UIApplication​Delegate launch​Options

Written by Mattt Thompson

AppDelegate is the dumping ground for functionality in iOS.

Application lifecycle management? URL routing? Notifications? Core Data incantations? Invasive and monolithic 3rd-party SDK initialization? Random functionality that doesn’t seem to fit anywhere else? Just stash it in AppDelegate.m!

And of all of the crowded, over-loaded parts in AppDelegate, -application:didFinishLaunchingWithOptions: is the most congested of all.

For many developers, the launchOptions parameter is akin to the String[] args argument of a Java main method—something ignored in the rush to building an application. Hiding in plain sight, launchOptions contains a wealth of keyed knowledge that speaks to the multitude of ways an app can be launched on iOS.

This week, all will be revealed in this NSHipster tell-all about the least understood parameter of the most important method in UIKit: launchOptions.


Every app begins with UIApplicationDelegate -application:didFinishLaunchingWithOptions: (or more accurately, -application:willFinishLaunchingWithOptions:, when implemented). It is called by the application to notify its delegate that the launch process is finishing, and nearly ready to run.

An app launches when its icon is tapped on Springboard, but there are several other occasions in which an app can be launched. For example, an app registered for a custom URL scheme, such as twitter://, could be launched as a result of opening a URL. An app could also be launched in response to a push notification, or a significant change in device location.

Determining why and how an app launched is the responsibility of the launchOptions parameter. Like a userInfo dictionary, -application:didFinishLaunchingWithOptions: can get information for particular named keys in launchOptions.

Many of these keys are also available in the UIApplicationDidFinishLaunchingNotification notification posted on application launch. Check the docs for additional details.

Numerous as they are, launchOptions keys can be more easily understood when organized into groups, corresponding to why the app was launched:

Opening from URL

Apps can launch other apps by passing URLs:

[[UIApplication sharedApplication] openURL:[NSURL URLWithString:@"app://..."]];

For example, an http:// URL would open in Safari, a mailto:// URL would open in Mail, and a tel:// URL would open in Phone.

In these circumstances, the UIApplicationLaunchOptionsURLKey key would be populated in launchOptions.

  • UIApplicationLaunchOptionsURLKey: Indicates that the app was launched in order to open a URL. The value of this key is an NSURL object containing the URL to open.

An app can also be launched through URLs with additional system information. When an app is launched from an UIDocumentInteractionController or via AirDrop, the following keys are set in launchOptions:

  • UIApplicationLaunchOptionsSourceApplicationKey: Identifies the app that requested the launch of your app. The value of this key is an NSString object that represents the bundle ID of the app that made the request
  • UIApplicationLaunchOptionsAnnotationKey: Indicates that custom data was provided by the app that requested the opening of the URL. The value of this key is a property-list object containing the custom data.
NSURL *fileURL = [[NSBundle mainBundle] URLForResource:@"Document" withExtension:@"pdf"];
if (fileURL) {
    UIDocumentInteractionController *documentInteractionController = [UIDocumentInteractionController interactionControllerWithURL:fileURL];
    documentInteractionController.annotation = @{@"foo": @"bar"};
    [documentInteractionController setDelegate:self];
    [documentInteractionController presentPreviewAnimated:YES];
}

Responding to Notification

Not to be confused with NSNotification, apps can be sent remote or local notifications.

Remote Notification

Introduced in iOS 3, remote, or “push” notifications are one of the defining features of the mobile platform.

To register for remote notifications, registerForRemoteNotificationTypes: is called in application:didFinishLaunchingWithOptions:.

[application registerForRemoteNotificationTypes:
    UIRemoteNotificationTypeBadge |
    UIRemoteNotificationTypeSound |
    UIRemoteNotificationTypeAlert];

…which, if successful, calls -application:didRegisterForRemoteNotificationsWithDeviceToken:. Once the device has been successfully registered, it can receive push notifications at any time.

