背景執行APP不死

在plist中加入Required Background Modes，然後在下面加入參數
1).App registers for location updates
2).App provides Voice over IP services
3).App plays audio
4).App processes Newsstand Kit downloads
5).App communicates using CoreBluetooth
6).App shares data using CoreBluetooth

在AppDelegate加入這段Code
- (void)applicationDidEnterBackground:(UIApplication *)application
{
    UIApplication *app = [UIApplication sharedApplication];
    __block UIBackgroundTaskIdentifier bgTask;
    bgTask = [app beginBackgroundTaskWithExpirationHandler:^{
        dispatch_async(dispatch_get_main_queue(), ^{
            if (bgTask != UIBackgroundTaskInvalid)
            {
                bgTask = UIBackgroundTaskInvalid;
            }
        });
    }];
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        dispatch_async(dispatch_get_main_queue(), ^{
            if (bgTask != UIBackgroundTaskInvalid)
            {
                bgTask = UIBackgroundTaskInvalid;
            }
        });
    });
}

//官方版

- (void)applicationDidEnterBackground:(UIApplication *)application

{

bgTask = [application beginBackgroundTaskWithExpirationHandler:^{

// Clean up any unfinished task business by marking where you

// stopped or ending the task outright.

[application endBackgroundTask:bgTask];

bgTask = UIBackgroundTaskInvalid;

}];

// Start the long-running task and return immediately.

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

// Do the work associated with the task, preferably in chunks.

[application endBackgroundTask:bgTask];

bgTask = UIBackgroundTaskInvalid;

});

}







//也可在App中加入檢查有無支援多工的程式碼

UIDevice* device = [UIDevice currentDevice];

    BOOL backgroundSupported = NO;

    if ([device respondsToSelector:@selector(isMultitaskingSupported)])

        backgroundSupported = device.multitaskingSupported;

//背景執行的原則

1. 不要在背景執行OpenGL ES
2. 在進入背景前取消所有跟Bonjour有關的服務
3. 
   

Being a Responsible Background App

The foreground app always has precedence over background apps when it comes to the use of system resources and hardware. Apps running in the background need to be prepared for this discrepancy and adjust their behavior when running in the background. Specifically, apps moving to the background should follow these guidelines:

• Do not make any OpenGL ES calls from your code. You must not create an EAGLContext object or issue any OpenGL ES drawing commands of any kind while running in the background. Using these calls causes your app to be killed immediately. Apps must also ensure that any previously submitted commands have completed before moving to the background. For information about how to handle OpenGL ES when moving to and from the background, see “Implementing a Multitasking-aware OpenGL ES Application” in OpenGL ES Programming Guide for iOS.
• Cancel any Bonjour-related services before being suspended. When your app moves to the background, and before it is suspended, it should unregister from Bonjour and close listening sockets associated with any network services. A suspended app cannot respond to incoming service requests anyway. Closing out those services prevents them from appearing to be available when they actually are not. If you do not close out Bonjour services yourself, the system closes out those services automatically when your app is suspended.
• Be prepared to handle connection failures in your network-based sockets. The system may tear down socket connections while your app is suspended for any number of reasons. As long as your socket-based code is prepared for other types of network failures, such as a lost signal or network transition, this should not lead to any unusual problems. When your app resumes, if it encounters a failure upon using a socket, simply reestablish the connection.
• Save your app state before moving to the background. During low-memory conditions, background apps may be purged from memory to free up space. Suspended apps are purged first, and no notice is given to the app before it is purged. As a result, apps should take advantage of the state preservation mechanism in iOS 6 and later to save their interface state to disk. For information about how to support this feature, see“State Preservation and Restoration.”
• Remove strong references to unneeded objects when moving to the background. If your app maintains a large in-memory cache of objects (especially images), remove all strong references to those caches when moving to the background. For more information, see “Memory Usage for Background Apps.”
• Stop using shared system resources before being suspended. Apps that interact with shared system resources such as the Address Book or calendar databases should stop using those resources before being suspended. Priority for such resources always goes to the foreground app. When your app is suspended, if it is found to be using a shared resource, the app is killed.
• Avoid updating your windows and views. While in the background, your app’s windows and views are not visible, so you should not try to update them. Although creating and manipulating window and view objects in the background does not cause your app to be killed, consider postponing this work until you return to the foreground.
• Respond to connect and disconnect notifications for external accessories. For apps that communicate with external accessories, the system automatically sends a disconnection notification when the app moves to the background. The app must register for this notification and use it to close out the current accessory session. When the app moves back to the foreground, a matching connection notification is sent, giving the app a chance to reconnect. For more information on handling accessory connection and disconnection notifications, see External Accessory Programming Topics.
• Clean up resources for active alerts when moving to the background. In order to preserve context when switching between apps, the system does not automatically dismiss action sheets (UIActionSheet) or alert views (UIAlertView) when your app moves to the background. It is up to you to provide the appropriate cleanup behavior prior to moving to the background. For example, you might want to cancel the action sheet or alert view programmatically or save enough contextual information to restore the view later (in cases where your app is terminated).
  For apps linked against a version of iOS earlier than 4.0, action sheets and alerts are still dismissed at quit time so that your app’s cancellation handler has a chance to run.
• Remove sensitive information from views before moving to the background. When an app transitions to the background, the system takes a snapshot of the app’s main window, which it then presents briefly when transitioning your app back to the foreground. Before returning from your applicationDidEnterBackground: method, you should hide or obscure passwords and other sensitive personal information that might be captured as part of the snapshot.
• Do minimal work while running in the background. The execution time given to background apps is more constrained than the amount of time given to the foreground app. Apps that spend too much time executing in the background can be throttled back by the system or terminated.

If you are implementing a background audio app, or any other type of app that is allowed to run in the background, your app responds to incoming messages in the usual way. In other words, the system may notify your app of low-memory warnings when they occur. And in situations where the system needs to terminate apps to free even more memory, the app calls its delegate’s applicationWillTerminate: method to perform any final tasks before exiting.

//iOS7特有種

- (void)applicationDidEnterBackground:(UIApplication *)application { UIApplication *app = [UIApplication sharedApplication]; __block UIBackgroundTaskIdentifier bgTask; bgTask = [app beginBackgroundTaskWithExpirationHandler:^{ dispatch_async(dispatch_get_main_queue(), ^{ if (bgTask != UIBackgroundTaskInvalid) { [app endBackgroundTask:bgTask]; bgTask = UIBackgroundTaskInvalid; } }); }]; dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ dispatch_async(dispatch_get_main_queue(), ^{ if (bgTask != UIBackgroundTaskInvalid) { [app endBackgroundTask:bgTask]; bgTask = UIBackgroundTaskInvalid; } }); }); }