Making MVVM Work in VBA Part 2: Event Propagation

Using a WithEvents variable to handle the MSForms.Control events of, say, a TextBox control has the irritating tendency to throw a rather puzzling run-time error 459 “Object or class does not support the set of events”. To be honest, I had completely forgotten about this when I started working on this MVVM framework. I had even posted an answer on Stack Overflow and my learning-it-the-hard-way is immortalized on that page.

…there’s a bit of COM hackery going on behind the scenes; there’s enough smokes & mirrors for VBA to successfully compile the above, but, basically, you’re looking at a glitch in The Matrix (Rubberduck’s resolver has similar “nope” issues with MSForms controls): there isn’t any obvious way to get VBA to bind a dynamic control object to its MSForms.Control events.

-Mathieu Guindon, Apr 18 ’19 

What I hadn’t noticed until today, was that another user had posted an answer to that question a few hours later that day – and that answer ultimately leads to the groundbreaking manual wiring-up of what VBA normally does automagically under the hood when we declare a WithEvents variable.

pUnk’d

The code I’m about to share is heavily based on the work shared on Stack Overflow by user Evr, and uses the ConnectToConnectionPoint Win32 API that, it must be mentioned, comes with a caveat:

This function is available through Windows XP and Windows Server 2003. It might be altered or unavailable in subsequent versions of Windows.

Regardless, it works (for now anyway, …if we lose Mac support for this specific capability).

Rubberduck uses similar connection points to handle a number of VBE events that aren’t otherwise exposed, so I knew this was going to work one way or another. The idea is to pass an IUnknown pointer to an object that exposes members with very specific VB_UserMemId attribute values, and have accordingly very specific member signatures.

This post lists a bunch of such attributes – however since there aren’t any problems with binding regular TextBox and CommandButton events (these do work with simple WithEvents event providers), I’m only interested in these:

Event	VB_UserMemId
AfterUpdate	-2147384832
BeforeUpdate	-2147384831
Enter	-2147384830
Exit	-2147384829

The VB_UserMemId attribute values for each of the MSForms.Control events.

This is going to be a little bit lower-level than usual, but every VBA user class has an IUnknown pointer, So we can use any class module that has the members with the appropriate VB_UserMemId attribute values, and pass that as the pUnk pointer argument.

So, here’s the punk in question, exactly as I currently have it:

VERSION 1.0 CLASS
BEGIN
  MultiUse = -1  'True
END
Attribute VB_Name = "ControlEventsPunk"
Attribute VB_GlobalNameSpace = False
Attribute VB_Creatable = False
Attribute VB_PredeclaredId = False
Attribute VB_Exposed = False
Attribute VB_Description = "Provides an event sink to relay MSForms.Control events."
'@Folder MVVM.Infrastructure.Win32
'@ModuleDescription "Provides an event sink to relay MSForms.Control events."
'based on https://stackoverflow.com/a/51936950
Option Explicit
Implements IControlEvents
Private Type GUID
    Data1 As Long
    Data2 As Integer
    Data3 As Integer
    Data4(0 To 7) As Byte
End Type
'[This function is available through Windows XP and Windows Server 2003. It might be altered or unavailable in subsequent versions of Windows.]
'https://docs.microsoft.com/en-us/windows/win32/api/shlwapi/nf-shlwapi-connecttoconnectionpoint
#If VBA7 Then
Private Declare PtrSafe Function ConnectToConnectionPoint Lib "shlwapi" Alias "#168" (ByVal Punk As stdole.IUnknown, ByRef riidEvent As GUID, ByVal fConnect As Long, ByVal PunkTarget As stdole.IUnknown, ByRef pdwCookie As Long, Optional ByVal ppcpOut As LongPtr) As Long
#Else
Private Declare Function ConnectToConnectionPoint Lib "shlwapi" Alias "#168" (ByVal punk As stdole.IUnknown, ByRef riidEvent As GUID, ByVal fConnect As Long, ByVal punkTarget As stdole.IUnknown, ByRef pdwCookie As Long, Optional ByVal ppcpOut As Long) As Long
#End If
Private Type TState
    RefIID As GUID 'The IID of the interface on the connection point container whose connection point object is being requested.
    Connected As Boolean
    PunkTarget As Object
    Cookie As Long
    
