逻辑分页是非常常用的一种性能优化的技术手段。虽然像ListView,ListBox有提供虚拟化的技术，能让我们加载大量数据而不会出现卡顿。但是……
实际上，虚拟化技术即便是开启后，我们在快速浏览和滚动或者是初始化加载数据的时候，会出现明显的卡顿，这并不符合我们的实际业务需要。因此，实现逻辑分页是非常有必要的。
实现逻辑分页也比较简单，主要是通过ScrollView来实现滚动加载，通常用于垂直滚动，分别可以像上滚动和像下滚动，水平滚动我们就不去考虑了。
常规的分页逻辑判断代码如下：
//向上滚动
if (e.VerticalOffset < 10 && e.VerticalChange < 0)
{
//do something
}
//向下滚动
if ((e.ExtentHeight - (e.VerticalOffset + e.ViewportHeight)) < 10)
{
//do something
}
//滚动条是否可见，可用于针对数据模版不确定的场景，动态需要计算初始分页的条数。
bool IsScrollBarVisible
{
get
{
return scrollViewer != null && scrollViewer.ExtentHeight > scrollViewer.ViewportHeight;
}
}
为了方便实现分页逻辑，我写了一个基于ItemsControl和ScrollViewer的基类，方便实现我们的分页逻辑：
/// <summary>
/// 提供逻辑分页的基础实现
/// 目标对象：ItemsControl
/// </summary>
public abstract class LogicPagingOperator : IDisposable
{
public LogicPagingOperator(ItemsControl itemsControl, int pageCapacity = 10)
{
ItemsSource = new ObservableCollection<object>();
Initialize(itemsControl, pageCapacity);
}
protected int _pageCapacity;
protected ScrollViewer ScrollViewer;
protected ItemsControl ItemsControl;
public IList OriginalSource { get; private set; }
protected readonly ObservableCollection<object> ItemsSource;
/// <summary>
/// 当前页
/// </summary>
protected int PageIndex
{
get; set;
}
/// <summary>
/// 逻辑源数据是否已经全部加载完成
/// </summary>
public bool IsLoaded
{
get; protected set;
}
private void Initialize(ItemsControl itemsControl, int pageCapacity)
{
ItemsControl = itemsControl;
if (pageCapacity < 1)
{
ExceptionHelper.ThrowForDebug(new ArgumentException("无效的LogicPagingOperatorBase.PageCapacity值，值不能小于1"));
pageCapacity = 1;
}
PageCapacity = pageCapacity;
PageIndex = -1;
ItemsControl.Loaded += ItemsControl_Loaded;
ItemsControl.Unloaded += ItemsControl_Unloaded;
}
private void InnerReset()
{
ItemsSource.Clear();
PageIndex = -1;
IsLoaded = false;
}
public void SetItemsSource(IEnumerable value)
{
if (value == null)
{
OriginalSource = null;
InnerReset();
return;
}
if (value is IList list)
{
UnregistryNotifyCollectionChanged();
InnerReset();
OriginalSource = list;
LoadNext();
RegistryNotifyCollectionChanged();
ItemsControl.ItemsSource = ItemsSource;
}
else
{
throw new NotSupportedException("只支持IList集合");
}
}
/// <summary>
/// 每逻辑页显示的数量
/// </summary>
public int PageCapacity
{
get
{
return _pageCapacity;
}
set
{
_pageCapacity = value;
InnerReset();
if (ItemsControl != null && ItemsControl.IsLoaded)
{
LoadNext();
}
}
}
#region 数据加载
/// <summary>
/// 加载下一页
/// </summary>
public void LoadNext()
{
if (OriginalSource == null)
{
return;
}
if (IsLoaded)
{
return;
}
PageIndex++;
OnLoadNext(OriginalSource);
}
protected virtual void OnLoadNext(IList list)
{
}
#endregion
#region 数据源变更处理
void UnregistryNotifyCollectionChanged()
{
if (OriginalSource is INotifyCollectionChanged notifyCollectionChanged)
{
notifyCollectionChanged.CollectionChanged -= NotifyCollectionChanged_CollectionChanged;
}
}
void RegistryNotifyCollectionChanged()
{
if (OriginalSource is INotifyCollectionChanged notifyCollectionChanged)
