Introduction

We will learn basic React-native elements needed to develop software graphical user interfaces including:

• Components
  • States
  • Props
• Hooks

Once we will learn this basic elements, we will develop the Restaurants list and the restaurant detail screen.

0. Setup

Clone template repository and download your copy. You can then run npm install and npm start. Check that the base project is working.

1. Components

In general, software components are some kind of artifacts that encapsulates a set of related functions so they can be reused. React components are the reusable building blocks that we can define to create our Apps' user interfaces. In the previous Lab, we used some components included in react-native library such as View, Text or Pressable. We also defined some components (such as our screens or a simple component named SystemInfo).

The preferred option to create components in React and React-native is the so-called Function Components. The defined function component takes as input some parameters which are called props, and returns a React element.

You can check out the RestaurantScreen.js file to understand how a component is defined:

import React from 'react'
import { StyleSheet, Button, Text, View } from 'react-native'

export default function RestaurantsScreen ({ navigation }) {
  return (
    <View>
      <Text>Random Restaurant</Text>
      <Button
        onPress={() => {
          navigation.navigate('RestaurantDetailScreen', { id: Math.floor(Math.random() * 100) })
        }}
        title='Go to Random Restaurant Details'
      />
    </View>
  )
}

2. States

Components usually needs to maintain some data in memory to remember things. In React and React-native this is called the state. In order to create and update the state we need to use the useState hook (we will learn about this in the next section):

const [state, setState] = useState(initialValue)

Notice that we define an array of elements [state, setState] including the state object and the method to change the state setState and we can define the initial value of this state object.

For instance, when we will make a request to the backend to retrieve the list of restaurants, the returned data should be kept in the state of the RestaurantsScreen component. So in this component, we need to define an state that will contain the array of restaurants (initially will be an empty array []) as:

const [restaurants, setRestaurants] = useState([])

3. Props

We can use props to pass data between components (we will see that Context API will help us for the same objective). Below you can find how we receive the route in the RestaurantDetailScreen component as a prop when navigating from RestaurantsScreen to RestaurantDetailScreen:

export default function RestaurantDetailScreen ({ route }) {
  const { id } = route.params
  return (
        <View style={styles.container}>
            <Text>Restaurant details. Id: {id}</Text>
        </View>
  )
}

4. Hooks

Hooks are specially-implemented functions that let us add functionality to React components beyond just creating and returning React elements. We will use hooks for three objectives:

1. Maintain the state of a component: useState hook
2. Update our interface when data is updated or retrieved: useEffect hook.
3. Share data between components by defining a context and retrieve that context using the useContext hook. We will learn about contexts in following labs.

4.1. useState hook

const [state, setState] = useState(initialValue)

The useState hook returns an array containing:

• the state object
• a function to update the state object (it has to be named set and the name given to the state object)

the setState function admits a new state object value, and provokes a re-render of the component

setState(newState)

4.2. useEffect hook

useEffect(() => {
   //code to be executed
  }, [object1, object2, ...])

The useEffect hook takes two arguments:

• the function to be run when the hook is triggered
• an optional array containing the dependency values that will trigger the hook when their values have changed. If the array is empty, it will be executed once the component is mount (inserted in the DOM tree). If the parameter is not present, will be executed when the component is mount and every time the component updates (after every re-render).

5. Developing RestaurantsScreen and RestaurantDetailScreen.

We want to develop two screens:

• RestaurantsScreen should render a list of restaurants that belongs to the owner. Each element should render at least the name of the restaurant, and if an element is clicked or tapped, should navigate to the restaurant detail screen of that restaurant.
• RestaurantDetailScreen should render the details of the restaurant selected in the previous screen, including description and the products (menu) of that restaurant.

In your project you will find a new folder api. In the future it will contain one file for each of the entities that have to be requested to the backend. In this lab, we included a mock RestaurantEndpoints.js file where you will find two methods:

• getAll will return some restaurants (at this point they are preloaded in the code)
• getDetail(id) will return all restaurant details of the given restaurant id, including the products (menu).

Notice that this is a mockup, not real API calls will be found. We will learn how to perform API calls in the next lab.

