Called when the output port has received a pull, and therefore ready to emit an element, i.e.
Called when the output port has received a pull, and therefore ready to emit an element, i.e. GraphStageLogic.push() is now allowed to be called on this port.
Called when the output port will no longer accept any new elements.
Called when the output port will no longer accept any new elements. After this callback no other callbacks will be called for this port.
Java API: callbacks for an output port where termination logic is predefined (completing when downstream cancels).