Technical Guide

Overview

This document provides a technical overview of the Qt Virtual Keyboard plugin.

Basic Concepts

The Qt Virtual Keyboard project is a Qt 5 input context plugin which implements QPlatformInputContextPlugin and QPlatformInputContext interfaces. These interfaces allow the plugin to be used as a platform input context plugin in Qt 5 applications.

The plugin itself provides an input framework supporting multiple input methods as well as a QML UI for the virtual keyboard. The input framework is extensible through a plugin interface, which allows third-party input methods and keyboard layouts to be loaded at runtime.

The input framework provides the following main interfaces:

• QVirtualKeyboardInputContext: provides contextual information for the virtual keyboard and other input components. Acts as an interface to the underlying text input component.
• QVirtualKeyboardInputEngine: exposes an API to integrate user input events (key presses, etc.) and acts as a host for input methods.
• QVirtualKeyboardAbstractInputMethod: a base type for C++-based input methods. The input method usually handles key events, but can also handle mouse and touch input events.
• InputMethod: a base type for QML-based input methods. The input method usually handles key events, but can also handle mouse and touch input events.
• QVirtualKeyboardExtensionPlugin: a plugin interface for virtual keyboard extensions that provide additional functionality.

Input Context

The input context is used by the keyboard as well as concrete input methods. InputContext is a singleton instance hosted by QML. An application should not directly interact with the input context.

Contextual Information

The input context provides access to contextual information that originates from the application. This information includes, but is not limited to:

• InputContext::cursorPosition
• InputContext::cursorRectangle
• InputContext::inputMethodHints
• InputContext::preeditText
• InputContext::selectedText
• InputContext::surroundingText

Locale

The Virtual Keyboard Engine generates the list of supported locales from locale-specific layout directories in layouts/. Each layout directory must contain a definition or fallback for the following layout types: dialpad, digits, handwriting, main, numbers, and symbols. Definitions are implemented in .qml-files, fallbacks are defined by a placeholder file with the .fallback file extension. The layouts/ directory must contain a fallback/ sub-directory that contains definitions for each layout type.

Each layout directory may contain the definition of one or more layout types. If the locale-specific layout is the same as that of the fallback locale, you can add a placeholder file for the layout called <layout type>.fallback. This instructs the virtual keyboard to use the fallback layout instead.

For example: you may add a locale-specific layout for Finnish, that defines the main layout type in main.qml. For the other layout types, you opt for the fallback mechanism. Your layouts/ tree will look like this:

.
├── fallback
│   ├── dialpad.qml
│   ├── digits.qml
│   ├── handwriting.qml
│   ├── main.qml
│   ├── numbers.qml
│   └── symbols.qml
└── fi_FI
    ├── dialpad.fallback
    ├── digits.fallback
    ├── handwriting.fallback
    ├── main.qml
    ├── numbers.fallback
    └── symbols.fallback

It's imperative that the layouts/fallback directory always contain a set of full implementation files.

The application can specify the initial layout by changing the default locale. However, this must be done before the application initializes and loads the input method plugin. If there are no changes to the default locale, the current system locale is used.

Matching the keyboard locale follows this sequence:

• layouts/<language>_<country>
• layouts/<language>_*
• layouts/fallback -- the default layout here is en_GB.

First, the locale is matched against the full locale name. If a there isn't a full match, then only the locale language is matched. Finally, the contents of layouts/fallback is used as a fallback when there's also no partial match.

After the locale selection is done, the keyboard updates the input locale and input direction to match the current layout. The application can receive this information through the QInputMethod interface.

Internally, the current input locale is also updated to QVirtualKeyboardInputEngine and the current input method instances.

Input Engine

The input engine object is owned by InputContext. Like the InputContext, there is only one instance of the QVirtualKeyboardInputEngine. The input engine contains API functions which the keyboard uses to map user interactions, such as key press and release events, to the input method.

For example, virtual keyboard key events are mapped through the following methods:

• QVirtualKeyboardInputEngine::virtualKeyPress
• QVirtualKeyboardInputEngine::virtualKeyRelease

The above-mentioned methods are intended for the integration of the virtual keyboard, hence the word "virtual" in the methods' names. This also means that the methods are not suitable for mapping the physical keystrokes. This is a consequence of the fact that the actual action is performed only when the key is released.

If the key press is interrupted before the key release event, the keyboard invokes the QVirtualKeyboardInputEngine::virtualKeyCancel method.