If an app receives a notification while in the foreground, its delegate will call application:didReceiveRemoteNotification:. However, if the app is launched, perhaps by swiping the alert in notification center, application:didFinishLaunchingWithOptions: is called with the UIApplicationLaunchOptionsRemoteNotificationKey launch option:

  • UIApplicationLaunchOptionsRemoteNotificationKey: Indicates that a remote notification is available for the app to process. The value of this key is an NSDictionary containing the payload of the remote notification. > - alert: Either a string for the alert message or a dictionary with two keys: body and show-view. > - badge: A number indicating the quantity of data items to download from the provider. This number is to be displayed on the app icon. The absence of a badge property indicates that any number currently badging the icon should be removed. > - sound: The name of a sound file in the app bundle to play as an alert sound. If “default” is specified, the default sound should be played.

Since this introduces two separate code paths for notification handling, a common approach is to have application:didFinishLaunchingWithOptions: manually call application:didReceiveRemoteNotification::

- (BOOL)application:(UIApplication *)application
didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // ...

    if (launchOptions[UIApplicationLaunchOptionsRemoteNotificationKey]) {
        [self application:application didReceiveRemoteNotification:launchOptions[UIApplicationLaunchOptionsRemoteNotificationKey]];
    }
}

Local Notification

Local notifications were added in iOS 4, and to this day, are still surprisingly misunderstood.

Apps can schedule UILocalNotifications to trigger at some future time or interval. If the app is active in the foreground at that time, the app calls -application:didReceiveLocalNotification: on its delegate. However, if the app is not active, the notification will be posted to Notification Center.

Unlike remote notifications, UIApplication delegate provides a unified code path for handling local notifications. If an app is launched through a local notification, it calls -application:didFinishLaunchingWithOptions: followed by -application:didReceiveLocalNotification: (that is, there is no need to call it from -application:didFinishLaunchingWithOptions: like remote notifications).

A local notification populates the launch options on UIApplicationLaunchOptionsLocalNotificationKey, which contains a payload with the same structure as a remote notification:

  • UIApplicationLaunchOptionsLocalNotificationKey: Indicates that a local notification is available for the app to process. The value of this key is an NSDictionary containing the payload of the local notification.

In the case where it is desirable to show an alert for a local notification delivered when the app is active in the foreground, and otherwise wouldn’t provide a visual indication, here’s how one might use the information from UILocalNotification to do it manually:

// .h
@import AVFoundation;

@interface AppDelegate ()
@property (readwrite, nonatomic, assign) SystemSoundID localNotificationSound;
@end

// .m
- (void)application:(UIApplication *)application
didReceiveLocalNotification:(UILocalNotification *)notification
{
    if (application.applicationState == UIApplicationStateActive) {
        UIAlertView *alertView =
            [[UIAlertView alloc] initWithTitle:notification.alertAction
                                       message:notification.alertBody
                                      delegate:nil
                             cancelButtonTitle:NSLocalizedString(@"OK", nil)
                             otherButtonTitles:nil];

        if (!self.localNotificationSound) {
            NSURL *soundURL = [[NSBundle mainBundle] URLForResource:@"Sosumi"
                                                      withExtension:@"wav"];
            AudioServicesCreateSystemSoundID((__bridge CFURLRef)soundURL, &_localNotificationSound);
        }
        AudioServicesPlaySystemSound(self.localNotificationSound);

        [alertView show];
    }
}

- (void)applicationWillTerminate:(UIApplication *)application {
    if (self.localNotificationSound) {
        AudioServicesDisposeSystemSoundID(self.localNotificationSound);
    }
}

Location Event

Building the next great geomobilelocalsocial check-in photo app? Well, you’re about 4 years late to the party.

But fear not! With iOS region monitoring, your app can be launched on location events:

  • UIApplicationLaunchOptionsLocationKey: Indicates that the app was launched in response to an incoming location event. The value of this key is an NSNumber object containing a Boolean value. You should use the presence of this key as a signal to create a CLLocationManager object and start location services again. Location data is delivered only to the location manager delegate and not using this key.