    Handlers As Collection
End Type
'from https://stackoverflow.com/a/61893857 (same user as #51936950!)
Private Const ExitEventID As Long = -2147384829
Private Const EnterEventID As Long = -2147384830
Private Const BeforeUpdateEventID As Long = -2147384831
Private Const AfterUpdateEventID As Long = -2147384832
Private This As TState
'@Description "Gets/sets the target MSForms.Control reference."
Public Property Get Target() As Object
Attribute Target.VB_Description = "Gets/sets the target MSForms.Control reference."
    Set Target = This.PunkTarget
End Property
Public Property Set Target(ByVal RHS As Object)
    Set This.PunkTarget = RHS
End Property
'@Description "Registers the listener."
Public Function Connect() As Boolean
Attribute Connect.VB_Description = "Registers the listener."
    GuardClauses.GuardNullReference This.PunkTarget, TypeName(Me), "Target is not set."
    ConnectToConnectionPoint Me, This.RefIID, True, This.PunkTarget, This.Cookie, 0&
    This.Connected = This.Cookie <> 0
    Connect = This.Connected
End Function
'@Description "De-registers the listener."
Public Function Disconnect() As Boolean
Attribute Connect.VB_Description = "De-registers the listener."
    If Not This.Connected Then Exit Function
    ConnectToConnectionPoint Me, This.RefIID, False, This.PunkTarget, This.Cookie, 0&
    This.Connected = False
    Disconnect = True
End Function
'@Description "A callback that handles MSForms.Control.AfterUpdate events for the registered target control."
Public Sub OnAfterUpdate()
Attribute OnAfterUpdate.VB_UserMemId = -2147384832
Attribute OnAfterUpdate.VB_Description = "A callback that handles MSForms.Control.AfterUpdate events for the registered target control."
    Dim Handler As IHandleControlEvents
    For Each Handler In This.Handlers
        Handler.HandleAfterUpdate
    Next
End Sub
'@Description "A callback that handles MSForms.Control.BeforeUpdate events for the registered target control."
Public Sub OnBeforeUpdate(ByVal Cancel As MSForms.ReturnBoolean)
Attribute OnBeforeUpdate.VB_UserMemId = -2147384831
Attribute OnBeforeUpdate.VB_Description = "A callback that handles MSForms.Control.BeforeUpdate events for the registered target control."
    Dim Handler As IHandleControlEvents
    For Each Handler In This.Handlers
        Handler.HandleBeforeUpdate Cancel
    Next
End Sub
'@Description "A callback that handles MSForms.Control.Exit events for the registered target control."
Public Sub OnExit(ByVal Cancel As MSForms.ReturnBoolean)
Attribute OnExit.VB_UserMemId = -2147384829
Attribute OnExit.VB_Description = "A callback that handles MSForms.Control.Exit events for the registered target control."
    Dim Handler As IHandleControlEvents
    For Each Handler In This.Handlers
        Handler.HandleExit Cancel
    Next
End Sub
'@Description "A callback that handles MSForms.Control.Enter events for the registered target control."
Public Sub OnEnter()
Attribute OnEnter.VB_UserMemId = -2147384830
Attribute OnEnter.VB_Description = "A callback that handles MSForms.Control.Enter events for the registered target control."
    Dim Handler As IHandleControlEvents
    For Each Handler In This.Handlers
        Handler.HandleEnter
    Next
End Sub
'@Description "Registers the specified object to handle the relayed control events."
Public Sub RegisterHandler(ByVal Handler As IHandleControlEvents)
Attribute RegisterHandler.VB_Description = "Registers the specified object to handle the relayed control events."
    This.Handlers.Add Handler
End Sub
Private Sub Class_Initialize()
    Set This.Handlers = New Collection
    This.RefIID.Data1 = &H20400
    This.RefIID.Data4(0) = &HC0
    This.RefIID.Data4(7) = &H46
End Sub
Private Sub IControlEvents_OnAfterUpdate()
    OnAfterUpdate
End Sub
Private Sub IControlEvents_OnBeforeUpdate(ByVal Cancel As MSForms.IReturnBoolean)
    OnBeforeUpdate Cancel
End Sub
Private Sub IControlEvents_OnEnter()
    OnEnter
End Sub
Private Sub IControlEvents_OnExit(ByVal Cancel As MSForms.IReturnBoolean)
    OnExit Cancel
End Sub
Private Sub IControlEvents_RegisterHandler(ByVal Handler As IHandleControlEvents)
    RegisterHandler Handler
End Sub

Let’s ignore the IControlEvents interface for now. The class has a Target – that’ll be our TextBox control instance. So we set the Target, and then we can invoke Connect, and when we’re done we can invoke Disconnect to explicitly undo the wiring-up.

