Outcurses

During the development of Growlight and Omphalos, I found myself implementing significant UI code atop ncurses. It's my goal to extract most of this, unify it, and make it available as liboutcurses. Code lives on github.

Why is it called 'outcurses'? Because there's more cursing, duh.

Panelreels

The Ncurses panels extension (originating in AT&T System V) facilitates management of a deck of possibly-overlapping window objects sharing a screen (there's little point in using panels if windows are strictly tiled among the screen, but not much reason not to, either). The various panels of a screen have a z-ordering, and higher panels obscure panels underneath. Using the panels extension requires linking in the extra library libpanel (or libpanelw for wide character support).

The panelreel is a UI abstraction supported by outcurses in which dynamically-created and -destroyed toplevel entities (referred to as tablets) are arranged in a torus (circular loop), allowing for infinite scrolling (infinite scrolling can be disabled, resulting in a line segment rather than a torus). This works naturally with keyboard navigation, mouse scrolling wheels, and touchpads (including the capacitive touchscreens of modern cell phones). The "panel" comes from the underlying ncurses objects (each entity corresponds to a single panel) and the "reel" from slot machines. A panelreel initially has no tablets; at any given time thereafter, it has zero or more tablets, and if there is at least one tablet, one tablet is focused (and on-screen). If the last tablet is removed, no tablet is focused. A tablet can support navigation within the tablet, in which case there is an in-tablet focus for the focused tablet, which can also move among elements within the tablet.

The panelreel object tracks the size of the screen, the size, number, information depth, and order of tablets, and the focuses. It also draws the optional borders around tablets and the optional border of the reel itself. It knows nothing about the actual content of a tablet, save the number of lines it occupies at each information depth. The typical control flow is that an application receives events (from the UI or other event sources), and calls into outcurses saying e.g. "Tablet 2 now has 40 valid lines of information". Outcurses might then call back into the application, asking it to draw some line(s) from some tablet(s) at some particular coordinate of that tablet's panel. Finally, control returns to the application, and the cycle starts anew.

Each tablet might be wholly, partially, or not on-screen. Outcurses always places as much of the focused tablet as is possible on-screen (if the focused tablet has more lines than the actual reel does, it cannot be wholly on-screen. In this case, the focused subelements of the tablet are always on-screen). The placement of the focused tablet depends on how it was reached (when moving to the next tablet, offscreen tablets are brought onscreen at the bottom. When moving to the previous tablet, offscreen tablets are brought onscreen at the top. When moving to an arbitrary tablet which is neither the next nor previous tablet, it will be placed in the center).

The controlling application can, at any time,

• Insert a new tablet somewhere in the reel (possibly off-screen)
• Delete a (possibly off-screen) tablet from the reel
• Change focus to the next or previous tablet, bringing it on-screen if it is off
• Change focus to some arbitrary other tablet, bringing it on-screen if it is off
• Expand or collapse the information depth of a tablet
• Change the content of a tablet, updating it if it is on-screen
  • Remove content from a tablet, possibly resizing it, and possibly changing focus within the tablet
  • Add content to the tablet, possibly resizing it, and possibly creating focus within the tablet
• Navigate within the focused tablet
• Create or destroy new panels atop the panelreel
• Indicate that the screen has been resized or needs be redrawn

A special case arises when moving among the tablets of a reel having multiple tablets, all of which fit entirely on-screen, and infinite scrolling is in use. Normally, upon moving to the next tablet from the bottommost tablet, the (offscreen) next tablet is pulled up into the bottom of the reel (the reverse is true when moving to the previous tablet from the topmost). When all tablets are onscreen with infinite scrolling, there are two possibilities: either the focus scrolls (moving from the bottom tablet to the top tablet, for instance), or the reel scrolls (preserving order among the tablets, but changing their order on-screen). In this latter case, moving to the next tablet from the bottommost tablet results in the tablet which is gaining focus being brought to the bottom of the screen from the top, and all other tablets moving up on the screen. Moving to the previous tablet from the topmost tablet results in the bottommost tablet moving to the top of the screen, and all other tablets moving down. This behavior matches the typical behavior precisely, and avoids a rude UI discontinuity when the tablets grow to fill the entire screen (or shrink to not fill it). If it is not desired, however, scrolling of focus can be configured instead.

See also

• My Xcurses project, which went exactly nowhere