Input Method Framework Overview

Contents

  1. Introduction
  2. Goals
  3. Input Method Support in Other Frameworks
  4. References

1. Introduction

Input methods are software components that interpret user operations such as typing keys, speaking, or writing using a pen device to generate text input for applications. The most common input methods are the ones that let users type text in Chinese, Japanese, or Korean, languages that use thousands of different characters, on a regular-sized keyboard. The text is typed in a form that can be handled by regular-sized keyboards, for example, in a romanized form, and then converted into the form that's really intended. Typically a sequence of several characters needs to be typed and then converted in one chunk, and conversion may have to be retried because there may be several possible translations. Similarly, for hand-writing recognition, the user may write a series of characters, convert them, and select the correct text from several possible conversion results. This process is called composition, and the text that the input method is working on is called composed text. It ends when the user confirms the final conversion result and the text is committed.

During composition, the composed text logically belongs to the input method, but still needs to be displayed to the user. Input method frameworks cooperate with input methods to provide at least two ways to do this: They enable modern text editing components to display the text in the context of the document that it will eventually belong to, albeit in a style that indicates that the text still needs to be converted or confirmed by the input method. This is called on-the-spot style input. And they provide a separate window to display the text as a fall-back if a not-so-modern text editing component can't deal with the text until it's confirmed. We call this root-window style input.

The input method framework enables the collaboration between text editing components and input methods in entering text. Its input method client API provides interfaces and classes that enable text editing components to communicate with input methods and implement a well-integrated text input user interface in the on-the-spot and below-the-spot styles; it also provides root-window style input as a fallback. Its service provider interface (SPI) for input method engines provides interfaces that enable the development of input methods in the Java programming language that can be used with any Java runtime environment; it also supports native input methods of the host platform.


2. Goals

"Write Once, Run Anywhere in the World" for Text Components

Text editing components that use the input method framework run on any Java application environment and support any text input methods available on that Java application environment without modifying or recompiling the text editing component. The framework isolates text editing components from differences in platforms and input method engines. It provides only one programming model for interacting with input methods. This lets developers write to a single API, and lets users choose whatever input method they prefer. When using native input methods, the framework maps to the platform input method APIs.

Although engine independence will be important to most developers, some high-end developers may want access to engine specific features. The framework allows this access.

"Write Once, Run Anywhere in the World" for Input Methods

Input methods written in the Java programming language and using the input method engine SPI can be installed into any Java application environment and support any text component running in that Java application environment. The framework isolates input methods from differences in platforms and text editing components. It provides only one programming model for interacting with text components. This lets developers write to a single SPI, and lets users choose whatever text editing component they prefer.

Language Independence

The input method framework is language independent in order to satisfy the needs of fully international applications. Although input methods are generally used for entering East Asian text, they may be useful for other languages as well. For example, a transliteration input method might be developed to allow Greek to be entered using Latin characters.

The framework handles input methods for different languages at the same time for truly multilingual text editing. While the choice of native input methods may be limited by the host operating system, any input method written in the Java programming language and using the input method engine SPI can be used at any time. Text created by an input method can have language attributes so that applications can perform language sensitive operations on the text.

Input Device Independence

The input method framework is designed to accommodate many possible input devices. Although keyboards are the predominant way of entering text today, other devices are gaining importance. For some platforms, pen-based input is the only way to enter text. Also, voice input is becoming more and more popular. The input method engine SPI opens the way to support these alternate devices.

Multiple Service Levels

Different programs have different needs for input method support. Tight integration with input methods provides the best user interface, but requires some additional programming. Some developers may not find that additional work worthwhile, but their applications still need to be able to receive East Asian text input. A few programs, such as games, want direct low-level keyboard input without interference from an input method. The framework lets programs choose one of the following levels of text input support.

