WPF, ListBox item-by-item scrolling to pixel based scrolling

Question

I'm using below code for fix my screen when i insert listboxItem.

public partial class KeepItemsInViewListBox : ListBox
    {
        private ScrollViewer ScrollViewer { get; set; }

        #region Overrides of FrameworkElement

        /// <inheritdoc />
        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();
            if (TryFindVisualChildElement(this, out ScrollViewer scrollViewer))
            {
                this.ScrollViewer = scrollViewer;
            }
        }

        #endregion

        #region Overrides of ListView

        /// <inheritdoc />
        protected override void OnItemsChanged(NotifyCollectionChangedEventArgs e)
        {
            base.OnItemsChanged(e);

            if (this.ScrollViewer == null)
            {
                return;
            }

            double verticalOffset;
            switch (e.Action)
            {
                case NotifyCollectionChangedAction.Add when e.NewItems != null:
                    // Check if insert or add
                    verticalOffset = e.NewStartingIndex < this.ScrollViewer.VerticalOffset
                      ? this.ScrollViewer.VerticalOffset + e.NewItems.Count
                      : this.ScrollViewer.VerticalOffset;
                    break;
                case NotifyCollectionChangedAction.Remove when e.OldItems != null:
                    verticalOffset = this.ScrollViewer.VerticalOffset - e.OldItems.Count;
                    break;
                default:
                    verticalOffset = this.ScrollViewer.VerticalOffset;
                    break;
            }

            this.ScrollViewer?.ScrollToVerticalOffset(verticalOffset);
        }


        #endregion

        public bool TryFindVisualChildElement<TChild>(DependencyObject parent, out TChild childElement)
          where TChild : FrameworkElement
        {
            childElement = null;
            if (parent == null)
            {
                return false;
            }

            for (int i = 0; i < VisualTreeHelper.GetChildrenCount(parent); i++)
            {
                DependencyObject child = VisualTreeHelper.GetChild(parent, i);
                if (child is TChild resultElement)
                {
                    childElement = resultElement;
                    return true;
                }

                if (TryFindVisualChildElement(child, out childElement))
                {
                    return true;
                }
            }

            return false;
        }
    }

that class makes my listbox's scroll to item-by-item.

see the below picture.

before upper Class(KeepItemsInViewListBox), my scrolling condition is Pixel. but after using that code my scrolling condition is changed.

because all of my listboxitem's size is diffrent, I added VirtualizingPanel.ScrollUnit="Pixel" or ScrollViewer.CanContentScroll = "False" in xaml, or ScrollViewer.CanContentScroll = false; in .cs file then my screen is also move...

charles kim charles kim · Accepted Answer · 2020-09-15T06:20:58

First thanks to Mr.Code

I modify Mr.Code's class, but my Code is not a perfect.

public class KeepItemsInViewListBox : ListBox
    {
        private ScrollViewer ScrollViewer { get; set; }

        private void ScrollViewer_ScrollChanged(object sender, ScrollChangedEventArgs e)
        {


            if (insertCheck)
            {
                ScrollViewer.ScrollToVerticalOffset(ScrollViewer.VerticalOffset + e.ExtentHeightChange);
                insertCheck = false;
            }
        }

        #region Overrides of FrameworkElement

        /// <inheritdoc />
        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();
            if (TryFindVisualChildElement(this, out ScrollViewer scrollViewer))
            {
                this.ScrollViewer = scrollViewer;
                ScrollViewer.ScrollChanged += ScrollViewer_ScrollChanged;
            }
        }

        #endregion
        private bool insertCheck = false;
        #region Overrides of ListView

        /// <inheritdoc />
        protected override void OnItemsChanged(NotifyCollectionChangedEventArgs e)
        {
            base.OnItemsChanged(e);
            switch (e.Action)
            {
                case NotifyCollectionChangedAction.Add when e.NewItems != null:
                    bool isInsert = this.ScrollViewer.CanContentScroll
                      ? e.NewStartingIndex < this.ScrollViewer.VerticalOffset
                      : e.NewStartingIndex + 1 != this.Items.Count - 1;

                    if (!isInsert)
                    {
                        return;
                    }
                    insertCheck = isInsert;
                    break;
            }
        }
        #endregion

        public bool TryFindVisualChildElement<TChild>(DependencyObject parent, out TChild childElement)
          where TChild : FrameworkElement
        {
            childElement = null;
            if (parent == null)
            {
                return false;
            }

            for (int i = 0; i < VisualTreeHelper.GetChildrenCount(parent); i++)
            {
                DependencyObject child = VisualTreeHelper.GetChild(parent, i);
                if (child is TChild resultElement)
                {
                    childElement = resultElement;
                    return true;
                }

                if (TryFindVisualChildElement(child, out childElement))
                {
                    return true;
                }
            }

            return false;
        }
    }

I just redefine "ScrollViewer_ScrollChanged" method and set verticalOffset to changed offset value.

It works in my test Project. but in my real Project doesn't work. Then i use ScrollViewer's ExtentHeight value. I checked the first ExtentHeight and it was saved. After scrolling, I compare two of ExtentHeight and set the diffrence to verticalOffset.

But it isn't work Beautifully

WPF, ListBox item-by-item scrolling to pixel based scrolling

1 Answers