Tutorial: Subscribing to changes

In JavaScript, subscribing relies on the on(...) method. The first parameter expects one or more subscribed types of event, while the second one expects the notification callback. The optional third parameters expects a callback for revocation events.

The notified events passed to the callback are instances of the DataSnapshot class: among the methods common to all subscribed types of events, acknowledged() indicates whether the data update represented by the event is acknowledged by the back end or local to the client cache, and the eventType() method returns the type of the notified event (it is useful when the callback is subscribed to several types of event at a time) (replace “<your-app>” with your actual application identifier):

const ref = new Webcom("<your-app>");
let node = ref.child("contacts");
node.on(["value", "value_ack"], // subscribed types of update events
    datasnapshot => { // the callback to call when an event is raised (except revocations)
        if (datasnapshot.eventType() === "value") {
            console.log("The data at CONTACTS has changed:", JSON.stringify(datasnapshot.val()));
        }
        console.log("The data at CONTACTS", datasnapshot.acknowledged() ? "is" : "is not yet", "acknowledged");
    },
    revocationError => { // optional callback for revocation events
        console.log("The subscription on CONTACTS has been revoked!", revocationError.message);
    });

Unsubscribing uses the off(...) method:

node.off(); // unsubscibe all callbacks on the node
node.off(["value", "value_ack"]); // unsubscribe all callbaks subscribed to "value" or "value_ack" on the node

In Kotlin, a subscription is registered using the subscribe() method of DatasyncNode objects, and is managed by the DatasyncManager object that has created this DatasyncNode instance:

• The first argument of this method represents the subscribed types of event, excluding “Value Acknowledgement” and “Child Acknowledgement”, which are separately requested using the boolean property includesAcknowledgements of the options parameter. The subscribed types of event are given as their common Kotlin type inheriting the SubscribableEvent interface: Value.Changed::class for the “Value Change” event type, Child.Added::class for the “Child Addition” event type, Child.Changed::class for the “Child Change” event type, etc...
• The last argument of the subscribe() method is a callback function, which is called for each raised event, including cancelation and revocation events.

The notifications received by the callback may be either:

• A cancelation or revocation notification (passed as instances of Notification.ControlNotification subclasses). In this case, it is the last notification to be received by the callback.
• A notification of one of the subscribed events (which are all subclasses of both Notification.DataNotification and SubscribableEvent). Among the properties common to all of these subclasses, isAcknowledgement indicates whether the event is one of the “Value Acknowledgement” or “Child Acknowledgement” events, and value gives the DatasyncValue associated to the event (including its acknowledgement status).

import com.orange.webcom.sdkv2.datasync.subscription.SubscribableEvent.*

val app = WebcomApplication.default
val manager = app.datasyncService.createManager()
val node = manager / "contacts"
val options = SubscriptionOptions(includesAcknowledgements = true)
val subscription = node.subscribe(Value.Changed::class, options = options) { // it: Notification<Value.Changed>
  when (it) {
    is Notification.DataNotification -> { // it.data: Value.Changed
      if (!it.data.isAcknowledgement) println("The data at CONTACTS has changed: ${it.data.value}")
      println("The data at CONTACTS ${if (it.data.value.acknowledged) "is" else "is not"} acknowledged")
    }
    is Notification.ControlNotification ->
      println("The subscription on CONTACTS has been canceled or revoked! ${it.error?.message}")
  }
}

Unsubscribing is done using the cancel() method of the Subscription object returned by the previous subscribe() method:

subscription.cancel()

Note that subscriptions are managed by DatasyncManager objects so that each manager automatically cancels its registered subscription when released (ie garbage-collected). A typical use-case is to create a dedicated manager for each Activity or Fragment of your Android app and bind their life-cycles. Thus, this avoids managing explicitly the cancelation of subscriptions, which is generally a tedious task.

In Swift, a subscription is registered using the subscribe() method of DatasyncNode objects and is managed by the DatasyncManager object that has created this DatasyncNode instance. A subscription is specified by a callback function that will be called each time the corresponding event is raised. The subscribed type of event is represented by a value of the dedicated DatasyncEventType enum.

The subscription callback receives as parameter a DatasyncEvent value that provides convenient methods to retrieve the kind of the just raised event, the value of the associated data node, as well as the corresponding DatasyncSubscription object (which makes it possible to individually revoke the subscription without waiting for the manager to do it).

let manager = Webcom.defaultApplication.datasyncService.createManager()
let contactsNode = manager / "contacts"
contactsNode?.subscribe(to: .valueChange) { event in
    guard let allContacts: [Contact] = event.value.decoded() else {
        return
    }
    print("The 'contacts' node has \(allContacts.count) child nodes")
}

Subscriptions are managed by DatasyncManager objects so that each manager automatically cancels its registered subscription when released. The idea is to create a manager for each view controller and bind their life-cycles, in order to avoid explicitly managing the cancelation of subscriptions, which is generally a tedious task.

