Promise

The Promise object represents the eventual completion (or failure) of an asynchronous operation and its resulting value.

Promise

A constructor and also a derived class of EventObject which can be fired only once (aka resolved or rejected) even before a corresponding event handler is attached

Promise[] _Promise (* constructor *)

To resolve or reject a promise - use EventFire

EventFire[p_Promise, Resolve | Reject, data_]

Then

An expression for asynchronous subscribing to promise resolution or rejection

Then[p_Promise | _List | _, resolve_]
Then[p_Promise | _List | _, resolve_, reject_]

where resolve and reject are any arbitrary functions, usually, non-blocking functions.

Being applied to a List of _Promise objects it will wait until all of them are resolved before evaluating resolve function and will return the list of results.

Any Wolfram Expressions, which is not a List of Promise counts as resolved, i.e. this is valid:

Then[1+1, Function[result, Print]];

p = Promise[];

Then[{1+1, p}, Function[result, Print]];
Pause[3];
EventFire[p, Resolve, 43];

Example

Let's try with a multiple promise events

p1 = Promise[];
p2 = Promise[];

SetTimeout[EventFire[p1, Resolve, Null], 1000];
SetTimeout[EventFire[p2, Resolve, Null], 1500];

Then[{p1,p2,Null}, Function[Null,
	Echo["Resolved!"];
]];

Here Null as a last element of a list was used just for demonstration purposes. It can also be any non _Promise | _List expression.

Relation to EventObject

Then[e_EventObject, resolve_]

It effectively assigns an event handler to e and calls resolve on any pattern captured.

WaitAll

A synchronous blocking function to wait until a promise has resolved and returns the result

WaitAll[p_Promise] _

WaitAll[p_Promise, timeout_:15] _

There is a timeout of 15 seconds, then $Failed is returned.