A Lazy Computed Property

Paul Lefebvre — Mon, 27 Jan 2020 10:00:00 +0000

Do you have a class that has an object property that needs to be initialized or instantiated? You can always instantiate the property in the Constructor (or Open event) like this:

MyClass1 = New MyClass

But you can also do this using a Computed Property on a Class (I’m using TestClass for this example).

First, create a private property such as mMyClass1 As MyClass. It will be Nil by default as all object properties are.

Then create a Computed Property called MyClass1 As MyClass.

In the Get, check if mMyClass1 is Nil. If it is, create it. Then return it.

If mMyClass1 Is Nil Then
   mMyClass1 = New MyClass
End If
Return mMyClass1

Most likely you’d want this property to be read-only so you would not implement Set (even though the property is read-only, since it is an object you’ll still be able to modify the properties it contains). But if you did want a way for the object reference to be changed, you can do this in the Set:

mMyClass1 = value

Now you can make use of MyClass1 on TestClass as shown below. The above technique has two advantages over instantiating the property in the Constructor:

It keeps the code more closely associated with the property.
The property is only initialized when it is first accessed.

Var tc As New TestClass
tc.MyClass1.TestNumProperty = 42

This technique is also referred to as Lazy Loading or Lazy Initialization.

Lazy Loading – Xojo Programming Blog

A Lazy Computed Property