source: trunk/doc/src/examples/inputpanel.qdoc@ 651

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/*!
    \example tools/inputpanel
    \title Input Panel Example

    The Input Panel example shows how to create an input panel that
    can be used to input text into widgets using only the pointer and
    no keyboard.

    \image inputpanel-example.png

    The input fields in the main window have no function other than
    to accept input. The main focus is on how the extra input panel
    can be used to input text without the need for a real keyboard or
    keypad.

    \section1 Main Form Class Definition

    Because the main window has no other function than to accept
    input, it has no class definition. Instead, its whole layout is
    made in Qt Designer. This emphasizes the point that no widget
    specific code is needed to use input panels with Qt.

    \section1 MyInputPanelContext Class Definition

    \snippet examples/tools/inputpanel/myinputpanelcontext.h 0

    The \c MyInputPanelContext class inherits QInputContext, which is
    Qt's base class for handling input methods.
    \c MyInputPanelContext is responsible for managing the state of
    the input panel and sending input method events to the receiving
    widgets.

    The \c inputPanel member is a pointer to the input panel widget
    itself; in other words, the window that will display the buttons
    used for input.

    The \c identifierName(), \c language(), \c isComposing() and
    \c reset() functions are there mainly to fill in the pure virtual
    functions in the base class, QInputContext, but they can be
    useful in other scenarios. The important functions and slots are
    the following:

    \list
    \o \c filterEvent() is where we receive events telling us to open
       or close the input panel.
    \o \c sendCharacter() is a slot which is called when we want to
       send a character to the focused widget.
    \o \c updatePosition() is used to position the input panel
       relative to the focused widget, and will be used when opening
       the input panel.
    \endlist

    \section1 MyInputPanelContext Class Implementation

    In the constructor we connect to the \c characterGenerated()
    signal of the input panel, in order to receive key presses. We'll
    see how it works in detail later on.

    \snippet examples/tools/inputpanel/myinputpanelcontext.cpp 0

    In the \c filterEvent() function, we must look for the two event
    types: \c RequestSoftwareInputPanel and \c CloseSoftwareInputPanel.

    \snippet examples/tools/inputpanel/myinputpanelcontext.cpp 1

    The first type will be sent whenever
    an input capable widget wants to ask for an input panel. Qt's
    input widgets do this automatically. If we receive that type of
    event, we call \c updatePosition() \mdash we'll see later on what it
    does \mdash then show the actual input panel widget. If we receive
    the \c CloseSoftwareInputPanel event, we do the opposite, and
    hide the input panel.

    \snippet examples/tools/inputpanel/myinputpanelcontext.cpp 2

    We implement the \c sendCharacter() function so that it sends the
    supplied character to the focused widget. All QInputContext based
    classes are always supposed to send events to the widget returned
    by QInputContext::focusWidget(). Note the QPointer guards to make
    sure that the widget does not get destroyed in between events.

    Also note that we chose to use key press events in this example.
    For more complex use cases with composed text it might be more
    appropriate to send QInputMethodEvent events.

    The \c updatePosition() function is implemented to position the
    actual input panel window directly below the focused widget.

    \snippet examples/tools/inputpanel/myinputpanelcontext.cpp 3

    It performs the positioning by obtaining the coordinates of the
    focused widget and translating them to global coordinates.

    \section1 MyInputPanel Class Definition

    The \c MyInputPanel class inherits QWidget and is used to display
    the input panel widget and its buttons.

    \snippet examples/tools/inputpanel/myinputpanel.h 0

    If we look at the member variables first, we see that there is
    \c form, which is made with Qt Designer, that contains the layout
    of buttons to click. Note that all the buttons in the layout have
    been declared with the \c NoFocus focus policy so that we can
    maintain focus on the window receiving input instead of the
    window containing buttons.

    The \c lastFocusedWidget is a helper variable, which also aids in
    maintaining focus.

    \c signalMapper is an instance of the QSignalMapper class and is
    there to help us tell which button was clicked. Since they are
    all very similar this is a better solution than creating a separate
    slot for each one.

    The functions that we implement in \c MyInputPanel are the
    following:

    \list
    \o \c event() is used to intercept and manipulate focus events,
       so we can maintain focus in the main window.
    \o \c saveFocusWidget() is a slot which will be called whenever
       focus changes, and allows us to store the newly focused widget
       in \c lastFocusedWidget, so that its focus can be restored
       if it loses it to the input panel.
    \o \c buttonClicked() is a slot which will be called by the
       \c signalMapper whenever it receives a \c clicked() signal
       from any of the buttons.
    \endlist

    \section1 MyInputPanel Class Implementation

    If we look at the constructor first, we have a lot of signals to
    connect to!

    We connect the QApplication::focusChanged() signal
    to the \c saveFocusWidget() signal in order to get focus updates.
    Then comes the interesting part with the signal mapper: the
    series of \c setMapping() calls sets the mapper up so that each
    signal from one of the buttons will result in a
    QSignalMapper::mapped() signal, with the given widget as a
    parameter. This allows us to do general processing of clicks.

    \snippet examples/tools/inputpanel/myinputpanel.cpp 0

    The next series of connections then connect each button's
    \c clicked() signal to the signal mapper. Finally, we create
    a connection from the \c mapped() signal to the
    \c buttonClicked() slot, where we will handle it.

    \snippet examples/tools/inputpanel/myinputpanel.cpp 3

    In the \c buttonClicked() slot, we extract the value of the
    "buttonValue" property. This is a custom property which was
    created in Qt Designer and set to the character that we wish the
    button to produce. Then we emit the \c characterGenerated()
    signal, which \c MyInputPanelContext is connected to. This will
    in turn cause it to send the input to the focused widget.

    In the \c saveFocusWidget() slot, we test whether the newly
    focused widget is a child of the input panel or not, using the
    QWidget::isAncestorOf() call.

    \snippet examples/tools/inputpanel/myinputpanel.cpp 2

    If it isn't, it means that the widget is outside the input panel,
    and we store a pointer to that widget for later.

    In the \c event() function we handle QEvent::WindowActivate
    event, which occurs if the focus switches to the input panel.

    \snippet examples/tools/inputpanel/myinputpanel.cpp 1

    Since we want avoid focus on the input panel, we immediately call
    QWidget::activateWindow() on the widget that last had focus, so
    that input into that widget can continue. We ignore any other events
    that we receive.

    \section1 Setting the Input Context

    The main function for the example is very similar to those for other
    examples. The only real difference is that it creates a
    \c MyInputPanelContext and sets it as the application-wide input
    context.

    \snippet examples/tools/inputpanel/main.cpp main

    With the input context in place, we set up and show the user interface
    made in Qt Designer before running the event loop.

    \section1 Further Reading

    This example shows a specific kind of input context that uses interaction
    with a widget to provide input for another. Qt's input context system can
    also be used to create other kinds of input methods. We recommend starting
    with the QInputContext documentation if you want to explore further.
*/