gtk2/docs/reference/gtk/input-handling.xml

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<refentry id="chap-input-handling">
<refmeta>
<refentrytitle>The GTK Input Model</refentrytitle>
<manvolnum>3</manvolnum>
<refmiscinfo>GTK Library</refmiscinfo>
</refmeta>

<refnamediv>
<refname>The GTK Input Model</refname>
<refpurpose>
    input and event handling in detail
</refpurpose>
</refnamediv>


  <refsect1 id="input-overview">
    <title>Overview of GTK input and event handling</title>

  <para>
   This chapter describes in detail how GTK handles input. If you are interested
   in what happens to translate a key press or mouse motion of the users into a
   change of a GTK widget, you should read this chapter. This knowledge will also
   be useful if you decide to implement your own widgets.
  </para>

  <refsect2>
    <title>Devices and events</title>

    <!-- input devices: master/slave, keyboard/pointer/touch -->
    <para>
     The most basic input devices that every computer user has interacted with are
     keyboards and mice; beyond these, GTK supports touchpads, touchscreens and
     more exotic input devices such as graphics tablets. Inside GTK, every such
     input device is represented by a #GdkDevice object.
    </para>

    <para>
     To simplify dealing with the variability between these input devices, GTK
     has a concept of master and slave devices. The concrete physical devices that
     have many different characteristics (mice may have 2 or 3 or 8 buttons,
     keyboards have different layouts and may or may not have a separate number
     block, etc) are represented as slave devices. Each slave device is
     associated with a virtual master device. Master devices always come in
     pointer/keyboard pairs - you can think of such a pair as a 'seat'.
    </para>
    <para>
     GTK widgets generally deal with the master devices, and thus can be used
     with any pointing device or keyboard.
    </para>

    <para>
     When a user interacts with an input device (e.g. moves a mouse or presses
     a key on the keyboard), GTK receives events from the windowing system.
     These are typically directed at a specific surface - for pointer events,
     the surface under the pointer (grabs complicate this), for keyboard events,
     the surface with the keyboard focus.
    </para>
    <para>
     GDK translates these raw windowing system events into #GdkEvents.
     Typical input events are:
     <simplelist>
       <member>#GdkEventButton</member>
       <member>#GdkEventMotion</member>
       <member>#GdkEventCrossing</member>
       <member>#GdkEventKey</member>
       <member>#GdkEventFocus</member>
       <member>#GdkEventTouch</member>
     </simplelist>
    </para>
    <para>
      Additionally, GDK/GTK synthesizes other signals to let know whether
      grabs (system-wide or in-app) are taking input away:
      <simplelist>
	<member>#GdkEventGrabBroken</member>
	<member>#GtkWidget::grab-notify</member>
      </simplelist>
    </para>
    <para>
      When GTK creates a GdkSurface, it connects to the ::event signal
      on it, which receives all of these input events. Surfaces have
      have signals and properties, e.g. to deal with window management
      related events.
    </para>
  </refsect2>

  <refsect2 id="event-propagation">
    <title>Event propagation</title>

    <para>
      The function which initially receives input events on the GTK
      side is gtk_main_do_event(). See its documentation
      for details of what it does: compression of enter/leave events,
      identification of the widget receiving the event, pushing the event onto a
      stack for gtk_get_current_event(), and propagating the event to the
      widget.
    </para>

    <para>
      When a GDK backend produces an input event, it is tied to a #GdkDevice and
      a #GdkSurface, which in turn represents a windowing system surface in the
      backend. If a widget has grabbed the current input device, or all input
      devices, the event is propagated to that #GtkWidget. Otherwise, it is
      propagated to the the #GtkWidget which called gtk_widget_register_surface()
      on the #GdkSurface receiving the event.
    </para>

    <para>
      Grabs are implemented for each input device, and globally. A grab for a
      specific input device (gtk_device_grab_add()), is sent events in
      preference to a global grab (gtk_grab_add()). Input grabs only have effect
      within the #GtkWindowGroup containing the #GtkWidget which registered the
      event’s #GdkSurface. If this #GtkWidget is a child of the grab widget, the
      event is propagated to the child — this is the basis for propagating
      events within modal dialogs.
    </para>

    <para>
      An event is propagated to a widget using gtk_propagate_event().
      Propagation goes down and up the widget hierarchy in three phases
      (see #GtkPropagationPhase) towards a target widget.
    </para>

    <para>
      For key events, the top-level window gets a first shot at activating
      mnemonics and accelerators. If that does not consume the events,
      the target widget for event propagation is window's current focus
      widget (see gtk_window_get_focus()).
    </para>

    <para>
      For pointer events, the target widget is determined by picking
      the widget at the events coordinates (see gtk_window_pick()).
    </para>

    <para>In the first phase (the “capture” phase) the event is
      delivered to each widget from the top-most (the top-level
      #GtkWindow or grab widget) down to the target #GtkWidget.
      <link linkend="event-controllers-and-gestures">Event
      controllers</link> that are attached with %GTK_PHASE_CAPTURE
      get a chance to react to the event.
    </para>

    <para>
      After the “capture” phase, the widget that was intended to be the
      destination of the event will run event controllers attached to
      it with %GTK_PHASE_TARGET. This is known as the “target” phase,
      and only happens on that widget.
    </para>

    <para>
      In the last phase (the “bubble” phase), the event is delivered
      to each widget from the target to the top-most, and event
      controllers attached with %GTK_PHASE_BUBBLE are run.
    </para>

    <para>
      Events are not delivered to a widget which is insensitive or
      unmapped.
    </para>

