Obtain a callback object that can be used asynchronously to re-enter the current GraphStageTShape with an asynchronous notification. The delegate returned is safe to be called from other threads and it will in the background thread-safely delegate to the passed callback function. I.e. it will be called by the external world and the passed handler will be invoked eventually in a thread-safe way by the execution environment. This object can be cached and reused within the same GraphStageLogic.

Obtain a callback object that can be used asynchronously to re-enter the current GraphStageTShape with an asynchronous notification. The delegate returned is safe to be called from other threads and it will in the background thread-safely delegate to the passed callback function. I.e. it will be called by the external world and the passed handler will be invoked eventually in a thread-safe way by the execution environment. This object can be cached and reused within the same GraphStageLogic.