我想实现以下目标：我想要一个显示可用COM端口的ComboBox。在启动时（并单击＆＃34;刷新＆＃34;按钮）我想获得可用的COM端口并将选择设置为最后选择的值（来自应用程序设置）。
如果设置（最后一个COM端口）中的值不在值列表（可用的COM端口）中，则会发生以下情况：
虽然ComboBox没有显示任何内容（它足够聪明＆＃34;知道新的SelectedItem不在ItemsSource中），但ViewModel更新了＆＃34;无效的价值＆＃34;。我实际上期望Binding具有与ComboBox显示的值相同的值。
用于演示目的的代码：
MainWindow.xaml：
<Window x:Class="DemoComboBinding.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525"
xmlns:local="clr-namespace:DemoComboBinding">
<Window.Resources>
<local:DemoViewModel x:Key="vm" />
</Window.Resources>
<StackPanel Orientation="Vertical">
<ComboBox SelectedItem="{Binding Source={StaticResource vm}, Path=Selected}" x:Name="combo"
ItemsSource="{Binding Source={StaticResource vm}, Path=Source}"/>
<Button Click="Button_Click">Set different</Button> <!-- would be refresh button -->
<Label Content="{Binding Source={StaticResource vm}, Path=Selected}"/> <!-- shows the value from the view model -->
</StackPanel>
</Window>
MainWindow.xaml.cs：
// usings removed
namespace DemoComboBinding
{
public partial class MainWindow : Window
{
//...
private void Button_Click(object sender, RoutedEventArgs e)
{
combo.SelectedItem = "COM4"; // would be setting from Properties
}
}
}
视图模型：
namespace DemoComboBinding
{
class DemoViewModel : INotifyPropertyChanged
{
string selected;
string[] source = { "COM1", "COM2", "COM3" };
public string[] Source
{
get { return source; }
set { source = value; }
}
public string Selected
{
get { return selected; }
set {
if(selected != value)
{
selected = value;
OnpropertyChanged("Selected");
}
}
}
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
void OnpropertyChanged(string propertyname)
{
var handler = PropertyChanged;
if(handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyname));
}
}
#endregion
}
}
我最初提出的解决方案是检查Selected setter内部是否设置的值在可用COM端口列表内（如果没有，设置为空字符串并发送OPC）。
我想知道： 为什么会这样？ 还有其他解决方案我没有看到吗？
答案 0 :(得分：5)
简而言之，您无法将SelectedItem设置为不在ItemsSource中的值。 AFAIK，这是所有Selector后代的默认行为，这是相当明显的：设置SelectedItem不仅是数据更改，这也会导致一些视觉后果，如生成项目容器和重新绘图项（所有这些操作ItemsSource）。你在这里做的最好的是这样的代码：
public DemoViewModel()
{
selected = Source.FirstOrDefault(s => s == yourValueFromSettings);
}
另一个选项是允许用户通过使其可编辑而在ComboBox中输入任意值。
答案 1 :(得分：2)
我意识到这对你有所帮助有点晚了，但我希望它至少对某人有帮助。对不起，如果有拼写错误，我必须在记事本中输入：
<强> ComboBoxAdaptor.cs：
using System;
using System.Collections;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Markup;
namespace Adaptors
{
[ContentProperty("ComboBox")]
public class ComboBoxAdaptor : ContentControl
{
#region Protected Properties
protected bool IsChangingSelection
{ get; set; }
protected ICollectionView CollectionView
{ get; set; }
#endregion
#region Dependency Properties
public static readonly DependencyProperty ComboBoxProperty =
DependencyProperty.Register("ComboBox", typeof(ComboBox), typeof(ComboBoxAdaptor),
new FrameworkPropertyMetadata(new PropertyChangedCallback(ComboBox_Changed)));
private static void ComboBox_Changed(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
var theComboBoxAdaptor = (ComboBoxAdaptor)d;
theComboBoxAdaptor.ComboBox.SelectionChanged += theComboBoxAdaptor.ComboBox_SelectionChanged;
}
public ComboBox ComboBox
{
get { return (ComboBox)GetValue(ComboBoxProperty); }
set { SetValue(ComboBoxProperty, value); }
}
public static readonly DependencyProperty NullItemProperty =
DependencyProperty.Register("NullItem", typeof(object), typeof(ComboBoxAdaptor),
new PropertyMetadata("(None)"));
public object NullItem
{
get { return GetValue(NullItemProperty); }
set { SetValue(NullItemProperty, value); }
}
public static readonly DependencyProperty ItemsSourceProperty =
DependencyProperty.Register("ItemsSource", typeof(IEnumerable), typeof(ComboBoxAdaptor),
new FrameworkPropertyMetadata(new PropertyChangedCallback(ItemsSource_Changed)));
public IEnumerable ItemsSource
{
get { return (IEnumerable)GetValue(ItemsSourceProperty); }
set { SetValue(ItemsSourceProperty, value); }
}
public static readonly DependencyProperty SelectedItemProperty =
DependencyProperty.Register("SelectedItem", typeof(object), typeof(ComboBoxAdaptor),
new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
new PropertyChangedCallback(SelectedItem_Changed)));
public object SelectedItem
{
get { return GetValue(SelectedItemProperty); }
set { SetValue(SelectedItemProperty, value); }
}
public static readonly DependencyProperty AllowNullProperty =
DependencyProperty.Register("AllowNull", typeof(bool), typeof(ComboBoxAdaptor),
new PropertyMetadata(true, AllowNull_Changed));
public bool AllowNull
{
get { return (bool)GetValue(AllowNullProperty); }
set { SetValue(AllowNullProperty, value); }
}
#endregion
#region static PropertyChangedCallbacks
static void ItemsSource_Changed(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
ComboBoxAdaptor adapter = (ComboBoxAdaptor)d;
adapter.Adapt();
}
static void AllowNull_Changed(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
ComboBoxAdaptor adapter = (ComboBoxAdaptor)d;
adapter.Adapt();
}
static void SelectedItem_Changed(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
ComboBoxAdaptor adapter = (ComboBoxAdaptor)d;
if (adapter.ItemsSource != null)
{
//If SelectedItem is changing from the Source (which we can tell by checking if the
//ComboBox.SelectedItem is already set to the new value), trigger Adapt() so that we
//throw out any items that are not in ItemsSource.
