我有以下转换器：
public class MyConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, string language)
{
Debug.WriteLine(value.GetType());
//The rest of the code
}
public object ConvertBack(object value, Type targetType, object parameter, string language)
{
throw new NotImplementedException();
}
}
以及尝试使用转换器的XAML：
<ListView ItemsSource="{x:Bind StickersCVS.View}" >
<ListView.ItemTemplate>
<DataTemplate x:DataType="models:StickerCategory">
<TextBlock Foreground="{x:Bind Converter={StaticResource MyConverter}}"/>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
这给了我一个value.GetType()的NPE,显然传入的值是null.
如果我更改以下部分：
< TextBlock Foreground =“ {x：Bind Converter = {StaticResource MyConverter}}” />
至
< TextBlock Foreground =“ {Binding Converter = {StaticResource MyConverter}}” />
然后就可以了.调试正确输出StickerCategory作为值的类型. x：Bind将null传递到转换器的任何原因以及如何使其与x：Bind一起使用？我正在尝试将DataContext传递给我的转换器.
解决方法:
{x：Bind}使用生成的代码来实现其优势,并且在{x：Bind}中使用不同的Path时,生成的代码具有一些差异.
在这里,我使用一个简单的示例.对于完整的样本,请检查GitHub.
在示例中,我有一个如下的ViewModel：
public class MyViewModel
{
public MyViewModel()
{
MyList = new List<Item>()
{
new Item {Name="1",Number=1 },
new Item {Name="2",Number=2 },
new Item {Name="3",Number=3 }
};
}
public List<Item> MyList { get; set; }
}
public class Item
{
public string Name { get; set; }
public int Number { get; set; }
public override string ToString()
{
return string.Format("Name: {0}, Number {1}", this.Name, this.Number);
}
}
当我们在MainPage.xaml中使用{x：绑定名称时,Converter = {StaticResource ItemConvert}}
<ListView ItemsSource="{x:Bind ViewModel.MyList}">
<ListView.ItemTemplate>
<DataTemplate x:DataType="local:Item">
<TextBlock Text="{x:Bind Converter={StaticResource ItemConvert}}" />
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
它在MainPage.g.cs中生成以下代码
public void DataContextChangedHandler(global::Windows.UI.Xaml.FrameworkElement sender, global::Windows.UI.Xaml.DataContextChangedEventArgs args)
{
global::xBindWithConverter.Item data = args.NewValue as global::xBindWithConverter.Item;
if (args.NewValue != null && data == null)
{
throw new global::System.ArgumentException("Incorrect type passed into template. Based on the x:DataType global::xBindWithConverter.Item was expected.");
}
this.SetDataRoot(data);
this.Update();
}
// IDataTemplateExtension
public bool ProcessBinding(uint phase)
{
throw new global::System.NotImplementedException();
}
public int ProcessBindings(global::Windows.UI.Xaml.Controls.ContainerContentChangingEventArgs args)
{
int nextPhase = -1;
switch(args.Phase)
{
case 0:
nextPhase = -1;
this.SetDataRoot(args.Item as global::xBindWithConverter.Item);
if (!removedDataContextHandler)
{
removedDataContextHandler = true;
((global::Windows.UI.Xaml.Controls.TextBlock)args.ItemContainer.ContentTemplateRoot).DataContextChanged -= this.DataContextChangedHandler;
}
this.initialized = true;
break;
}
this.Update_((global::xBindWithConverter.Item) args.Item, 1 << (int)args.Phase);
return nextPhase;
}
...
public void Update()
{
this.Update_(this.dataRoot, NOT_PHASED);
this.initialized = true;
}
和
global::Windows.UI.Xaml.Controls.TextBlock element3 = (global::Windows.UI.Xaml.Controls.TextBlock)target;
MainPage_obj3_Bindings bindings = new MainPage_obj3_Bindings();
returnValue = bindings;
bindings.SetDataRoot((global::xBindWithConverter.Item) element3.DataContext);
bindings.SetConverterLookupRoot(this);
element3.DataContextChanged += bindings.DataContextChangedHandler;
global::Windows.UI.Xaml.DataTemplate.SetExtensionInstance(element3, bindings);
初始化Page时,element3.DataContextChanged = bindings.DataContextChangedHandler;将首先执行.此后,将在初始化时引发DataContextChanged事件,从而调用DataContextChangedHandler方法.然后将执行ProcessBindings方法,以使用绑定数据更新列表项容器元素.
在DataContextChangedHandler方法中,它调用this.Update();.最后调用Update_(global :: xBindWithConverter.Item obj,int phase)方法的方法.但是,当调用DataContextChangedHandler方法时,它的args.NewValue值为null,因此Update_(global :: xBindWithConverter.Item obj,int阶段)方法中的obj也为null.
并且在XAML中使用{x：Bind Converter = {StaticResource ItemConvert}}时,Update_(global :: xBindWithConverter.Item obj,int阶段)的生成代码为：
// Update methods for each path node used in binding steps.
private void Update_(global::xBindWithConverter.Item obj, int phase)
{
if((phase & ((1 << 0) | NOT_PHASED )) != 0)
{
XamlBindingSetters.Set_Windows_UI_Xaml_Controls_TextBlock_Text(this.obj3.Target as global::Windows.UI.Xaml.Controls.TextBlock, (global::System.String)this.LookupConverter("ItemConvert").Convert(obj, typeof(global::System.String), null, null), null);
}
}
由于obj为null,因此Convert中的值为null,最后它将在value.GetType()处抛出NPE.
但是,如果我们在{x：Bind}中使用另一个路径,例如{x：Bind Name,Converter = {StaticResource ItemConvert}},则Update_(global :: xBindWithConverter.Item obj,int阶段)的生成代码是不同的：
// Update methods for each path node used in binding steps.
private void Update_(global::xBindWithConverter.Item obj, int phase)
{
if (obj != null)
{
if ((phase & (NOT_PHASED | (1 << 0))) != 0)
{
this.Update_Name(obj.Name, phase);
}
}
}
private void Update_Name(global::System.String obj, int phase)
{
if((phase & ((1 << 0) | NOT_PHASED )) != 0)
{
XamlBindingSetters.Set_Windows_UI_Xaml_Controls_TextBlock_Text(this.obj3.Target as global::Windows.UI.Xaml.Controls.TextBlock, (global::System.String)this.LookupConverter("ItemConvert").Convert(obj, typeof(global::System.String), null, null), null);
}
}
它将确定obj是否为null.因此,此处不会调用XamlBindingSetters.Set_Windows_UI_Xaml_Controls_TextBlock_Text方法,并且不会发生NullReferenceException.
要解决此问题,就像@MarkusHütter所说的那样,您可以在转换器中添加一个空检查,例如：
public object Convert(object value, Type targetType, object parameter, string language)
{
if (value != null)
{
System.Diagnostics.Debug.WriteLine(value.GetType());
return value.ToString();
}
else
{
System.Diagnostics.Debug.WriteLine("value is null");
return null;
}
}