Creating a simple Login function with Redux and Thunk in React Native
Tags: reactnative, tutorial, beginners, javascript
Published: 2020-04-13 07:26:44

This is my first post here :)

In this post, we will see how an action can be dispatched using Redux on login, and set the app state accordingly.

*Assuming familiarity with React Native and Redux concepts*

We have the following Login component to begin with :

```js
import React, { useState } from 'react';
import { View, Button, Text } from 'react-native';

import CustomButton from '../../components/CustomButton';
import InputField from '../../components/InputField';

import { styles } from './style';

const Login = (props) => {
  
  // remove these initial assignments after testing
  const [username, setUsername] = useState('');
  const [password, setPassword] = useState('');

  return (
    <View>
      <InputField
        placeholder='Enter username'
        value={username}
        onChangeText={(text) => setUsername(text)}
      />
      <InputField
        placeholder='Enter password'
        secureTextEntry={true}
        value={password}
        onChangeText={(text) => setPassword(text)}
      />
      <CustomButton 
        title='Sign In' 
        onPress={() => }  
      />
  );
};

export default Login;
```
Right now it doesn't do anything, it's just barebones UI.

To make it "react" to user action we should update the `onPress` paramter in the SignIn button.

```js 
<CustomButton title='Sign In' onPress={() => } />
```
We use redux here, so pressing the button should `dispatch` and `action` to the reducer which should in turn update the overall app's state.

For sake of simplicity, all redux code is placed in a 'redux' folder, while the components are in 'src/components/< ComponentName >/index.js'.

This is how our redux folder looks like.
redux
├── actions.js
├── actionTypes.js
├── initialState.js
├── reducer.js
└── store.js

Let's set `initialState` as follows. These are all the fields that our login API will return (yours may differ).
`userId` and `isLogged` in are flags that we will set on our own (these are not part of API response)

```js
export const initialState = {
  isLoggedIn: false,
  userId: '',
  token: '',
  refreshToken: '',
  expiresOn: '',
  data: '',
};
```

Define action type in `actionTypes.js`
```js 
export const SET_LOGIN_STATE = "SET_LOGIN_STATE"
```

Let's now create our `loginReducer` in `reducer.js`
```js
import { initialState } from './initialState';
import * as t from './actionTypes';

export const loginReducer = (state = initialState, action) => {
  switch (action.type) {
    case t.SET_LOGIN_STATE:
      return {
        ...state,
        ...action.payload, // this is what we expect to get back from API call and login page input
        isLoggedIn: true, // we set this as true on login
      };
    default:
      return state;
  }
};
```
We can now generate our redux store using all the available information and `thunk` as middleware to handle API calls.

```js
import thunkMiddleware from 'redux-thunk';
import { createStore, combineReducers, applyMiddleware } from 'redux';
import { composeWithDevTools } from 'redux-devtools-extension/developmentOnly'; // this is for debugging with React-Native-Debugger, you may leave it out
import { loginReducer } from './reducer';

const rootReducer = combineReducers({
  loginReducer: loginReducer,
});

export const store = createStore(
  rootReducer,
  composeWithDevTools(applyMiddleware(thunkMiddleware))
);
```

We have these things in place but we still have not figured how to set the state from the `Login` component. For this we need to define some `actions` in `actions.js`

What we are looking at is a function that can call the login API and return the result back to us.

Something like :

```js
return fetch(LoginUrl, {
      method: 'POST',
      headers: {
        Accept: 'application/json',
        'Content-Type': 'application/json',
      },
      body: JSON.stringify(loginInput),
    })
    .then()
...................
```
But we also need to ensure that the action is "connected" to the "reducer" in order to update the redux state or store.

Since API call is considered unpredictable, it should not dispatch the action object directly to reducer, but through a helper. 

Dispatching action can return only an Object. If it returns a promise the app will break. We need to make sure of this.

In `actions.js` :

```js
import * as t from './actionTypes';
import { LoginUrl } from '../constants/Api';

// this is what our action should look like which dispatches the "payload" to reducer
const setLoginState = (loginData) => {
  return {
    type: t.SET_LOGIN_STATE,
    payload: loginData,
  };
};
```
To fetch this `loginData` in the action above, we create another function using the `fetch` operation discussed above:

```js
import { Alert } from 'react-native'; // to show alerts in app

export const login = (loginInput) => {
  const { username, password } = loginInput;
  return (dispatch) => {  // don't forget to use dispatch here!
    return fetch(LoginUrl, {
      method: 'POST',
      headers: {  // these could be different for your API call
        Accept: 'application/json',
        'Content-Type': 'application/json',
      },
      body: JSON.stringify(loginInput),
    })
      .then((response) => response.json())
      .then((json) => {
        if (json.msg === 'success') { // response success checking logic could differ
          dispatch(setLoginState({ ...json, userId: username })); // our action is called here
        } else {
          Alert.alert('Login Failed', 'Username or Password is incorrect');
        }
      })
      .catch((err) => {
        Alert.alert('Login Failed', 'Some error occured, please retry');
        console.log(err);
      });
  };
};
```
You can see how our action is dispatched from this function, which in turn will return a payload object to the reducer in order to perform state update.

Only thing remaining now is connecting this function to the UI. Let's go back to our `Login` component in the Submit button section and specify `onPress`

```js
import { useDispatch } from 'react-redux';
import { login } from '../../redux/actions';

...............

<CustomButton 
        title='Sign In' 
        onPress={() => useDispatch(login({'username': username, 'password': password }))}  
      />
...............
```

Since we are using redux here, all our functions should ideally be in form of some action, which will be caught in the `redux-thunk` middleware first and then passed on appropriately to reducer.

On successful login, the `initialState` values will all be populated. On failure, an alert will show up stating error.