index.js

/**
 * Expose
 */
module.exports = GestureRecognizer;

/**
 * Constructor
 */
function GestureRecognizer() {
  this.pairs = [];
}

/**
 * Prototype
 */
var prototype = GestureRecognizer.prototype = Object.create(null);
var states = prototype.states = Object.create(null);

states.POSSIBLE = 'possible';
states.BEGAN = 'began';
states.CHANGED = 'changed';
states.ENDED = 'ended';
states.CANCELLED = 'cancelled';
states.FAILED = 'failed';
states.RECOGNIZED = states.ENDED;

prototype.state = states.POSSIBLE;

prototype.addTarget = function (target, action) {
  var pairs = this.pairs;

  if (!target)
    throw new Error("You must specify a target");

  if (!action)
    throw new Error("You must specify an action");

  if (typeof action !== "string")
    throw new Error("Action must be a string");

  if (typeof target[action] !== "function")
    throw new Error("The specified action must be a function of the target");

  if (!hasTargetActionPair(target, action, pairs))
    pairs.push([target, action]);
};

prototype.removeTarget = function (target, action) {
  var pairs = this.pairs;

  if (target === null && action === null) {
    pairs.length = 0;
  } else if (target === null) {
    var i = pairs.length;

    while (i--) if (pairs[i][1] === action) pairs.splice(i, 1);
  } else if (action === null) {
    var i = pairs.length;

    while (i--) if (pairs[i][0] === target) pairs.splice(i, 1);
  } else {
    var i = indexOfTargetActionPair(target, action, pairs);

    if (typeof i === 'number') pairs.splice(i, 1);
  }
};

/**
 * Helper functions
 */
function indexOfTargetActionPair(target, action, pairs) {
  for (var i = 0, l = pairs.length; i < l; i ++)
    if (pairs[i][0] === target && pairs[i][1] === action) return i;
}

function hasTargetActionPair(target, action, pairs) {
  return typeof indexOfTargetActionPair(target, action, pairs) === "number";
}