expo/src/AppLoading.js

/* @flow */
import React from 'react';
import { EventEmitter } from 'fbemitter';
import NativeAppLoading from './AppLoadingNativeWrapper';

type Props =
  | {
      startAsync: () => Promise<any>,
      onError?: (error: Error) => void,
      onFinish: () => void,
    }
  | {
      startAsync: null,
      onError: null,
      onFinish: null,
    };

export default class AppLoading extends React.Component<Props> {
  _isMounted: boolean;

  componentDidMount() {
    this._isMounted = true;
    _emitEvent('componentDidMount');

    // startAsync is optional, you can do this process manually if you prefer
    // (this is mainly for backwards compatibility and it is not recommended)
    if (this.props.startAsync) {
      /* $FlowFixMe */
      this._startLoadingAppResourcesAsync().done();
    }
  }

  componentWillUnmount() {
    this._isMounted = false;
    _emitEvent('componentWillUnmount');
  }

  _startLoadingAppResourcesAsync = async () => {
    if (!this.props.onFinish) {
      throw new Error(
        'AppLoading onFinish prop is required if startAsync is provided'
      );
    }

    try {
      await this.props.startAsync();
    } catch (e) {
      if (!this._isMounted) return;

      if (this.props.onError) {
        this.props.onError(e);
      } else {
        throw e;
      }
    } finally {
      if (!this._isMounted) return;

      // If we get to this point then we know that either there
      // was no error, or the error was handled.
      if (this.props.onFinish) {
        this.props.onFinish();
      }
    }
  };

  render() {
    return <NativeAppLoading />;
  }
}

let _lifecycleEmitter: ?EventEmitter;
function _emitEvent(event: string) {
  if (_lifecycleEmitter) {
    _lifecycleEmitter.emit(event);
  }
}

export function getAppLoadingLifecycleEmitter() {
  if (!_lifecycleEmitter) {
    _lifecycleEmitter = new EventEmitter();
  }
  return _lifecycleEmitter;
}