Here’s an example of how an app might go about monitoring for significant location change to determine launch behavior:

// .h
@import CoreLocation;

@interface AppDelegate () <CLLocationManagerDelegate>
@property (readwrite, nonatomic, strong) CLLocationManager *locationManager;
@end

// .m
- (BOOL)application:(UIApplication *)application
didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // ...

    if (![CLLocationManager locationServicesEnabled]) {
        [[[UIAlertView alloc] initWithTitle:NSLocalizedString(@"Location Services Disabled", nil)
                                    message:NSLocalizedString(@"You currently have all location services for this device disabled. If you proceed, you will be asked to confirm whether location services should be reenabled.", nil)
                                   delegate:nil
                          cancelButtonTitle:NSLocalizedString(@"OK", nil)
                          otherButtonTitles:nil] show];
    } else {
        self.locationManager = [[CLLocationManager alloc] init];
        self.locationManager.delegate = self;
        [self.locationManager startMonitoringSignificantLocationChanges];
    }

    if (launchOptions[UIApplicationLaunchOptionsLocationKey]) {
        [self.locationManager startUpdatingLocation];
    }
}

Newsstand

All of the Newsstand developers in the house: say “Yeah!”

crickets.aiff

Well alright, then.

Newsstand can launch when newly-downloaded assets are available.

This is how you register:

[application registerForRemoteNotificationTypes:
    UIRemoteNotificationTypeNewsstandContentAvailability];

And this is the key to look out for in launchOptions:

  • UIApplicationLaunchOptionsNewsstandDownloadsKey: Indicates that newly downloaded Newsstand assets are available for your app. The value of this key is an array of string identifiers that identify the NKAssetDownload objects corresponding to the assets. Although you can use the identifiers for cross-checking purposes, you should obtain the definitive array of NKAssetDownload objects (representing asset downloads in progress or in error) through the downloadingAssets property of the NKLibrary object representing the Newsstand app’s library.

Not too much more to say about that.

Bluetooth

iOS 7 introduced functionality that allows apps to be relaunched by Bluetooth peripherals.

If an app launches, instantiates a CBCentralManager or CBPeripheralManager with a particular identifier, and connects to other Bluetooth peripherals, the app can be re-launched by certain actions from the Bluetooth system. Depending on whether it was a central or peripheral manager that was notified, one of the following keys will be passed into launchOptions:

  • UIApplicationLaunchOptionsBluetoothCentralsKey: Indicates that the app previously had one or more CBCentralManager objects and was relaunched by the Bluetooth system to continue actions associated with those objects. The value of this key is an NSArray object containing one or more NSString objects. Each string in the array represents the restoration identifier for a central manager object.
  • UIApplicationLaunchOptionsBluetoothPeripheralsKey: Indicates that the app previously had one or more CBPeripheralManager objects and was relaunched by the Bluetooth system to continue actions associated with those objects. The value of this key is an NSArray object containing one or more NSString objects. Each string in the array represents the restoration identifier for a peripheral manager object.
// .h
@import CoreBluetooth;

@interface AppDelegate () <CBCentralManagerDelegate>
@property (readwrite, nonatomic, strong) CBCentralManager *centralManager;
@end

// .m
self.centralManager = [[CBCentralManager alloc] initWithDelegate:self queue:nil options:@{CBCentralManagerOptionRestoreIdentifierKey:(launchOptions[UIApplicationLaunchOptionsBluetoothCentralsKey] ?: [[NSUUID UUID] UUIDString])}];

if (self.centralManager.state == CBCentralManagerStatePoweredOn) {
    static NSString * const UID = @"7C13BAA0-A5D4-4624-9397-15BF67161B1C"; // generated with `$ uuidgen`
    NSArray *services = @[[CBUUID UUIDWithString:UID]];
    NSDictionary *scanOptions = @{CBCentralManagerScanOptionAllowDuplicatesKey:@YES};
    [self.centralManager scanForPeripheralsWithServices:services options:scanOptions];
}

Keeping track of all of the various ways and means of application launching can be exhausting. So it’s fortunate that any given app will probably only have to handle one or two of these possibilities.

Knowing what’s possible is often what it takes to launch an app from concept to implementation, so bear in mind all of your options when the next great idea springs to mind.