5.1 RestaurantsScreen

First, open RestaurantsScreen.js. We will need to use some hooks (useState and useEffect), components and functions from other files. To this end, we have added the following import statements:

import React, { useState, useEffect } from 'react'
import { StyleSheet, Text, FlatList, Pressable } from 'react-native'
import { getAll } from '../../api/RestaurantEndpoints'
import { brandSecondary } from '../../styles/GlobalStyles'

Next, we will define the component state, which will be a restaurants object array where we will store the list of restaurants. To this end, you can add the following line inside the RestaurantsScreen function component:

  const [restaurants, setRestaurants] = useState([])

At first render, the restaurants state object will be empty (its initial value is an empty array []). We need to load our restaurants in the state. To this end, we will define a new effect with the useEffect hook. As explained before, this hook takes two arguments: the function to be triggered, and an optional array of objects which triggers the function when their values have been changed. To load the restaurants, we do not need the optional dependencies array, as no dependency is needed to trigger the loading. To imitate the loading from a real API, a two-seconds timeout is established so restaurants will load after these two seconds delay. To do this, you can include the following inside the RestaurantsScreen function component:

useEffect(() => {
    console.log('Loading restaurants, please wait 2 seconds')
    setTimeout(() => {
      setRestaurants(getAll) // getAll function has to be imported
      console.log('Restaurants loaded')
    }, 2000)
  }, [])

At this point, we have the necessary elements to load the list of restaurants as an state object. Next, we will render them by using one of the core components included with React-native: FlatList. More info about FlatLists can be found in the official documentation: https://reactnative.dev/docs/flatlist

In order to use this FlatList component we will need to pass some props to it:

• data: the array of elements to be rendered.
• renderItem: the function that receives each element (composed of the item itself, the index within the flatlist and separator elements).
• keyExtractor: the function that extracts a unique key for each element. Keys are required to be of type string. Our ids are defined as numerics, so we will just transform these ids to string.

Let us first define how to render each item (restaurant). We will define a new function called renderRestaurant. This function will receive the item to be rendered (a restaurant) and have to return the graphical components that will render each restaurant on the FlatList. We propose to define a Pressable area and print the restaurant name inside. When pressed, it should navigate to the RestaurantDetailScreen, passing the restaurant id as a prop included the route object.

const renderRestaurant = ({ item }) => {
    return (
      <Pressable
        style={styles.row}
        onPress={() => {
          navigation.navigate('RestaurantDetailScreen', { id: item.id })
        }}>
          <Text>
              {item.name}
          </Text>
      </Pressable>
    )
  }

Finally, we will overwrite the return statement of our component. We will include the Flatlist defining the following props:

• data: as our restaurants state object.
• renderItem: the renderRestaurant function.
• keyExtractor: the function that extracts a unique key for each element (it needs to be a String): item => item.id.toString()

return (
      <FlatList
        style={styles.container}
        data={restaurants}
        renderItem={renderRestaurant}
        keyExtractor={item => item.id.toString()}
      />
  )

At this point, your RestaurantsScreen should show a list with two restaurants, and when one is clicked or tapped it should navigate to the RestaurantDetailScreen which only shows the id of the pressed restaurant.

5.2 RestaurantDetailScreen

Next, we will develop our RestaurantDetailScreen so it queries all the details of a restaurant, including its products, and will render its name, description and the list of products. To this end we will follow the same approach: defining the state object, defining a useEffect so it retrieves the restaurant details from de mock API, and render the FlatList component. Define the state object:

  const [restaurant, setRestaurant] = useState({})

Define the useEffect hook to load restaurant details:

useEffect(() => {
    console.log('Loading restaurant details, please wait 1 second')
    setTimeout(() => {
      setRestaurant(getDetail(route.params.id))
      console.log('Restaurant details loaded')
    }, 1000)
  }, [])

Define a renderProduct function:

const renderProduct = ({ item }) => {
    return (
      <Pressable
        style={styles.row}
        onPress={() => { }}>
          <Text>
              {item.name}
          </Text>
      </Pressable>
    )
  }

Notice that at this moment, it does not navigate when pressed. We will change this behaviour in future labs.

