A notification is the alert's pushy cousin. With your permission, an app can nudge messages onto your screen even when the app isn't front and center on the device. Text messages, calendar alarms, and voicemail alerts are familiar examples of the boldest form of notification: an alert box that pops up to exclaim its message, interrupting whatever you might be doing on the phone. (Notifications can also take the quieter form of badges, a topic you'll explore in a moment.) At their best, notifications deliver important information in real time—flight changes, news headlines, sports scores, to-do list reminders, instant-message alerts, and so on. If your app deals with personally urgent and time-sensitive information, offering notifications can be a truly helpful way to keep your audience up to date on events that matter most to them.
Figure 10-4. The Phone app (left) triggers a notification when you've missed a call or received a voicemail. At your request, the Umbrella app (right) can notify you when it's going to rain today.
Thing is, notification alerts always arrive in the same abrupt manner, no matter how urgent or mundane the message. Just like a regular alert box, every notification briefly takes over your phone, blocking your current task until you dismiss the message. If a notification arrives during a phone call, you can't even hang up until you dismiss it. Some notifications come with sound and vibration, too. The effect is like talking to someone who shouts everything they say without discrimination. There's no way for apps to "whisper" a notification; every message arrives as a noisy interruption. It's one of the few false notes in the otherwise elegant iPhone OS.
Since notifications always shout, good etiquette at least requires some discerning choices about when you choose to send them. A potentially useful service becomes irritating and then outright rude when it spams people with more messages than they want. Spew too much, and people will shut you down. Push notifications for individual apps can be turned off in the built-in Settings app, but it's just as common for people to delete a chatty app outright. Software engineer and back-pocket iPhone user Michael "Rands" Lopp summed it up via Twitter:
The dilemma is that one person's trickle is another person's fire hose. It's tough, even impossible, to adjust the notification flow to a rate that will please everyone. You have to leave it up to individual users. If you choose to offer notification alerts, always give people options to fine-tune the timing and content:
Offer detailed content control. Let people specify exactly what kinds of notifications they receive. Social networking apps should let people "subscribe" to notifications from a special few. Sports apps should send scores only for requested teams or even specific games.
Establish "quiet hours." Don't bleep and bloop notifications to your audience in the middle of the night unless they specifically ask for it. Boxcar, for example, is an app that sends notifications about activity in your Twitter, Facebook, and email accounts. To prevent your night-owl pals from waking you up when they tweet, the app lets you turn off alerts during specific hours.
Make sound optional. Notifications can include custom sounds, a clever way to add a little extra personality to the notification. Weather app Umbrella, for example, makes the sound of an umbrella unfurling when it sends a notification of imminent rain. Inevitably, though, not everyone will find the sound as cute as you do. Offer an option to turn it off.
