Guides/Scripting Guide/Tutorials/Create Presence

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Creating presences

The previous tutorials explained how to move an entity's presence. This tutorial describes how to create new presences for an entity.

The create presence call

All a scripter does to create a new entity is to execute the createPresence call.

createPresence requires two arguments: a uri for the mesh to load for the newly created presence (as a string) and a callback function to execute once the presence connects.

The createPresence call is non-blocking. What this means is that a presence is not created as soon as the call returns. Instead, a presence is connected to the world and appears some time (usually less than one second) after the call has been made. When the presence does get connected, the system automatically executes the callback, passing in the newly-created presence as an argument to this callback.

Let's combine this information with the previous example on moving presences to create a new presence, and then have it move forever in one direction:

   function presCreatedCallback(presCreated) {
       //the new presence's velocity is set to
       //1 m/s in the x direction.
       presCreated.setVelocity(<1,0,0>);
   }
   
   //the new presence will have the mesh of a kiosk.
   var newPresMesh = 'meerkat:///kittyvision/kiosk/flyers.dae/optimized/0/flyers.dae';
   system.createPresence(newPresMesh,presCreatedCallback);

Managing presences: system.presences and system.self

As you can imagine, an entity with very many presences can become difficult to manage. To address this, Emerson automatically manages two global variables: system.presences and system.self. Any time a new presence is created, it gets loaded into system.presences, an array that contains all the presences connected to an entity. If, for instance, a scripter wanted to set the velocity for all of his/her presences, he/she could enter the following code:

   for (var s in system.presences) {
       system.presences[s].setVelocity(<1,0,0>);
   }

While, system.presences allows a scripter to track all the presences he/she has connected, system.self is intended to automatically track the most "relevant" presence for an event.

Emerson is event-based, and the majority of events in the virtual world are associated with a single presence. system.self gets set to the presence associated with that event. This may seem a little confusing, and may make more sense through a series of examples:

  • If an entity's presence receives a message, system.self is automatically set to the presence that received the message.
  • If an entity receives a timeout event, system.self is automatically set to the presence that originally set the timeout.
  • If an entity receives a presence connected event, system.self is automatically set to the presence that was connected.

To provide an example of system.self in action, recall our earlier task of creating a new presence and setting its velocity to be 1 m/s. We can re-write presCreatedCallback to use system.self instead of presCreated:

   function presCreatedCallback(presCreated) {
       //the new presence's velocity is set to
       //1 m/s in the x direction.
       system.self.setVelocity(<1,0,0>);
       //now using system.self
   }