class EventEmitter {
  constructor() {
    this.events = Object.create(null);
  }

  on(event, handler) {
    if (!this.events[event])
      this.events[event] = [handler];
    else
      this.events[event].push(handler);
    return this;
  }

  emit(event, data) {
  	console.log('emit event ' + event);
    if (!(event in this.events))
      return;

    for (let handler of this.events[event])
      handler(data);
  }
}

class FormValidator extends EventEmitter {
	submitGuard(formID) {
		var form = document.getElementById(formID);
		this.form = form;
		var that = this;
		form.addEventListener('submit', function(event) {
			that.form.isValid = true;
			var fields = form.getElementsByTagName('input');
			for(var i = 0; i < fields.length; i++) {
				var input = fields[i];
    			if(input.type !== 'radio') {

      				that.emit(input.id + '_changed', {value: input.value, field: input});
      			}
			}
			var fields = form.getElementsByTagName('select');
			for(var i = 0; i < fields.length; i++) {
				var select = fields[i];
				var value = select.options[select.selectedIndex].value;

      			that.emit(input.id + '_changed', {value: value, field: select});
			}
			if(that.form.isValid == false) {
				event.preventDefault();
			}
		})
		return this;
	}
	withValidator(validator, errorHandler) {
		var that = this;
		console.log('should validate ' + validator.id);
		this.on(validator.id + '_changed', function(data) {
			console.log('validating field ' + validator.id);
			for(var i = 0; i < validator.checkers.length; i++) {
				if(validator.checkers[i](data.value) == false) {
					errorHandler(data.field);
					that.form.isValid = false;
					break;
				}
			}
		})
		return this;
	}
}

function validator(elementID) {
	return {
		id: elementID,
		checkers: [],
		withChecker: function(checker) {
			this.checkers.push(checker);
			return this;
		}
	}
}

function submitGuard(formID) {
	var formValidator = new FormValidator();
	formValidator.submitGuard(formID);
	return formValidator;
}

var titleChecker = function(userInput) {
	return true;
}
function lengthChecker(length) {
	return function(userInput) {
		return true;
	}
}

function wrongTitleHandler() {

}

function wrongPriceHandler() {

}

let titleValidator = validator('title').withChecker(titleChecker);
let priceValidator = validator('price').withChecker(lengthChecker(6));
submitGuard('create_ad_form')
    .withValidator(titleValidator, wrongTitleHandler)
    .withValidator(priceValidator, wrongPriceHandler);