Switch from a function based implementation to making Promise a object. Maps straightforward to a javascript promise, except for this: We need to do cleanup when the promise has been fully processed. We do this by adding a final handler. However, the handlers are called in the order they are added, so the final handler we add in the constructor will be executed too early. We solve this, in a non optimal way, by adding a final handler after each then, and catch, handler. This makes the interface easy to use, even if it is suboptimal Fixes: QTBUG-142138 Change-Id: I6be3f102e838467cc98f4a9825fd5f6fe0e1e1a2 Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>