CODEWITHSUNDEEP
androidsingletonkotlin
johnny_crq
johnny_crq

Reputation: 4391

Kotlin singleton application class

On Android I want to make my application class a singleton.

Making it like this:

object MyApplication: Application(){}

won't work. The following error is thrown at runtime:

java.lang.IllegalAccessException: private com....is not accessible from class android.app.Instrumentation.

Doing this is also not possible:

class MyApp: Application() {

    private val instance_: MyApp

    init{
        instance_ = this
    }

    override fun onCreate() {
        super.onCreate()
        if (BuildConfig.DEBUG) {
            Timber.plant(Timber.DebugTree());
        }
    }

    companion object{
        fun getInstance() = instance_
    }
}

How can I get an instance of my application class everywhere in my app? I would like to use MyApp.instance() instead of (applicationContext as MyApp).

Also an explanation why I want this: I have classes in my app. For example, a SharedPreference Singleton which is initialised with a context, and as it’s a singleton, it can't have arguments.

Upvotes: 56

Views: 58156

Answers (4)

Raja Jawahar
Raja Jawahar

Reputation: 6972

class AppController : Application() {

    init {
        instance = this
    }

    companion object {
        private var instance: AppController? = null

        fun applicationContext() : AppController {
            return instance as AppController
        }
    }

    override fun onCreate() {
        super.onCreate()
        
    }
}

Upvotes: 21

nitrico
nitrico

Reputation: 623

If you want to use it to access some static properties you have there: You will only have one instance of your Application, so simply use the name you gave to the class. Don't worry about it not being an actual singleton, you can use it the same way.

Example:

class MyApp : Application() {

    companion object {
        const val CONSTANT = 12
        lateinit var typeface: Typeface
    }

    override fun onCreate() {
        super.onCreate()
        typeface = Typeface.createFromAsset(assets, "fonts/myFont.ttf")
    }

}

Then you can use MyApp.CONSTANT and MyApp.typeface anywhere in your app.

-

If what you want is to use it as an application context you can create an extension property for Context:

val Context.myApp: MyApp
        get() = applicationContext as MyApp

Then you can use myApp to get the the application context anywhere you have a context.

Upvotes: 50

Kirill Rakhman
Kirill Rakhman

Reputation: 43861

You can do the same thing you would do in Java, i.e. put the Application instance in a static field. Kotlin doesn't have static fields, but properties in objects are statically accessible.

class MyApp: Application() {

    override fun onCreate() {
        super.onCreate()
        instance = this
    }

    companion object {
        lateinit var instance: MyApp
            private set
    }
}

You can then access the property via MyApp.instance.

Upvotes: 123

Michael
Michael

Reputation: 54725

You cannot do that because Android creates an Application instance using its parameterless constructor.

The problem you want to solve can be easily solved with DI. Just create instances with an injector so that the Context can be injected into objects as a dependency.

Upvotes: 3

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