Input Method

The input method is a concrete implementation of the keypress handler. It's main function is to handle keypress events and maintain state information for the user input. It interacts with the text editor through QVirtualKeyboardInputContext via pre-edit text or key events.

The input method instance can be created in various ways, depending on the use case:

• KeyboardLayout::inputMethod: the keyboard layout can create an input method instance solely for this keyboard layout. It should be noted that this instance will be destroyed when the keyboard layout changes. Therefore, this method is usually limited to very narrow use cases.
• KeyboardLayout::createInputMethod(): the keyboard layout can dynamically create an input method that can be used with this layout and also with the shared layouts (e.g. symbol layout). This is the preferred way for creating specialized input methods, such as those involving complex languages or handwriting.
• DefaultInputMethod: the virtual keyboard attempts to create this type of input method at startup. This instance will be used as the default input method across all keyboard layouts, unless the keyboard layout uses a custom input method. This instance outlives keyboard layout changes across languages and is the preferred way of creating and overriding the default input method.

Virtual Keyboard Extension Plugin

An extension plugin allows third-party providers to enhance the functionality of the virtual keyboard without recompiling it. Currently the extension plugin can provide keyboard layouts and custom input methods. Creating a plugin for Qt Virtual Keyboard is similar to the standard way of creating a Qt plugin:

• Link against the virtualkeyboard library.

  The virtualkeyboard module must be linked against the plugin:

  QT += virtualkeyboard

  This allows the plugin to access QVirtualKeyboardInputContext and other interfaces.

• Implement a subclass of QVirtualKeyboardExtensionPlugin.

  The plugin provides the virtual keyboard with the necessary metadata, which allows it to sort and load the plugins.

Adding Keyboard Layouts

The plugin can add keyboard layouts for the virtual keyboard by including the layout files in the Qt resources of the plugin binary.

The virtual keyboard searches for the keyboard layouts (per language) from a specific path /QtQuick/VirtualKeyboard/content/layouts/<language_COUNTRY>, so it is essential to use this exact path also in the plugin. Qt resource paths can be overlapped, however, the plugin cannot override the existing layouts in the virtual keyboard plugin.

If the extension plugin needs to override the built-in keyboard layout, the virtual keyboard must be recompiled with only specific languages (e.g. CONFIG+=lang-en) or without keyboard layouts at (e.g. CONFIG+=disable-layouts). Alternatively, if recompiling is not an option, it is possible to override the built-in keyboard layouts and have them loaded directly from the file system by using the QT_VIRTUALKEYBOARD_LAYOUT_PATH environment variable.

Adding Input Method

The plugin can register an input method that other keyboard layouts can use by default (such as DefaultInputMethod) or an input method that is used privately in the plugin.

The input method must implement QVirtualKeyboardAbstractInputMethod (C++) or InputMethod (QML) interface and the plugin must register it as a QML type in {QVirtualKeyboardExtensionPlugin::registerTypes} method.

Metadata for the Extension Plugin

Metadata allows the virtual keyboard to inspect the plugin before loading the actual library. It also prevents the virtual keyboard from loading conflicting extension plugins, for example, two handwriting extensions.

The following metadata example is from the Lipi-Toolkit extension plugin (lipi.json):

{
    "Name": "handwriting",
    "Provider": "Qt Lipi-Toolkit Extension",
    "InputMethod": "HandwritingInputMethod",
    "Version": 100
}

Implementing a Custom Input Method

The implementation of input methods starts by deciding which interface is used; QML or C++. In this example the QML interface is used.

The following example shows the minimum functionality that is required from an input method:

/****************************************************************************
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** Copyright (C) 2021 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/
**
** This file is part of the Qt Virtual Keyboard module of the Qt Toolkit.
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** accordance with the commercial license agreement provided with the
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** a written agreement between you and The Qt Company. For licensing terms
** and conditions see https://www.qt.io/terms-conditions. For further
** information use the contact form at https://www.qt.io/contact-us.
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** $QT_END_LICENSE$
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****************************************************************************/

import QtQuick 2.0
import QtQuick.VirtualKeyboard 1.0

// file: CustomInputMethod.qml

InputMethod {
    function inputModes(locale) {
        return [InputEngine.Latin];
    }

    function setInputMode(locale, inputMode) {
        return true
    }

    function setTextCase(textCase) {
        return true
    }

    function reset() {
        // TODO: reset the input method without modifying input context
    }

    function update() {
        // TODO: commit current state and update the input method
    }

    function keyEvent(key, text, modifiers) {
        var accept = false
        // TODO: Handle key and set accept or fallback to default processing
        return accept;
    }
}

The InputMethod::inputModes() method is called by the input engine before an input mode is set. The method returns a list of input modes available in the given locale.

