The runtime environment of iPhone OS is designed for the fast and secure execution of programs. The following sections describe the key aspects of this runtime environment and provide guidance on how best to operate within it.
Fast Launch, Short Use
The strength of iPhone OS–based devices is their immediacy. A typical user pulls a device out of a pocket or bag and uses it for a few seconds, or maybe a few minutes, before putting it away again. The user might be taking a phone call, looking up a contact, changing the current song, or getting some piece of information during that time.
In iPhone OS, only one foreground application runs at a time. This means that every time the user taps your application’s icon on the Home screen, your application must launch and initialize itself quickly to minimize the delay. If your application takes a long time to launch, the user may be less inclined to use it.
In addition to launching quickly, your application must be prepared to exit quickly too. Whenever the user leaves the context of your application, whether by pressing the Home button or by using a feature that opens content in another application, iPhone OS tells your application to quit. At that time, you need to save any unsaved changes to disk and exit as quickly as possible. If your application takes more than 5 seconds to quit, the system may terminate it outright.
Even though your application does not run in the background when the user switches to another application, you are encouraged to make it appear as if that is the case. When your application quits, you should save out information about your application’s current state in addition to any unsaved data. At launch time, you should look for this state information and use it to restore your application to the state it was in when it was last used. Doing so provides a more consistent user experience by putting the user right back where they were when they last used your application. Saving the user’s place in this way also saves time by potentially eliminating the need to navigate back through multiple screens’ worth of information each time an application is launched.
The Application Sandbox
For security reasons, iPhone OS restricts an application (including its preferences and data) to a unique location in the file system. This restriction is part of the security feature known as the application’ s “sandbox.” The sandbox is a set of fine-grained controls limiting an application’s access to files, preferences, network resources, hardware, and so on. In iPhone OS, an application and its data reside in a secure location that no other application can access. When an application is installed, the system computes a unique opaque identifier for the application. Using a root application directory and this identifier, the system constructs a path to the application’s home directory. Thus an application home directory could be depicted as having the following structure:
During the installation process, the system creates the application’s home directory and several key subdirectories, configures the application sandbox, and copies the application bundle to the home directory.
The use of a unique location for each application and its data simplifies backup-and-restore operations, application updates, and uninstallation. For more information about the application-specific directories created for each application and about application updates and backup-and-restore operations, see “File and Data Management”).
The Virtual Memory System
To manage program memory, iPhone OS uses essentially the same virtual memory system found in Mac OS X. In iPhone OS, each program still has its own virtual address space, but (unlike Mac OS X) its usable virtual memory is constrained by the amount of physical memory available. This is because iPhone OS does not write volatile pages to disk when memory gets full. Instead, the virtual memory system frees up nonvolatile memory, as needed, to make sure the running application has the space it needs. It does this by removing memory pages that are not being used and that contain read-only contents, such as code pages. Such pages can always be loaded back into memory later if they are needed again.
If memory continues to be constrained, the system may also send notifications to the running applications, asking them to free up additional memory. All applications should respond to this notification and do their part to help relieve the memory pressure. For information on how to handle such notifications in your application,see “Observing Low- Memory Warnings”
The Automatic Sleep Timer
One way iPhone OS attempts to save power is through the automatic sleep timer. If the system does not detect touch events for an extended period of time, it dims the screen initially and eventually turns it off altogether. Although most developers should leave this timer on, game developers and developers whose applications do not use touch inputs can disable this timer to prevent the screen from dimming while their application is running. To disable the timer, set the idleTimerDisabled property of the shared UIApplication object to YES.
Because it results in greater power consumption, disabling the sleep timer should be avoided at all costs. The only applications that should consider using it are mapping applications, games, or applications that do not rely on touch inputs but do need to display visual content on the device’s screen. Audio applications do not need to disable the timer because audio content continues to play even after the screen dims. If you do disable the timer, be sure to reenable it as soon as possible to give the system the option to conserve more power.
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