Kotlin syntax for LiveData observer?

Question

I have the following bit of code in my HomeActivity to use LiveData.

override fun onCreate(savedInstanceState: Bundle?) {
    sup         


        

Answer 1

To omit the Observer { ... } part just add import androidx.lifecycle.observe and use it like this:

this.viewModel.user.observe(this) { user: User? ->
    // ...
}

Answer 2

in Kotlin the Observer { } lambda gives you param it, you can rename it as you want and use. by default data will be available with it.something() etc...

JAVA:

... new Observer {
  void onChanged(User user){
     user.something()
  }
}

KOTLIN

... object : Observer<User> {
   fun onChanged(user: User?){
        user.something()
   }
}

OR

... Observer {
   it.something()
}

you can rename it to whatever you want like

... Observer { myUser ->
   myUser.something()
}

Answer 3

This is called SAM Conversion, a concept that helps interacting with Java Single Abstract Method Interfaces like in your example.

The following creates an implementation of Runnable, where the single abstract method is run():

val runnable = Runnable { println("This runs in a runnable") }

It’s described in the docs: https://kotlinlang.org/docs/reference/java-interop.html#sam-conversions

Alternatively, but more verbose, would be to use an object:

val runnable2 = object : Runnable {
        override fun run() {
            println("This runs in a runnable")
        }
}

Both are examples of anonymous implementations of that interface. It's of course also possible to create a concrete subclass and instantiate it then.

class MyRunnable : Runnable {
    override fun run() {
        println("This runs in a runnable")
    }
}

val runnable3 = MyRunnable()