后端有两个表：一个是组件,一个是制造商.每个组件都有一个制造商.根本不是一个不寻常的外键查找关系.
我是Silverlight,我通过WCF服务访问数据.我将调用Components_Get(id)来获取Current组件(查看或编辑)以及调用Manufacturers_GetAll()以获取制造商的完整列表,以填充ComboBox的可能选择.然后,我将ComboBox上的SelectedItem绑定到当前组件的制造商,将ComboBox上的ItemSource绑定到可能的制造商列表.像这样：
<UserControl.Resources>
<data:WebServiceDataManager x:Key="WebService" />
</UserControl.Resources>
<Grid DataContext={Binding Components.Current,mode=OneWay,Source={StaticResource WebService}}>
<ComboBox Grid.Row="2" Grid.Column="2" Style="{StaticResource ComboBoxStyle}" Margin="3"
ItemsSource="{Binding Manufacturers.All,Mode=OneWay,Source={StaticResource WebService}}"
SelectedItem="{Binding Manufacturer,Mode=TwoWay}" >
<ComboBox.ItemTemplate>
<DataTemplate>
<Grid>
<TextBlock Text="{Binding Name}" Style="{StaticResource DefaultTextStyle}"/>
</Grid>
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>
</Grid>
这是最好的工作时间,直到我变得聪明并做了一点客户端缓存组件(我计划为制造商打开).当我打开组件的缓存并获得缓存命中时,所有数据都将正确存在于对象中,但SelectedItem将无法绑定.原因是,在Silverlight中调用是异步的,并且在缓存的帮助下,在制造商之前不会返回Component.因此,当SelectedItem尝试在ItemsSource列表中找到Components.Current.Manufacturer时,它不存在,因为此列表仍然为空,因为尚未从WCF服务加载Manufacturers.All.同样,如果我关闭组件缓存,它会再次运行,但感觉不对 – 就像我很幸运时机正在运行.正确的修复恕我直言,MS用于修复ComboBox / ItemsControl控件,以了解这将发生Asynch调用是常态.但在那之前,我需要一种方法来解决它…
以下是我想到的一些选项：
>消除缓存或将其全面打开以再次掩盖问题.不好恕我直言,因为这将再次失败.不太愿意把它扫回地毯下.
>创建一个可以为我同步的中间对象(应该在ItemsControl本身中完成).当两者都到达时,它将接受和Item和ItemsList,然后输出和ItemWithItemsList属性.我会将ComboBox绑定到结果输出,以便它永远不会在另一个之前得到一个项目.我的问题是,这似乎是一种痛苦,但它会确保竞争条件不再发生.
任何想法/评论？
@Joe：非常感谢你的回复.我知道只需要从UI线程更新UI.这是我的理解,我想我已经通过调试器确认了这一点,在SL2中,服务参考生成的代码为您解决了这个问题.即当我调用Manufacturers_GetAll_Asynch()时,我通过Manufacturers_GetAll_Completed事件获得结果.如果查看生成的服务引用代码,它可确保从UI线程调用* Completed事件处理程序.我的问题不是这个,而是我做了两个不同的调用(一个用于制造商列表,一个用于引用制造商id的组件),然后将这两个结果绑定到一个ComboBox.它们都绑定在UI线程上,问题是如果列表在选择之前没有到达那里,则忽略选择.
另请注意,这仍是if you just set the ItemSource and the SelectedItem in the wrong order的问题！
另一个更新：
虽然仍然存在组合框竞争条件,但我发现了其他一些有趣的东西.您永远不应该从该属性的“getter”中生成PropertyChanged事件.示例：在我的类型为ManufacturerData的SL数据对象中,我有一个名为“All”的属性.在Get {}中,它会检查它是否已被加载,如果没有,它会加载它：
public class ManufacturersData : DataServiceAccessbase
{
public ObservableCollection<Web.Manufacturer> All
{
get
{
if (!AllLoaded)
LoadAllManufacturersAsync();
return mAll;
}
private set
{
mAll = value;
OnPropertyChanged("All");
}
}
private void LoadAllManufacturersAsync()
{
if (!mCurrentlyLoadingall)
{
mCurrentlyLoadingall = true;
// check to see if this component is loaded in local Isolated Storage,if not get it from the webservice
ObservableCollection<Web.Manufacturer> all = IsoStorageManager.GetDataTransferObjectFromCache<ObservableCollection<Web.Manufacturer>>(mAllManufacturersIsoStoreFilename);
if (null != all)
{
Updateall(all);
mCurrentlyLoadingall = false;
}
else
{
Web.SystemBuilderClient sbc = GetSystemBuilderClient();
sbc.Manufacturers_GetAllCompleted += new EventHandler<hookitupright.com.silverlight.data.Web.Manufacturers_GetAllCompletedEventArgs>(sbc_Manufacturers_GetAllCompleted);
sbc.Manufacturers_GetAllAsync(); ;
}
}
}
private void Updateall(ObservableCollection<Web.Manufacturer> all)
{
All = all;
AllLoaded = true;
}
private void sbc_Manufacturers_GetAllCompleted(object sender,hookitupright.com.silverlight.data.Web.Manufacturers_GetAllCompletedEventArgs e)
{
if (e.Error == null)
{
Updateall(e.Result.Records);
IsoStorageManager.CacheDataTransferObject<ObservableCollection<Web.Manufacturer>>(e.Result.Records,mAllManufacturersIsoStoreFilename);
}
else
OnWebServiceError(e.Error);
mCurrentlyLoadingall = false;
}
}
请注意,此代码在“缓存命中”时失败,因为它将在All {Get {}}方法中为“All”生成PropertyChanged事件,这通常会导致绑定系统再次调用All {get {}}. .我从ScottGu博客发布的方式复制了这种创建可绑定silverlight数据对象的模式,它总体上对我很有帮助,但是这样的东西使得它非常棘手.幸运的是,修复很简单.希望这有助于其他人.
