text: class Callback {
    constructor(collection, callback) {
      this.collection = collection;
      this.callback = callback;
  
      this.handle = Util.guid();
  
      this.register();
    }
  
    stop() {
      delete this.collection.onChangeCallbacks[this.handle];
    }
  
    // Private
  
    register() {
      this.collection.onChangeCallbacks[this.handle] = this.callback;
    }
  }
  