Then we have an OnEnter method with VB_UserMemId = -2147384830, which makes it an event handler procedure for MSForms.Control.Enter. The name of the procedure isn’t relevant, but it’s important that the procedure is parameterless.

Similarly, the name of the OnExit procedure has no importance, but it must have a single ByVal Cancel As MSForms.ReturnBoolean parameter (only ByVal and the data type matter). For events that have more than one parameter, the order is also important.

In theory that’s all we need: we could go on and handle Control.Exit in this OnExit procedure, and call it a day. In fact you can probably do that right away – however I need another step for my purposes, because I’m going to need my PropertyBindingBase class to propagate these events “up” to, say, some TextBoxPropertyBinding class that can implement some TextBox-specific behavior for the Control events.

Propagating Events

I had already a working pattern for my INotifyPropertyChange requirements to propagate property changes across objects, and the pattern is applicable here too. See, I could have declared a Public Event Exit(ByRef Cancel As MSForms.ReturnBoolean) on the ControlEventsPunk class, and then I could have used a WithEvents variable to handle them – and that would have worked too. Except I don’t want to use events here, because events work well as implementation details… but they can’t be exposed on an interface, which makes them actually more complicated to work with.

There are two interfaces: one that defines the “events” and exposes a method to register “handlers”, and the other mandates the presence of a callback for each “event”. For INotifyPropertyChange the handler interface was named IHandlePropertyChange, so I went with IControlEvents and IHandleControlEvents.

So, the “provider” interface looks like this:

'@Folder MVVM.Infrastructure.Bindings.Abstract
'@ModuleDescription "Provides the infrastructure to relay MSForms.Control events."
Option Explicit
Public Sub RegisterHandler(ByVal Handler As IHandleControlEvents)
End Sub
Public Sub OnEnter()
End Sub
Public Sub OnExit(ByVal Cancel As MSForms.ReturnBoolean)
End Sub
Public Sub OnAfterUpdate()
End Sub
Public Sub OnBeforeUpdate(ByVal Cancel As MSForms.ReturnBoolean)
End Sub

And then the “handler” interface looks like this:

'@Folder MVVM.Infrastructure.Bindings.Abstract
'@ModuleDescription "An object that can be registered as a handler for IControlEvents callbacks."
Option Explicit
Public Sub HandleEnter()
End Sub
Public Sub HandleExit(ByVal Cancel As MSForms.ReturnBoolean)
End Sub
Public Sub HandleAfterUpdate()
End Sub
Public Sub HandleBeforeUpdate(ByVal Cancel As MSForms.ReturnBoolean)
End Sub

So, looking back at the ControlEventsPunk class, we find that the implementation for RegisterHandler consists in adding the provided Handler object to an encapsulated Collection that holds all the registered handlers; when we “handle” a control event, we iterate all registered handlers and invoke them all in a sequence. When an event has a Cancel parameter, the last handler that ran gets the final say on whether the parameter should be True or False, and each handler receives the Cancel value that was set by the previous handler than ran.

This is a slightly different paradigm than your regular VBA/VB6 auto-wired events, where one event only ever has one handler: now these work more like the multicast delegates that events are in .NET, with an “invocation list” and the ability to add/remove (although, I haven’t implemented the removal) handlers dynamically at run-time – except the “handlers” are full-fledged VBA objects here, rather than .NET delegates.

Whenever the MVVM infrastructure needs to propagate events, I use this pattern instead. This was my first time actually implementing an Observer Pattern, and hadn’t even realized! (thanks Max!) – that isn’t a pattern you see often in event-capable languages, but I can definitely see this proven, solid abstraction (Java developers would probably be rather familiar with that one) become my new favorite go-to pattern to expose events on an interface in VBA… But there’s probably a reason the first time I come across a situation where that pattern is really handy (and actually needed, for testability), is when I’m writing framework-level (i.e. an API intended to be used by code that isn’t written yet) code that’s very much as deep into the OOP rabbit hole as I’ve ever been in VBA (or any other language for that matter)… and there’s still no rock bottom in sight.

