Can you explain what the keyword crossinline means in Kotlin and in what cases it is used?

In Kotlin, the keyword crossinline is used in the context of inline functions and lambdas. It is applied to function parameters to prohibit the use of non-local return operators.

When a function is marked as inline, lambda parameters can also be inlined into the calling function's code. Usually, in such lambdas, you can use return to exit the outer function (non-local return). But if the lambda is passed to another function or executed asynchronously, such a return is not possible.

crossinline prevents non-local returns, ensuring that the lambda cannot interrupt the execution of the outer function. This is useful when the lambda is called not directly, but for example, passed further or executed in another context.

Example:

inline fun doSomething(crossinline action: () -> Unit) {
    val runnable = Runnable { action() }
    runnable.run()
}

Here, crossinline is needed so that return cannot be used inside action to exit doSomething, since action is called inside a Runnable.