How to detect ListView is scrolling up or down

问题:

Is there a way to detect that ScrollViwer of ListView is in scrolling mode and stopped scrolling. In windows phone 8.1 ListView we can not get reference of the scrollviewer.

Any one done it in windows phone 8.1 WinRT app?

回答1:

Once the ListView is Loaded you can get the ScrollViewer like this:

var sv = (ScrollViewer)VisualTreeHelper.GetChild(VisualTreeHelper.GetChild(this.ListV, 0), 0);

Edit

As Romasz suggested, once you get the ScrollViewer, you can use its ViewChanged event, to monitor when it is scrolling and when it stops.

Also, here's the generic extension method that I use for traversing the visual tree:

// The method traverses the visual tree lazily, layer by layer
// and returns the objects of the desired type
public static IEnumerable<T> GetChildrenOfType<T>(this DependencyObject start) where T : class 
{
    var queue = new Queue<DependencyObject>();
    queue.Enqueue(start);

    while (queue.Count > 0) {
        var item = queue.Dequeue();

        var realItem = item as T;
        if (realItem != null) {
             yield return realItem;
        }

        int count = VisualTreeHelper.GetChildrenCount(item);
        for (int i = 0; i < count; i++) {
            queue.Enqueue(VisualTreeHelper.GetChild(item, i));
        }
    }
}

To get the ScrollViewer using this methos, do this:

var sv = yourListView.GetChildrenOfType<ScrollViewer>().First();

回答2:

You can find the ScrollViewer of your ListView by using VisualTreeHelper. For example like this:

// method to pull out a ScrollViewer
public static ScrollViewer GetScrollViewer(DependencyObject depObj)
{
    if (depObj is ScrollViewer) return depObj as ScrollViewer;

    for (int i = 0; i < VisualTreeHelper.GetChildrenCount(depObj); i++)
    {
        var child = VisualTreeHelper.GetChild(depObj, i);

        var result = GetScrollViewer(child);
        if (result != null) return result;
    }
    return null;
}

Once you have a ScrollViewer you can subscribe to its events:

GetScrollViewer(yourListView).ViewChanged += yourEvent_ViewChanged;

回答3:

You must load data to listview before getting scrollview. If listview has empty row then scrollview you get will be null.