Not available in REST API

In JavaScript, the “Value Change” event type to subscribe to is represented by the "value" string. The val() method of the DataSnapshot object passed to the callback gets the subscriber node value.

const ref = new Webcom("<your-app>");
let contactsNode = ref.child("contacts");
// Display the count of contacts in real time
contactsNode.on("value",
    snapshot => console.log("There are now", Object.keys(snapshot.val()).length, "contacts"),
    revocationError => console.log("Contact watching revoked:", revocationError.message)
);

In Kotlin, there are 2 variants of the “Value Change” event type:

• Value.Changed, which conveys the value of the subscriber node when raised,
• Value.ChangedHidingData, which doesn't convey the value of the subscriber node when raised.

import com.orange.webcom.sdkv2.datasync.subscription.SubscribableEvent.*

val app = WebcomApplication.default
val manager = app.datasyncService.createManager()
val contactsNode = manager / "contacts"
// Display the count of contacts in real time
val subscription = contactsNode.subscribe(Value.Changed::class) { // it: Notification<Value.Changed>
  when (it) {
    is Notification.DataNotification -> // it.data: Value.Changed
      println("There are now ${it.data.value.asMap.size} contacts")
    is Notification.ControlNotification -> println("Contact watching revoked: ${it.error?.message}")
  }
}

let app = Webcom.defaultApplication
let manager = app.datasyncService.createManager()
let contactsNode = manager / "contacts"
// Display the count of contacts in real time
contactsNode?.subscribe(to: .valueChange) { event in
    print("There are now \(event.value.decoded(as: [String: Contact].self)?.count ?? 0) contacts")
} onCompletion: { completion in
    if case let .revocation(reason: reason) = completion {
        print("Contact watching revoked: \(reason)")
    }
}

In JavaScript, the “Value Acknowledgement” event type to subscribe to is represented by the "value_ack" string.

const ref = new Webcom("<your-app>");
let contactsNode = ref.child("contacts");
// Display a message each time the contact list is acknowledged by the back end
contactsNode.on(["value", "value_ack"], snapshot => {
  if (snapshot.acknowledged()) { console.log("The contact list is acknowledged"); }
});

As explained above, in Kotlin, you don't subscribe explicitly to the “Value Acknowledgement” event type. Instead, you must subscribe to the Value.Changed (or Value.ChangedHidingData) event and pass SubscriptionOptions with the includesAcknowledgement property set to true to the subscribe() method.

import com.orange.webcom.sdkv2.datasync.subscription.SubscribableEvent.*

val app = WebcomApplication.default
val manager = app.datasyncService.createManager()
val contactsNode = manager / "contacts"
// Display a message each time the contact list is acknowledged by the back end
val options = SubscriptionOptions(includesAcknowledgements = true)
contactsNode.subscribe(Value.Changed::class, options = options) { // it: Notification<Value.Changed>
  if (it is Notification.DataNotification && it.data.value.acknowledged) println("The contact list is acknowledged")
}

As a consequence, contrary to the JavaScript SDK, it is not possible to subscribe to the “Value Acknowledgement” event alone, without subscribing to the “Value Change” event. It is not really a limitation, since this use case is quite useless (see the tip corner below).

let app = Webcom.defaultApplication
let manager = app.datasyncService.createManager()
let contactsNode = manager / "contacts"
// Display a message each time the contact list is acknowledged by the back end
contactsNode?.subscribe(to: .valueEvent) {
    if $0.value.isAcknowledged {
        print("The contact list is acknowledged")
    }
}

In JavaScript, the “Child Addition” event type to subscribe to is represented by the "child_added" string. The first argument passed to the callback is a DataSnapshot object, whose name() method gives the key of the added child, and the val() method gets its value. The second argument passed to the callback is a string representing the key of the child that precedes the added child or null if it is the 1st child of the subscriber node.

const ref = new Webcom("<your-app>");
let contactsNode = ref.child("contacts");
// Update a GUI in real-time each time a new contact is added
contactsNode.on("child_added",
        (snapshot, leftChild) => myView.addItem({name: snapshot.name(), value: snapshot.val(), after: leftChild}),
        revocationError => console.log("Contact watching revoked:", revocationError.message)
);

