How to split UIElement and add pieces in several FixedPage

Question

For example, with this:

var grid = new Grid() { Children = { listBox} };
grid.Measure(new Size(page.Width, double.PositiveInfinity));
grid.Arrange(new Rect(grid.DesiredSize));

I see that the listBox needs to span several pages and with grid.DesiredSize.Height / page.Height, I know that it need 3 pages. How can I split the grid/listBox and access each segments to add into pages?

Answer 1

Ended up with ICollectionView and GroupStyle. In addition to what I've here, I've these in OnApplyTemplate():

internalList = new TestList<Item>();  
//internalList = new ObservableCollection<Item>();  
internalView = CollectionViewSource.GetDefaultView(internalList);  
internalView.GroupDescriptions.Add(new PropertyGroupDescription("Group"));  
content.ItemsSource = internalView;  

and in AddItem/RemoveItem, instead of content.Items.Add/Remove, I've to have internalList.Add/Remove and one more method is necessary to refresh the internalView when I'm done with a page:

public void RefreshInternalView() => internalView.Refresh();  

a 114 page pdf report, with 3,600 entries in 4 groups takes 30-35 seconds to generate, not bad, right? I thought that instead of ObservableCollection<T>, a subclass of IList<T>, that implements INotifyCollectionChanged, would reduce time BUT that's a joke I think! Spent a few hours on that and ended up with these:

public class TestList<T> : IList<T>, INotifyCollectionChanged  
{  
    T[] _array = new T[50];  
    int itemCount = 0;  
    public void Add(T item)  
    {  
        _array[itemCount++] = item;  
        OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, item, itemCount - 1));  
    }  
    public bool Remove(T item)  
    {  
        Array.Resize(ref _array, --itemCount);  
        OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Remove, item, itemCount));  
        return true;  
    }  
    public IEnumerator<T> GetEnumerator()  
    {  
        for (int i = 0; i < itemCount; i++)  
            yield return _array[i];  
    }  
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();  
    public event NotifyCollectionChangedEventHandler CollectionChanged;  
    void OnCollectionChanged(NotifyCollectionChangedEventArgs e) => CollectionChanged?.Invoke(this, e);  

    public T this[int index] { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }  
    public bool IsReadOnly => throw new NotImplementedException();  
    public int Count => throw new NotImplementedException();  
    public int IndexOf(T item) => throw new NotImplementedException();  
    public void Insert(int index, T item) => throw new NotImplementedException();  
    public void RemoveAt(int index) => throw new NotImplementedException();  
    public void Clear() => throw new NotImplementedException();  
    public bool Contains(T item) => throw new NotImplementedException();  
    public void CopyTo(T[] array, int arrayIndex) => throw new NotImplementedException();         
}  

the ObservableCollection<T> beats it always by 1/2 second.