In any case, now that we have a way to handle and propagate control events, we can have MVVM property bindings that can format TextBox.Text on exit, i.e. we can have a ViewModel that knows SomeProperty has a value of 25.59, and the Text property of the bound textbox control can say $25.59 just by specifying a FormatString (like “Currency”, for example) when we create the binding.

For the next post in this series I think we’re ready to deep-dive into the actual binding mechanics, and I’ll have the updated MVVM infrastructure code on GitHub by then.

Everything You Ever Wanted To Know About Events

VBA is often said to be an event-driven language: a lot of worksheet automation involves executing code in response to such or such workbook or worksheet event. ActiveX controls such as MSForms.CommandButton are trivially double-clicked, and code is written in some CommandButton1_Click procedure.

But how does it all work, and can we leverage this event-driven paradigm in our own code too? But first, what is this paradigm all about?

Asynchronous Code

In a procedural paradigm, code executes one statement at a time, top to bottom, in sequence: procedure A gets invoked, procedure A calls procedure B, procedure B completes, execution returns to procedure A, procedure A completes, execution ends. That’s a paradigm you’re likely already familiar with.

In an event-driven paradigm, code still executes one statement at a time, in the very same way – except now procedures are being invoked by an external object, and there isn’t always a way to tell at compile-time what the run-time sequence will be. If you’re writing the code-behind for a UserForm module with Button1 and Button2 controls, there is no way to know whether Button1_Click will run before Button2_Click, or if either are even going to run at all: what code gets to run, is driven by what events get raised – hence, event-driven.

Event-driven code is asynchronous, meaning you could be in the middle of a loop, and then a DoEvents statement is encountered, and suddenly you’re not in the loop body anymore, but (as an example) in some worksheet event handler that gets invoked when the selected cell changes. And when that handler completes, execution resumes in the loop body, right where it left off.

This mechanism is extremely useful, especially in an object-oriented project, since only objects (class modules) are allowed to raise and handle events. It is being put to extensive use in the OOP BattleShip project (see GridViewAdapter and WorksheetView classes for examples of event forwarding and how to define an interface that exposes events), which I’m hoping makes a good advanced-level study on the matter.

But let’s start at the beginning.

Host Document & Other Built-In Events

Whether you’re barely a week into your journey to learn VBA, or several years into it, unless all you ever did was record a macro, you’ve been exposed to VBA events.

VBA code lives in the host document, waiting to be executed: any standard module public procedure that can be invoked without parameters can be an entry point to begin code execution – these are listed in the “macros” list, and you can attach them to e.g. some Shape and have the host application invoke that VBA code when the user clicks it.

But macros aren’t the only possible entry points: the host document often provides “hooks” that you can use to execute VBA code when the host application is performing some specific operation – like opening a new document, saving it, modifying it, etc.: different hosts allow for various degrees of granularity on “hooking” a varying amount of operations. These “hooks” are the events exposed by the host application’s object model, and the procedures that are executed when these events are raised are event handler procedures.

Document Modules

In Excel the host document is represented by a Workbook module, named ThisWorkbook; every worksheet in this workbook is represented by a Worksheet module. These document modules are a special type of class module in that they inherit a base class: the ThisWorkbook class is a Workbook class; the Sheet1 class is a Worksheet class – and when classes relate to each other with an “is-a” relationship, we’re looking at class inheritance (“has-a” being composition). So document modules are a very special kind of module, first because their instantiation is in the hands of the host application (you can’t do Set foo = New Worksheet to create a new sheet), and second because like UserForm modules, they are inheriting the members of a base class – that’s how you can type Me. in a procedure inside an otherwise empty document module, and get plenty of members to pick from: if you’re in the Sheet1 module, you have access to Me.Range, and that Range property is inherited from the Worksheet “base class”. Or Me.Controls in the UserForm1 module, inherited from the UserForm class.

Wait I thought VBA didn’t support inheritance?

Indeed, VBA user code doesn’t have any mechanism in the language to support this: there’s no Inherits keyword in VBA. But VBA creates and consumes COM types, and these types can very well be pictured as having an inheritance hierarchy.

Or something like it. Picturing the ThisWorkbook : Workbook relationship as though there was a hidden Private WithEvents Workbook As Workbook field in the ThisWorkbook module, i.e. more like composition than inheritance, wouldn’t be inaccurate either.

Fair enough. So what does this have to do with events?