An input method is initialized in InputMethod::setInputMode() method with a locale and input mode. After setting the locale and input mode, the input method should be ready for use.

InputMethod::reset() is called when an input method needs to be reset. The reset must only reset the internal state of the input method, and not the user text.

InputMethod::update() is called when the input context is updated and the input state is possibly out of sync. The input method should commit the current text.

The keystroke events are handled in InputMethod::keyEvent(). This method handles a single keystroke event and returns true if the event was processed. Otherwise the keystroke is handled by the default input method.

Selection Lists

Selection lists are an optional feature that can be integrated into the input method. The input framework supports various types of lists, such as the word candidate list. Responsibilities in implementing the lists are handled such that the input method is responsible for the content and activities, such as the click behavior. The input framework is responsible for maintaining the list model and delivery to the user interface.

Allocating the Selection Lists

Selection lists are allocated when the input method is activated. The InputMethod::selectionLists() method returns a list of the required selection list types:

function selectionLists() {
    return [SelectionListModel.Type.WordCandidateList];
}

In the above example, the input method allocates the word candidate list for its use.

Updating the Selection Lists

When the input method requires the UI to update the contents of a selection list, it will emit the InputMethod::selectionListChanged signal. Likewise, if the input method requires the UI to highlight an item in the list, it will emit the InputMethod::selectionListActiveItemChanged signal.

selectionListChanged(SelectionListModel.Type.WordCandidateList)
selectionListActiveItemChanged(SelectionListModel.Type.WordCandidateList, wordIndex)

Populating Items in the Selection Lists

The items are populated with method callbacks which will provide the number of items in a list as well as the data for individual items.

The InputMethod::selectionListItemCount callback requests the number of items in the list identified by the given type.

function selectionListItemCount(type) {
    if (type == SelectionListModel.Type.WordCandidateList) {
        return wordList.length
    }
    return 0
}

The InputMethod::selectionListData callback requests the data for items.

function selectionListData(type, index, role) {
    var result = null
    if (type == SelectionListModel.Type.WordCandidateList) {
        switch (role) {
        case SelectionListModel.Role.Display:
            result = wordList[index]
            break
        default:
            break
        }
    }
    return result
}

The role parameter identifies which data is requested for an item. For example, the SelectionListModel.Role.Display requests the display text data.

Responding to User Actions

When the user selects an item in the list, the input method responds to the event in the InputMethod::selectionListItemSelected method callback.

function selectionListItemSelected(type, index) {
    if (type == SelectionListModel.Type.WordCandidateList) {
        inputContext.commit(wordlist[index])
        update()
    }
}

Integrating Handwriting Recognition

Since version 2.0 of the virtual keyboard, input methods can consume touch input data from touch screens or other input devices. This allows integration of a handwriting recognition engine seamlessly to the virtual keyboard, without any changes to the existing keyboard layouts (as the virtual keyboard already provides the handwriting keyboard layouts for most languages).

Towards an input method, handwriting recognition works on the same principle as handling of normal keyboard input, i.e. input data is collected by the keyboard layout and transferred by the input engine to the input method for processing.

Data Model for Handwriting Input

Virtual keyboard collects the handwriting data in a special data model QVirtualKeyboardTrace. Each trace represents a collection of data sampled from one touch (e.g. a swipe on the screen). There will be as many instances of QVirtualKeyboardTrace as there are touches on the handwriting input area.

By definition, trace is a set of data sampled from one touch. In addition to the basic point data, it can also include other types of data, such as the time of each point. The input method can define the desired input channels in the beginning of a trace event.

The input method does not participate in the actual collection of trace data. However, the input method has full control over the input since it can either accept or reject a QVirtualKeyboardTrace (e.g. if there are too many instances to handle). This also allows for precise control over how many fingers can be used simultaneously.

The input method can collect as many traces as it sees fit and it can begin processing them when necessary. The processing can even be performed in parallel while sampling the data, although it is not recommended because of the potential performance issues. The recommended way is to start processing in a background thread after a suitable delay from the last input, so that the processing does not affect negatively to the user interface.

Trace API for Input Methods

The trace API consists of the following virtual methods, which the input method must implement in order to receive and process trace input data.

• patternRecognitionModes
• traceBegin
• traceEnd

By implementing these methods, the input method can receive and process data from a variety of input sources (e.g. keyboard layout or full screen).

