Technical Guide


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 provides the following main interfaces:

  • InputContext: provides contextual information for the virtual keyboard and other input components.
  • InputEngine: exposes an API to integrate user input events (key presses, etc.) and acts as a host for input methods.
  • InputMethod: a base type for QML based input methods.
  • AbstractInputMethod: a base type for C++ based input methods.

The Input Context

The input context is used by the keyboard as well as concrete input methods.

Contextual Information

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


The list of supported locales is specified by the existence of a locale specific layout directory in "layouts/*". Each layout directory may contain one or more layouts, for example fi_FI/main.qml or symbols.qml.

The application can specify the initial layout by changing the default locale. However, this needs to be done before the application is initialized and the input method plugin is loaded. If no changes are made to the default locale, the current system locale is used.

The keyboard locale matching is performed in the following sequence:

  • layouts/language_country
  • layouts/language_*
  • layouts/en_GB

The locale is first matched against the full locale name. If a full match is not found, then only the locale language is matched. If a partial match is not found, then the "en_GB" locale is used as a fallback.

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 the InputEngine and the current InputMethod instances.

UI Animations

The keyboard should notify the input context about UI transitions and animations. The InputContext::animating property sets the animating property of the input context.

The Input Engine

The input engine object is owned by InputContext. The input engine contains API functions which the keyboard can use to map user interactions such as key press and key release events.

The input events are mapped through the following methods:

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 user releases the key without having to perform the actual action, the key can be interrupted by using the InputEngine::virtualKeyCancel method.

Activating an Input Method

Activating an input method is straightforward. The required steps are:

When the input method is active, it receives key events from the input engine and can produce text.

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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** information to ensure the GNU General Public License requirements will
** be met:

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 keystorke 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.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.

selectionListActiveItemChanged(SelectionListModel.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.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.WordCandidateList) {
        switch (role) {
        case SelectionListModel.DisplayRole:
            result = wordList[index]
    return result

The role parameter identifies which data is requested for an item. For example, the SelectionListModel.DisplayRole 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.WordCandidateList) {

Integrating Selection Lists into the UI

The input engine provides a list model for each selection list type. The model is null while the list is not allocated, allowing the UI to hide the list if necessary.

The list model's word candidate list is provided by the InputEngine::wordCandidateListModel property.

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.

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 further processing.

In case of a regular keyboard, the amount of data transferred from the keyboard to input method is minimal (namely the keycode and text), but in the case of handwriting recognition the data volume is much bigger. Therefore, the touch input is stored in a particular data model.

The input method does not participate in the actual collection of touch data. However, the input method has full control over touch input since it can either accept or reject touch. This allows for precise control over how many fingers can be used simultaneously.

The input method can collect as many traces as it deems necessary and begin processing them at will. The processing can even be performed in parallel with the touch input, although it is not recommended because of the potential side effects. A recommended way is to start processing in a background thread after a suitable delay, so that it does not negatively affect the performance of the user interface.

Data Model for the Handwriting Input

The data collected from the input source is stored in an object named QtVirtualKeyboard::Trace (C++) or Trace (QML).

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

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 touch input data.

By implementing these methods, the input method can receive and process data from a variety of input sources.

The patternRecognitionModes method returns a list of pattern recognition modes, which are supported by the input method. A pattern recognition mode, such as HandwritingRecoginition , 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:

property var inputMethodSpecifies an input method for this layout. If the input method is not defined, then the current input method is used.
property int inputModeSpecifies an input mode for this layout.
property real keyWeightSpecifies the default key weight used for all keys in this keyboard layout. The key weight is a proportional value which affects the size of individual keys in relation to each other.

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:

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.

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:


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 the virtual keyboard Styles directory. 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:


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

Note: The style.qml file should not be renamed, or otherwise the virtual keyboard cannot load the style.

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

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