Finally, render all these elements:

return (
        <View style={styles.container}>
            <Text style={styles.textTitle}>{restaurant.name}</Text>
            <Text style={styles.text}>{restaurant.description}</Text>
            <Text style={styles.text}>shippingCosts: {restaurant.shippingCosts}</Text>
            <FlatList
              style={styles.container}
              data={restaurant.products}
              renderItem={renderProduct}
              keyExtractor={item => item.id.toString()}
            />
        </View>
  )

6. Extra exercises

Restaurants, Products and other entities include some image properties. These properties store relative path to the images. For instance, a restaurant logo property value could be: logo: 'public/restaurants/100MontaditosLogo.jpeg'. This is the relative path of this image in the server. In order to load the image, we need to add the backend server ip. To do this, copy the .env.example and rename the copy as .env file. Check that the following property points to your backend deployment server and port: API_BASE_URL=http://localhost:3000

Your project includes the needed packages for reading the .env file. To access a property you can just use the following: process.env.PROPERTY_NAME. For instance the API_BASE_URL property can be read by process.env.API_BASE_URL

6.1: List header

Modify the RestaurantDetailScreen component so now the FlatList renders the header that includes the information about the restaurant. See https://reactnative.dev/docs/flatlist#listheadercomponent for more information.

You can use the following renderHeader function (you will need to add some imports).

const renderHeader = () => {
    return (
      <ImageBackground source={(restaurant?.heroImage) ? { uri: process.env.API_BASE_URL + '/' + restaurant.heroImage, cache: 'force-cache' } : undefined } style={styles.imageBackground}>
        <View style={styles.restaurantHeaderContainer}>
            <TextSemiBold textStyle={styles.textTitle}>{restaurant.name}</TextSemiBold>
            <Image style={styles.image} source={restaurant.logo ? { uri: process.env.API_BASE_URL + '/' + restaurant.logo, cache: 'force-cache' } : undefined} />
            <TextRegular textStyle={styles.text}>{restaurant.description}</TextRegular>
        </View>
      </ImageBackground>
    )
  }

And you can add the following styles:

const styles = StyleSheet.create({
  container: {
    flex: 1
  },
  row: {
    padding: 15,
    marginBottom: 5,
    backgroundColor: brandSecondary
  },
  restaurantHeaderContainer: {
    height: 250,
    padding: 20,
    backgroundColor: 'rgba(0,0,0,0.5)',
    flexDirection: 'column',
    alignItems: 'center'
  },
  imageBackground: {
    flex: 1,
    resizeMode: 'cover',
    justifyContent: 'center'
  },
  image: {
    height: 100,
    width: 100,
    margin: 10
  },
  text: {
    color: 'white'
  },
  textTitle: {
    fontSize: 20,
    color: 'white'
  }
})

6.2: Card components

Card components are a very popular solution to render information about items. Learn more about cards here: https://material.io/components/cards

You have been provided with an ImageCard component at the components folder. You may use it to render restaurant items or product items. For instance, we can render the restaurant item at the RestaurantsScreen with the following renderer function:

const renderRestaurantWithImageCard = ({ item }) => {
    return (
      <ImageCard
        imageUri={item.logo ? { uri: process.env.API_BASE_URL + '/' + item.logo } : undefined}
        title={item.name}
        onPress={() => {
          navigation.navigate('RestaurantDetailScreen', { id: item.id })
        }}
      >
          <TextRegular numberOfLines={2}>{item.description}</TextRegular>
          {item.averageServiceMinutes !== null &&
            <TextSemiBold>Avg. service time: <TextSemiBold textStyle={{ color: brandPrimary }}>{item.averageServiceMinutes} min.</TextSemiBold></TextSemiBold>
          }
          <TextSemiBold>Shipping: <TextSemiBold textStyle={{ color: brandPrimary }}>{item.shippingCosts.toFixed(2)}€</TextSemiBold></TextSemiBold>
      </ImageCard>
    )
  }

You may need to add the needed imports.

Once you have rendered your restaurants with the ImageCard component, use this component to render the products of a restaurant as well.