IPhone 4.0

Multitasking! On phones! Everybody does it now. However each smartphone platform does it differently. Here are the various tricky ways that the major platforms try to juggle multiple apps.

Smartphone is a computer that fits in your pocket

A smartphone is a computer that fits in your pocket, nevertheless it has some constraints that a desktop doesn't, which make multitasking far trickier. Namely, these four resources are constricted, and they're what on the whole shape the way multitasking works on phones:

• Screen real-estate: There's only so much you can do with a 3-to-4 inch screen at one time • Battery life: You want your phone to to tell the truth last all day, or come close to it, right? • CPU: It's slower than anything in a laptop, because of size and battery life • Memory: There's not that much of it, also in some cases because of battery life

This fourth item, a lack of memory, means you can do far fewer things then; there's as well no disk swap in any of the major OSes, so you can't use the phone's actual storage as virtual memory, either. Lack of plentiful RAM is like as not the most pernicious constraint afterwards battery life, in fact.

User perspective

From a user perspective, multitasking doesn't work so differently if you move from Windows to Linux to Mac on a desktop. Nevertheless on a phone, everybody has their own ideas about how to make it work, ranging from leaving things near wide open for third-party apps to run willy-nilly to making things pretty damn restrictive. Even the common smartphone components—notifications and fast app switching—can differ wildly from platform to platform. So here's the gist of how multitasking works on each major smartphone.

The Android model is like as not the most interesting, a hybrid system that's fairly open about allowing stuff running in the background, but together aims to be completely invisible to you, the end user, so that you don't have to actively manage whether an app is open or closed. It's a little complicated so we talked to an Android engineer about what's going on inside.

What happens when I switch to another app?

What happens when I switch to another app? The app you switched from doesn't stop—the process stays open. Anyway, as long as it can. When Android detects it's running low on memory, at the time it kills processes to free up resources. It asks apps to save their state when you switch, so that any can be killed at any point, at once reopen like it was never killed when you return to it.

What can apps do in the background? Android has two basic facilities for third-party multitasking—broadcast receivers and services. With a broadcast receiver, an app that goes into the background is deep down asking to be told about an event, like if you move 500 meters, or your battery level hits a certain percentage. That way the app can go away, and not use resources, however restart when something happens that it needs to act on.

What can't apps do in the background?

What can't apps do in the background? Remember Android's garbage battery life when it first came out? That's because there in effect were no restrictions on what kind of resources an app could consume in the background. Since Android 1.5, apps running in the background are capped at using 5-10 percent of the CPU altogether—which is the only major restriction placed on apps in the background. The other is that it's not easy for app to push itself to the foreground—they're supposed to the window shade notifications system.

iPhone has always been a multitasking OS, nevertheless only for first-party apps. That changes with iPhone 4.0, which delivers multitasking and background processes for third-party apps, yet in a characteristically tight-gripped way. Like Android, the idea is that users won't to tell the truth manage applications themselves, however that the system will take care of it for them. Unfortunately, a lot of the technical details are after all wrapped up by non-disclosure agreements, yet we have a basic idea of how stuff works.

What happens when I switch to another app?

What happens when I switch to another app? Of the seven new multitasking services, two can come into play when you switch to another app: With fast-app switching, an app is frozen instead of killed, allowing users to right away pick up where they left off, without having to reload the app. Since the user isn't actively managing whether applications completely quit or not, the system presumably kills apps when it has too many hanging around. The other is task finishing, where, if you leave an app when it's mid-task, the app can tell the OS it needs to keep running in the background to finish whatever it was doing.

What can apps do in the background? Furthermore finishing a task, a third-party app is able to perform a limited set of functions in the background via Apple's new multitasking services. The difference between iPhone and Android is that with iPhone, the app isn't full-blown running, however just operating via these services. For users, that mainly comes down to VoIP (Voice over Internet Protocol), background audio, and background location, which comes in two flavors, cell tower location checking, to save battery life, and full GPS tracking, for navigation apps.

What can't apps do in the background?

What can't apps do in the background? At heart anything not set forth in the seven services. So if it's not VoIP, or background audio or location, it probably ain't running in the background.

What can apps do in the background? There's a couple different ways that apps can run in the background. As a minimized card, an app can "normally do what it wants to do in terms of data and network access" says Allan, because "our desire was a fairly open architecture." A "pure background app" can schedule itself to be woken up and run for up to 30 seconds to generate data requests, or to schedule another wake up. If you do close an app utterly, it can open what Palm calls a dashboard, which is "like a traffic monitor," which deep down means it can listen for notifications.

Oh, irony—Microsoft goes from having one of the most open multitasking smartphone OSes with Windows Mobile to one of the most restrictive. Their goal in designing the way multitasking works with Windows Phone 7 is to stick to delivering the "core experiences" people want out of multitasking, without going full bore on it. Sounds like it'shmmmm...so we talked to Microsoft's Aaron Woodman, director of the mobile communications business, about it.

What happens when I switch to another app?

What happens when I switch to another app? Apps don't die when you leave them in Windows Phone 7. So when you switch from an app, it saves its state in memory and is frozen, for fast-app switching—like Android and iPhone 4.0. Woodman says "as you get furthermore away from an app, and resources are needed, it frees them, nevertheless you can go right back." When it needs to free up resources, an app is "dehydrated," which is "one level above being killed" and at the time when you open it back up, with that saved state, it's "rehydrated."

What can apps do in the background? Um, not a lot—it's very much back to the olden iPhone days, where notifications are an app's primary way of getting anything done when it's not running. The fancy demos of Pandora running in the background? That's for an elite group of third-party partners only, and Woodman says access of that sort, it's "hard to know if that'll scale in the broad term." In terms of notifications, Microsoft has an impressively wide variety even though: A tile notification, which pushes new info to a Live Tile on the home screen; a toast notification, which is a pop-up, like an incoming call; or a raw notification, which is raw data sent to the application.

And that's about it, or for the time being, what you as a matter of fact need to know. We left off old-school Windows Mobile because it's effectively obsolete, along with BlackBerry OS because it's in flux with OS 6, as then as Symbian, which is undergoing its own contortions with S^3—not to mention their models aren't complicated as the ones here.