Searching for an equivalent in Swift, I narrowed my choice to:
|Library||# contributors||tests||last update?||documentation||issues resolved|
|Promise Kit||51||yes||< week||yes||31 open, 227 closed|
|Future Kit||9||yes||10 days||Some, but limited||3 open, 6 closed|
|Bright Futures||18||yes & Travis||1 month||yes||9 open, 56 closed|
|Bolts||4||yes & Travis||< week||yes||1 open, 16 closed|
On a purely analytical survey, Promise Kit appears to be the winner.
My choice was Bright Futures