[Reading Notes] The program enters the background, and the Reading Notes program is in the background.

[Reading Notes] The program enters the background, and the Reading Notes program is in the background.

When an iOS app is sent to the background, its main thread is paused. The thread you created using the detachNewThreadSelector: toTar get: withObject: Class Method of NSThread is also suspended. If you want to complete a long-term task in the background, you must call the beginBackgroundTaskWithExpirationHandler of UIApplication to borrow time from iOS. The backgroundTimeRemaining attribute of UIApplication contains the number of seconds that a program can use to complete its tasks. If a long-term task is not completed within this period, iOS terminates the program. For each beginBackgroundTaskWithExpirationHandler: method call, endBackgroundTask must be called accordingly: method (another instance method of UIApplication ).