Position of `@` label in Kotlin when denoting receiver with `this`

Question

I am a newbie in Kotlin. I am curious about the difference of labeled this in Kotlin with prefix @ or postfix @.

I have just seen a code which writes SignInActivity@this, which seems to work exactly same as this@SignInActivity.

Are these two exactly the same thing? If not, what is the difference between the two?

I was trying to do some research on *@this form, but I couldn't find any reference on it. All I could find was this official doc which demonstrates this@*. It will be nice if anybody could share me with the correct reference I should go to.

Answer 1

SignInActivity@this means SignInActivity.this (Java) this@SignInActivity means - using the SignInActivity context instead a local context (usually is in closures).

Answer 2

SignInActivity@ this is just another expression for this, with the functionality of defining an unnecessary label called SignInActivity(which has nothing to do with actual class name) for this.

According to Kotlin grammar documentation:

labelReference (used by atomicExpression, jump)
   : "@" ++ LabelName
   ;
labelDefinition (used by prefixUnaryOperation, annotatedLambda)
  : LabelName ++ "@"
  ;

hello@ is just a label with the name "hello" (for Returns and Jumps) ,

whereas @hello is a reference for the labeled loop or block.

These expressions combined can be used as below:

loop@ for (i in 1..100) {
    for (j in 1..100) {
        if (...) break@loop //jump to loop@
   }
}