Undo feature - stack, collection of collections or what?

[A.P.Gumby]

Hi,
I am writing a simple drawing application where the user places a series of data points on the screen.
Each of the data points has a number of properties as defined by :

    <Serializable()> _
    Public Class objDataPoint
        Public Location As Point
        Public Moveable As Boolean
        Public Time As Single
        Public Power As Single
        Public Selected As Boolean

        Public Sub New(ByVal Location As Point, ByVal Moveable As Boolean, ByVal Time As Single, ByVal Power As Single, ByVal Selected As Boolean)
            'Set the attributes 
            Me.Location = Location
            Me.Moveable = Moveable
            Me.Time = Time
            Me.Power = Power
            Me.Selected = Selected
        End Sub
    End Class

The whole drawing is defined by a collection of data points so I have :

    Private colDataPoints As New Collection

and as the user adds a data point to the drawing canvas I just add another member to the collection.

Now I want to implement an Undo function and I am not sure how to do this.
My first idea was to create another collection and have a collection of collections but I could not get this to work.
I have done a lot of searching and come up with a number of suggestions including serialization and using a Stack object.
This is my attempt at using the Stack object but it doesn't work either.

    Private HistoryStack As New Stack

    Private Sub SaveUndoInformation()
        HistoryStack.Push(colDataPoints)
        tsbUndo.Enabled = True
    End Sub

    Private Sub tsbUndo_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles tsbUndo.Click
        'Remove all current data points.
        colDataPoints.Clear()
        'Make sure that no point is currently selected.
        m_intSelectedDataPoint = -1
        'Extract the data.
        Dim colTemp As New Collection
        colTemp = HistoryStack.Pop
        colDataPoints = HistoryStack.Pop
        If HistoryStack.Count = 0 Then tsbUndo.Enabled = False
        'Force the display area to redraw after all new data points have been loaded.
        picCanvas.Invalidate()
    End Sub

I have been scratching my head on this all day so any help is appreciated.

[Atheist]

First off, i wouldnt use a 'Collection' class for the objDataPoints, i would use a List(Of T).
For the undo feature I would create a separate Stack, and upon undoing, i would remove the last element in the objDataPoints List and push it onto the stack. That way you could also have a 'redo' feature, where you would simply pop the first element of the stack again and back into the List(Of T).

[A.P.Gumby]

Thanks for the quick reply!

So I can learn from my mistakes. Why is using a Collection not a good way to go go and why is List(Of T) better?

I was trying to stop some of the complexities of this system from confusing the question but, because of the way the system works, sometimes when the user adds a single point, the program will automatically add more than one point.
Therefore, I really do need to save snapshots of the entire drawing (i.e. all of the data points).

[Atheist]

Collection is a leftover from VB6, and it is not strongly-typed. The List(Of T) is strongly-typed and performs better.

[A.P.Gumby]

Thanks for the explanation.

I have migrated from VB6 to .NET so am still trying to shake off some of the old ways of doing things and learning the new ways.

I will change from Collection to List(Of T).

How about the second part of my reply.
Won't I have the same problem saving and retreiving the entire List(Of T) as I am having when using the Collection?

[Atheist]

Do you mean that you want to be able to save the entire List of objDataPoints to a file, and load it back again? You could definitely use serialization for that. If thats what you need I'll give you an example.

[jmcilhinney]

A Stack is the right option for an Undo feature. That said, you should use a generic stack rather than a standard Stack. i.e. use a Stack(Of T) just as you're using a List(Of T). You could also use a second Stack to provide a Redo feature, but I'd get the first bit right first.

[A.P.Gumby]

Thanks for the replies,

Atheist,

Yes, because more than one data point can be changed with each user action, I want to save the entire list of data points.

I am already using Serialization in a file Save/Load feature but was not sure if this was the right was to go with the Undo feature. Would I be saving copies to the hard disk or is there some way to serialize and save to a Stack?

jmcilhinney,

All of this is just showing me that I have a lot more to learn about .NET. Atheist suggested List(Of T) rather than the Collection that I am currently using. First thing that I have to do is change my implementation to use that method. Then I will look at using Stack(Of T) for the Undo.