The patternRecognitionModes method returns a list of pattern recognition modes, which are supported by the input method. A pattern recognition mode, such as Handwriting , defines the method by which the input method processes the data.

The trace interaction is started when an input source detects a new contact point, and calls the traceBegin method for a new trace object. If the input method accepts the interaction, it creates a new trace object and returns it to the caller. From this point on, trace data is collected until the traceEnd method is called.

When the traceEnd method is called, the input method may begin processing of the data contained in the trace object. After processing the data, the input method should destroy the object. This also removes the trace rendered to the screen.

Keyboard Layouts

Keyboard layouts are located in the src/virtualkeyboard/content/layouts directory. Each subdirectory of the layout directory represents a locale. The locale directory is a string of the form "language_country", where language is a lowercase, two-letter ISO 639 language code, and country is an uppercase, two or three-letter ISO 3166 country code.

Layout Types

Different keyboard layout types are used in different input modes. The default layout which is used for regular text input, is called the "main" layout. The layout type is determined by the layout file name. Therefore, the "main" layout file is called the "main.qml".

List of supported layout types:

• main The main layout for normal text input
• symbols Symbol layout for special characters etc. (activated from main layout)
• numbers Numeric layout for formatted numbers (activated by Qt::ImhFormattedNumbersOnly)
• digits Digits only layout (activated by Qt::ImhDigitsOnly)
• dialpad Dialpad layout for phone number input (activated by Qt::ImhDialableCharactersOnly)
• handwriting Handwriting layout for handwriting recognition (activated from main layout)

Adding New Keyboard Layouts

The keyboard layout element must be based on the KeyboardLayout QML type. This type defines the root item of the layout. The root item has the following optional properties which can be set if necessary:

New rows are added to the keyboard layout by using the KeyboardRow type. The KeyboardRow can also specify the default key weight for its child elements. Otherwise, the key weight is inherited from its parent element.

New keys are added to the keyboard row using the Key type or one of the specialized key types. Below is the list of all key types:

• Key
• BackspaceKey
• ChangeLanguageKey
• EnterKey
• FillerKey
• HideKeyboardKey
• NumberKey
• ShiftKey
• SpaceKey
• SymbolModeKey
• HandwritingModeKey
• TraceInputKey

For example, to add a regular key which sends a key event to the input method:

import QtQuick 2.0
import QtQuick.Layouts 1.0
import QtQuick.VirtualKeyboard 2.1

// file: layouts/en_GB/main.qml

KeyboardLayout {
    keyWeight: 160
    KeyboardRow {
        Key {
            key: Qt.Key_Q
            text: "q"
        }
    }
}

Key Size Calculation

The keyboard layouts are scalable, which means that there cannot be any fixed sizes set for any items in the layout. Instead, the key widths are calculated from key weight in relation to each other and the height by dividing the space equally among the keyboard rows.

In the above example, the key size is inherited from parent elements in this order:

Key > KeyboardRow > KeyboardLayout

The effective value for the key weight will be 160. For the sake of the example, we add another key which specifies a custom key weight:

import QtQuick 2.0
import QtQuick.Layouts 1.0
import QtQuick.VirtualKeyboard 2.1

// file: layouts/en_GB/main.qml

KeyboardLayout {
    keyWeight: 160
    KeyboardRow {
        Key {
            key: Qt.Key_Q
            text: "q"
        }
        Key {
            key: Qt.Key_W
            text: "w"
            keyWeight: 200
        }
    }
}

Now the total key weight of a row is 160 + 200 = 360. When the keyboard layout is activated, the width of an individual key is calculated as follows:

key width in pixels = key weight / SUM(key weights in a row) * row width in pixels

This means that the keyboard can be scaled to any size, while the relative key sizes remain the same.

Alternative Keys

Key can specify an alternativeKeys property, which results in a popup that lists alternative keys when the user presses and holds the key. The alternativeKeys can specify either a string, or a list of strings. If alternativeKeys is a string, the user can select between the characters in the string.

Styles and Layouts

The keyboard layouts cannot specify any visual elements. Instead, the layout is visualized by the keyboard style. On the other hand, the keyboard style cannot affect the size of the keyboard layout.

Keyboard Layouts with Multiple Pages of Keys

Some keyboard layouts, such as symbol layouts, may contain more keys than it is feasible to present on a single keyboard layout. A solution is to embed multiple keyboard layouts into the same context by using the KeyboardLayoutLoader.

