Best way to swap Listview Items?

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Nov 15th, 2018, 11:23 PM
JimAvanti

Best way to swap Listview Items?

As part of a bubble sort of listview items I call a routine that swaps row idx with row idx+1. What I have works, but I know there has to be a much better and quicker way of doing this than the way I am doing it.

Code:

Private Sub SwapItems(ByVal idx As Integer) Dim Temp0 As String = "" Dim Temp1 As String = "" Dim Temp2 As String = "" Dim Temp3 As String = "" Dim Temp4 As String = "" Dim Temp5 As String = "" Dim Temp6 As String = "" Temp0 = ListView1.Items(idx).Text Temp1 = ListView1.Items(idx).SubItems(1).Text Temp2 = ListView1.Items(idx).SubItems(2).Text Temp3 = ListView1.Items(idx).SubItems(3).Text Temp4 = ListView1.Items(idx).SubItems(4).Text Temp5 = ListView1.Items(idx).SubItems(5).Text Temp6 = ListView1.Items(idx).SubItems(6).Text ListView1.Items(idx).Text = ListView1.Items(idx + 1).Text ListView1.Items(idx).SubItems(1).Text = ListView1.Items(idx + 1).SubItems(1).Text ListView1.Items(idx).SubItems(2).Text = ListView1.Items(idx + 1).SubItems(2).Text ListView1.Items(idx).SubItems(3).Text = ListView1.Items(idx + 1).SubItems(3).Text ListView1.Items(idx).SubItems(4).Text = ListView1.Items(idx + 1).SubItems(4).Text ListView1.Items(idx).SubItems(5).Text = ListView1.Items(idx + 1).SubItems(5).Text ListView1.Items(idx).SubItems(6).Text = ListView1.Items(idx + 1).SubItems(6).Text ListView1.Items(idx + 1).Text = Temp0 ListView1.Items(idx + 1).SubItems(1).Text = Temp1 ListView1.Items(idx + 1).SubItems(2).Text = Temp2 ListView1.Items(idx + 1).SubItems(3).Text = Temp3 ListView1.Items(idx + 1).SubItems(4).Text = Temp4 ListView1.Items(idx + 1).SubItems(5).Text = Temp5 ListView1.Items(idx + 1).SubItems(6).Text = Temp6 End Sub

I tried using a temporary ListViewItem to hold the ListView1 item shown below, but I guess you can’t directly assign one ListView item to another. It seems you have to completely remove one listview item and then add the other after it as a new item.

Code:

Dim TempItem As ListViewItem TempItem = ListView1.Items(idx) ListView1.Items(idx) = ListView1.Items(idx + 1) ListView1.Items(idx + 1) = TempItem
Nov 16th, 2018, 12:15 AM
jmcilhinney

Re: Best way to swap Listview Items?
You should absolutely not be swapping items in the ListView. What you should do is create an array and populate it with all the ListViewItems and then sort the array the same way you would any other array. I would hope that you already know how to bubble sort an array of Integers or Strings. Sorting an array is exactly the same. The only thing that would change is the way you compare two elements of the array. Once you have sorted the array, you could Clear the Items of the ListView and then repopulate it with a single call to AddRange.

That said, if you really do want to sort in-place then you should actually be swapping items, not the contents of items. Swapping the items at indexes (i) and (i+1) is as simple as this:
vb.net Code:
```
Dim item As ListViewItem = ListView1.Items(i + 1)
 
ListView1.Items.Remove(item)
ListView1.Items.Insert(i, item)
```
That moves the lower item up. The result is the same but you can also move the upper item down:
vb.net Code:
```
Dim item As ListViewItem = ListView1.Items(i)
 
ListView1.Items.Remove(item)
ListView1.Items.Insert(i + 1, item)
```
Nov 16th, 2018, 12:52 AM
JimAvanti

Re: Best way to swap Listview Items?
Thanks for the quick reply. Both examples make sense. I didn't think of using an array and rebuilding the listview. Is there any reason you are against sorting in-place? does it take longer to sort that way? It would require less code in your second example.