In Kotlin, the “Child Addition” event type is represented by the Child.Added interface:

import com.orange.webcom.sdkv2.datasync.subscription.SubscribableEvent.*

val app = WebcomApplication.default
val manager = app.datasyncService.createManager()
val contactsNode = manager / "contacts"
// Update a GUI in real-time each time a new contact is added
val subscription = contactsNode.subscribe(Child.Added::class) { // it: Notification<Child.Added>
  when (it) {
    is Notification.DataNotification -> // it.data: Child.Added
      myView.addItem(name = it.data.node.key, value = it.data.value.asA(Contact::class))
    is Notification.ControlNotification -> println("Contact watching revoked: ${it.error?.message}")
  }
}

If needed, the previousKey property gives the key of the preceding child node or null if the subscriber node is in first position.

let app = Webcom.defaultApplication
let manager = app.datasyncService.createManager()
let contactsNode = manager / "contacts"
// Update a GUI in real-time each time a new contact is added
contactsNode?.subscribe(to: .childAddition) { event in
    myView.addItem(name: event.key, value: event.value.decoded())
} onCompletion: { completion in
    if case let .revocation(reason: reason) = completion {
        print("Contact watching revoked: \(reason)")
    }
}

In JavaScript, the “Child Change” event type to subscribe to is represented by the "child_changed" string. The first argument passed to the callback is a DataSnapshot object, whose name() method gives the key of the changed child, and the val() method gets its new value. The second argument passed to the callback is a string representing the key of the child that precedes the changed child or null if it is the 1st child of the subscriber node.

const ref = new Webcom("<your-app>");
let contactsNode = ref.child("contacts");
// Update a GUI in real-time each time a contact is updated
contactsNode.on("child_changed",
        (snapshot, leftChild) => myView.updateItem({name: snapshot.name(), value: snapshot.val(), after: leftChild}),
        revocationError => console.log("Contact watching revoked:", revocationError.message)
);

In Kotlin, the “Child Change” event type is represented by the Child.Changed interface:

import com.orange.webcom.sdkv2.datasync.subscription.SubscribableEvent.*

val app = WebcomApplication.default
val manager = app.datasyncService.createManager()
val contactsNode = manager / "contacts"
// Update a GUI in real-time each time a new contact is added
val subscription = contactsNode.subscribe(Child.Changed::class) { // it: Notification<Child.Changed>
  when (it) {
    is Notification.DataNotification -> // it.data: Child.Changed
      myView.updateItem(name = it.data.node.key, value = it.data.value.asA(Contact::class), after = it.data.previousKey)
    is Notification.ControlNotification -> println("Contact watching revoked: ${it.error?.message}")
  }
}

As with Child.Added, the previousKey property gives the key of the preceding child node or null if the subscriber node is in first position.

let app = Webcom.defaultApplication
let manager = app.datasyncService.createManager()
let contactsNode = manager / "contacts"
// Update a GUI in real-time each time a new contact is added
contactsNode?.subscribe(to: .childChange) { event in
    myView.updateItem(name: event.key, value: event.value.decoded())
} onCompletion: { completion in
    if case let .revocation(reason: reason) = completion {
        print("Contact watching revoked: \(reason)")
    }
}

In JavaScript, the “Child Removal” event type to subscribe to is represented by the "child_removed" string. The argument passed to the callback is a DataSnapshot object, whose name() method gives the key of the changed child, and the val() method gets its deleted value.

const ref = new Webcom("<your-app>");
let contactsNode = ref.child("contacts");
// Update a GUI in real-time each time a contact is deleted
contactsNode.on("child_removed",
        snapshot => myView.deleteItem({name: snapshot.name()}),
        revocationError => console.log("Contact watching revoked:", revocationError.message)
);

In Kotlin, the “Child Removal” event type is represented by the Child.Removed interface:

import com.orange.webcom.sdkv2.datasync.subscription.SubscribableEvent.*

val app = WebcomApplication.default
val manager = app.datasyncService.createManager()
val contactsNode = manager / "contacts"
// Update a GUI in real-time each time a new contact is added
val subscription = contactsNode.subscribe(Child.Removed::class) { // it: Notification<Child.Removed>
  when (it) {
    is Notification.DataNotification -> // it.data: Child.Removed
      myView.deleteItem(name = it.data.node.key)
    is Notification.ControlNotification -> println("Contact watching revoked: ${it.error?.message}")
  }
}

let app = Webcom.defaultApplication
let manager = app.datasyncService.createManager()
let contactsNode = manager / "contacts"
// Update a GUI in real-time each time a new contact is added
contactsNode?.subscribe(to: .childRemoval) { event in
    myView.deleteItem(name: event.key)
} onCompletion: { completion in
    if case let .revocation(reason: reason) = completion {
        print("Contact watching revoked: \(reason)")
    }
}

