Static extension methods in Kotlin

问题:

How do you define a static extension method in Kotlin? Is this even possible? I currently have an extension method as shown below.

public fun Uber.doMagic(context: Context) {
    // ...
}

The above extension can be invoked on an instance.

uberInstance.doMagic(context) // Instance method

but how do I make it static method like shown below.

Uber.doMagic(context)         // Static or class method

回答1:

To achieve Uber.doMagic(context), you can write an extension to the companion object of Uber (the companion object declaration is required):

class Uber {
    companion object {}
}

fun Uber.Companion.doMagic(context: Context) { }

回答2:

This is what the official documentation says:

Kotlin generates static methods for package-level functions. Kotlin can also generate static methods for functions defined in named objects or companion objects if you annotate those functions as @JvmStatic. For example:

Kotlin static methods

class C {
  companion object {
    @JvmStatic fun foo() {}
    fun bar() {}
  }
}

Now, foo() is static in Java, while bar() is not:

C.foo(); // works fine
C.bar(); // error: not a static method

回答3:

I actually had this exact question 30 minutes ago, so I started digging around and couldn't find any solution or workaround for this, BUT while searching I found this section on the Kotlinglang website that states that:

Note that extensions can be defined with a nullable receiver type. Such extensions can be called on an object variable even if its value is null.

So then I had the craziest idea ever, why not define an extension function with a nullable receiver (without actually using that receiver) and then call it on a null object! So I tried that, and it worked pretty well, but it looked so ugly. It was like this:

(null as Type?).staticFunction(param1, param2)

So I went around that by creating a val in my extensions file of the receiver type that had a value of null and then use it in my other class. So, as an example, here is how I implemented a "static" extension function for the Navigation class in Android: In my NavigationExtensions.kt file:

val SNavigation: Navigation? = null
fun Navigation?.createNavigateOnClickListener(@IdRes resId: Int, args: Bundle? = null, navOptions: NavOptions? = null,
                                                navigationExtras: Navigator.Extras? = null) : (View) -> Unit {
    //This is just implementation details, don't worry too much about them, just focus on the Navigation? part in the method declaration
    return { view: View -> view.navigate(resId, args, navOptions, navigationExtras) }
}

In the code that uses it:

SNavigation.createNavigateOnClickListener(R.id.action_gameWonFragment_to_gameFragment)

Obviously, this isn't a class name, it is just a variable of the class type that has a null value. This is obviously ugly on the extension maker side (because they have to create the variable) and on the developer side (because they have to use the SType format instead of the actual class name), but it is the closest that can be achieved right now compared to actual static functions. Hopefully, the Kotlin language makers will respond to the issue that was created and add that feature in the language.

回答4:

You can create a static method with using Companion object like:

class Foo {
    // ...
    companion object {
        public fun bar() {
            // do anything
        }
    }
}

and then you can call it like:

class Baz {
    // ...
    private fun callBar() {
        Foo.bar()
    }
}

回答5:

Recomend you to look at this link. As you can see there, you just should declare method at the top-level of the package (file):

package strings
public fun joinToString(...): String { ... }

This is equal to

package strings;

public class JoinKt {
    public static String joinToString(...) { ... }
}

With constans everything are the same. This declaration

val UNIX_LINE_SEPARATOR = "\n"

is equal to

public static final String UNIX_LINE_SEPARATOR = "\n";

回答6:

To create an extension method in kotlin you have to create a kotlin file(not a class) then declare your method in the file Eg:

public fun String.toLowercase(){
    // **this** is the string object
}

Import the function in the class or file you are working on and use it.

回答7:

I'm also quite fond of having the possibility to add static extension methods in Kotlin. As a workaround for now I'm adding the exntension method to multiple classes instead of using one static extension method in all of them.

class Util    

fun Util.isDeviceOnline(context: Context): Boolean {
    val connMgr = context.getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
    val networkInfo = connMgr.activeNetworkInfo
    return networkInfo != null && networkInfo.isConnected
}

fun Activity.isDeviceOnline(context: Context) = { Util().isDeviceOnline(context) }
fun OkHttpClient.isDeviceOnline(context: Context) = { Util().isDeviceOnline(context) }