Qt Toolkit - Character input in Qt/Embedded

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# Character input in Qt/Embedded

Internally in the client/server protocol, each key press and key release is sent as a `QWSKeyEvent`. A QWSKeyEvent contains the following fields:

- `unicode`: Unicode value
- `keycode`: Qt keycode value as defined in qnamespace.h
- `modifier`: A bitfield consisting of some of Qt::ShiftButton, Qt::ControlButton, and Qt::AltButton.
- `is_press`: TRUE if this is a key press, FALSE if it is a key release.
- `is_auto_repeat`: TRUE if this event is caused by auto repeat.

When the server receives a key event, it is sent to each client process, which is responsible for processing the key event and sending it to the right window, if any. Key events may come from several different sources.

### Keyboard drivers

A keyboard driver reads data from a device and gives key events to the server.

Keyboard drivers are currently compiled into the library. They are defined in the file src/kernel/qkeyboard_qws.cpp. At the time of writing, the following drivers are defined:

`QWSTtyKeyboardHandler`
  Input from the system console (tty)
`QWSVr41xxButtonsHandler`
  Input handler for the buttons on Cassiopeia-style PDAs
`QWSVFbKeyboardHandler`
  Virtual framebuffer key input

The keyboard drivers all follow the same pattern. They read keyboard data from a device, find out which keys were pressed, and then call the static function QWSServer::processKeyEvent() with the key information.

At present, the console keyboard driver also handles console switching (Ctrl-Alt-F1...F10) and termination (Ctrl-Alt-Backspace).

To add a keyboard driver for a new device, make a subclass of `QWSKeyboardHandler` and instantiate it in `QWSServer::newKeyboardHandler()` (in qkeyboard_qws.cpp).

### Key event filters (input methods)

When the server receives a key event from a keyboard driver, it first passes it through a filter.

This can be used to implement input methods, providing input of characters that are not on the keyboard.

To make an input method, subclass QWSServer::KeyboardFilter (in src/kernel/qwindowsystem_qws.h) and implement the virtual function filter(). If filter() returns FALSE, the event will be sent to the clients (using QWSServer::sendKeyEvent()). If filter() returns TRUE, the event will be stopped. To generate new key events, use QWSServer::sendKeyEvent(). (Do not use processKeyEvent(), since this will lead to infinite recursion.)

To install a keyboard event filter, use `QWSServer::setKeyboardFilter()`. Currently, only one filter can be installed at a time.

Filtering must be done in the server process.

The launcher example contains an example of a simple input method, `SimpleIM` which reads a substitution table from a file.

### Pen input

Key events do not need to come from a keyboard device. The server process may call QWSServer::sendKeyEvent() at any time.

Typically, this is done by popping up a widget, and letting the user specify characters with the pointer device.

Note: the key input widget should not take focus, since the server would then just send the key events back to the input widget. One way to make sure that the input widget never takes focus is to set the `WStyle_Customize` and `WStyle_Tool` widget flags in the QWidget constructor.

The compact example contains three example input widgets:

- A simple handwriting recognition example.
- A virtual keyboard example.
- An example Unicode input widget.