object adapterValue = (e.NewValue ?? adapter.NullItem);
object comboboxValue = (adapter.ComboBox.SelectedItem ?? adapter.NullItem);
if (!object.Equals(adapterValue, comboboxValue))
{
adapter.Adapt();
adapter.ComboBox.SelectedItem = e.NewValue;
}
//If the NewValue is not in the CollectionView (and therefore not in the ComboBox)
//trigger an Adapt so that it will be added.
else if (e.NewValue != null && !adapter.CollectionView.Contains(e.NewValue))
{
adapter.Adapt();
}
}
}
#endregion
#region Misc Callbacks
void ComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
if (ComboBox.SelectedItem == NullItem)
{
if (!IsChangingSelection)
{
IsChangingSelection = true;
try
{
int selectedIndex = ComboBox.SelectedIndex;
ComboBox.SelectedItem = null;
ComboBox.SelectedIndex = -1;
ComboBox.SelectedIndex = selectedIndex;
}
finally
{
IsChangingSelection = false;
}
}
}
object newVal = (ComboBox.SelectedItem == null ? null : ComboBox.SelectedItem);
if (!object.Equals(SelectedItem, newVal))
{
SelectedItem = newVal;
}
}
void CollectionView_CurrentChanged(object sender, EventArgs e)
{
if (AllowNull && (ComboBox != null) && (((ICollectionView)sender).CurrentItem == null) && (ComboBox.Items.Count > 0))
{
ComboBox.SelectedIndex = 0;
}
}
#endregion
#region Methods
protected void Adapt()
{
if (CollectionView != null)
{
CollectionView.CurrentChanged -= CollectionView_CurrentChanged;
CollectionView = null;
}
if (ComboBox != null && ItemsSource != null)
{
CompositeCollection comp = new CompositeCollection();
//If AllowNull == true, add a "NullItem" as the first item in the ComboBox.
if (AllowNull)
{
comp.Add(NullItem);
}
//Now Add the ItemsSource.
comp.Add(new CollectionContainer { Collection = ItemsSource });
//Lastly, If Selected item is not null and does not already exist in the ItemsSource,
//Add it as the last item in the ComboBox
if (SelectedItem != null)
{
List<object> items = ItemsSource.Cast<object>().ToList();
if (!items.Contains(SelectedItem))
{
comp.Add(SelectedItem);
}
}
CollectionView = CollectionViewSource.GetDefaultView(comp);
if (CollectionView != null)
{
CollectionView.CurrentChanged += CollectionView_CurrentChanged;
}
ComboBox.ItemsSource = comp;
}
}
#endregion
}
}
如何在Xaml中使用
<adaptor:ComboBoxAdaptor
NullItem="Please Select an Item.."
ItemsSource="{Binding MyItemsSource}"
SelectedItem="{Binding MySelectedItem}">
<ComboBox Width="100" />
</adaptor:ComboBoxAdaptor>
一些笔记
如果SelectedItem更改为ComboBox以外的值，则会将其添加到ComboBox（但不会添加到ItemsSource）。下次SelectedItem通过Binding进行更改时，ItemsSource以外的所有项目都将从ComboBox中删除。
此外，ComboBoxAdaptor允许您将Null项插入ComboBox。这是一项可选功能，您可以通过在xaml中设置AllowNull="False"来关闭。
答案 2 :(得分：0)
您可以通过创建单单元格Grid，然后将ComboBox放入网格中，并将TextBlock放在ComboBox顶部上，以实现类似的目的。只需通过绑定到ComboBox的Text.IsEmpty属性来控制TextBlock的可见性。
您可能需要调整文本框的边距，对齐方式，大小和其他属性，以使其看起来不错。
<Grid>
<ComboBox Name="MyComboBox"
ItemsSource="{Binding Options}"
SelectedIndex="{Binding SelectedIndex}" />
<TextBlock Text="{Binding EmptySelectionPromptText}"
Margin="4 3 0 0"
Visibility="{Binding ElementName=MyComboBox, Path=Text.IsEmpty, Converter={StaticResource BoolToVis}}">
</TextBlock>
</Grid>