offline-mobx.mo

Owner: Maxime Sraïki

Control Points

If you want to make your application work offline, you'll need to check that

• Data are persisted accross application restart

• Your user is aware of its connectivity state

• When you're offline, you gracefully handle the network interraction for the user

Motivation

• Your application has to be used in a low connectivity context (abroad, far from towns, in the subway, in Darius's house, ...)

Prerequisites

• Working with a React-Native app

• Using MobX as an application state manager

Redux User ?

If you're using Redux, check this out

Steps

Knowing wether or not you're offline (~20min)

Control points

• Create an observable reflecting the state of your connectivity

• Turn your app to plane mode

• Check the observable is saying you're offline

• Turn plane mode off

• Check the observable is saying you're online

Create a connectivity store

import { NetInfo } from "react-native";
import { observable, action } from "mobx";

class ConnectivityStore {
  constructor() {
    NetInfo.isConnected.addEventListener("connectionChange", this.checkConnection);
  }

  @observable isConnected = null;

  checkConnection = isConnected => {
    if (this.isConnected !== isConnected) {
      NetInfo.isConnected.removeEventListener("connectionChange", this.checkConnection);
      NetInfo.isConnected.addEventListener("connectionChange", this.checkConnection);
    }
    this.setIsConnected(isConnected);
  };

  @action
  setIsConnected = isConnected => {
    this.isConnected = isConnected;
  };
}

const connectivityStore = new ConnectivityStore();

export default connectivityStore;

Reading data offline (~ 20min)

Control points

• Persist some data

• Kill your app

• Turn it to plane mode

• Open your app

• See your data

Here we will do it by ourselves by using AsyncStorage, you can refer to this MO to know how to use AsyncStorage

Another Solution

You can also use mobx-persist but in my experience, it's not simpler.

Writing data offline

Defensive (~1h)

Control points

• Your user knows that he's offline and that he needs to get his connection back before the operation is tried again.

• You show your user a temporary state of the data he was supposed to modify.

• You only update your application state once the remote operation is succesfully done.

• Define what call you want to make work offline

• Store the payload to your application state as pending

• Persist it

• Whenever there is something stored as pending in your store, display a warning message to the user to let him know

• When he wants to, let him do the call again with the stored payload

• Clear the pending observable when your call is succesful

In your offline store:

import { observable, action } from "mobx";
import { AsyncStorage } from "react-native";
import momentTz from "moment-timezone";
import moment from "moment/min/moment-with-locales";
import { asyncStorageKeys } from "../services/asyncStorage";

momentTz.locale("fr");

class OfflineStore {
  constructor() {
    AsyncStorage.getItem(asyncStorageKeys.DEPARTURE_INVENTORY_PENDING).then(isDepartureInventoryPending => {
      if (isDepartureInventoryPending) {
        this.isDepartureInventoryPending = JSON.parse(isDepartureInventoryPending);
      }
    });
    AsyncStorage.getItem(asyncStorageKeys.PENDING_DEPARTURE_INVENTORY).then(pendingDepartureInventory => {
      if (pendingDepartureInventory) {
        const pendingDepartureInventoryFromAsyncStorage = JSON.parse(pendingDepartureInventory);
        if (pendingDepartureInventoryFromAsyncStorage.inventoryDate) {
          pendingDepartureInventoryFromAsyncStorage.inventoryDate = moment.utc(
            pendingDepartureInventoryFromAsyncStorage.inventoryDate
          );
        }
        this.pendingDepartureInventory = pendingDepartureInventoryFromAsyncStorage;
      }
    });
  }

  @observable isDepartureInventoryPending = false;
  @observable pendingDepartureInventory = {};

  @action
  setIsDepartureInventoryPending = boolean => {
    this.isDepartureInventoryPending = boolean;
    AsyncStorage.setItem(
      asyncStorageKeys.DEPARTURE_INVENTORY_PENDING,
      JSON.stringify(this.isDepartureInventoryPending)
    );
  };

  @action
  setPendingDepartureInventory = inventory => {
    this.pendingDepartureInventory = inventory;
    AsyncStorage.setItem(asyncStorageKeys.PENDING_DEPARTURE_INVENTORY, JSON.stringify(this.pendingDepartureInventory));
  };

  storePendingDepartureInventory = inventory => {
    this.setPendingDepartureInventory(inventory);
    this.setIsDepartureInventoryPending(true);
  };

  clearPendingDepartureInventory = () => {
    this.setPendingDepartureInventory({});
    this.setIsDepartureInventoryPending(false);
  };
}

const offlineStore = new OfflineStore();

export default offlineStore;

In your component

sendPendingDepartureInventory = () =>
  confirmDepartureInventory(
    this.props.pendingDepartureInventory.bookingId,
    this.props.pendingDepartureInventory.damages,
    this.props.pendingDepartureInventory.mileageStart,
    this.props.pendingDepartureInventory.energyStartInLiters,
    this.props.accessToken,
    this.props.pendingDepartureInventory.energyType,
    this.props.pendingDepartureInventory.inventoryDate,
    this.props.pendingDepartureInventory.isKeyCardPresent
  )
    .then(() => {
      this.props.booking.isDepartureInventoryConfirmed = true;
      this.props.clearPendingDepartureInventory();
      this.setState({ shouldDisplaySuccessModal: true });
    })

  return Promise.resolve();
});

[...]

{this.props.isDepartureInventoryPending && (
    <EDLAlert
      sendInventory={this.sendPendingDepartureInventory}
      title={I18n.t('departureInventoryOffline_alertSend.title')}
      text={I18n.t('departureInventoryOffline_alertSend.text')}
      onMenuPress={() => this.drawer.openDrawer()}
    />
  )
}

We implemented a working example of it @BAM, let me know if you want to know more about it.

Go further

This is a working example for a one shot call but you can also write a queue of calls letting you store multiple payloads with multiple calls.

Optimistic (~2h00)

Control points

• Your user knows that he's offline and that he needs to get his connection back before the operation is tried again.

• You update your application state directly after the user action.

• You rollback the modification if something goes wrong in the remote operation.

• Define what call you want to make work offline

• Store the payload to your application state as pending

• Persist it

• Whenever there is something stored as pending in your store display a discret message/icon to the user to let him know

• When you get online again (you can use your connectivity observable) try to do the call again

• Define a fail detection strategy (for instance after n tries, decide that the called is failed and rollback the state modification you made)

Tips & TroubleShoot

• In order to be able to detect this, you can use the react-native's NetInfo.isConnected function to get the connectivity status your phone thinks it has.
• There are ways to check user's connectivity by pinging one of your own server routes instead of basing it on the phone self awareness of its connectivity.

• We did not implement offline calls that would need conflict management (several users able to access the same data at the same time) yet.