Past experience conditions people to expect certain gestures even when the app designer hasn't anticipated them. A popular yarn among interface designers (and landscapers!) describes a university campus that was built without footpaths. Instead, the founders just let students walk where they might, wearing dirt paths across the lawn, and the university paved sidewalks only after these "cowpaths" emerged. The idea of paving cowpaths is appealingly simple: design according to the patterns people already follow. When you substitute touchscreen fingers for the meandering undergrads in this tale, you get a useful method to discover if your app could put gestures to better use. Watch first-timers use your app and look for two things: unsuccessful gesture attempts and repetitive, time-consuming actions.
Unsuccessful gesture attempts. Experience with other apps creates expectations for how new apps should work. People anticipate that gestures will work in a certain way and become frustrated when they don't deliver. New users of the built-in Calendar app, for example, routinely try to swipe to change months or days. Alas, the only way to do that is by tapping the arrow buttons at the top of the screen. The arrow controls are clearly labeled and designed to invite touch, but the swipe is even more irresistible. Even these iPhone newcomers already expect that they can swipe to flip through adjacent views. Paving the cowpaths suggests adding a swipe gesture to navigate the calendar.
Figure 8-2. The only way to switch months in Calendar is to tap the arrow buttons at the top of the screen, but that doesn't stop new users from routinely trying to swipe to a new month. The big calendar spread looks like something you could grab and move, while the small arrow buttons are understated and out of the way.
Repetitive, time-consuming actions. When people constantly repeat the same multitap sequence over and over again, the situation demands a new cowpath—a shortcut to speed through the tedium. The standard left-to-right swipe across a list item, for example, is a shortcut for deleting. Instead of tapping the Edit button, then the list item's delete icon, and then its Delete button, the gesture saves time by simply letting you swipe and tap to delete. There's a similar gesture shortcut in Reeder, an elegant app for browsing Google Reader news feeds: instead of opening each article to mark it as read (like you might do with the built-in Mail app), Reeder lets you slide the item to change its status, letting you skip past articles that don't interest you. (This is also an example of piggybacking on a standard gesture, a topic you'll explore in a moment.)
Figure 8-3. The swipe-to-delete shortcut lets you delete a list item in a two-step process. Swipe left to right across the item to delete, then tap the Delete button.
Figure 8-4. Reeder uses a shortcut gesture to let you mark an article as read by sliding it to the right. A pointer appears, and sliding it to the empty disc icon marks it as read. You can mark the article unread by repeating the gesture, and the disc icon changes to solid. Or, slide the item left to move the pointer to the star and mark it as a favorite.