    <para>
      Any time during the propagation phase, a controller may indicate
      that a received event was consumed and propagation should
      therefore be stopped. If gestures are used, this may happen
      when the gesture claims the event touch sequence (or the
      pointer events) for its own. See the “gesture states” section
      below to learn more about gestures and sequences.
    </para>
  </refsect2>

  <refsect2>
    <title>Touch events</title>

    <para>
      Touch events are emitted as events of type %GDK_TOUCH_BEGIN,
      %GDK_TOUCH_UPDATE or %GDK_TOUCH_END, those events contain an
      “event sequence” that univocally identifies the physical touch
      until it is lifted from the device.
    </para>
  </refsect2>

  <refsect2>
    <title>Grabs</title>

    <para>
      Grabs are a method to claim all input events from a device,
      they happen either implicitly on pointer and touch devices,
      or explicitly. Implicit grabs happen on user interaction, when
      a #GdkEventButtonPress happens, all events from then on, until
      after the corresponding #GdkEventButtonRelease, will be reported
      to the widget that got the first event. Likewise, on touch events,
      every #GdkEventSequence will deliver only events to the widget
      that received its %GDK_TOUCH_BEGIN event.
    </para>

    <para>
      Explicit grabs happen programatically (both activation and
      deactivation), and can be either system-wide (GDK grabs) or
      application-wide (GTK grabs). On the windowing platforms that
      support it, GDK grabs will prevent any interaction with any other
      application/window/widget than the grabbing one, whereas GTK grabs
      will be effective only within the application (across all its
      windows), still allowing for interaction with other applications.
    </para>

    <para>
      But one important aspect of grabs is that they may potentially
      happen at any point somewhere else, even while the pointer/touch
      device is already grabbed. This makes it necessary for widgets to
      handle the cancellation of any ongoing interaction. Depending on
      whether a GTK or GDK grab is causing this, the widget will
      respectively receive a #GtkWidget::grab-notify signal, or a
      #GdkEventGrabBroken event.
    </para>

    <para>
      On gestures, these signals are handled automatically, causing the
      gesture to cancel all tracked pointer/touch events, and signal
      the end of recognition.
    </para>
  </refsect2>

  <refsect2>
    <title>Keyboard input</title>

    <para>
      Every #GtkWindow maintains a single focus location (in
      the ::focus-widget property). The focus widget is the
      target widget for key events sent to the window. Only
      widgets which have ::can-focus set to %TRUE can become
      the focus. Typically these are input controls such as
      entries or text fields, but e.g. buttons can take the
      focus too.
    </para>

    <para>
      Input widgets can be given the focus by clicking on them,
      but focus can also be moved around with certain key
      events (this is known as “keyboard navigation”). GTK
      reserves the Tab key to move the focus to the next location,
      and Shift-Tab to move it back to the previous one. In addition
      many containers allow “directional navigation” with the
      arrow keys.
    </para>

    <!-- mnemonics, accelerators, bindings -->
  </refsect2>

  <refsect2 id="event-controllers-and-gestures">
    <title>Event controllers and gestures</title>

    <para>
      Event controllers are standalone objects that can perform
      specific actions upon received #GdkEvents. These are tied
      to a #GtkWidget, and can be told of the event propagation
      phase at which they will manage the events.
    </para>

    <para>
      Gestures are a set of specific controllers that are prepared
      to handle pointer and/or touch events, each gesture
      implementation attempts to recognize specific actions out the
      received events, notifying of the state/progress accordingly to
      let the widget react to those. On multi-touch gestures, every
      interacting touch sequence will be tracked independently.
    </para>

    <para>
      Since gestures are “simple” units, it is not uncommon to tie
      several together to perform higher level actions, grouped
      gestures handle the same event sequences simultaneously, and
      those sequences share a same state across all grouped
      gestures. Some examples of grouping may be:

      <simplelist>
	<member>
	  A “drag” and a “swipe” gestures may want grouping.
          The former will report events as the dragging happens,
          the latter will tell the swipe X/Y velocities only after
          recognition has finished.
	</member>
	<member>
	  Grouping a “drag” gesture with a “pan” gesture will only
          effectively allow dragging in the panning orientation, as
          both gestures share state.
	</member>
	<member>
	  If “press” and “long press” are wanted simultaneously,
          those would need grouping.
	</member>
      </simplelist>
    </para>
  </refsect2>

  <refsect2>
    <title>Gesture states</title>
    <para>
      Gestures have a notion of “state” for each individual touch
      sequence. When events from a touch sequence are first received,
      the touch sequence will have “none” state, this means the touch
      sequence is being handled by the gesture to possibly trigger
      actions, but the event propagation will not be stopped.
    </para>

    <para>
      When the gesture enters recognition, or at a later point in time,
      the widget may choose to claim the touch sequences (individually
      or as a group), hence stopping event propagation after the event
      is run through every gesture in that widget and propagation phase.
      Anytime this happens, the touch sequences are cancelled downwards
      the propagation chain, to let these know that no further events
      will be sent.
    </para>

    <para>
      Alternatively, or at a later point in time, the widget may choose
      to deny the touch sequences, thus letting those go through again
      in event propagation. When this happens in the capture phase, and
      if there are no other claiming gestures in the widget,
      a %GDK_TOUCH_BEGIN/%GDK_BUTTON_PRESS event will be emulated and
      propagated downwards, in order to preserve consistency.
    </para>

    <para>
      Grouped gestures always share the same state for a given touch
      sequence, so setting the state on one does transfer the state to
      the others. They also are mutually exclusive, within a widget
      there may be only one gesture group claiming a given sequence.
      If another gesture group claims later that same sequence, the
      first group will deny the sequence.
    </para>
  </refsect2>

  </refsect1>
</refentry>