VBA Object-Oriented VBA Encapsulation

Example

Encapsulation hides implementation details from client code.

The Handling QueryClose example demonstrates encapsulation: the form has a checkbox control, but its client code doesn't work with it directly - the checkbox is an implementation detail, what the client code needs to know is whether the setting is enabled or not.

When the checkbox value changes, the handler assigns a private field member:

Private Type TView
    IsCancelled As Boolean
    SomeOtherSetting As Boolean
    'other properties skipped for brievety
End Type
Private this As TView

'...

Private Sub SomeOtherSettingInput_Change()
    this.SomeOtherSetting = CBool(SomeOtherSettingInput.Value)
End Sub

And when the client code wants to read that value, it doesn't need to worry about a checkbox - instead it simply uses the SomeOtherSetting property:

Public Property Get SomeOtherSetting() As Boolean
    SomeOtherSetting = this.SomeOtherSetting
End Property

The SomeOtherSetting property encapsulates the checkbox' state; client code doesn't need to know that there's a checkbox involved, only that there's a setting with a Boolean value. By encapsulating the Boolean value, we've added an abstraction layer around the checkbox.

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Using interfaces to enforce immutability

Let's push that a step further by encapsulating the form's model in a dedicated class module. But if we made a Public Property for the UserName and Timestamp, we would have to expose Property Let accessors, making the properties mutable, and we don't want the client code to have the ability to change these values after they're set.

The CreateViewModel function in the Abstraction example returns an ISomeModel class: that's our interface, and it looks something like this:

Option Explicit

Public Property Get Timestamp() As Date
End Property

Public Property Get UserName() As String
End Property

Public Property Get AvailableItems() As Variant
End Property

Public Property Let AvailableItems(ByRef value As Variant)
End Property

Public Property Get SomeSetting() As String
End Property

Public Property Let SomeSetting(ByVal value As String)
End Property

Public Property Get SomeOtherSetting() As Boolean
End Property

Public Property Let SomeOtherSetting(ByVal value As Boolean)
End Property

Notice Timestamp and UserName properties only expose a Property Get accessor. Now the SomeModel class can implement that interface:

Option Explicit
Implements ISomeModel

Private Type TModel
    Timestamp As Date
    UserName As String
    SomeSetting As String
    SomeOtherSetting As Boolean
    AvailableItems As Variant
End Type
Private this As TModel

Private Property Get ISomeModel_Timestamp() As Date
    ISomeModel_Timestamp = this.Timestamp
End Property

Private Property Get ISomeModel_UserName() As String
    ISomeModel_UserName = this.UserName
End Property

Private Property Get ISomeModel_AvailableItems() As Variant
    ISomeModel_AvailableItems = this.AvailableItems
End Property

Private Property Let ISomeModel_AvailableItems(ByRef value As Variant)
    this.AvailableItems = value
End Property

Private Property Get ISomeModel_SomeSetting() As String
    ISomeModel_SomeSetting = this.SomeSetting
End Property

Private Property Let ISomeModel_SomeSetting(ByVal value As String)
    this.SomeSetting = value
End Property

Private Property Get ISomeModel_SomeOtherSetting() As Boolean
    ISomeModel_SomeOtherSetting = this.SomeOtherSetting
End Property

Private Property Let ISomeModel_SomeOtherSetting(ByVal value As Boolean)
    this.SomeOtherSetting = value
End Property

Public Property Get Timestamp() As Date
    Timestamp = this.Timestamp
End Property

Public Property Let Timestamp(ByVal value As Date)
    this.Timestamp = value
End Property

Public Property Get UserName() As String
    UserName = this.UserName
End Property

Public Property Let UserName(ByVal value As String)
    this.UserName = value
End Property

Public Property Get AvailableItems() As Variant
    AvailableItems = this.AvailableItems
End Property

Public Property Let AvailableItems(ByRef value As Variant)
    this.AvailableItems = value
End Property

Public Property Get SomeSetting() As String
    SomeSetting = this.SomeSetting
End Property

Public Property Let SomeSetting(ByVal value As String)
    this.SomeSetting = value
End Property

Public Property Get SomeOtherSetting() As Boolean
    SomeOtherSetting = this.SomeOtherSetting
End Property

Public Property Let SomeOtherSetting(ByVal value As Boolean)
    this.SomeOtherSetting = value
End Property

The interface members are all Private, and all members of the interface must be implemented for the code to compile. The Public members are not part of the interface, and are therefore not exposed to code written against the ISomeModel interface.

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Using a Factory Method to simulate a constructor

Using a VB_PredeclaredId attribute, we can make the SomeModel class have a default instance, and write a function that works like a type-level (Shared in VB.NET, static in C#) member that the client code can call without needing to first create an instance, like we did here:

Private Function CreateViewModel() As ISomeModel
    Dim result As ISomeModel
    Set result = SomeModel.Create(Now, Environ$("UserName"))
    result.AvailableItems = GetAvailableItems
    Set CreateViewModel = result
End Function

This factory method assigns the property values that are read-only when accessed from the ISomeModel interface, here Timestamp and UserName:

Public Function Create(ByVal pTimeStamp As Date, ByVal pUserName As String) As ISomeModel
    With New SomeModel
        .Timestamp = pTimeStamp
        .UserName = pUserName
        Set Create = .Self
    End With
End Function

Public Property Get Self() As ISomeModel
    Set Self = Me
End Property

And now we can code against the ISomeModel interface, which exposes Timestamp and UserName as read-only properties that can never be reassigned (as long as the code is written against the interface).