Take any old Sheet1 module: because it “inherits” the Worksheet class, it has access to the events defined in that class. You can easily see what events are available in any class, using the Object Browser (F2) – all events are members represented with a lightning bolt icon:

The Worksheet.Activate event is raised when the Worksheet.Activate method is invoked.

So when you’re in a Worksheet module, you can implement event handlers for any events fired by the base Worksheet class. Because of how events work under the hood, in order for an event handler to “hook” the event it means to handle, it must have the correct signature or at least, a compatible one: the name of the procedure, the number, order, and type of its parameters must match exactly with the signature/definition of the event to be handled… and ensuring that isn’t as complicated as it sounds:

All available event sources (left) and corresponding events (right) can be selected from dropdowns at the top of the code pane: the VBE generates a procedure with the correct signature automatically!

Notice the VBE generates Private procedures: there is no reason whatsoever to ever make an event handler public. Event handler procedures are meant to handle events, i.e. they’re callbacks whose intent is to be invoked by the VBA runtime, not by user code! That’s why I recommend limiting the amount of logic that you put into an event handler procedure, and having the bulk of the work into a separate, dedicated procedure that can be made public if it needs to be invoked from elsewhere. This is especially important for UserForm modules, which tend to be accessed from outside the form’s code-behind module.

Event handler procedures are always named in a very specific way, just like interface implementations:

Private Sub EventProvider_EventName()

Note the underscore: it matters, on a syntactical level. This is why you should avoid underscores in procedure names, and name all procedures in PascalCase. Adhering to this naming standard will spare you many headaches later, when you start defining and impementing your own interfaces (spoiler: your project will refuse to compile if you try implementing an interface that has members with an underscore in their name).

Custom Events

Any VBA class module can define its own events, and events may only be defined in a class module (remember: document and userform modules are classes). Defining events is done using the (drumroll) Event keyword:

Public Event BeforeSomething(ByRef Cancel As Boolean)
Public Event AfterSomething()

You’ll want the events Public, so they can be handled in other classes. Now all that’s left to do is to raise these events. That’s done using the RaiseEvent keyword:

Public Sub DoSomething()
    Dim cancelling As Boolean
    RaiseEvent BeforeSomething(cancelling)
    If Not cancelling Then
        'do stuff...
        RaiseEvent AfterSomething
    End If
End Sub

Here are a few guidelines (that word is chosen) for sane event design:

• DO define Before/After event pairs that are raised before and after a given operation. This leaves the handlers the flexibility to execute preparatory/cleanup code around that operation.
• DO provide a ByRef Cancel As Boolean parameter in Before events. This lets the handlers determine whether an operation should be cancelled or not.
• CONSIDER using ByVal Cancel As MSForms.ReturnBoolean if the MSForms type library is referenced. Being a simple object encapsulating the cancel state, it can be passed by value, and the handler code can treat it as a Boolean if it wants to, because the object’s Value is the class’ default member.
• CONSIDER exposing a public On[EventName] procedure with the same signature as the event, whose purpose is simply to raise said event; events can only be raised by the class they are defined in, so such methods are very useful for making an object raise an event, notably for testing purposes.
• DO use past tense to indicate that an event occurs after a certain operation has completed, when there is no need for an event to occur before. For example: Changed instead of Change.
• DO use future tense to indicate that an event occurs before a certain operation has started, when there is no need for an event to occur after. For example: WillConnect.
• DO NOT use present tense (be it indicative or progressive/continuous), it’s ambiguous and unclear exactly when the event is raised in the operation. For example, a lot of standard library events use this naming scheme, and it’s easy to second-guess whether the event is fired before or after said Change or Changing has actually happened.
• DO NOT use Before or After without also exposing a corresponding After/Before event.
• DO NOT mistake guidelines for gospel, what really matters is consistency.

The class that defines an event is the provider; a class that handles that event is a client. The client code needs to declare a WithEvents field – these fields must be early-bound, and the only types available for the As clause are event providers, i.e. classes that expose at least one event.

Option Explicit
Private WithEvents foo As Something

Private Sub foo_BeforeDoSomething(ByRef Cancel As Boolean)
    'handles Something.DoSomething
End Sub

Every WithEvents field adds a new item in the left-hand code pane dropdown, and that class’ events are displayed in the right-hand code pane dropdown – exactly like any plain old workbook or worksheet event!