{
notifyCollectionChanged.CollectionChanged -= NotifyCollectionChanged_CollectionChanged;
notifyCollectionChanged.CollectionChanged += NotifyCollectionChanged_CollectionChanged;
}
}
void NotifyCollectionChanged_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
switch (e.Action)
{
case NotifyCollectionChangedAction.Add:
OnSourceAdd(e);
break;
case NotifyCollectionChangedAction.Remove:
OnSourceRemove(e);
break;
case NotifyCollectionChangedAction.Replace:
OnSourceReplace(e);
break;
case NotifyCollectionChangedAction.Move:
OnSourceMove(e);
break;
case NotifyCollectionChangedAction.Reset:
OnSourceClear(e);
break;
default:
break;
}
}
protected virtual void OnSourceAdd(NotifyCollectionChangedEventArgs e)
{
if (e.NewStartingIndex < ItemsSource.Count && e.NewStartingIndex > -1)
{
ItemsSource.Insert(e.NewStartingIndex, e.NewItems[0]);
}
else
{
foreach (var item in e.NewItems)
{
ItemsSource.Add(item);
}
}
}
protected virtual void OnSourceRemove(NotifyCollectionChangedEventArgs e)
{
if (e.OldStartingIndex >= 0 && e.OldStartingIndex < ItemsSource.Count)
ItemsSource.RemoveAt(e.OldStartingIndex);
}
protected virtual void OnSourceMove(NotifyCollectionChangedEventArgs e)
{
OnSourceRemove(e);
OnSourceAdd(e);
}
protected virtual void OnSourceReplace(NotifyCollectionChangedEventArgs e)
{
if (e.NewStartingIndex < ItemsSource.Count && e.NewStartingIndex >= 0 && e.NewItems != null)
{
ItemsSource[e.NewStartingIndex] = e.NewItems[0];
}
}
protected virtual void OnSourceClear(NotifyCollectionChangedEventArgs e)
{
ItemsSource.Clear();
PageIndex = -1;
}
#endregion
#region 滚动条逻辑处理
void FindScrollView(ItemsControl itemsControl)
{
if (itemsControl != null && ScrollViewer == null)
{
ScrollViewer = itemsControl.VisualAncestor<ScrollViewer>();
if (ScrollViewer == null)
{
ScrollViewer = itemsControl.VisualDescendant<ScrollViewer>();
}
}
}
void RegistryScrollChanged(ScrollViewer scrollViewer)
{
if (scrollViewer != null)
{
scrollViewer.ScrollChanged -= ScrollViewer_ScrollChanged;
scrollViewer.ScrollChanged += ScrollViewer_ScrollChanged;
}
}
void UnregistryScrollChanged()
{
if (ScrollViewer != null)
{
ScrollViewer.ScrollChanged -= ScrollViewer_ScrollChanged;
}
}
void ScrollViewer_ScrollChanged(object sender, ScrollChangedEventArgs e)
{
OnScrollChanged(e);
}
protected virtual void OnScrollChanged(ScrollChangedEventArgs e)
{
}
#endregion
#region 事件处理
private void ItemsControl_Unloaded(object sender, System.Windows.RoutedEventArgs e)
{
UnregistryNotifyCollectionChanged();
UnregistryScrollChanged();
OnUnloaded();
}
private void ItemsControl_Loaded(object sender, System.Windows.RoutedEventArgs e)
{
FindScrollView(ItemsControl);
RegistryNotifyCollectionChanged();
RegistryScrollChanged(ScrollViewer);
OnLoaded();
}
protected virtual void OnLoaded()
{
}
protected virtual void OnUnloaded()
{
}
#endregion
public void Dispose()
{
ItemsSource.Clear();
ItemsControl = null;
}
}
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