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24 <p>Android is fairly unique in the ways it allows multiple applications to run at the same time. Developers coming from a different platform may find the way it operates surprising. Understanding its behavior is important for designing applications that will work well and integrate seamlessly with the rest of the Android platform. This article covers the reasons for Android's multitasking design, its impact on how applications work, and how you can best take advantage of Android's unique features.</p>
41 <p>The first two requirements highlight an interesting conflict. We don't want users to worry about closing their apps, but rather make it appear that all of the applications are always running. At the same time, mobile devices have hard limits on memory use, so that a system will degrade or even start failing very quickly as it needs more RAM than is available; a desktop computer, with swap, in contrast will simply start slowing down as it needs to page RAM to its swap space. These competing constraints were a key motivation for Android's design.</p>
44 <p>The fact that you can see an application's process "running" does not mean the application is running or doing anything. It may simply be there because Android needed it at some point, and has decided that it would be best to keep it around in case it needs it again. Likewise, you may leave an application for a little bit and return to it from where you left off, and during that time Android may have needed to get rid of the process for other things.</p>
45 <p>A key to how Android handles applications in this way is that processes don't shut down cleanly. When the user leaves an application, its process is kept around in the background, allowing it to continue working (for example downloading web pages) if needed, and come immediately to the foreground if the user returns to it. If a device never runs out of memory, then Android will keep all of these processes around, truly leaving all applications "running" all of the time.</p>
48 <p>If a user later returns to an application that's been killed, Android needs a way to re-launch it in the same state as it was last seen, to preserve the "all applications are running all of the time" experience. This is done by keeping track of the parts of the application the user is aware of (the Activities), and re-starting them with information about the last state they were seen in. This last state is generated each time the user leaves that part of the application, not when it is killed, so that the kernel can later freely kill it without depending on the application to respond correctly at that point.</p>
55 <p>A BroadcastReceiver allows an application to run, for a brief amount of time, in the background as a result of something else happening. It can be used in many ways to build higher-level facilities: for example the AlarmManager allows an application to have a broadcast sent at a certain time in the future, and the LocationManager can send a broadcast when it detects interesting changes in location. Because information about the receiver is part of an application's manifest, Android can find and launch the application even if it isn't running; of course if it already has its process available in the background, the broadcast can very efficiently be directly dispatched to it.</p>
56 <p>When handling a broadcast, the application is given a fixed set of time (currently 10 seconds) in which to do its work. If it doesn't complete in that time, the application is considered to be misbehaving, and its process immediately tossed into the background state to be killed for memory if needed.</p>
57 <p>Broadcast receivers are great for doing small pieces of work in response to an external stimulus, such as posting a notification to the user after being sent a new GPS location report. They are very lightweight, since the application's process only needs to be around while actively receiving the broadcast. Because they are active for a deterministic amount of time, fairly strong guarantees can be made about not killing their process while running. However they are not appropriate for anything of indeterminate length, such as networking.</p>
61 <p>Process management for services is different than broadcast receivers, because an unbounded number of services can ask to be running for an unknown amount of time. There may not be enough RAM to have all of the requesting services run, so as a result no strong guarantees are made about being able to keep them running.</p>
62 <p>If there is too little RAM, processes hosting services will be immediately killed like background processes are. However, if appropriate, Android will remember that these services wish to remain running, and restart their process at a later time when more RAM is available. For example, if the user goes to a web page that requires large amounts of RAM, Android may kill background service processes like sync until the browser's memory needs go down.</p>
71 The alarm clock schedules a broadcast receiver with the alarm manager, to go off at the next set alarm time.
74 The calendar application likewise schedules an alarm to display or update its notification at the appropriate time for the next calendar event.
