Delegating Android SharedPreferences in Kotlin

The Problem: Boilerplate Code

SharedPreferences provide a relatively simple way of storing key-value pairs on Android. The application might not only use them to save settings, but also to keep its persistent state.

However, when the app grows more complex, using SharedPreferences might become painful. The boilerplate code required to store and retrieve a preference takes more lines than the actual operation performed on its value.

For example, consider a LoginActivity that only displays input fields for username and password. As a user, I want the application to remember my username so that I don’t need to type it again the next time I want to log in—true story (pun intended).

How can this be done in Java?

First, we need to obtain an instance of SharedPreferences object. Assuming our code is in the Activity, we can do it by calling getSharedPreferences():

SharedPreferences sharedPreferences =
        getSharedPreferences(
            "it.czerwinski.myapp.MY_PREFERENCES",
            Context.MODE_PRIVATE);

Or, if multiple preferences files are not needed, we may use the default shared preferences instead:

SharedPreferences sharedPreferences =
        PreferenceManager.getDefaultSharedPreferences(this);

We may then retrieve a String by calling the getString() method, which requires two arguments: the name of the preference, and the default value, returned if the preference does not exist:

usernameEditText.setText(
        sharedPreferences.getString("USERNAME", ""));

Storing a value, however, is not that straightforward. SharedPreferences object only provides methods to get preferences. To modify their values, we need to use an Editor, returned by the edit() method:

sharedPreferences.edit()
        .putString("USERNAME", usernameEditText.getText().toString())
        .apply();

We shouldn’t forget to call apply(). Otherwise, the value would not be saved.

This code looks simple and clean… as long as we only need two or three calls to SharedPreferences. But typically, that is not the case, is it?

Anko Comes To The Rescue

Kotlin language is probably the best thing that has happened to Android developers since first Android phones hit the market. It allowed creation of powerful yet lightweight tools that simplify our lives.

One of those tools, Anko, provides an extension property of the Context class, called defaultSharedPreferences, that eliminates the need to explicitly initialize the SharedPreferences object. Nonetheless, we still need to retrieve and save the values in the same inconvenient manner:

usernameEditText.setText(
        defaultSharedPreferences.getString("USERNAME", ""))

defaultSharedPreferences.edit()
        .putString("USERNAME", usernameEditText.text.toString())
        .apply()

Utilize Kotlin Properties

If the same statements occur in code more than once, they should be extracted into a separate method, or in this case, two methods—getUsername and setUsername.

But since we are using Kotlin, why not declare a property with defined custom getter and setter? We can also make it an extension property, so it might be referred from any Activity in our app:

var Context.username: String
    get() = defaultSharedPreferences.getString("USERNAME", "")
    set(value) = defaultSharedPreferences.edit()
            .putString("USERNAME", value)
            .apply()

As a result, the calls to save and restore the username become much cleaner:

usernameEditText.setText(username)

username = usernameEditText.text.toString()

However simple, this approach still has its drawbacks. The declaration of the username property, even though a bit complicated, at least keeps SharedPreferences in one place. But imagine that there are 10 similar properties in the app… or 20… or even 50…

Delegate

The above quotation perfectly conveys our case. And Kotlin offers us a solution—delegated properties.

So let’s create a delegate that handles values stored in SharedPreferences:

class StringSharedPreferenceDelegate(
        context: Context,
        private val key: String,
        private val defaultValue: String) {

    private val sharedPreferences by lazy {
        PreferenceManager.getDefaultSharedPreferences(context)
    }

    operator fun getValue(ref: Any?, property: KProperty<*>): String =
            sharedPreferences.getString(key, defaultValue)

    operator fun setValue(ref: Any?, property: KProperty<*>, value: String): Unit {
        sharedPreferences.edit()
                .putString(key, value)
                .apply()
    }
}

Now, the property can be delegated to an instance of StringSharedPreferenceDelegate in a single line of code:

var username: String by StringSharedPreferenceDelegate(context, "USERNAME", "")

Reuse

Probably most Android applications use SharedPreferences at some point. So I decided to implement generalized version of delegates and put them into a reusable library.

To use the delegates simply add a dependency to your build.gradle (the package is available in jcenter repository):

dependencies {
    compile 'it.czerwinski.android:delegates-shared-preferences:0.1'
}

A delegated property can then be defined in your Activity class:

var username by stringSharedPreference("USERNAME", "")

or in any other class containing a reference to the Context:

var username by context.stringSharedPreference("USERNAME", "")

It might be a good idea to define an additional extension function, at least for those preferences, which are used in many different classes:

fun Context.usernameSharedPreference() =
        stringSharedPreference("USERNAME", "")

var username by usernameSharedPreference()

What’s Next?

So far, the library only supports several basic data types: Int, Long, Float, Double, Boolean, String and Set<String>. But in the future, I plan to implement delegates for other common types, e.g. Array, List, Date, as well as data classes.

In the meantime, feel free to use the existing features. The library is still in development stage, but it has been fully tested.

Credits

• Social media image by Gerd Altmann from Pixabay