// @flow
import FormInputComponent from './FormInputComponent';

type LoadingListener = (isLoading: boolean) => void;

export default class FormModel {
  fields = {};
  errors = {};
  handlers: LoadingListener[] = [];
  renderErrors: boolean;
  _isLoading: boolean;

  /**
   * @param {object} options
   * @param {bool} [options.renderErrors=true] - whether the bound filed should
   *                                             render their errors
   */
  constructor(options: { renderErrors?: boolean } = {}) {
    this.renderErrors = options.renderErrors !== false;
  }

  /**
   * Connects form with React's component
   *
   * Usage:
   * <input {...this.form.bindField('foo')} type="text" />
   *
   * @param {string} name - the name of field
   *
   * @returns {object} - ref and name props for component
   */
  bindField(name: string) {
    this.fields[name] = {};

    const props: Object = {
      name,
      ref: (el: ?FormInputComponent<any>) => {
        if (el) {
          if (!(el instanceof FormInputComponent)) {
            throw new Error('Expected FormInputComponent component');
          }

          this.fields[name] = el;
        } else {
          delete this.fields[name];
        }
      },
    };

    if (this.renderErrors && this.getError(name)) {
      props.error = this.getError(name);
    }

    return props;
  }

  /**
   * Focuses field
   *
   * @param {string} fieldId - an id of field to focus
   */
  focus(fieldId: string) {
    if (!this.fields[fieldId]) {
      throw new Error(
        `Can not focus. The field with an id ${fieldId} does not exists`,
      );
    }

    this.fields[fieldId].focus();
  }

  /**
   * Get a value of field
   *
   * @param {string} fieldId - an id of field to get value of
   *
   * @returns {string}
   */
  value(fieldId: string) {
    const field = this.fields[fieldId];

    if (!field) {
      throw new Error(
        `Can not get value. The field with an id ${fieldId} does not exists`,
      );
    }

    if (!field.getValue) {
      return ''; // the field was not initialized through ref yet
    }

    return field.getValue();
  }

  /**
   * Add errors to form fields
   *
   * errorType may be string or object {type: string, payload: object}, where
   * payload is additional data for errorType
   *
   * @param {object} errors - object maping {fieldId: errorType}
   */
  setErrors(errors: { [key: string]: string }) {
    if (typeof errors !== 'object' || errors === null) {
      throw new Error('Errors must be an object');
    }

    const oldErrors = this.errors;
    this.errors = errors;

    Object.keys(this.fields).forEach(fieldId => {
      if (this.renderErrors) {
        if (oldErrors[fieldId] || errors[fieldId]) {
          this.fields[fieldId].setError(errors[fieldId] || null);
        }
      }

      if (this.hasErrors()) {
        this.fields[fieldId].onFormInvalid();
      }
    });
  }

  /**
   * @returns {object|string|null}
   */
  getFirstError() {
    return this.errors ? Object.values(this.errors).shift() : null;
  }

  /**
   * Get error by id
   *
   * @param {string} fieldId - an id of field to get error for
   *
   * @returns {string|object|null}
   */
  getError(fieldId: string) {
    return this.errors[fieldId] || null;
  }

  /**
   * @returns {bool}
   */
  hasErrors() {
    return Object.keys(this.errors).length > 0;
  }

  /**
   * Convert form into key-value object representation
   *
   * @returns {object}
   */
  serialize(): { [key: string]: any } {
    return Object.keys(this.fields).reduce((acc, fieldId) => {
      const field = this.fields[fieldId];

      if (field) {
        acc[fieldId] = field.getValue();
      } else {
        console.warn('Can not serialize %s field. Because it is null', fieldId);
      }

      return acc;
    }, {});
  }

  /**
   * Bind handler to listen for form loading state change
   *
   * @param {Function} fn
   */
  addLoadingListener(fn: LoadingListener) {
    this.removeLoadingListener(fn);
    this.handlers.push(fn);
  }

  /**
   * Remove form loading state handler
   *
   * @param {Function} fn
   */
  removeLoadingListener(fn: LoadingListener) {
    this.handlers = this.handlers.filter(handler => handler !== fn);
  }

  /**
   * Switch form in loading state
   */
  beginLoading() {
    this._isLoading = true;
    this.notifyHandlers();
  }

  /**
   * Disable loading state
   */
  endLoading() {
    this._isLoading = false;
    this.notifyHandlers();
  }

  /**
   * @api private
   */
  notifyHandlers() {
    this.handlers.forEach(fn => fn(this._isLoading));
  }
}