When the KeyboardLayoutLoader is used as a root item of a keyboard layout, the actual keyboard layouts are wrapped inside Component elements. The keyboard layout is activated by assigning the id of an active component to the sourceComponent property.

For example:

import QtQuick 2.0
import QtQuick.Layouts 1.0
import QtQuick.VirtualKeyboard 2.1

// file: layouts/en_GB/symbols.qml

KeyboardLayoutLoader {
    property bool secondPage
    onVisibleChanged: if (!visible) secondPage = false
    sourceComponent: secondPage ? page2 : page1
    Component {
        id: page1
        KeyboardLayout {
            KeyboardRow {
                Key {
                    displayText: "1/2"
                    functionKey: true
                    onClicked: secondPage = !secondPage
                }
            }
        }
    }
    Component {
        id: page2
        KeyboardLayout {
            KeyboardRow {
                Key {
                    displayText: "2/2"
                    functionKey: true
                    onClicked: secondPage = !secondPage
                }
            }
        }
    }
}

Handwriting Keyboard Layout

Each language which supports handwriting recognition must provide a special keyboard layout named handwriting.qml.

This type of keyboard layout must meet the following requirements:

• contains a TraceInputKey in the keyboard layout
• provides an instance of HandwritingInputMethod as the input method.

The handwriting layout may also include ChangeLanguageKey. For this purpose, it is important to use the customLayoutsOnly attribute, which will filter out languages that do not use handwriting.

Both the main and handwriting layouts should contain a key to activate and deactivate the handwriting input mode. This can be done by adding a HandwritingModeKey to the layout.

Adding Custom Layouts

The virtual keyboard layouts system supports built-in layouts as well as custom layouts. The built-in layouts are embedded as Qt Resources into the plugin binary. Custom layouts are located in the file system, so that they can be installed without recompiling the virtual keyboard itself, or they can be located in a resource file.

The selection of layouts at runtime is affected by the QT_VIRTUALKEYBOARD_LAYOUT_PATH environment variable.

In case the environment variable is not set, or contains an invalid directory, the virtual keyboard falls back to the default built-in layouts.

To prevent the built-in layouts from being built into the virtual keyboard plugin when using custom layouts, add disable-layouts to the CONFIG qmake variable. For more information, see Advanced Configuration Options.

Keyboard Styles

The virtual keyboard styling system supports built-in styles as well as custom styles. The built-in styles are embedded as Qt Resources into the plugin binary and the custom styles are located in the file system and can be installed without recompiling the virtual keyboard itself.

The selection of the runtime style is affected by an environment variable QT_VIRTUALKEYBOARD_STYLE, which can be set to the name of the built-in style, e.g. "retro", or any of the custom styles installed into the Styles directory:

$$[QT_INSTALL_QML]/QtQuick/VirtualKeyboard/Styles

In case the environment variable is not set, or contains an invalid style name, the virtual keyboard falls back in the default built-in style.

Adding Custom Styles

The process of creating a new style begins by creating a new subdirectory for the style in a QML import path under the URL-based directory structure QtQuick/VirtualKeyboard/Styles/. See QML Import Path for information about QML import paths. The directory name can not contain spaces or special characters other than underscore. Also, the directory name can not be the same as one of the built-in style, which currently includes "default" and "retro".

A good starting point for creating a new style is to use an existing built-in style as a template and edit it. You can find the built-in styles from the virtual keyboard sources directory src/virtualkeyboard/content/styles. Copy one of the directories containing a built-in style into the Styles directory and rename it to "test". The directory structure should now be as follows:

test/default_style.qrc
test/style.qml
test/images
test/images/backspace.png
test/images/check.png
test/images/enter.png
test/images/globe.png
test/images/hidekeyboard.png
test/images/search.png
test/images/shift.png

The QRC configuration file, which is unnecessary in this case, can be safely removed.

Next, open the style.qml in your favorite editor and set the resourcePrefix property to an empty string. The resource prefix is not needed as the resources are contained in the same directory as the style.qml file.

Also, to make it more obvious to see that the custom style is actually being loaded and used, set the keyboard background to a different color:

keyboardBackground: Rectangle {
    color: "gray"
}

The final step is to run the example application with your custom style:

QT_VIRTUALKEYBOARD_STYLE=test virtualkeyboard

Using Qt Virtual Keyboard with QQuickWidget

When using Qt Virtual Keyboard in a QQuickWidget on a touch device, it is necessary to set the Qt::WA_AcceptTouchEvents attribute via QWidget::setAttribute(). Without this attribute set, events from a touch device will be converted into synthesized mouse events.