Gets the area this can start a swipe from for the given direction and gesture type.
This can be used to restrict swipes to only be possible from a certain area, for example, to only allow edge swipes, or to have a draggable element and ignore swipes elsewhere.
If not implemented, the default implementation returns the allocation of this, allowing swipes from anywhere.
| this |
a swipeable |
| navigation_direction |
the direction of the swipe |
| is_drag |
whether the swipe is caused by a dragging gesture |
| rect |
a pointer to a rectangle to store the swipe area |