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The Observer feature is a new API concept that was introduced in release 15.04. The Observer is a convenience API that aims to simplify the task of discovering all objects on the bus that implement a given set of interfaces, and interacting with those objects.

In a nutshell, it handles:

  • About notifications (calling BusAttachment::WhoImplements, installing AboutListeners, interpreting AboutListener::Announced messages, ...)
  • session and presence management (setting up sessions with peers hosting bus objects of interest, periodic ping requests to check whether the peer is still present on the bus, ...)
  • ProxyBusObject creation and bookkeeping

on behalf of the application.

As is evident from the above summary, the Observer is a consumer-side (client-side) API for AllJoyn: it is designed to facilitate the consumption of information or services offered by peers on the AllJoyn bus, not to aid your application in exposing functionality of its own.

When to Use an Observer

The Observer API is most useful if you're interested in all bus objects that implement a given set of interfaces. The Observer will automatically maintain a session with each peer that hosts at least one bus object implementing the set of interfaces.

The Observer lends itself well to a publish-subscribe-like approach to AllJoyn interaction. The set of interfaces the Observer looks for can be considered the "topic" the application is subscribed to. The Observer offers notifications to inform the application that "instances" (bus objects implementing the set of interfaces) have appeared on the topic, or have disappeared from the topic. The ProxyBusObject::PropertiesChangedListener mechanism can be used to subscribe to notifications of state updates for the topic instances (i.e., property value updates for the discovered bus objects).

If the interaction model of your application is more service-oriented, i.e. the application enumerates all peers that offer a given service, selects one, and then interacts exclusively with the selected peer, the Observer API is probably not for you. Manual About discovery and session management will be more efficient, albeit less convenient, in this case.


The Observer API is currently available in four language bindings:

  • C++
  • C
  • Java
  • Objective-C (iOS & Mac OS X)

The API is very similar in all language bindings. Please refer to the respective API references for the language bindings for details on Observer usage in the various supported programming languages. In this guide, we'll limit ourselves mainly to the C++ language binding.

Creating an Observer

To create an Observer, you need to supply a BusAttachment and the minimal set of interfaces bus objects must implement to be considered eligible for discovery by the Observer.

Note: the interface names passed to the Observer must correspond with interfaces that have previously been registered with the BusAttachment (via BusAttachment::CreateInterface and InterfaceDescription::Activate). If this is not the case, the Observer creation will fail silently, and the Observer will not discover any objects on the bus.

BusAttachment bus;
const char* mandatory[] = { "org.alljoyn.example.Foo", "org.alljoyn.example.Bar" };

Observer obs(bus, mandatory, sizeof(mandatory)/sizeof(mandatory[0]));

It is possible to create different Observers side-by-side, even Observers that have the same set of mandatory interfaces pose no problem.

Once created, the Observer will monitor the About announcements emitted by the peers on the bus for objects that implement at least the set of mandatory interfaces. The Observer will set up a session with peers that host at least one such bus object, and create ProxyBusObjects for each of the discovered objects. These ProxyBusObject instances have support for all discovered interfaces in the remote object, not just the mandatory interfaces.

Note: due to the design of the Java language binding, it is not possible to make the Java ProxyBusObjects support all interfaces in the discovered object. Therefore, the Java Observer constructor allows you to pass in a second set of interfaces, the so-called optional interfaces. The ProxyBusObjects created by the Java Observer will support all mandatory interfaces, and those optional interfaces that are implemented by the corresponding remote object.

Getting Notifications for Discovered and Lost Objects

The Observer supports asynchronous notification of applications via the Observer::Listener class. The listener defines two callbacks:

  • ObjectDiscovered(ProxyBusObject& proxy) is invoked whenever a new remote object is discovered. The proxy object that is passed along into this callback can be used for the registration of PropertiesChanged listeners. The Observer will keep this proxy object around until the corresponding remote object is removed from the bus.
  • ObjectLost(ProxyBusObject& proxy) is invoked whenever a previously discovered object is lost. Objects are considered lost if the peer that hosts them issues an About announcement that no longer includes that object, if the session to the hosting peer is lost, or if that peer becomes unresponsive to ping requests.

It is possible to register multiple listeners to a single Observer. To register a listener, call Observer::RegisterListener. This method takes two parameters: a reference to the listener object, and an optional boolean parameter (true by default) that states whether the application wants to receive notifications for already-discovered objects.

The latter parameter is necessary because it is possible that the Observer has already discovered various remote objects in the time window between its construction and the registration of the first listener.

class MyListener : public Observer::Listener {
    virtual void ObjectDiscovered(ProxyBusObject& proxy) {
        std::cout << "Discovered object with path " << proxy.GetPath();
        std::cout << " from peer " << proxy.GetUniqueName() << std::endl;
    virtual void ObjectLost(ProxyBusObject& proxy) {
        std::cout << "Lost object with path " << proxy.GetPath();
        std::cout << " from peer " << proxy.GetUniqueName() << std::endl;

MyListener listener;
obs.RegisterListener(listener, true);

/* ... */

/* when you're done, there are two ways to unregister a listener */
/* or, alternatively */

Retrieving a Specific Proxy Object

Between the invocation of Observer::Listener::ObjectDiscovered and Observer::Listener::ObjectLost for the same ProxyBusObject, the Observer keeps a reference to that proxy object around internally. You can iterate over all proxies in the Observer (discussed later), or retrieve a specific proxy object.

A remote object is uniquely identified by the pair (unique bus name, object path). In the C++ language binding, the ajn::ObjectId type encapsulates this pair in a convenient class.

To retrieve a specific proxy object from the Observer, call Observer::Get(ObjectId id).

ProxyBusObject proxy = obs.Get(ObjectId(unique_name, "/some/path"));
if (proxy.IsValid()) {
    // OK, the Observer knows about an object with this identity
} else {
    // ouch, the Observer does not know an object with this identity

Iterating Over All Discovered Objects

To iterate over all discovered objects, use the Observer::GetFirst() and Observer::GetNext() methods.

for (ProxyBusObject iter = obs.GetFirst(); iter.IsValid(); iter = obs.GetNext(iter)) {
    // do something fun with the proxy

Best Practices

Threading Model

All Observer::Listener callbacks are invoked from the BusAttachment's dispatcher threads, as is the case for all other application-facing callbacks in the AllJoyn framework (asynchronous method replies, session listener callbacks, etc.). If you want to perform a blocking or long-running operation in a callback, you should first call BusAttachment::EnableConcurrentCallbacks.

The Observer has been designed in such a way that it is never possible to have two ObserverListener callbacks in flight at the same time, not even callbacks from different listeners associated with different Observer instances.

Service-side Requirements

The Observer functionality only works well if the service side (i.e. the peer that provides the bus object) plays along nicely. The requirements on the service side are simple:

  • announce bus objects and their interfaces through About
  • keep your application's About announcements up to date: if bus objects are registered or unregistered, be sure to do the appropriate re-announcement
  • accept sessions on the session port your application announces in About. The Observer will attempt to establish a point-to-point session on that port. If that fails, the Observer will ignore the bus objects your application announces.
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