Can we use Interfaces and Events together at the same time?

Question

I\'m still trying to wrap my head around how Interfaces and Events work together (if at all?) in VBA. I\'m about to build a large application in Microsoft Access, and I want

Answer 1

This is a perfect use-case for an Adapter: internally adapting the semantics for a set of contracts (interfaces) and exposing them as its own external API; possibly according to some other contract.

Define class modules IViewEvents:

Option Compare Database
Option Explicit

Private Const mModuleName   As String = "IViewEvents"

Public Sub OnBeforeDoSomething(ByVal Data As Object, ByRef Cancel As Boolean):  End Sub
Public Sub OnAfterDoSomething(ByVal Data As Object):                            End Sub

Private Sub Class_Initialize()
    Err.Raise 5, mModuleName, AccessError(5) & "-Interface class must not be instantiated."
End Sub

IViewCommands:

Option Compare Database
Option Explicit

Private Const mModuleName   As String = "IViewCommands"

Public Sub DoSomething(ByVal arg1 As String, ByVal arg2 As Long):   End Sub

Private Sub Class_Initialize()
    Err.Raise 5, mModuleName, AccessError(5) & "-Interface class must not be instantiated."
End Sub

ViewAdapter:

Option Compare Database
Option Explicit

Private Const mModuleName   As String = "ViewAdapter"

Public Event BeforeDoSomething(ByVal Data As Object, ByRef Cancel As Boolean)
Public Event AfterDoSomething(ByVal Data As Object)

Private mView       As IViewCommands

Implements IViewCommands
Implements IViewEvents

Public Function Initialize(View As IViewCommands) As ViewAdapter
    Set mView = View
    Set Initialize = Me
End Function

Private Sub IViewCommands_DoSomething(ByVal arg1 As String, ByVal arg2 As Long)
    mView.DoSomething arg1, arg2
End Sub

Private Sub IViewEvents_OnBeforeDoSomething(ByVal Data As Object, ByRef Cancel As Boolean)
    RaiseEvent BeforeDoSomething(Data, Cancel)
End Sub
Private Sub IViewEvents_OnAfterDoSomething(ByVal Data As Object)
    RaiseEvent AfterDoSomething(Data)
End Sub

and Controller:

Option Compare Database
Option Explicit

Private Const mModuleName       As String = "Controller"

Private WithEvents mViewAdapter As ViewAdapter

Private mData As Object

Public Function Initialize(ViewAdapter As ViewAdapter) As Controller
    Set mViewAdapter = ViewAdapter
    Set Initialize = Me
End Function

Private Sub mViewAdapter_AfterDoSomething(ByVal Data As Object)
    ' Do stuff
End Sub

Private Sub mViewAdapter_BeforeDoSomething(ByVal Data As Object, ByRef Cancel As Boolean)
    Cancel = Data Is Nothing
End Sub

plus Standard Modules Constructors:

Option Compare Database
Option Explicit
Option Private Module

Private Const mModuleName   As String = "Constructors"

Public Function NewViewAdapter(View As IViewCommands) As ViewAdapter
    With New ViewAdapter:   Set NewViewAdapter = .Initialize(View):         End With
End Function

Public Function NewController(ByVal ViewAdapter As ViewAdapter) As Controller
    With New Controller:    Set NewController = .Initialize(ViewAdapter):   End With
End Function

and MyApplication:

Option Compare Database
Option Explicit

Private Const mModuleName   As String = "MyApplication"

Private mController As Controller

Public Function LaunchApp() As Long
    Dim frm As IViewCommands 
    ' Open and assign frm here as instance of a Form implementing 
    ' IViewCommands and raising events through the callback interface 
    ' IViewEvents. It requires an initialization method (or property 
    ' setter) that accepts an IViewEvents argument.
    Set mController = NewController(NewViewAdapter(frm))
End Function

Note how use of the Adapter Pattern combined with programming to interfaces results in a very flexible structure, where different Controller or View implementations can be substituted in at run time. Each Controller definition (in the case of different implementations being required) uses different instances of the same ViewAdapter implementation, as Dependency Injection is being used to delegate the event-source and command-sink for each instance at run time.

The same pattern can be repeated to define the relationship between the Controller/Presenter/ViewModel and the Model, though implementing MVVM in COM can get rather tedious. I have found MVP or MVC is usually better suited for COM-based applications.

A production implementation would also add proper error handling (at a minimum) to the extent supported by VBA, which I have only hinted at with the definition of the mModuleName constant in each module.