In JavaScript, the “Child Acknowledgement” event type to subscribe to is represented by the "child_ack" string.

const ref = new Webcom("<your-app>");
let contactsNode = ref.child("contacts");
// Update a GUI in real-time each time the list of contacts is updated
contactsNode.on(["child_added", "child_changed", "child_removed", "child_ack"],
        (snapshot, leftChild) => {
          switch (snapshot.eventType()) {
            case "child_added":
              myView.addItem({name: snapshot.name(), value: snapshot.val(), after: leftChild});
              break;
            case "child_changed":
              myView.updateItem({name: snapshot.name(), value: snapshot.val()});
              break;
            case "child_removed":
              myView.deleteItem({name: snapshot.name()}); // mark the item removed without deleting it
              break;
            default: // "child_ack" events do nothing here
              break;
          }
          // darken, lighten or delete the item depending on its acknowledged and removed statuses
          myView.ackItem({name: snapshot.name(), acknowledged: snapshot.acknowledged()});
        },
        revocationError => console.log("Contact watching revoked:", revocationError.message)
);

Like with the “Value Acknowledgement” event type, in Kotlin, you don't subscribe explicitly to the “Child Acknowledgement” event type. Instead, you must subscribe to one or more child events and pass SubscriptionOptions with the includesAcknowledgement property set to true to the subscribe() method.

import com.orange.webcom.sdkv2.datasync.subscription.SubscribableEvent.*

val app = WebcomApplication.default
val manager = app.datasyncService.createManager()
val contactsNode = manager / "contacts"
// Update a GUI in real-time each time a new contact is added
val options = SubscriptionOptions(includesAcknowledgements = true)
val subscription = contactsNode.subscribe(Child.AddedChangedRemoved::class, options = options) { // it: Notification<Child.AddedChangedRemoved>
  when (it) {
    is Notification.DataNotification -> with(it.data) { // this: Child.AddedChangedRemoved
      // this is an instance of either Child.Added, Child.Changed, Child.Removed or Child.Acknowledged
      if (!isAcknowledgement) when (this) { // Child.Acknowledged inherits all Child.* interfaces, so it must be checked first
        is Child.Added -> myView.addItem(name = node.key, value = value.asA(Contact::class))
        is Child.Changed -> myView.updateItem(name = node.key, value = value.asA(Contact::class), after = previousKey)
        is Child.Removed -> myView.deleteItem(name = node.key) // mark the item removed without deleting it
      }
      // darken, lighten or delete the item depending on its acknowledged and removed statuses
      myView.markItem(name = node.key, acknowledged = value.acknowledged)
    }
    is Notification.ControlNotification -> println("Contact watching revoked: ${it.error?.message}")
  }
}

As the Child.Acknowledged interface inherits all Child.Added, Child.Changed and Child.Removed ones. Consequently, you have to check whether an event is Child.Acknowledged BEFORE performing any pattern matching against one of these interfaces.
To do so, simply test the isAcknowledgement property.

Note also how you can subscribe to several child events: there is one interface available for each possible combination: Child.AddedChanged for subscribing to both “Child Addition” and “Child Change” events, Child.AddedRemoved for both “Child Addition” and “Child Removal”, etc...

let app = Webcom.defaultApplication
let manager = app.datasyncService.createManager()
let contactsNode = manager / "contacts"
// Update a GUI in real-time each time the list of contacts is updated
contactsNode?.subscribe(to: .childEvent) { event in
    switch event.eventType {
    case .childAddition:
        myView.addItem(name: event.key, value: event.value.decoded())
    case .childChange:
        myView.updateItem(name: event.key, value: event.value.decoded())
    case .childRemoval:
        myView.deleteItem(name: event.key) // mark the item removed without deleting it
    case .childAcknowledgement:
        break // childAcknowledgement vents do nothing here
    default:
        break // other events are not subscribed
    }
    // darken, lighten or delete the item depending on its acknowledged and removed statuses
    myView.markItem(name: event.key, isAcknowledged: event.value.isAcknowledged) 
} onCompletion: { completion in
    if case let .revocation(reason: reason) = completion {
        print("Contact watching revoked: \(reason)")
    }
}