解决方法
在设置SelectedItem之后设置ItemSource时存在问题.当发生这种情况时,Combobx将其视为选择的完全重置并清除SelectedItem / Selectedindex.你可以在System.Windows.Controls.Primitives.Selector(ComboBox的基类)中看到这个：
protected override void OnItemsChanged(NotifyCollectionChangedEventArgs e)
{
base.OnItemsChanged(e);
int selectedindex = this.Selectedindex;
bool flag = this.IsInit && this._initializingData.IsIndexSet;
switch (e.Action)
{
case NotifyCollectionChangedAction.Add:
if (!this.AddedWithSelectionSet(e.NewStartingIndex,e.NewStartingIndex + e.NewItems.Count))
{
if ((e.NewStartingIndex <= selectedindex) && !flag)
{
this._processingSelectionPropertyChange = true;
this.Selectedindex += e.NewItems.Count;
this._processingSelectionPropertyChange = false;
}
if (e.NewStartingIndex > this._focusedindex)
{
return;
}
this.SetFocusedItem(this._focusedindex + e.NewItems.Count,false);
}
return;
case NotifyCollectionChangedAction.Remove:
if (((e.OldStartingIndex > selectedindex) || (selectedindex >= (e.OldStartingIndex + e.OldItems.Count))) && (e.OldStartingIndex < selectedindex))
{
this._processingSelectionPropertyChange = true;
this.Selectedindex -= e.OldItems.Count;
this._processingSelectionPropertyChange = false;
}
if ((e.OldStartingIndex <= this._focusedindex) && (this._focusedindex < (e.OldStartingIndex + e.OldItems.Count)))
{
this.SetFocusedItem(-1,false);
return;
}
if (e.OldStartingIndex < selectedindex)
{
this.SetFocusedItem(this._focusedindex - e.OldItems.Count,false);
}
return;
case NotifyCollectionChangedAction.Replace:
if (!this.AddedWithSelectionSet(e.NewStartingIndex,e.NewStartingIndex + e.NewItems.Count))
{
if ((e.OldStartingIndex <= selectedindex) && (selectedindex < (e.OldStartingIndex + e.OldItems.Count)))
{
this.Selectedindex = -1;
}
if ((e.OldStartingIndex > this._focusedindex) || (this._focusedindex >= (e.OldStartingIndex + e.OldItems.Count)))
{
return;
}
this.SetFocusedItem(-1,false);
}
return;
case NotifyCollectionChangedAction.Reset:
if (!this.AddedWithSelectionSet(0,base.Items.Count) && !flag)
{
this.Selectedindex = -1;
this.SetFocusedItem(-1,false);
}
return;
}
throw new InvalidOperationException();
}
注意最后一种情况 – 重置…当你加载一个新的ItemSource时,你最终会在这里被任何SelectedItem / Selectedindex吹走？！？！
那么解决方案最终非常简单.我只是将错误的ComboBox子类化,并为此方法提供和覆盖如下.虽然我确实需要添加：
public class FixedComboBox : ComboBox { public FixedComboBox() : base() { // This is here to sync the dep properties (Onselecteditemchanged is private is the base class - thanks M$) base.SelectionChanged += (s,e) => { FixedSelectedItem = SelectedItem; }; } // need to add a safe dependency property here to bind to - this will store off the "requested selectedItem" // this whole this is a kludgy wrapper because the Onselecteditemchanged is private in the base class public readonly static DependencyProperty FixedSelectedItemProperty = DependencyProperty.Register("FixedSelectedItem",typeof(object),typeof(FixedComboBox),new PropertyMetadata(null,new PropertyChangedCallback(FixedSelectedItemPropertyChanged))); private static void FixedSelectedItemPropertyChanged(DependencyObject obj,DependencyPropertyChangedEventArgs e) { FixedComboBox fcb = obj as FixedComboBox; fcb.mLastSelection = e.NewValue; fcb.SelectedItem = e.NewValue; } public object FixedSelectedItem { get { return GetValue(FixedSelectedItemProperty); } set { SetValue(FixedSelectedItemProperty,value);} } protected override void OnItemsChanged(System.Collections.Specialized.NotifyCollectionChangedEventArgs e) { base.OnItemsChanged(e); if (-1 == Selectedindex) { // if after the base class is called,there is no selection,try if (null != mLastSelection && Items.Contains(mLastSelection)) SelectedItem = mLastSelection; } } protected object mLastSelection = null; }
所有这一切都是(a)保存旧的SelectedItem然后(b)检查是否在ItemsChanged之后,如果我们没有做出选择并且旧的SelectedItem存在于新列表中……那么……选择它！