example AudioObserver consumer

observer.js

/**
 * Mock up of observers consuming the notifications for the web audio API
 * explaining in:
 * https://bugzilla.mozilla.org/show_bug.cgi?id=980506#c25
 */

// Use SDK observer
// resource://gre/modules/commonjs/sdk/system/events.js
const { on, off } = require("sdk/system/events");


// Let's say this object is instantiated on every new document load
// and takes a `nsIDOMWindow` argument.
function AudioTool (window) {
  
  // `contexts` will be a hash of context IDs that map to a `Set` of audio nodes
  // related to that context. This is for illustrative purposes of showing
  // all the data returned from the observer service
  let contexts = {};
  
  // Called when `new AudioContext()` occurs in any document
  on("web-audio:create-audio-context", ({subject, type, data}) => {
    // If subject does not match the window for the current document's toolbox,
    // ignore it. Same goes for all the listeners here for the most part.
    if (subject !== window) return;
  
    // `data` from the Observer just has the ID of the AudioContext
    // NICE TO HAVE: data for `sampleRate` and `currentTime` (currentTime not useful
    // in this context, but if we have a `get-context-data` notification, it'd have
    // the same information).
    //
    // We also need the destination node ID as after context creation,
    // another node would be created and connect to the destination, which
    // will require an ID for that node
    let { contextID, destinationID } = data;
    
    // Create a new Set for audio node IDs within this context.
    contexts[data.contextID] = new Set();
    contexts[contextID].add(destinationID);
  });
  
  // Called when a new AudioNode is created from a context.
  on("web-audio:create-audio-node", ({subject, type, data}) => {
    if (subject !== window) return;
    
    // Returns values of both the AudioContext ID and the AudioNode ID
    // from the ObserverService. Also a `type` which is a string referring
    // to the type of node created, i.e. 'OscillatorNode'
    let { contextID, nodeID, type } = data;
    
    contexts[contextID].add(nodeID);
  });
  
  // Called when an AudioNode connects to either another AudioNode
  // or an AudioParam
  on("web-audio:connect-audio-node", ({subject, type, data}) => {
    if (subject !== window) return;
    
    // Returns values of both the AudioContext ID, the ID of the
    // source node connecting, and the ID of the target of that connection.
    // If `paramName` defined, then this is connecting to `targetID`'s 
    // AudioParam by the name of `paramName`.
    //
    // Maybe AudioParams should have their own ID, but they can also be referenced
    // by their Node's ID along with the property name.
    //
    // osc.connect(gain);
    //   -> contextID = "ID-for-context"
    //   -> sourceID = "ID-for-Osc-Node"
    //   -> targetID = "ID-for-Gain-Node"
    //   -> paramName = undefined
    //
    // osc.connect(biquadFilter.frequency)
    //   -> contextID = "ID-for-context"
    //   -> sourceID = "ID-for-Osc-Node"
    //   -> targetID = "ID-for-BiquadFilter-Node"
    //   -> paramName = "frequency"
    
    // `output` and `input` refer to index used during `node.connect(target, output, input)`
    let { contextID, sourceID, targetID, paramName, output, input } = data;

    if (paramName === undefined) {
      doSomethingWithConnectToNode(sourceID, targetID)
    } 
    else {
      doSomethingWithConnectToParam(sourceID, targetID, paramName)
    }
  });
  
  // Called when an AudioNode disconnects from all of it's connections
  on("web-audio:disconnect-audio-node", ({subject, type, data}) => {
    if (subject !== window) return;
    
    // Same return information as `web-audio:connect-audio-node`, minus `input`
    let { contextID, sourceID, targetID, paramName, output } = data;

    if (paramName === undefined) {
      doSomethingWithDisconnectToNode(sourceID, targetID)
    } 
    else {
      doSomethingWithDisconnectToParam(sourceID, targetID, paramName)
    }
  });
  
  // Called when an AudioNode is destroyed/GC'd (AudioNode::DestroyMediaStream?)
  on("web-audio:destroy-audio-node", ({subject, type, data}) => {
    if (subject !== window) return;
    
    let { contextID, nodeID } = data;
    
    contexts[contextID].remove(nodeID);
  });
  
  /**
   * Follow up Bugs/notifications:
   * `web-audio:get-context-properties`
   * `web-audio:get-node-properties`
   * `web-audio:set-node-properties`
   * `web-audio:automation-scheduled`
   */
}