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Skygear provides pubsub services for building apps required real-time updates. A typical pubsub services can be divided into two parts: Publish and Subscription.

Skygear will automatically re-connect on connection drop. Skygear will also re-subscribe all existing handler on connection restore. So in normal case, you don't need to re-subscribe all your handler on re-connect.

Publish

The following code snippet shows how to publish a data object (in JSON format) to a specific channel:

Pubsub pubsub = Container.defaultContainer(this).getPubsub();

JSONObject jsonObject = new JSONObject();
jsonObject.put("msg", "Hello World");

pubsub.publish("chatroom-001", jsonObject);

To publish a message to the channel through cloud code, please refer to the Cloud Functions Guide: Calling Skygear API - Pubsub Events.

Subscribe

You may need to subscribe to a channel, so that you will be notified if there are updates:

Pubsub pubsub = Container.defaultContainer(this).getPubsub();
pubsub.subscribe("chatroom-001", new Pubsub.Handler() {
    @Override
    public void handle(JSONObject data) {
        String msg = data.getString("msg");
        messageList.add(msg);  // Your own logic to handle the incoming message
    }
});

By default, the handler you provide will be dispatched to main thread. If you want your handler to run in the background, you can update the pubsub settings:

Pubsub pubsub = Container.defaultContainer(this).getPubsub();
pubsub.setHandlerExecutionInBackground(true);

Unsubscribe

You may also need to unsubscribe to a channel if you don't need the notification from that channel:

Pubsub pubsub = Container.defaultContainer(this).getPubsub();

// you may need the return value for unsubscription
Pubsub.Handler handler = pubsub.subscribe("chatroom-001", new Pubsub.Handler() {
    // Pubsub.Handler implementation ...
});

pubsub.unsubscribe("chatroom-001", handler);

Of cause, you can add multiple handlers and unsubscribe all of them:

Pubsub pubsub = Container.defaultContainer(this).getPubsub();

pubsub.subscribe("chatroom-001", new Pubsub.Handler() {
    // Pubsub.Handler implementation A ...
});

pubsub.subscribe("chatroom-001", new Pubsub.Handler() {
    // Pubsub.Handler implementation B ...
});

pubsub.subscribe("chatroom-001", new Pubsub.Handler() {
    // Pubsub.Handler implementation C ...
});

pubsub.unsubscribeAll("chatroom-001");