Jim
Quote:
Originally Posted by jmcilhinney

You should absolutely not be swapping items in the ListView. What you should do is create an array and populate it with all the ListViewItems and then sort the array the same way you would any other array. I would hope that you already know how to bubble sort an array of Integers or Strings. Sorting an array is exactly the same. The only thing that would change is the way you compare two elements of the array. Once you have sorted the array, you could Clear the Items of the ListView and then repopulate it with a single call to AddRange.

That said, if you really do want to sort in-place then you should actually be swapping items, not the contents of items. Swapping the items at indexes (i) and (i+1) is as simple as this:

vb.net Code:

Dim item As ListViewItem = ListView1.Items(i + 1)

ListView1.Items.Remove(item)
ListView1.Items.Insert(i, item)

That moves the lower item up. The result is the same but you can also move the upper item down:

vb.net Code:

Dim item As ListViewItem = ListView1.Items(i)

ListView1.Items.Remove(item)
ListView1.Items.Insert(i + 1, item)

Re: Best way to swap Listview Items?

Quote:

Originally Posted by JimAvanti

Is there any reason you are against sorting in-place? does it take longer to sort that way?

I haven't tested this scenario specifically but calling Add repeatedly to add multiple items to a ListView gets exponentially slower because the control redraws after each Add call. I'd expect the same to happen at least once - possibly twice - when you call Remove and Insert. If you really wanted to swap data between items then I'd suggest writing a method or two to do it. You should already have the items themselves because you would have to have compared them to determine that they need to be swapped, so you shouldn't need to work with indexes:
vb.net Code:
Private Sub SwapListViewItems(item1 As ListViewItem, item2 As ListViewItem)
    Dim text As String
 
    For i = 0 To item1.SubItems.Count - 1
        Dim subitem1 = item1.SubItems(i)
        Dim subitem2 = item2.SubItems(i)
 
        text = subitem1.Text
        subitem1.Text = subitem2.Text
        subitem2.Text = text
    Next
End Sub
If you must use indexes:
vb.net Code:
Private Sub SwapListViewItems(index1 As Integer)
    Dim item1 = ListView1.Items(index1)
    Dim item2 = ListView1.Items(index1 + 1)
 
    Dim text As String
 
    For i = 0 To item1.SubItems.Count - 1
        Dim subitem1 = item1.SubItems(i)
        Dim subitem2 = item2.SubItems(i)
 
        text = subitem1.Text
        subitem1.Text = subitem2.Text
        subitem2.Text = text
    Next
End Sub

Nov 17th, 2018, 11:40 AM
JimAvanti

Re: Best way to swap Listview Items?
jmcilhinney,

I used your first example and it worked great. It is quicker than how I had it. I don't quite understand how it works though. It seems passing the Listview items into item1 and item2 and swapping them along with the subitems should not actually change the ListView1, but it does. I would have never figured that one out on my own.

Thanks,
Jim
Quote:
Originally Posted by jmcilhinney

I haven't tested this scenario specifically but calling Add repeatedly to add multiple items to a ListView gets exponentially slower because the control redraws after each Add call. I'd expect the same to happen at least once - possibly twice - when you call Remove and Insert. If you really wanted to swap data between items then I'd suggest writing a method or two to do it. You should already have the items themselves because you would have to have compared them to determine that they need to be swapped, so you shouldn't need to work with indexes:

vb.net Code:

Private Sub SwapListViewItems(item1 As ListViewItem, item2 As ListViewItem)
Dim text As String

For i = 0 To item1.SubItems.Count - 1
Dim subitem1 = item1.SubItems(i)
Dim subitem2 = item2.SubItems(i)

text = subitem1.Text
subitem1.Text = subitem2.Text
subitem2.Text = text
Next
End Sub