9 Handling Shared Preferences

Picture this: You’re browsing recipes and find one you like. You’re in a hurry and want to bookmark it to check it later. Can you build a Flutter app that does that? You sure can! Read on to find out how.

In this chapter, your goal is to learn how to use the shared_preferences plugin to save important pieces of information to your device.

You’ll start with a new project that shows three tabs at the bottom of the screen for three different views: Recipes, Bookmarks and Groceries.

The first screen is where you’ll search for recipes you want to prepare. Once you find a recipe you like, just bookmark it and the app will add the recipe to your Bookmarks page and also add all the ingredients you need to your shopping list. You’ll use a web API to search for recipes and store the ones you bookmark in a local database.

The completed app will look something like:

This shows the Recipes tab with the results you get when searching for Pasta. It’s as easy as typing in the search text field and pressing the Search icon. The app stores your search term history in the combo box to the right of the text field.

When you tap a card, you’ll see something like:

To save a recipe, just tap the Bookmark button. When you navigate to the Bookmarks tab, you’ll see that the recipe has been saved:

If you don’t want the recipe anymore, swipe left or right and you’ll see a delete button that allows you to remove it from the list of bookmarked recipes.

The Groceries tab shows the ingredients you need to make the recipes you’ve bookmarked.

You’ll build this app over the next few chapters. In this chapter, you’ll use shared preferences to save simple data like the selected tab and also to cache the searched items in the Recipes tab.

By the end of the chapter, you’ll know:

• What shared preferences are.
• How to use the shared_preferences plugin to save and retrieve objects.

Now that you know what your goal is, it’s time to jump in!

Getting started

Open the starter project for this chapter in Android Studio, run flutter pub get, then run the app.

Notice the three tabs at the bottom — each will show a different screen when you tap it. Only the Recipes screen currently shows any UI. It looks like this:

App libraries

The starter project includes the following libraries in pubspec.yaml:

dependencies:
  ...
  cached_network_image: ^3.2.2
  flutter_slidable: ^2.0.0
  platform: ^3.1.0
  flutter_svg: ^1.1.4

Here’s what they help you do:

• cached_network_image: Download and cache the images you’ll use in the app.
• flutter_slidable: Build a widget that lets the user slide a card left and right to perform different actions, like deleting a saved recipe.
• platform: For accessing Platform specific information.
• flutter_svg: Load SVG images without the need to use a program to convert them to vector files.

Now that you’ve had a look at the libraries, take a moment to think about how you save data before you begin coding your app.

Saving data

There are three primary ways to save data to your device:

1. Write formatted data, like JSON, to a file.
2. Use a library or plugin to write simple data to a shared location.
3. Use a SQLite database.

Writing data to a file is simple, but it requires you to handle reading and writing data in the correct format and order.

You can also use a library or plugin to write simple data to a shared location managed by the platform, like iOS and Android. This is what you’ll do in this chapter.

For more complex data, you can save the information to a local database. You’ll learn more about that in Chapter 15, “Saving Data With SQLite”.

Why save small bits of data?

There are many reasons to save small bits of data. For example, you could save the user ID when the user has logged in — or if the user has logged in at all. You could also save the onboarding state or data that the user has bookmarked to consult later.

Note that this simple data saved to a shared location is lost when the user uninstalls the app.

The shared_preferences plugin

shared_preferences is a Flutter plugin that allows you to save data in a key-value format so you can easily retrieve it later. Behind the scenes, it uses the aptly named SharedPreferences on Android and the similar UserDefaults on iOS.

For this app, you’ll learn to use the plugin by saving the search terms the user entered as well as the tab that’s currently selected.

One of the great things about this plugin is that it doesn’t require any setup or configuration. Just create an instance of the plugin and you’re ready to fetch and save data.

Note: The shared_preferences plugin gives you a quick way to persist and retrieve data, but it only supports saving simple properties like strings, numbers, and boolean values.

In later chapters, you’ll learn about alternatives that you can use when you want to save complex data.

Be aware that shared_preferences is not a good fit to store sensitive data. To store passwords or access tokens, check out the Android Keystore for Android and Keychain Services for iOS, or consider using the flutter_secure_storage plugin.

To use shared_preferences, you need to first add it as a dependency. Open pubspec.yaml and, underneath the flutter_svg library, add:

shared_preferences: ^2.0.15

Make sure you indent it the same as the other libraries.

Now, click the Pub Get button to get the shared_preferences library.

You can also run pub get from the command line:

flutter pub get

You’re now ready to store data. You’ll start by saving the searches the user makes so they can easily select them again in the future.

Saving UI states

You’ll use shared_preferences to save a list of saved searches in this section. Later, you’ll also save the tab that the user has selected so the app always opens to that tab.

You’ll start by preparing your search to store that information.

Adding an entry to the search list

First, you’ll change the UI so that when the user presses the search icon, the app will add the search entry to the search list.

