B4J Tutorial [B4X]Cloning Class Instances, Semi-Automatic Self-Replication, and a Trick

Disclaimer:
None of the techniques used here are new to our community and you may have seen the 'Trick' used by me before.

The Problem:
We are using classes more frequently, especially with B4XPages. After laboriously creating an instance
of a class, we often need another one just like it (except prehaps for some small differences).

We know how to create a Clone Sub in the Class that will spawn another instance.

B4X:
'In use:    Dim Instance2 as ClassName = Instance1.Clone
'In ClassName:
Public Sub Clone As ClassName
    Dim newInstance as ClassName
    newInstance.Initialize
    newInstance.propertyX = propertyX
    'etc.
    return NewInstance
End Sub

If the class has few properties we can do this manually.
However, Classes can become complex with numerous properties and variables.
These can be arrays, and other structures that can be difficult to copy.

The Trick:
In the IDE we copy the properties section and paste it into a Smart String

B4X:
Sub Class_Globals
    Private fx As JFX
    Private xui As XUI
    Private ser As B4XSerializator

    'Properties    and variables for cloning
    Private mNames() As String
    Private mAge, mHeight As Int
    Private mChildren As List
    Private mBox As B4XView
    Public OnState As Boolean
    Private i = 0, j = 2, k = 5 As Int                                                'ignore
    Private txt = "test" As String, value = 1.05 As Double, flag = False As Boolean 'ignore
   
    'Smart string version
    Private propsToClone As String = $"
    Private mNames() As String
    Private mAge, mHeight As Int
    Private mChildren As List
    Private mBox As B4XView
    Public OnState As Boolean
    Private i = 0, j = 2, k = 5 As Int                                                'ignore
    Private txt = "test" As String, value = 1.05 As Double, flag = False As Boolean 'ignore
    "$
End Sub

By parsing and processing "propsToClone" we can generate "copying code" for the Clone Sub
This code is put on the Clipboard, available for pasting in the Clone Sub.

B4X:
Public Sub Initialize(FirstName As String, LastName As String)
    mChildren.Initialize
    mNames = Array As String(FirstName, LastName)
    OnState = True
End Sub

Public Sub Clone As classA
    CloningCodeToClipboard    'The Smart string version is parsed and code is generated and placed on clipboard.
    Dim newInstance As classA
    newInstance.Initialize(mNames(0), mNames(1))
   
    'After 1st attempt at cloning, paste clipboard contents below.
    '_____________________________I have pasted the contents of clipboard, before I did that this section was empty
    newInstance.mNames = copyStringArray(mNames)
    newInstance.mAge = mAge
    newInstance.mHeight = mHeight
    newInstance.mChildren = copyStructure(mChildren)
    newInstance.mBox = copyPanel(mBox)
    newInstance.OnState = OnState
    newInstance.i = i
    newInstance.j = j
    newInstance.k = k
    newInstance.txt = txt
    newInstance.value = value
    newInstance.flag = flag
    '_____________________________

    Return newInstance
End Sub

You'll see that the generated code relies on a number of helper functions. One requires the RandomAccessFile library.
The Example with the CloningCodeToClipboard Sub, and Helper Subs is attached as a .zip file.

The use of the example above...
B4X:
Private Sub demonstrate
    Dim InstanceA As classA
    InstanceA.Initialize("Alpha", "Beta")
    InstanceA.Age = 45
    InstanceA.Height = 181
    InstanceA.MakeBox
   
    Dim InstanceB As classA = InstanceA.Clone      
    'If the class is a B4XPage then you need to register it with B4XPages.AddPage now.
    InstanceB.Age = 75
    InstanceB.AddChild("William")
    InstanceB.AddChild("Suzanna")
   
    Log(InstanceA.Names(0) & TAB & InstanceA.Names(1) & TAB & InstanceA.Age & TAB & InstanceA.Height & TAB & InstanceA.Children.Size)
    Log(InstanceB.Names(0) & TAB & InstanceB.Names(1) & TAB & InstanceB.Age & TAB & InstanceA.Height & TAB & InstanceB.Children.Size)
    Log(InstanceB.Box.Width)
   
'    Alpha    Beta    45    181    0
'    Alpha    Beta    75    181    2
'    300
End Sub

For Consideration:
Manipulating code like this should be considered as a possible time saver, but is not generally recommended.
It could lead to errors (for example: was there or will there be something unexpected on the Clipboard?).
I'll use it, since I am very methodical in how I code, and I check and test every step of the way.

Finally, the attachment has a series of Helper Subs that are useful in their own right. Take a look.
Feedback is welcome.
 

Attachments

  • cloning.zip
    12 KB · Views: 112
Last edited:
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