • pre-edit buffer indication should be clear and salient
    • the pre-edit string should have some different rendering attributes (such as underline) from its surrounding text
    • a candidate window should be displayed next to the current segment of the pre-edit being converted, not just simply at the beginning of the pre-edit buffer
  • IM-specific keyboard shortcuts should be accessible, and should not be overriden by the application shortcuts (current GTK2 problem)

System Integrator Requirements

There should be a mechanism to limit the IM systems available. For example, a distributor may wish to limit support to the IIIMF framework.

The reason for this requirement is that there is no global switching framework for the linux desktop, and different input method systems attempt to provide switching in different ways. For example, IIIMF adds a level of indirection by running a user-privileged IM server. Desktop-level switching is accomplished by creating an applet that communicates directly with the server instance which is unique for each desktop instance.

A library such as UIM on the other hand, would require an applet to communicate with each application directly to change the current input method. Because there is no trivial way to unify all these switching mechanisms, the distributor may be limited to forcing only one mechanism, by supporting only one input method library.

In the GTK2 case, this amounts to setting the GTK_DEFAULT_IMMODULE variable as well as disabling the right-click input method selection sub-menu, in order to avoid confusing the user with an inconsistent set of choices. (GTK immodule may support more methods than IIIMF, therefore the menu and a menu provided by an IIIMF only applet may differ)

Until the situation stabilizes, Qt will need an equivalent mechanism to allow distributors to provide a consistent set of choices across various applications.

API Requirements / Issues

Changes to QIMEvent

During the experience of actually writing input system modules, we have discovered that the current structure of IMStart, IMCompose, and IMEnd does not map well to many IM's and thus is an easy source for bugs.

More details later..

Error handling

Semantics and API for error handling need to be carefully defined.

Candidate window positioning

The setMicroFocusHint() type method needs to be well defined and possible extended. Some widgets may like to exert more control over how the candidate window (or any other input method-controlled window) is displayed.

Multiple input contexts and widget to input context mapping

In many cases, it is useful to have several input context instances within the same application associated to different widgets. Some simple examples:

  • 1 chat applications: In an instant messenger, a user would likely talk to different users in different languages. Switching the IM language back and forth manually every time the user talks to a different person is cumbersome and error-prone. The application should be able to remember which input context for which conversation. In general, if an application has multiple ``language contexts_, then it is often if not always desireable to have an separate input context associated with each language context.

event handling and key compression

The interaction between the event handling mechanism and the input method need to be clearly defined. It appears that the simplest method for doing this is to give the input method the option to filter a key event before anything else sees it. The interaction between the input method and Qt's key compression features also needs to be defined.

language identification of input methods

An application should be able to query the language of each input method. semantics need to be defined for those input methods that can handle multiple languages.

language tagging for inputted text

With han unification in unicode, it is often required to know the language of a string of characters to be able to choose the right font. The language can be determined from the method, as usually an input method is designed to input for one specific language. It would be useful if QIMEvent could be modified so that along with the text, the language of the text be carried along as well, so the client widget can act accordingly. Another approach is to add a language() query function to the QInputContext so that the "current" language can be identified, and the text recieving widget can handle the information accordingly.

Surrounding text

It can be extremely useful for some input methods to obtain text that is immediately before or after the current input positions. For example, in Japanese, this feature can be used to recognize particles and auxilary verbs that are right before or after the cursor position, allowing the system to narrow down conjugation choices by applying the appropriate grammatical rules.

Pluggable Popup Context Menu

This allows input methods to provide integrated direct UI controls in QT's text widgets.

Default Input Method Specification

qtrc, environment variable, etc.

QT_IM_SWITCHER idea proposed by YAMAKEN: 

In our current implementation, there is a usability problem. We
can configure default IM plugin using QT''IM''MODULE env var (or
the equivalent in qtrc), but there is no way to control default
IM on QMultiInputContext because it is an ordinary plugin. The
user interface is not useful.

application    <- QT''IM''MODULE="multi"
  |
QMultiInputContext
  |
QFooInputContext


To resolve such problem, we can introduce additional
configuration variable for QMultiInputContext. The Qt itself is
not aware of QT''IM''MODULE''FOR''SWITCHER variable.

application    <- QT''IM''MODULE="multi"
  |
QMultiInputContext <- QT''IM''MODULE''FOR''SWITCHER="bar"
  |
QBarInputContext


Although the problem itself can be resolved as above,
unnecessary confusion is also introduced by the solution. Most
users will configure QT''IM''MODULE to choose their own favorite
IM because they always configure so in GTK+ environment. They
will be confused if the configuration QT''IM''MODULE="bar" makes
IM-switching menu vanish.

application    <- QT''IM''MODULE="bar"
  |
QBarInputContext


To avoid the confusion, I propose following naming
convention. The name QT''IM''SWITCHER is just a trick for users to
form appropriate mental model. Qt itself is not treat
IM-switchers specially. It is still ordinary IM plugin.

application    <- QT''IM''SWITCHER="multi" (default)
  |
QMultiInputContext <- QT''IM''MODULE="bar"
  |
QBarInputContext


The convention is upward compatible with GTK+. In GTK+, the
equivalent of QMultiInputContext is hardcoded as default and not
replaceable. But IM-switcher should be replaceable as I said
recently, so I propose the new convention.

application   <- gtk''im''multicontext_new()
  |
GtkIMMulticontext       <- GTK''IM''MODULE="bar"
         |
GtkIMBarContext


To accomplish the new convention, I've renamed the identifier
name of QMultiInputContext "multi" as "imsw-multi". The "imsw-"
prefix prevents IM-switchers from being listed in popup menu as
input method. All other IM-switcher implementation is also
expected to follow this "imsw-" convention. I think that it
should not be API such as QInputContextPlugin::isIMSwitcher()
because Qt API should be isolated from IM-switcher issues.

If a system integrator such as RedHat want to make a specific
IM-framework the switcher, the patch offers following
configuration.

application                      <- QT''IM''SWITCHER="iiimqcf"
         |
IIIMInputContext
         |
(IIIMF's own switching framework)

-- Main.?KenDeeter - 27 Jun 2004