我注意到ListBox
es和LongListSelector
s,项目绑定属性请求被推迟到滚动为止。
是否可以对ScrollViewer
中的自定义定位项执行相同的操作,以便项目在向其滚动之前不会对其绑定属性值发出任何请求?是否有任何简单的设置可以自动实现?
如果没有,那么最简单的方法是什么呢?
答案 0 :(得分:0)
它叫做Lazy Load 这里有一个懒惰列表的例子:
public class LazyCollection<T> : IList<T>, IList, INotifyCollectionChanged, INotifyPropertyChanged
{
private const int LOAD_THRESHOLD = 3;
private ObservableCollection<T> _innerCollection;
public LazyCollection(Func<int, IEnumerable<T>> fetch)
: this(fetch, null)
{ }
public LazyCollection(Func<int, IEnumerable<T>> fetch, IEnumerable<T> items)
{
_fetch = fetch;
_innerCollection = new ObservableCollection<T>(items ?? new T[0]);
this.AttachEvents();
this.HasMoreItems = true;
}
private void AttachEvents()
{
_innerCollection.CollectionChanged += (s, e) => OnCollectionChanged(e);
((INotifyPropertyChanged)_innerCollection).PropertyChanged += (s, e) => OnPropertyChanged(e);
}
#region INotifyPropertyChanged
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(PropertyChangedEventArgs e)
{
if (this.PropertyChanged != null)
this.PropertyChanged(this, e);
}
#endregion
#region INotifyCollectionChanged
public event NotifyCollectionChangedEventHandler CollectionChanged;
protected virtual void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
{
if (this.CollectionChanged != null)
this.CollectionChanged(this, e);
}
#endregion
#region IList<T>
public int IndexOf(T item)
{
return _innerCollection.IndexOf(item);
}
public void Insert(int index, T item)
{
_innerCollection.Insert(index, item);
}
public void RemoveAt(int index)
{
_innerCollection.RemoveAt(index);
}
public T this[int index]
{
get
{
Debug.WriteLine("LazyCollection - Reading item {0}", index);
return _innerCollection[index];
}
set { _innerCollection[index] = value; }
}
public void Add(T item)
{
_innerCollection.Add(item);
}
public void Clear()
{
_innerCollection.Clear();
}
public bool Contains(T item)
{
return _innerCollection.Contains(item);
}
public void CopyTo(T[] array, int arrayIndex)
{
_innerCollection.CopyTo(array, arrayIndex);
}
public int Count
{
get { return _innerCollection.Count; }
}
public bool IsReadOnly
{
get { return ((IList<T>)_innerCollection).IsReadOnly; }
}
public bool Remove(T item)
{
return _innerCollection.Remove(item);
}
public IEnumerator<T> GetEnumerator()
{
Debug.WriteLine("LazyCollection - GetEnumerator");
return _innerCollection.GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator()
{
Debug.WriteLine("LazyCollection - GetEnumerator explicit");
return ((IEnumerable)_innerCollection).GetEnumerator();
}
#endregion
#region IList
int IList.Add(object value)
{
return ((IList)_innerCollection).Add(value);
}
bool IList.Contains(object value)
{
return ((IList)_innerCollection).Contains(value);
}
int IList.IndexOf(object value)
{
return ((IList)_innerCollection).IndexOf(value);
}
void IList.Insert(int index, object value)
{
((IList)_innerCollection).Insert(index, value);
}
bool IList.IsFixedSize
{
get { return ((IList)_innerCollection).IsFixedSize; }
}
bool IList.IsReadOnly
{
get { return ((IList)_innerCollection).IsReadOnly; }
}
void IList.Remove(object value)
{
((IList)_innerCollection).Remove(value);
}
object IList.this[int index]
{
get
{
if (index > this.Count - LOAD_THRESHOLD)
{
this.TryLoadMoreItems();
}
Debug.WriteLine("LazyCollection - Reading item {0} IList", index);
return ((IList)_innerCollection)[index];
}
set { ((IList)_innerCollection)[index] = value; }
}
void ICollection.CopyTo(Array array, int index)
{
((IList)_innerCollection).CopyTo(array, index);
}
bool ICollection.IsSynchronized
{
get { return ((IList)_innerCollection).IsSynchronized; }
}
object ICollection.SyncRoot
{
get { return ((IList)_innerCollection).SyncRoot; }
}
#endregion
public T[] GetLoadedItems()
{
return _innerCollection.ToArray();
}
public void ResetLoadedItems(IEnumerable<T> list)
{
_innerCollection.Clear();
foreach (var i in list)
_innerCollection.Add(i);
this.HasMoreItems = true;
}
public bool HasMoreItems { get; set; }
private bool _isLoading = false;
private Func<int, IEnumerable<T>> _fetch;
private async Task TryLoadMoreItems()
{
if (_isLoading || !this.HasMoreItems)
return;
try
{
_isLoading = true;
Debug.WriteLine("LazyCollection - Loading more items skip {0}", this.Count);
List<T> items = _fetch != null ? (_fetch(Count)).ToList() : new List<T>();
if (items.Count == 0)
{
Debug.WriteLine("LazyCollection - No items returned, Loading disabled", this.Count);
this.HasMoreItems = false;
}
items.ForEach(x => _innerCollection.Add(x));
Debug.WriteLine("LazyCollection - Items added. Total count: {0}", this.Count);
}
finally
{
_isLoading = false;
}
}
}
public class SearchResults : LazyCollection<Verse>
{
public SearchResults()
: base(count => GetSearchResult(), GetSearchResult())
{
}
public static int _index = 0;
public static string CurrentSearch = "";
public static List<Verse> AllVerses = new List<Verse>();
private static List<Verse> GetSearchResult()
{
List<Verse> results = new List<Verse>();
string lower = CurrentSearch.ToLower();
bool resultsChanged = false;
for (int index = _index; index < AllVerses.Count; index++)
{
Verse verse = AllVerses[index];
if (verse.Content.ToLower().Contains(lower) || verse.Header.ToLower().Contains(lower))
{
results.Add(verse);
resultsChanged = true;
}
if ((index >= (AllVerses.Count / 200) + _index || index + 1 == AllVerses.Count) && resultsChanged && (results.Count > 10 || AllVerses.Count == index + 1))
{
_index = index + 1;
return results;
}
}
return results;
}
}