epub.js/lib/epubjs/hooks.js

EPUBJS.Hook = function(context){
  this.context = context || this;
  this.hooks = [];
};

//-- Hooks allow for injecting async functions that must all complete in order before finishing 
//   Functions must return a promise.

// this.beforeDisplay = new EPUBJS.Hook();
// this.beforeDisplay.register(function(){});
// this.beforeDisplay.trigger(args).then(function(){});

// Adds a function to be run before a hook completes
EPUBJS.Hook.prototype.register = function(func){
  this.hooks.push(func);
};

// Triggers a hook to run all functions
EPUBJS.Hook.prototype.trigger = function(){
  var length = this.hooks.length;
  var current = 0;
  var executing;
  var defer = new RSVP.defer();
  var args = arguments;

  if(length) {

    executing = this.hooks[current].apply(this.context, args);
    executing.then(function(){
      current += 1;
      if(current < length) {
        return this.hooks[current].apply(this.context, args);
      }
    }.bind(this));
    
  } else {
    executing = defer.promise;
    defer.resolve();
  }

  return executing;
};