在WPF MVVM中使用按钮删除列表框中的项目 [英] Delete an item in a listbox with a button, in wpf MVVM
问题描述
我有问题.看起来很简单，但并不是那么容易.我有两个列表框，其中包含对象.一个充满了可用的对象，另一个充满了，我可以通过拖放来填充它.如果您填充此空白列表框，则这些项也将添加到的列表中，这是另一个对象的属性.但是现在我的问题是，如何通过单击按钮可以轻松地删除此列表框中的对象.
I have a problem. It seems a simple thing but it isn't that easy. I have two listboxes, with objects in it. One is full with available objects and the other is empty, and I can fill this up with drag and drop. If you fill this empty listbox, the items are also added to a list of, this is a property of another object. But now my question is how I can easily delete an object in this listbox by clicking on a button.
我尝试了一些操作，但无法运行. 这是我的xaml:
I tried something but it wouldn't run. Here is my xaml:
<ListBox ItemsSource="{Binding AvailableQuestions}"
DisplayMemberPath="Description"
dd:DragDrop.IsDragSource="True"
dd:DragDrop.IsDropTarget="True" Margin="0,34,0,339" Background="#CDC5CBC5"
dd:DragDrop.DropHandler="{Binding}"/>
<ListBox SelectedItem="{Binding Path=SelectedQuestionDropList, UpdateSourceTrigger=PropertyChanged,Mode=TwoWay}" ItemsSource="{Binding SelectedQuestions}"
DisplayMemberPath="Description"
dd:DragDrop.IsDragSource="True"
dd:DragDrop.IsDropTarget="True" Margin="0,201,0,204" Background="#CDC5CBC5"
dd:DragDrop.DropHandler="{Binding}" />
<Button Content="Delete" Height="23"
HorizontalAlignment="Left"
Margin="513,322,0,0"
Name="button1"
VerticalAlignment="Top" Width="75" Command="{Binding Commands}"
CommandParameter="Delete" />
这是我的视图模型:
// other stuff and properties..
public ICommand Commands { get; set; }
public bool CanExecute(object parameter)
{
return true;
}
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested += value; }
}
public void Execute(Object parameter)
{
foreach (ExaminationQuestion exaq in this.Examination.ExaminationQuestions)
{
if (exaq.Question.Guid == SelectedQuestionDropList.Guid)
{
this.Examination.ExaminationQuestions.Remove(exaq);
SelectedQuestions.Remove(SelectedQuestionDropList);
OnPropertyChanged("SelectedQuestions");
}
}
}
有人可以帮我吗?
推荐答案
您需要了解两件事:
- 首先，不要忘记将
ListBox绑定到ObservableCollection而不是经典的IList.否则，如果您的命令实际上被触发并从列表中删除了一项，则用户界面将完全不变 - 第二，您的命令在这里永远不会被创建.让我解释.您正在将删除"按钮绑定到命令命令".它有吸气剂.但是，如果您调用get，它将返回什么?没什么，命令为空...
- First, Don't forget to bind your
ListBoxto anObservableCollectionrather than to a classic IList. Otherwise, if your command is actually fired and removes an item from the list, the UI wouldn't change at all - Second, your command here is never created. Let me explain. You are binding the Delete button to your command Commands. It has a getter. However, if you call get, what will it return? Nothing, the command is null...
这是我建议您工作的方式:创建一个通用命令类(按照教程，我将其称为RelayCommand，但名称无关紧要):
Here is the way I'd advise you to work: Create a generic command class (I called it RelayCommand, following a tutorial... but the name doesn't matter):
/// <summary>
/// Class representing a command sent by a button in the UI, defines what to launch when the command is called
/// </summary>
public class RelayCommand : ICommand
{
#region Fields
readonly Action<object> _execute;
readonly Predicate<object> _canExecute;
#endregion // Fields
#region Constructors
public RelayCommand(Action<object> execute)
: this(execute, null)
{
}
public RelayCommand(Action<object> execute, Predicate<object> canExecute)
{
if (execute == null)
throw new ArgumentNullException("execute");
_execute = execute;
_canExecute = canExecute;
}
#endregion // Constructors
#region ICommand Members
//[DebuggerStepThrough]
/// <summary>
/// Defines if the current command can be executed or not
/// </summary>
/// <param name="parameter"></param>
/// <returns></returns>
public bool CanExecute(object parameter)
{
return _canExecute == null ? true : _canExecute(parameter);
}
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
public void Execute(object parameter)
{
_execute(parameter);
}
#endregion // ICommand Members
}
然后，在您的ViewModel中，创建一个属性命令，并定义实际的getter(您不必使用setter，您可以将其忘记):
Then, in your ViewModel, create a property command, and define the actual getter (you don't have to use the setter, you can just forget it):
private RelayCommand _deleteCommand;
public ICommand DeleteCommand
{
get
{
if (_deleteCommand == null)
{
_deleteCommand = new RelayCommand(param => DeleteItem());
}
return _deleteCommand;
}
}
这意味着当您调用命令时，它将调用函数DeleteItem
This means that when you call your command, it will call the function DeleteItem
剩下要做的就是:
private void DeleteItem() {
//First step: copy the actual list to a temporary one
ObservableCollection<ExaminationQuestion> tempCollection = new ObservableCollection<ExaminationQuestion>(this.Examination.ExaminationQuestions);
//Then, iterate on the temporary one:
foreach (ExaminationQuestion exaq in tempCollection)
{
if (exaq.Question.Guid == SelectedQuestionDropList.Guid)
{
//And remove from the real list!
this.Examination.ExaminationQuestions.Remove(exaq);
SelectedQuestions.Remove(SelectedQuestionDropList);
OnPropertyChanged("SelectedQuestions");
}
}
}
然后就可以了:)
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