Integrated Text Input User Interface

Modern programs intended to be used with East Asian languages usually provide a fully integrated user interface for text input operations and show the text being composed embedded in the document text. While converting text, the current candidate is highlighted, and when the user chooses a different candidate, the previous candidate text is replaced with the new one. This user interface support for input methods is known as on-the-spot style (or inline style). An alternative that Chinese-speaking users often prefer is the below-the-spot style, where composed text is shown in a separate composition window that is automatically positioned close to the insertion point where text will be inserted after being committed. The input method client API lets text editing components implement these integrated text input user interfaces.

Integration does not only mean displaying the composed text in the context of the target document. It also means that the text editing component understands "input method events", which let the input method communicate more information about the text than simple key events (for example, grammatical information), and that it can responds to requests for information from the input method that let the input method improve its functionality (for example, its accuracy).

Non-Integrated Text Input User Interface

For programs that do not want to deal with the user interface for text input operations, only final input text is sent to the application. In this case, the framework provides a user interface for input operations in a separate composition window outside the application. This is known as root-window style. It is less convenient because the user either has to manually position the window near the insertion point or to move his/her eyes between the composition window and the application window.

In this case, the application receives the input text as a sequence of key events. Therefore, there is no way to receive any information associated with the input text, such as grammatical information. Also, surrogate pairs (as defined in the Unicode Standard, version 2.0) are received as two separate key events.

No Input Method Support

Some applications, such as game software, may need only raw key input and no input method support. The framework provides a way to explicitly disable input method support.

Building Block Reuse

An important theme of object-oriented programming is reuse of program building blocks. The same building block may be used in many different applications, and several times within the same application. To make it easy to combine different text editing building blocks of possibly different capabilities, the input method framework interacts with each text editing component directly and individually.

As a consequence, many applications will not interact with the input method framework directly at all. Instead, they will use ready-made text editing components, such as the Swing text components, which will handle the interaction for them.

Close Integration with Other Frameworks

The input method framework is designed as an integral part of the Java platform. Interfaces that are necessary to exchange text between input methods and text components are designed to also support communication between other text-related frameworks, such as Java 2D and the Swing text components. Support for input method highlight drawing is integrated into Java 2D, so that text components can treat composed text just like any other text and the highlights just like any other style. Java 2D interacts with the input method framework to determine appropriate visual styles for input method highlights depending on the platform. The Swing text components use the input method framework to implement the best possible user interface with minimal additional programming.


3. Input Method Support in Other Frameworks

This chapter summarizes functionality in other Java platform frameworks that support input methods.

Text

The AttributedCharacterIterator interface provides a standard way of communicating text information with attributes between frameworks. It lets text readers access text without having to know how the text is stored in the information provider. Attribute information may include font and style attributes as well as language tags and grammatical annotations.

Abstract Window Toolkit

The Window class creates an initial input context for the window, and disposes this input context for the window when the window is disposed. Explicitly disposing input contexts allows to free up the (often substantial) resources allocated by native input methods.

The Component class has methods to handle input contexts and input method request handlers. Newly created Component instances other than windows initially share the input context of their containing window. The event handling in the Component class redirects incoming events to the input context associated with the component, and only passes them on to the component's listeners if they have not been consumed by an input method. Focus changes are communicated to the input context so it can activate or deactivate input methods.

The AWT classes that have knowledge of event or listener classes handle the InputMethodEvent and InputMethodListener classes.

The Graphics class defines a drawString method that accepts AttributedCharacterIterator instances as input. Instances of the TextLayout class can be constructed from AttributedCharacterIterator instances and used to draw text with input method highlights. Both ways of drawing text recognize input method highlights as attributes of the text and render them.

Swing

The Swing text components are by default active clients of the input method framework. That means, applications using these text components will use on-the-spot or below-the-spot style input by default. Application developers are still responsible for ending input operations when some other operation is started that requires the text to be committed. They may also use the input method framework's methods to create a private input context, to select an input method, or to set an expected character subset.


4. References

Information about the implementation of input methods on different platforms can be found in the following literature:


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