Hey guys. I know a little bit about `qualified this` but I am trying to wrap my brains around this s...

# android

[Chaiwa Berian]

Hey guys. I know a little bit about qualified this but I am trying to wrap my brains around this syntax right here:

Observer { loginFormState ->
                if (loginFormState == null) {
                    return@Observer
                }
                loginButton.isEnabled = loginFormState.isDataValid
                loginFormState.usernameError?.let {
                    usernameEditText.error = getString(it)
                }
                loginFormState.passwordError?.let {
                    passwordEditText.error = getString(it)
                }
            })

I understand the rest of the code except what is happening here:

if (loginFormState == null) {
     return@Observer
  }

What would be the interpretation of what return@Observer mean? Semantics?

[ephemient]

https://kotlinlang.org/docs/returns.html#return-to-labels

[Chaiwa Berian]

Thanks @ephemient I remember doing this type of thing in Pascal with a goto. Same principal right?

[ephemient]

no

[Chaiwa Berian]

Is it like continue in other languages?

[ephemient]

Observer { ... } is an anonymous object like

object Observer {
    fun observe(loginFormState) {
        if (loginFormState == null) {
            return

or a function taking a lambda, can't tell without more context, but the code behavior is similar either way

[Chaiwa Berian]

Here is the full context:

loginViewModel.loginFormState.observe(viewLifecycleOwner,
            Observer { loginFormState ->
                if (loginFormState == null) {
                    return@Observer
                }
                loginButton.isEnabled = loginFormState.isDataValid
                loginFormState.usernameError?.let {
                    usernameEditText.error = getString(it)
                }
                loginFormState.passwordError?.let {
                    passwordEditText.error = getString(it)
                }
            })

[Chaiwa Berian]

If I understand the docs, it is a way of returning to a specific point in code. So in this case it returns back to that place labelled Observer and then keeps waiting.

[Chaiwa Berian]

Am I right?

[ephemient]

no

[Chaiwa Berian]

Is it like exiting a named anonymous function?

[ephemient]

a bare return always means "return from the enclosing `fun`". there are cases where this is either not the intended behavior (intentional early return within an inline lambda) or impossible (non-inline lambda, SAM/`fun interface`), where you would use return@. this case is a SAM

[Chaiwa Berian]

What is SAM?

[ephemient]

https://kotlinlang.org/docs/fun-interfaces.html

[Chaiwa Berian]

I see. With your explanation, I can now notice that Observer is not a label but some functional interface and it is being instantiated via lambda expression hence the need to use the return@ in order to return early. Did I now get you right?

[ephemient]

yes. in this case you cannot use return at all, because the observer outlives the function

[Chaiwa Berian]

Tough stuff! Let me read a little bit more, but thank you very much for the help.