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List Box Infinite Loop
f#

I'm creating a windows form application similar to textpad and I've got a list box on the right featuring all currently opened documents (including unnamed new documents). I'm using a SelectedIndexChanged function to change the documents of the main window but it keeps getting caught in an infinite loop. I think I understand why it is but first the code:

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    do lstProject.SelectedIndexChanged.Add(fun _ ->
            documentList.Item(prevSelection) <- {filename=documentList.Item(prevSelection).filename; text=txtEditor.Text;};
            prevSelection = lstProject.SelectedIndex;
            txtEditor.Text <- documentList.Item(lstProject.SelectedIndex).text
        )

I think the problem is I'm setting the item in the documentList and since the documentList is bound to the list box, it takes away the item and resets it, thus seeming like it's 'adding' a new element and resetting the selected index over and over (I get overflows on the documentList.Item(prevSelection) <- line).

I tried documentList.SetItem() but it says I can't use that within there. Am I right in thinking that is the problem? How could I go about solving this?

.

I think you are correct, in your implementation you put a new item in the list, replacing an old one I would guess this raises the SelectedIndexChanged event. There are a couple of solutions, you could make your document object mutable so you can just change the text itself rather than the whole object:

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    type Document = {filename: string; mutable text: string;}

    do lstProject.SelectedIndexChanged.Add(fun _ ->
            documentList.Item(prevSelection).text <- txtEditor.Text;
            prevSelection = lstProject.SelectedIndex;
            txtEditor.Text <- documentList.Item(lstProject.SelectedIndex).text
        )

Or you could create sort of an update lock:

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    let mutable updatingDoc = false

    do lstProject.SelectedIndexChanged.Add(fun _ ->
            if not updatingDoc then begin
                updatingDoc <- true
                use updateLock = { new IDisposable with Dispose() = updatingDoc <- false };
                documentList.Item(prevSelection).text <- txtEditor.Text;
                prevSelection = lstProject.SelectedIndex;
                txtEditor.Text <- documentList.Item(lstProject.SelectedIndex).text
            end
        )

Be aware I haven't tested either idea.

By on 3/7/2008 3:05 AM ()

I've implemented the first and it works. I also needed to make prevSelection mutable as well (which it wasn't before). I haven't tried the second but the logic sounds about right (famous last words :) ).

By on 3/7/2008 8:19 AM ()

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