import React, { Component } from 'react';
import PropTypes from 'prop-types';

class DataFetcher extends Component {
  constructor(props) {
    super(props);

    this._activePromises = 0;
    this.result = {};
    this.errors = null;
    this._toBeExecuted = Object.keys(props.loader).length;
    this.checkState = this.checkState.bind(this);
    this.spawn = this.spawn.bind(this);

    this.state = {
      loading: true,
    };
  }
  checkState() {
    if (this._activePromises === 0 && this._toBeExecuted === 0) {
      this.setState({
        loading: false,
        result: { ...this.result },
        errors: this.errors || null,
      });
    }
  }
  spawn(name, fetcher) {
    this._activePromises++;
    this._toBeExecuted--;
    fetcher()
      .then(data => {
        this.result[name] = data;
        this._activePromises--;
      })
      .catch(error => {
        if (!this.errors) this.errors = {};
        this.errors[name] = error;
        this._activePromises--;
      })
      .then(() => {
        this.checkState();
      });
  }
  componentDidMount() {
    const { loader } = this.props;
    Object.keys(loader).map(x => {
      this.spawn(x, loader[x]);
    });
  }
  render() {
    const { result, loading, errors } = this.state;
    const { render, children } = this.props;
    if (typeof render === 'function') {
      return render({ result, loading, errors });
    }
    if (typeof children === 'function') {
      return children({ result, loading, errors });
    }
  }
}

DataFetcher.propTypes = {
  loader: PropTypes.object.isRequired,
  render: PropTypes.func,
  children: PropTypes.func,
};

export default DataFetcher;