Open lib/ui/recipes/recipe_list.dart, locate // TODO: Add imports and replace it with:

import 'package:shared_preferences/shared_preferences.dart';
import '../widgets/custom_dropdown.dart';
import '../colors.dart';

That imports the shared_preferences plugin, a custom widget to display a drop-down menu and a helper class to set colors.

Next, you’ll give each search term a unique key. Find // TODO: Add key and replace it with:

static const String prefSearchKey = 'previousSearches';

All preferences need to use a unique key or they’ll be overwritten. Here, you’re simply defining a constant for the preference key.

Next, replace // TODO: Add searches array with

List<String> previousSearches = <String>[];

This clears the way for you to save the user’s previous searches and keep track of the current search.

Running code in the background

To understand the code you’ll add next, you need to know a bit about running code in the background.

Most modern UI toolkits have a main thread that runs the UI code. Any code that takes a long time needs to run on a different thread or process so it doesn’t block the UI. Dart uses a technique similar to JavaScript to achieve this. The language includes these two keywords:

• async
• await

async marks a method or code section as asynchronous. You then use the await keyword inside that method to wait until an asynchronous process finishes in the background.

Dart also has a class named Future, which indicates that the method promises a future result. SharedPreferences.getInstance() returns Future<SharedPreferences>, which you use to retrieve an instance of the SharedPreferences class. You’ll see that in action next.

Saving previous searches

Now that you’ve laid some groundwork, you’re ready to implement saving the searches.

Still in recipe_list.dart, find // TODO: Add savePreviousSearches and replace it with:

void savePreviousSearches() async {
  // 1
  final prefs = await SharedPreferences.getInstance();
  // 2
  prefs.setStringList(prefSearchKey, previousSearches);
}

Here, you use the async keyword to indicate that this method will run asynchronously. It also:

1. Uses the await keyword to wait for an instance of SharedPreferences.
2. Saves the list of previous searches using the prefSearchKey key.

Next, replace // TODO: Add getPreviousSearches with the following method:

void getPreviousSearches() async {
  // 1
  final prefs = await SharedPreferences.getInstance();
  // 2
  if (prefs.containsKey(prefSearchKey)) {
    // 3
    final searches = prefs.getStringList(prefSearchKey);
    // 4
    if (searches != null) {
      previousSearches = searches;
    } else {
      previousSearches = <String>[];
    }
  }
}

This method is also asynchronous. Here, you:

1. Use the await keyword to wait for an instance of SharedPreferences.
2. Check if a preference for your saved list already exists.
3. Get the list of previous searches.
4. If the list is not null, set the previous searches, otherwise initialize an empty list.

Finally, find // TODO: Call getPreviousSearches and substitute it with:

getPreviousSearches();

This loads any previous searches when the user restarts the app.

Adding the search functionality

To perform a search, you need to clear any of your variables and save the new search value. This method will not do an actual search just yet. Do this by replacing // TODO: Add startSearchwith:

void startSearch(String value) {
  // 1
  setState(() {
    // 2
    currentSearchList.clear();
    currentCount = 0;
    currentEndPosition = pageCount;
    currentStartPosition = 0;
    hasMore = true;
    value = value.trim();

    // 3
    if (!previousSearches.contains(value)) {
      // 4
      previousSearches.add(value);
      // 5
      savePreviousSearches();
    }
  });
}

In this method, you:

1. Tell the system to redraw the widgets by calling setState().
2. Clear the current search list and reset the count, start and end positions.
3. Check to make sure the search text hasn’t already been added to the previous search list.
4. Add the search item to the previous search list.
5. Save the new list of previous searches.

Adding a search button

Next, you’ll add a search button that saves terms each time the user performs a search.

In _buildSearchCard(), replace the const Icon(Icons.search), with the following:

IconButton(
  icon: const Icon(Icons.search),
  // 1
  onPressed: () {
    // 2
    startSearch(searchTextController.text);
    // 3
    final currentFocus = FocusScope.of(context);
    if (!currentFocus.hasPrimaryFocus) {
      currentFocus.unfocus();
    }
  },
),

This replaces the icon with an IconButton that the user can tap to perform a search.

1. Add onPressed to handle the tap event.
2. Use the current search text to start a search.
3. Hide the keyboard by using the FocusScope class.

