Flutter Mobile Development · Lezione

Shared Preferences per i dati

Utilizzi `shared_preferences` per memorizzare localmente piccole quantità di dati, come impostazioni e preferenze dell'utente, in coppie chiave-valore.

Shared Preferences per i dati è una lezione Flutter Mobile Development gratuita su CoddyKit. Questa è la lezione 1 di 4. Puoi leggere la lezione completa qui gratuitamente — poi esercitati direttamente nel browser con un editor di codice integrato e un tutor IA disponibile 24/7. Fa parte del percorso di apprendimento Flutter Mobile Development, e i tuoi progressi si sincronizzano tra il web e l'app CoddyKit. Il corso Flutter Mobile Development include 4 lezioni in totale.

Parti di questa lezione non sono ancora state tradotte e vengono mostrate in inglese.

What are Shared Preferences?

When building mobile apps, you often need to store small bits of data locally on the device. This could be anything from a user's dark mode preference to their last logged-in status.

shared_preferences is a Flutter package that helps you do just that! It provides a simple way to store key-value pairs of primitive data types.

Why Use Shared Preferences?

Simple Data: Ideal for storing simple data types like booleans, integers, doubles, strings, and lists of strings.
User Settings: Perfect for saving user preferences, app settings, or theme choices.
Lightweight: It's a quick and easy way to persist small amounts of data without needing a full database.

Think of it like a small digital notepad for your app's temporary memory!

Installing the Package

To use shared_preferences, you first need to add it to your project's dependencies.

Open your pubspec.yaml file and add the following line under dependencies:

dependencies:
  flutter:
    sdk: flutter
  shared_preferences: ^2.2.2 # Add this line

Saving Your First Preference

Once the package is added, you can start saving data. You'll need to get an instance of SharedPreferences and then use methods like setBool(), setString(), etc.

Remember to use await because these operations are asynchronous!

import 'package:shared_preferences/shared_preferences.dart';

Future<void> saveSetting() async {
  SharedPreferences prefs = await SharedPreferences.getInstance();
  await prefs.setBool('isDarkMode', true);
  print('Dark mode setting saved!');
}

Run It: Saving a Boolean

Try running this example to see how a boolean value is saved. We'll set a 'welcomeShown' flag to true.

import 'package:flutter/widgets.dart';
import 'package:shared_preferences/shared_preferences.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized(); 
  SharedPreferences prefs = await SharedPreferences.getInstance();

  // Save a boolean value
  await prefs.setBool('welcomeShown', true);
  print('welcomeShown saved: ${prefs.getBool('welcomeShown')}');
}

Retrieving Saved Data

To get data back, you use corresponding get methods like getBool(), getString(), etc. These methods return a nullable type (e.g., bool?) because the key might not exist.

It's good practice to provide a default value using the ?? operator if the key is not found.

import 'package:shared_preferences/shared_preferences.dart';

Future<bool> getSetting() async {
  SharedPreferences prefs = await SharedPreferences.getInstance();
  // Get 'isDarkMode', default to false if not found
  bool isDark = prefs.getBool('isDarkMode') ?? false;
  return isDark;
}

Run It: Loading a Boolean

Let's load the welcomeShown preference we saved earlier. Notice how we handle the case where it might not exist by providing a default of false.

import 'package:flutter/widgets.dart';
import 'package:shared_preferences/shared_preferences.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  SharedPreferences prefs = await SharedPreferences.getInstance();

  // Load 'welcomeShown', default to false if not present
  bool welcomeShown = prefs.getBool('welcomeShown') ?? false;
  print('Is welcome screen already shown? $welcomeShown');

  // Let's try to load a key that doesn't exist
  String? userName = prefs.getString('userName');
  print('User Name: ${userName ?? 'Guest'}');
}

Saving Other Data Types

shared_preferences supports several basic data types:

setString(key, value) for text
setInt(key, value) for whole numbers
setDouble(key, value) for decimal numbers
setStringList(key, value) for lists of strings

There are corresponding get methods for each type.

Run It: Saving Mixed Data

This example demonstrates saving and loading a string, an integer, and a list of strings. See how different types are handled.

import 'package:flutter/widgets.dart';
import 'package:shared_preferences/shared_preferences.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  SharedPreferences prefs = await SharedPreferences.getInstance();

  // Save different data types
  await prefs.setString('userName', 'Coddy');
  await prefs.setInt('score', 12345);
  await prefs.setStringList('favoriteFoods', ['Pizza', 'Sushi', 'Tacos']);
  print('All data saved!');

  // Load and print them
  String? name = prefs.getString('userName');
  int? score = prefs.getInt('score');
  List<String>? foods = prefs.getStringList('favoriteFoods');

  print('User Name: ${name ?? 'N/A'}');
  print('Score: ${score ?? 0}');
  print('Favorite Foods: ${foods ?? []}');
}

Deleting Preferences

You can also remove stored preferences:

remove(key): Deletes a specific key-value pair.
clear(): Deletes ALL data stored by your app using shared_preferences. Use with caution!

Both methods are asynchronous and return a Future<bool> indicating success.

Quick Check

You want to store a user's preferred language, which is a single string like 'en_US' or 'es_ES'. Which shared_preferences method is the most appropriate for saving this data?

Recap & Next Steps

Great job! You've learned how to use shared_preferences to store and retrieve small, simple data locally in your Flutter applications.

It's ideal for user settings and preferences.
Supports basic types: bool, int, double, string, and List<String>.
Operations are asynchronous (use await).
Remember to handle null values when retrieving data.

Next, you'll explore more structured data storage with SQLite databases!

Domande Frequenti

La lezione «Shared Preferences per i dati» è gratuita?

Sì — il testo completo di «Shared Preferences per i dati» è gratuito qui sul web. Per esercitarvi in modo interattivo (un editor di codice integrato e un tutor IA 24/7) e sbloccare il resto del corso Flutter Mobile Development, passa a CoddyKit PRO. Il corso Flutter Mobile Development include 4 lezioni in totale.

Cosa imparerò in «Shared Preferences per i dati»?

Utilizzi `shared_preferences` per memorizzare localmente piccole quantità di dati, come impostazioni e preferenze dell'utente, in coppie chiave-valore. Eserciti Flutter Mobile Development con codice pratico che esegui direttamente nel browser, e un tutor IA 24/7 risponde alle tue domande mentre lavori sulla lezione.

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Non è richiesta alcuna esperienza precedente. Flutter Mobile Development su CoddyKit è strutturato per principianti e studenti avanzati, quindi puoi iniziare da qui o dall'inizio e procedere al tuo ritmo. Questa è la lezione 1 di 4.

Quanto tempo richiede la lezione «Shared Preferences per i dati»?

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