How can I share information between my iOS and Watch apps using WatchConnectivity?

At the time of writing this answer (watchOS3 is the current stable release and watchOS4 is in beta stage), the only option for direct communication between an iOS app and its WatchKit extension is the WatchConnectivity framework. (I said direct, because this Q&A is not concerned with using cloud technologies such as CloudKit to upload files to the internet from one device and download them on the other one.)

First, let's discuss which function of WCSession should be used for what purpose. For the code examples, please scroll down.

A quick, one-liner about each function and when to use them before diving deep into details:

updateApplicationContext: synchronise states between the apps, send data to be displayed on the UI (only use it to send small pieces of data)
transferUserInfo: send a dictionary of data in the background
transferFile: send a file in the background
sendMessage: send an instant message between at least the watch app is running in foreground

Detailed description

updateApplicationContext(_:) should be used if you want to synchronise your apps (such as keep the UI updated or send state information, like user logged in, etc.). You can send a dictionary of data. Subsequent calls to this function replace the previously sent dictionary, so the counterpart app only receives the last item sent using updateApplicationContext. The system tries to call this function at an appropriate time for it to receive data by the time it is needed and meanwhile minimising power usage. For this reason, the function can be called when neither app is running in the foreground, but WCSession needs to be activated for the transfer to succeed. Frequent calls trying to transfer large amount of data using updateApplicationContext might fail, so for this usage call transferUserInfo instead.

transferUserInfo(:) and transferCurrentComplicationUserInfo(:) should be used if you want to send data in the background that needs to be received by the other application. Subsequent calls to this method are queued and all information sent from one app to the other is received. transferCurrentComplicationUserInfo might be used to send complication-related data to the WatchKit extension using a high-priority message and waking up the WatchKit app if needed. However, be aware that this function has a daily limit and once it's exceeded, data is transferred using the transferUserInfo function.

transferFile(_:metadata:) is similar in implementation and nature to transferUserInfo, but it accepts a fileURL instead of a dictionary as its input parameter and hence it should be used to send files local to the device to its counterpart. Subsequent calls are queued. Received files must be saved to a new location in the session(_:didReceive:) method, otherwise they are deleted.

sendMessage(:replyHandler:errorHandler:) and sendMessageData(:replyHandler:errorHandler:) send data immediately to a counterpart app. The counterpart app must be reachable before calling this method. The iOS app is always considered reachable, and calling this method from your Watch app wakes up the iOS app in the background as needed. The Watch app is considered reachable only while it is installed and running. The transfer must be initiated in the foreground. Subsequent calls to this method are queued.

For more information please see App programming guide for watchOS - Sharing Data.

Now some code examples:

Set up WatchConnectivity in the iOS app's AppDelegate:

import UIKit
import WatchConnectivity

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        if WCSession.isSupported() {
            let session = WCSession.default()
            session.delegate = self
            session.activate()
        }
        return true
    }
}

extension AppDelegate: WCSessionDelegate {

    func session(_ session: WCSession, didReceiveMessage message: [String : Any]) {
        print("Message received: ",message)
    }

    //below 3 functions are needed to be able to connect to several Watches
    func session(_ session: WCSession, activationDidCompleteWith activationState: WCSessionActivationState, error: Error?) {}

    func sessionDidDeactivate(_ session: WCSession) {}

    func sessionDidBecomeInactive(_ session: WCSession) {}
}

Make your WatchKit class conform to WCSessionDelegate:

extension InterfaceController: WCSessionDelegate {
    func session(_ session: WCSession, activationDidCompleteWith activationState: WCSessionActivationState, error: Error?) {}
}

Using the instant messaging function, sendMessage:

In the WatchKit app use this code when you want to send information immediately to the iOS app.

if WCSession.isSupported() {
    print("WCSession supported")
    let session = WCSession.default()
    session.delegate = self
    session.activate()
    if session.isReachable {
        session.sendMessage(["Instant":"Message"], replyHandler: nil, errorHandler: { error in
            print("Error sending message",error)
        })
    }
}

Detailed description

Now some code examples:

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