windows phone listbox虚拟化(下)

   之前写过一篇关于listbox虚拟化的文章,那里采用的方法都是自己早期研究的一些思路,然后发现当数据很大的时候,其实性能效果还是不太理想,下面让我们来仔细想一想到底是基于什么原因,我们回去破坏默认的虚拟化呢?首先一个原因就是下来刷新的问题:由于listbox默认是不支持刷新效果的,这个时候我们可能回去重新自定义listbox,但是我们一旦重写listbox就有可能带来虚拟化的问题,针对这一问题,现在比较好的解决方法就是:乱世经典博客,这种方法并不是像现在很多网路通用的listbox刷新,重写三个自定义控件,它里面只是通过切换不同的VisualStateGroup,这样的话listbox的itemplans就还是支持虚拟化的面板。既然刷新的问题,我们有很好的办法解决虚拟化。那么还有其他的原因会让我们破坏虚拟化吗?肯定是有的,就是当整个页面列表的布局很复杂时候,这种情况下我们可能需要在listbox里面放置自定义控件,一旦你放置usercontrol的话,虚拟化就被破坏了,比如有的时候,我们可能需要listbox里面嵌套listbox这个时候虚拟化肯定是不行的,其实这种情况也有一种很好的解决办法就是:阿干@.NET的博客,在这里面我们可以看到针对listbox里面嵌套listbox的情况,我们可以使用itemcontrols,这样一来的话,它的itempanels就还是虚拟化面板,因此虚拟化还是没有被破坏掉的。

  其实还有一种变态的情况就是有的时候我们需要listbox显示的数据是横排的,通常我们的做法就是:

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<ListBox.ItemsPanel>
               <ItemsPanelTemplate>                 
                   <toolkit:WrapPanel></toolkit:WrapPanel>                
               </ItemsPanelTemplate>
           </ListBox.ItemsPanel>

 这样一来的话,就必然会导致虚拟化被破坏,因为你连面板都换掉了。那么针对这种情况有没有好的解决办法呢?其实阿干那篇博客里面就提供了好的解决办法,我们可以自己去实现分列显示的效果,核心的代码是:

windows phone listbox虚拟化(下)
windows phone listbox虚拟化(下)
using System;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Ink;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Diagnostics;

namespace MoHoo.MultiColumnListBox.Demo
{
    public class RowAdapter<TItemType> : IList<IEnumerable<TItemType>>, INotifyCollectionChanged
    {
        private readonly IList<TItemType> _sourceList;
        private readonly int _columns;

        public IList<TItemType> SourceList
        {
            get { return _sourceList; }
        } 

        private class RowObject : IEnumerable<TItemType>
        {
            internal readonly RowAdapter<TItemType> Parent;
            internal readonly int StartIndex;

            public RowObject(RowAdapter<TItemType> parent, int startIndex)
            {
                Parent = parent;
                StartIndex = startIndex;
            }

            #region IEnumerable<TItemType> Members

            public IEnumerator<TItemType> GetEnumerator()
            {
                int limit = Parent._sourceList.Count;
                int end = Math.Min(StartIndex + Parent._columns, limit);

                for (int pos = StartIndex; pos < end; ++pos)
                {
                    yield return Parent._sourceList[pos];
                }
            }

            #endregion

            #region IEnumerable Members

            System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
            {
                return GetEnumerator();
            }

            #endregion
        }

        public RowAdapter(IList<TItemType> sourceList, int columns)
        {
            if (null == sourceList)
                throw new ArgumentNullException("sourceList", "Resource.RowAdapter_RowAdapter_sourceList_is_null");
            if (columns <= 0)
                throw new ArgumentOutOfRangeException("columns", "Resource.RowAdapter_RowAdapter_ColumnsGreaterOne");

            // We require the source list to implement IList because we
            // need to know how many item there are
            _sourceList = sourceList;
            _columns = columns;

            var sourceNotify = sourceList as INotifyCollectionChanged;
            if (null != sourceNotify)
            {
                sourceNotify.CollectionChanged += OnSourceCollectionChanged;
            }
        }

        #region IList<IEnumerable<TItemType>> Members

        public int IndexOf(IEnumerable<TItemType> item)
        {
            var realItem = item as RowObject;
            if (null == realItem || !ReferenceEquals(realItem.Parent, this))
                return -1;          // It does not belong to this collection

            Debug.Assert(0 == realItem.StartIndex % _columns, "RowObject item has a wierd index");
            return realItem.StartIndex / _columns;
        }

        public void Insert(int index, IEnumerable<TItemType> item)

        {
            throw new NotSupportedException();
        }

        public IEnumerable<TItemType> this[int index]
        {
            get
            {
                if (index < 0 || index > Count)
                    return null;

                return InternalGetRow(index);
            }
            set
            {
                throw new NotSupportedException();
            }
        }

        public void RemoveAt(int index)
        {
            throw new NotSupportedException();
        }

        #endregion

        #region ICollection<IEnumerable<TItemType>> Members

        public void Add(IEnumerable<TItemType> item)
        {
            throw new NotSupportedException();
        }

        public bool Contains(IEnumerable<TItemType> item)
        {
            var realItem = item as RowObject;
            return null != realItem && object.ReferenceEquals(realItem.Parent, this);
        }

        public void CopyTo(IEnumerable<TItemType>[] array, int arrayIndex)
        {
            // I haven‘t implemented this. It is easy to implement if you need it
            throw new NotImplementedException();
        }

        public bool Remove(IEnumerable<TItemType> item)
        {
            throw new NotSupportedException();
        }
        public void Clear()
        {
            throw new NotSupportedException();
        }

        public int Count
        {
            get
            {
                return (_sourceList.Count + (_columns - 1)) / _columns;
            }
        }

        public bool IsReadOnly
        {
            get { return true; }
        }

        #endregion

        #region IEnumerable<IEnumerable<TItemType>> Members

        public IEnumerator<IEnumerable<TItemType>> GetEnumerator()
        {
            for (int i = 0; i < Count; ++i)
            {
                yield return InternalGetRow(i);
            }
        }

        #endregion

        #region IEnumerable Members

        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }

        #endregion

        #region INotifyCollectionChanged Members

        public event NotifyCollectionChangedEventHandler CollectionChanged;

        private void FireCollectionChanged()
        {

            var handler = CollectionChanged;
            if (null != handler)
            {
                handler(this, new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
            }
        }

        private void OnSourceCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            FireCollectionChanged();
        }

        #endregion

        private RowObject InternalGetRow(int index)
        {
            return new RowObject(this, index * _columns);
        }
    }
}
View Code
windows phone listbox虚拟化(下)

这里面的核心思想就是自定义一个索引器。

windows phone listbox虚拟化(下),布布扣,bubuko.com

windows phone listbox虚拟化(下)

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