GCC has some optimization for the inclusion guard, but they only work if
the check is the outermost one.
We're fairly inconsistent because of historical reasons, so we should
ensure that we follow the same pattern in every public header.
TapAction is a GestureAction-subclass that handles clicks and
tap gestures. It is meant to provide a replacement for ClickAction
using GestureAction:
• it handles events trasparently without capturing them, so that it
can coexists with other GestureActions;
• the ::tap signal is not emitted if the drag threshold is exceeded;
• building upon GestureAction the amount of code is greatly reduced.
TapAction provides:
• tap signal, notifying users when a tap has been performed.
The image-content example program has been updated replacing its
ClickAction usage with TapAction.
https://bugzilla.gnome.org/show_bug.cgi?id=683948
