Promises in VBA - yes we can. VBA is not ideal for dealing with multiple things in paralell. Event processing and some asynchronous capabilities exist, but orchestration of them is complex. Here's an introduction to a basic VBA implementation of some of the orchestration improvedments given by deferred promises available in a number of javascript based frameworks

1. Orchestrating asynchronous Excel
VBA operations with promises
cPromise primer from Excel Liberation

2. Excel Liberation for details
Waiting for stuff to finish
 JavaScript is good at doing something else in the
meantime
doSomething ( function (result) {
report(result);
} );
doSomethingElse();
function doSomething (callback) {
var result = process();
callback (result);
}
 VBA prefers waiting for things to finish before doing the
next
result = process()
report (result)
doSomethingElse()

3. Excel Liberation for details
Orchestration with promises
 JavaScript can quickly become a mess with multiple callbacks. Many
developers are now using Deferred promises for better orchestration. For
example in jQuery
var promise = doSomething();
doSomethingElse();
promise.done ( function (result) {
report(result);
})
.fail ( function (error) {
report (error);
});
function doSomething (callback) {
var d = $.Deferred();
process ( function (result) { d.resolve(result);},
function (error) { d.reject (error);} );
return d.promise();
}

4. Excel Liberation for details
Orchestration in VBA
 One of the most common is to use withEvents, for example
you can execute ADO asynchronously and handle connection
events by declaring your connection with events
Private WithEvents prConnection As ADODB.Connection
 Other functions allow a callback function name, but put
restrictions on what that can be. For example the XMLHTTP
object has an onReadyStateChange property, but first you
have to figure out how to create a class with a default member.
pResponse.OnReadyStateChange = pAsynch
 Custom classes can declare and raise custom events, but they
are underused and complicated
 VBA needs syntactic gymnastics to deal with callbacks and
often forces the need for a network of global variables

5. Excel Liberation for details
Promises in VBA
 We’ve seen that VBA has various capabilities. Could
they be used to create a deferred/promise structure
?
 Yes they can!
 Here’s a snippet that will get data from a web site
asynchronously, and populate a range with the result
when its done.
Dim callbacks As New yourCallbacks
loadData("http://www.mcpher.com", Range("rest!a1"))
.done(callbacks, "populate")
.fail callbacks, "show“
doSomethingElseInTheMeantime()

6. Excel Liberation for details
Difference between cPromise and cDeferred
A new cDeferred is created by some task that will complete later. Each cDeferred has a
single cPromise
3 simple rules.
 Deferred is used by the function doing the work
 Promise is used by the function receiving the results.
 Function doing the work returns the promise method of the deferred
Function doing the work
return deferred.promise()
And later
deferred.reject( array(.. Some arguments..))
deferred.resolve( array(..some arguments..))
Function receiving the results
promise.fail (callbacks, “handleit”)
promise.done(callbacks,”processit”)

7. Excel Liberation for details
Error handling
An issue with asynch and event processing in VBA is how
to communicate errors to the caller. Using promises its
easy. A promise either fails or succeeds. On completion,
if it fails the .fail() method is executed, otherwise the
.done() method is executed.
And it used like this, where you’ve written a handler in your
callbacks class
promise.fail (callbacks, “handleit”)
The arguments you registered at the time of rejection will
be passed to your handler
deferred.reject (Array(“it failed”, statusCode,
someOtherInfo))

8. Excel Liberation for details
Success handling
Success handling and error handling techniques turn
out to be the same when using promises.
You’ve written a handler in your callbacks class
promise.done (callbacks, “processIt”)
The arguments you registered at the time of resolution
will be passed to your handler
deferred.resolve (Array(“it worked”,data))

9. Excel Liberation for details
Setup – cDeferred and cPromise classes
 Here’s the classes that are provided
 cDeferred – like $.Deferred() – is used to .resolve() or
.reject() a promise, and to provide the promise() instance
 cPromise – the .done() and .fail() methods set up what to
do on resolution or rejection, very much like the jQuery
.done() and .fail() methods. I dont provide .then() or
$.Where() but may add them later

10. Excel Liberation for details
Setup – creating a function that returns a
promise
 Every one of these will be different since VBA has
multiple ways of dealing with asynchronicity.
However they must follow this structure
Set d = new cDeferred (create an instance)
.. Do something asyncronous (passing a reference to d)
return d.promise()

11. Excel Liberation for details
Setup – doing something asynchronous
 The asynchronous function must
Signal completion using the deferred instance that returned
the promise in the caller function like this
d.resolve(..arguments...)
Or
d.reject (..arguments)

12. Excel Liberation for details
Setup – arguments
 The asynchronous function should also return some
arguments. These will be passed on to the function
that is eventually called on completion of the task
Arguments can be of any type and number, but should be
wrapped in an array like this. This protects them from
being incorrectly handled when passing through the
chain.
d.resolve (Array(.url, .response.responseText, .optionalArgument))

13. Excel Liberation for details
Memory and scope
One of the problems with asynch and event processing in VBA is that you can end up with many global or module level
variables to communicate. Using promises to pass arguments avoids this need, since the promise resolution
records a reference to the results data to be passed, and thus prevents it going out of scope.
When you create an proc that is going to behave asynchronously though, there is a chance that you will find local
variables going out of scope and therefore events not firing – this is nothing to do with promises – but a regular
issue with orchestration in VBA. One solution is to use module level variables. Another is to make reference to a
transient variable in a single, module level object.
I provide a register class to register asynch variables with, which is declared at module level
Private register As cDeferredRegister
And used like this
Dim ca As cHttpDeferred
Set ca = New cHttpDeferred
register.register ca
You should create a teardown method in any classes you create that have special memory leak prevention needs.
Register.teardown will clean itself up and execute any teardowns with any object instances that have been
registered
register.tearDown()

14. Excel Liberation for details
Summary
These examples start to address how asynchronicity
might be better orchestrated in VBA within the
limitations of the available syntax. Over time I will
add other promise related functions such as when()
Memory leaks relating to asynchronicity are not
resolved by this, but can at least be identified and
mitigated by this cleaner orchestration.
For more detail, examples, and to download see Excel
Liberation