Gtk – 4.0: Overview of GTK input and event handling
Description: Reference for Gtk-4.0: Overview of GTK input and event handling

#### Overview of GTK input and event handling [src]

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.

## Devices and events

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.

To simplify dealing with the variability between these input devices, GTK has a concept of logical and physical 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 physical devices. Each physical device is associated with a virtual logical device. Logical devices always come in pointer/keyboard pairs - you can think of such a pair as a ‘seat’.

GTK widgets generally deal with the logical devices, and thus can be used with any pointing device or keyboard.

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.

GDK translates these raw windowing system events into `GdkEvents`. Typical input events are button clicks, pointer motion, key presses or touch events. These are all represented as `GdkEvents`, but you can differentiate between different events by looking at their type, using gdk_event_get_event_type().

Some events, such as touch events or button press-release pairs, are connected in to each other in an “event sequence” that univocally identifies events that are related to the same interaction.

When GTK creates a `GdkSurface`, it connects to the ::event signal on it, which receives all of these input events. Surfaces have signals and properties, e.g. to deal with window management related events.

## Event propagation

The function which initially receives input events on the GTK side is responsible for a number of tasks.

1. Find the widget which got the event.
2. Generate crossing (i.e. enter and leave) events when the focus or hover location change from one widget to another.
3. Send the event to widgets.

An event is propagated down and up the widget hierarchy in three phases towards a target widget.

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()).

For pointer events, the target widget is determined by picking the widget at the events coordinates (see gtk_widget_pick()).

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 widget. Event controllers that are attached with `GTK_PHASE_CAPTURE` get a chance to react to the event.

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.

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.

Events are not delivered to a widget which is insensitive or unmapped.

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.

## Keyboard input

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 :focusable 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.

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.

Many widgets can be “activated” to trigger and action. E.g., you can activate a button or switch by clicking on them, but you can also activate them with the keyboard, by using the `Enter` or `Space` keys.

Apart from keyboard navigation, activation and directly typing into entries or text views, GTK widgets can use key events for activating “shortcuts”. Shortcuts generally act as a quick way to move the focus around or to activate a widget that does not currently have the focus.

GTK has traditionally supported different kinds of shortcuts:

- Accelerators are any other shortcuts that can be activated regardless of where the focus is, and typically trigger global actions, such as `Ctrl`+`Q` to quit an application.
- Mnemonics are usually triggered using `Alt` as a modifier for a letter. They are used in places where a label is associated with a control, and are indicated by underlining the letter in the label. As a special case, inside menus (i.e. inside `GtkPopoverMenu`), mnemonics can be triggered without the modifier.
- Key bindings are specific to individual widgets, such as `Ctrl`+`C` or `Ctrl`+`V` in an entry copy to or paste from the clipboard. They are only triggered when the widget has focus.

GTK handles accelerators and mnemonics in a global scope, during the capture phase, and key bindings locally, during the target phase.

Under the hood, all shortcuts are represented as instances of `GtkShortcut`, and they are managed by `GtkShortcutController`.

## Text input

When actual text input is needed (i.e. not just keyboard shortcuts), input method support can be added to a widget by connecting an input method context and listening to its `::commit` signal. To create a new input method context, use gtk_im_multicontext_new(), to provide it with input, use gtk_event_controller_key_set_im_context().

## Event controllers and gestures

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

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.

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:

- 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.
- Grouping a “drag” gesture with a “pan” gesture will only effectively allow dragging in the panning orientation, as both gestures share state.
- If “press” and “long press” are wanted simultaneously, those would need grouping.

Shortcuts are handled by `GtkShortcutController`, which is a complex event handler that can either activate shortcuts itself, or propagate them to another controller, depending on its scope.

## Gesture states

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.

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.

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.

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 where 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.