Android SharedPreferences in Kotlin

In Android, SharedPrefences had always been favorite to store small data but sometimes big too, in a JSON string format.

Today I am going to simplify the data storing and retrieving method via
SharedPreferences in Kotlin way.

Let's start by observing some examples to store/retrieve data from shared preferences.

// get stored value
val myString = prefs.myString
// store value
prefs.myString = "My String Value"
// get stored array
val myStringArray = prefs.myStringArray
// store array
prefs.myStringArray = arrayOf("String 1","String 2","String 3")
// store value
prefs.myObject = MyObject("hello")
// retrieve my object
val myObject : MyObject? = prefs.myObject

First, we need to create a class Prefs with context as the constructor parameter.

class Prefs(context: Context) {

    companion object {
        private const val PREFS_FILENAME = "shared_prefs_name"

        private const val KEY_MY_STRING = "my_string"
        private const val KEY_MY_BOOLEAN = "my_boolean"
        private const val KEY_MY_ARRAY = "string_array"
        private const val KEY_MY_OBJECT = "my_object"
    }
    private val gson = Gson()
    private val sharedPrefs: SharedPreferences =
        context.getSharedPreferences(PREFS_FILENAME, Context.MODE_PRIVATE)

    var myString: String
        get() = sharedPrefs.getString(KEY_MY_STRING, "") ?: ""
        set(value) = sharedPrefs.edit { putString(KEY_MY_STRING,       value) }

    var myBoolean: Boolean
        get() = sharedPrefs.getBoolean(KEY_MY_BOOLEAN, false)
        set(value) = sharedPrefs.edit { putBoolean(KEY_MY_BOOLEAN, value) }
    var myStringArray: Array<String>
        get() = sharedPrefs.getStringSet(KEY_MY_ARRAY, emptySet())?.toTypedArray()?: emptyArray()
       set(value) = sharedPrefs.edit { putStringSet(KEY_MY_ARRAY, value.toSet()) }
var myObject: MyObject?
        get() {
           val jsonString = sharedPrefs.getString(KEY_USER_DATA, null) ?: return null
           return gson.fromJson(jsonString, object : TypeToken<MyObject>() {}.type)
    }
        set(value) = sharedPrefs.edit { putString(KEY_MY_OBJECT, gson.toJson(value)) }
}
data class MyObject(val name: String)

Here, `sharedPrefs.edit{…}` is provided by the android core ktx library and should be implemented by adding core ktx dependency in appliation level `build.gradle`.
implementation “androidx.core:core-ktx:1.0.2”
and for Gson
implementation “com.google.code.gson:gson:2.8.5”

We can get the instance of `SharedPreferences` by using code:

val prefs = Prefs(context)

Furthermore, you can create the Singleton object of Prefs and use from anywhere within the app.

val prefs: Prefs by lazy {
 Prefs(App.instance)
}

where App extends Application and should be included in AndroidManifest.

//App.kt
class App : Application() {
    companion object {
        lateinit var instance: App
    }

    override fun onCreate() {
        super.onCreate()
        instance = this
    }
}
//AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest .....

   <application
        android:name=".App"
        ....
   </application>
</manifest>