Let's see what this looks like in code. The following code sample demonstrates proper setup of a background transfer session using the CreateBackgroundSessionConfiguration method and a unique string identifier: CreateBackgroundSessionConfiguration "com.
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Working With Tasks and Delegates Now that we have configured a background session, let's kick off tasks to handle the transfer. DidFinishEventsForBackgroundSession - This method gets called when all tasks have finished, and the transfer is complete. DidReceiveChallenge - Called to request credentials when authorization is required. Background sessions are limited to two types of tasks: This delegate provides additional methods to track upload progress, handle HTTP redirection, and more.
This delegate provides all the methods for upload tasks, as well as download-specific methods to track the download progress and determine when a download task has resumed or completed. CreateDownloadTask request ; Next, we'll create a new session download delegate to keep track of all download tasks in this session: WriteLine string.
Format "DownloadTask: We'd love to hear your thoughts. Choose the type you'd like to provide: Product feedback Sign in to give documentation feedback. You may also leave feedback directly on GitHub. Content feedback. From here, edit or add a new scheme and check the Launch due to a background fetch event checkbox as shown below. Remote notifications allow you to notify your app when important events occur. Remote notifications are great for sporadic but immediately important content, where the delay between background fetches might not be acceptable.
Remote Notifications can also be much more efficient than Background Fetch, as your application only launches when necessary. A Remote Notification is really just a normal Push Notification with the content-available flag set. You might send a push with an alert message informing the user that something has happened, while you update the UI in the background. If your push notifications are throttled, they might be delayed until the next time the device sends a keep-alive packet or receives another notification. To send a remote notification, set the content-available flag in a push notification payload.
The content-available flag is the same key used to notify Newsstand apps, so most push scripts and libraries already support remote notifications. This could save you a few networking requests and increase the responsiveness of your app. Here the -n flag specifies that the content-available key should be included, and -d allows us to add our own data keys to the payload. Again, the app is launched into the background and given 30 seconds to fetch new content and update its UI, before calling the completion handler.
There are three types of session tasks — data, download, and upload — each of which add syntactic sugar to NSURLSessionTask , so you should use the appropriate one for your use case. Tasks added to a background session are run in an external process and continue even if your app is suspended, crashes, or is killed.
One option is the discretionary flag, which allows the system to schedule tasks for optimal performance. What this means is that your transfers will only go over Wifi when the device has sufficient power. The discretionary flag only has an effect if the session configuration object has been constructed by calling the backgroundSessionConfiguration: If the transfer is initiated from the background the transfer will always run in discretionary mode.
We create a download task using the NSURLSession class method and configure its request, and provide a description for use later. You must remember to call [task resume] to actually start the task, as all session tasks begin in the suspended state.
Background Transfer Service in iOS 7 SDK: How To Download File in Background
If your app is still running in the foreground when the background session task completes, the above code will be sufficient. In these cases, you must implement two application delegates methods so the system can wake your application. Unlike previous delegate callbacks, the application delegate is called twice, as your session and task delegates may receive several messages. The app delegate method application: In the former method, you store a background completionHandler , and in the latter you call it to update your UI:. To act as a Bluetooth accessory, you must enable support for that mode from the Background modes section of the Capabilities tab in your Xcode project.
Enabling this mode lets the Core Bluetooth framework wake the app up briefly in the background so that it can handle accessory-related requests. Apps woken up for these events should process them and return as quickly as possible so that the app can be suspended again. Any app that supports the background processing of Bluetooth data must be session-based and follow a few basic guidelines:. Apps must provide an interface that allows the user to start and stop the delivery of Bluetooth events.
That interface should then open or close the session as appropriate. For example, an alarm clock app might use local notifications to play an alarm sound and display an alert to disable the alarm. When a notification is delivered to the user, the user must decide if the information warrants bringing the app back to the foreground. If the app is already running in the foreground, local notifications are delivered quietly to the app and not to the user.
To schedule the delivery of a local notification, create an instance of the UILocalNotification class, configure the notification parameters, and schedule it using the methods of the UIApplication class.
Configuring a Background Session
The local notification object contains information about the type of notification to deliver sound, alert, or badge and the time when applicable at which to deliver it. The methods of the UIApplication class provide options for delivering notifications immediately or at the scheduled time. Listing shows an example that schedules a single alarm using a date and time that is set by the user. This example configures only one alarm at a time and cancels the previous alarm before scheduling a new one.
Your own apps can have no more than local notifications active at any given time, any of which can be configured to repeat at a specified interval. The alarm itself consists of an alert box and a sound file that is played if the app is not running or is in the background when the alarm fires. Sound files used with local notifications have the same requirements as those used for push notifications.
When the notification is sent and the sound is played, the system also triggers a vibration on devices that support it. You can cancel scheduled notifications or get a list of notifications using the methods of the UIApplication class. For additional information about configuring local notifications, see Local and Remote Notification Programming Guide.
Apps that support background execution may be relaunched by the system to handle incoming events. If an app is terminated for any reason other than the user force quitting it, the system launches the app when one of the following events happens:. The device entered or exited a registered region. Regions can be geographic regions or iBeacon regions. For audio apps, the audio framework needs the app to process some data.
Audio apps include those that play audio or use the microphone. A push notification arrives for an app and the payload of the notification contains the content-available key with a value of 1. For apps downloading content in the background using the NSURLSession class, all tasks associated with that session object either completed successfully or received an error.
In most cases, the system does not relaunch apps after they are force quit by the user. One exception is location apps, which in iOS 8 and later are relaunched after being force quit by the user. In other cases, though, the user must launch the app explicitly or reboot the device before the app can be launched automatically into the background by the system. When password protection is enabled on the device, the system does not launch an app in the background before the user first unlocks the device. 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:. 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. 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.
Remove strong references to unneeded objects when moving to the background.
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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 Reduce Your Memory Footprint. 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.
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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. The exception is in cases where you need to update the contents of a window prior to having a snapshot of your app taken. 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.
Remove sensitive information from views before moving to the background. Before returning from your applicationDidEnterBackground: 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.
Background Transfer and NSURLSession in Xamarin.iOS
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. When an app opts out, it cycles between the not-running, inactive, and active states and never enters the background or suspended states. When the user presses the Home button to quit the app, the applicationWillTerminate: Opting out of background execution is strongly discouraged but may be the preferred option under certain conditions.
Specifically, if coding for background execution adds significant complexity to your app, terminating the app might be a simpler solution. Also, if your app consumes a large amount of memory and cannot easily release any of it, the system might kill your app quickly anyway to make room for other apps.
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