Send messages between iOS and WatchOS with WatchConnectivity in watchOS2

WatchConnectivity

First the two classes that are supposed to communicate with each other (iOS and watchOS) need to conform the <WCSessionDelegate> and #import the WatchConnectivity framework

Before you can send data you need to check if your device is able to send data

if ([WCSession isSupported]) {
      WCSession *session = [WCSession defaultSession];
      session.delegate = self;
      [session activateSession];
      NSLog(@"WCSession is supported");
}

Then if you wish to use “interactive messaging” (sendMessage APIs) you will need to see if the other device is reachable first:

if ([[WCSession defaultSession] isReachable]) {
    //Here is where you will send you data
}

The “background operations” APIs do not require the counterpart device to be reachable at the point in time you call the WCSession API.

You have several options when it comes to transferring data between your apps, in the Apple Documentation they are described like this:

• Use the updateApplicationContext:error: method to communicate only the most recent state information to the counterpart. When the counterpart wakes, it can use this information to update its own state and remain in sync. Sending a new dictionary with this method overwrites the previous dictionary.

• Use the sendMessage:replyHandler:errorHandler: or sendMessageData:replyHandler:errorHandler: method to transfer data immediately to the counterpart. These methods are intended for immediate communication when your iOS app and WatchKit extension are both active.

• Use the transferUserInfo: method to transfer a dictionary of data in the background. The dictionaries you send are queued for delivery to the counterpart and transfers continue when the current app is suspended or terminated.

• Use the transferFile:metadata: method to transfer files in the background. Use this method in cases where you want to send more than a simple dictionary of values. For example, use this method to send images or file-based documents.

I will give you an example how to send/receive data with Application Context

Send data:

WCSession *session = [WCSession defaultSession];
NSError *error;

[session updateApplicationContext:@{@"firstItem": @"item1", @"secondItem":[NSNumber numberWithInt:2]} error:&error];

Receive data:

- (void) session:(nonnull WCSession *)session didReceiveApplicationContext:(nonnull NSDictionary<NSString *,id> *)applicationContext {

    NSLog(@"%@", applicationContext);


    NSString *item1 = [applicationContext objectForKey:@"firstItem"];
    int item2 = [[applicationContext objectForKey:@"secondItem"] intValue];
}

For more information about WatchConnectivity I really recommend watching the WWDC2015 session video and reading the Apple Documentation on WatchConnectivity