Next, replace everything in between // *** Start Replace and // *** End Replace with:

Expanded(
  child: Row(
    children: <Widget>[
      Expanded(
          // 3
          child: TextField(
        decoration: const InputDecoration(
            border: InputBorder.none, hintText: 'Search'),
        autofocus: false,
        // 4
        textInputAction: TextInputAction.done,
        // 5
        onSubmitted: (value) {
          startSearch(searchTextController.text);
        },
        controller: searchTextController,
      )),
      // 6
      PopupMenuButton<String>(
        icon: const Icon(
          Icons.arrow_drop_down,
          color: lightGrey,
        ),
        // 7
        onSelected: (String value) {
          searchTextController.text = value;
          startSearch(searchTextController.text);
        },
        itemBuilder: (BuildContext context) {
          // 8
          return previousSearches
              .map<CustomDropdownMenuItem<String>>((String value) {
            return CustomDropdownMenuItem<String>(
              text: value,
              value: value,
              callback: () {
                setState(() {
                  // 9
                  previousSearches.remove(value);
                  savePreviousSearches();
                  Navigator.pop(context);
                });
              },
            );
          }).toList();
        },
      ),
    ],
  ),
),

In this code, you:

1. Add a TextField to enter your search queries.
2. Set the keyboard action to TextInputAction.done. This closes the keyboard when the user presses the Done button.
3. Start the search when the user finishes entering text.
4. Create a PopupMenuButton to show previous searches.
5. When the user selects an item from previous searches, start a new search.
6. Build a list of custom drop-down menus (see widgets/custom_dropdown.dart) to display previous searches.
7. If the X icon is pressed, remove the search from the previous searches and close the pop-up menu.

To show the list of previous text searches, you used a text field with a drop-down menu. That is a row with a TextField and a CustomDropDownMenuItem. The menu item shows the search term and an icon on the right. It will look something like:

Clicking the X will delete the corresponding entry from the list.

Test the app

It’s time to test the app. Because you added a new dependency, quit the running instance and run it again (Note that you do not always need to restart when adding dependencies). You’ll see something like this:

The PopupMenuButton displays a menu when tapped and calls the method onSelected()when the user selects a menu item.

Enter a food item like pastas and make sure that, when you hit the search button, the app adds your search entry to the drop-down list.

Don’t worry about the progress circle running — that happens when there’s no data. Your app should look like this when you tap the drop-down arrow:

Now, stop the app by tapping the red stop button.

Run the app again and tap the drop-down button. The pastas entry is there. It’s time to celebrate :]

The next step is to use the same approach to save the selected tab.

Saving the selected tab

In this section, you’ll use shared_preferences to save the current UI tab that the user has navigated to.

Open main_screen.dart and add the following import:

import 'package:shared_preferences/shared_preferences.dart';

Next, replace // TODO: Add index key with:

static const String prefSelectedIndexKey = 'selectedIndex';

That is the constant you will use for the selected index preference key.

Next, add this new method after the initState method by replacing // TODO: Add saveCurrentIndex with this:

void saveCurrentIndex() async {
  // 1
  final prefs = await SharedPreferences.getInstance();
  // 2
  prefs.setInt(prefSelectedIndexKey, _selectedIndex);
}

Here, you:

1. Use the await keyword to wait for an instance of the shared preference plugin.
2. Save the selected index as an integer.

Now, find and replace // TODO: Add getCurrentIndex with this method:

void getCurrentIndex() async {
  // 1
  final prefs = await SharedPreferences.getInstance();
  // 2
  if (prefs.containsKey(prefSelectedIndexKey)) {
    // 3
    setState(() {
      final index = prefs.getInt(prefSelectedIndexKey);
      if (index != null) {
        _selectedIndex = index;
      }
    });
  }
}

With this code, you:

1. Use the await keyword to wait for an instance of the shared preference plugin.
2. Check if a preference for your current index already exists.
3. Get the current index and update the state accordingly.

Now, replace // TODO: Call getCurrentIndex with:

getCurrentIndex();

That will retrieve the currently-selected index when the page is loaded.

Finally, you need to call saveCurrentIndex() when the user taps a tab.

To do this, substitute // TODO: Call saveCurrentIndex with:

saveCurrentIndex();

This saves the current index every time the user selects a different tab.

Now, hot reload the app and select either the second or the third tab.

Quit the app and run it again to make sure the app uses the saved index when it starts.

At this point, your app should show a list of previously-searched items and also take you to the last selected tab when you start the app again. Here’s a sample:

Congratulations! You’ve saved the state for both the current tab and any previous searches the user made.

Key points

• There are multiple ways to save data in an app: to files, in shared preferences and to a SQLite database.
• Shared preferences are best used to store simple, key-value pairs of primitive types like strings, numbers and Booleans.
• An example of when to use shared preferences is to save the tab a user is viewing, so the next time the user starts the app, they’re brought to the same tab.
• The async/await keyword pair let you run asynchronous code off the main UI thread and then wait for the response. An example is getting an instance of SharedPreferences.
• The shared_preferences plugin should not be used to hold sensitive data. Instead, consider using the flutter_secure_storage plugin.

Where to go from here?

In this chapter, you learned how to persist simple data types in your app using the shared_preferences plugin.

If you want to learn more about Android SharedPreferences, go to https://developer.android.com/reference/kotlin/android/content/SharedPreferences?hl=en.

For iOS, check UserDefaultshttps://developer.apple.com/documentation/foundation/userdefaults.

In the next chapter, you’ll continue building the same app and learn how to serialize JSON in preparation for getting data from the internet. See you there!