我有两个组合框,A& B,每个都绑定到一个Observable Collection。每个都附有一个SelectionChanged触发器,用于在用户更改选择时捕获。触发器将选择传递给命令。
集合实现了INotifyPropertyChanged,在每个Setter中,会触发NotifyPropertyChanged事件。这需要(在MVVM方法中)通知UI(View)ComboBox的内容已经改变。
两个组合框是相互依赖的 - 更改A中的选择会导致B重新填充新项目。
现在,问题在于B的SelectionChanged触发器会响应其重新填充的集合(以及用户更改选择)而触发。由于命令中代码的复杂性,这极大地浪费了资源。
理论上我可以通过在设置B集合时不提升NotifyPropertyChanged事件来阻止这种情况(因为,查看调用堆栈,这似乎会导致SelectionChanged触发器触发),但是MVVM方法依赖于此保持用户界面刷新。
有什么建议吗?
答案 0 :(得分:2)
为什么ComboB需要SelectionChanged事件?您可以直接将所选项目绑定到VM上的属性中。
我之前解决这个问题的方法是将ComboA的选定项目绑定到VM中。在该属性的setter中,我重新计算ComboB的可用项并将它们分配给VM上的另一个属性,并且ComboB的ItemsSource绑定到此属性。当然该属性将通知(使用INotifyPropertyChanged),但不需要做任何其他事情,我的ComboB没有SelectionChanged事件。通过使用这种方法,我也不需要在ComboA上使用SelectionChanged,这使得视图的代码保持良好和稀疏 - 一切都在VM中处理,常规数据绑定负责其余部分。
以下是从属性设置器中调整所需列表的示例:
public class MyViewModel : INotifyPropertyChanged
{
//ItemsSource of ComboA is bound to this list
public List<SomeObject> ComboAList
{
get { return _comboAList; }
set { _comboAList = value; }
}
//ItemsSource of ComboB is bound to this list
public List<SomeObject> ComboBList
{
get { return _comboBList; }
set
{
_comboBList = value;
OnPropertyChanged("ComboBList");
}
}
//ItemsSource of the dataGrid is bound to this list
public List<SomeObject> DataGridList
{
get { return _datagridList; }
set
{
_datagridList = value;
OnPropertyChanged("DataGridList");
}
}
//SelectedItem of ComboA is bound to this property
public SomeObject FirstSelectedItem
{
get { return _firstSelectedItem; }
set
{
_firstSelectedItem = value;
RefreshListForComboB();
}
}
//SelectedItem of ComboB is bound to this property
public SomeObject SecondSelectedItem
{
get { return _secondSelectedItem; }
set
{
_secondSelectedItem = value;
RefreshListForDataGrid();
}
}
private void RefreshListForComboB()
{
//do whatever is necessary to filter or create a list for comboB
ComboBList = doSomethingThatReturnsAListForComboB();
}
private void RefreshListForDataGrid()
{
//do whatever is necessary to filter or create the list for the DataGrid
DataGridList = doSomethingThatReturnsAListForDataGrid();
}
protected void OnPropertyChanged(string propertyName)
{
if (this.PropertyChanged != null)
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
#endregion
private List<SomeObject> _comboAList, _comboBList, _datagridList;
private SomeObject _firstSelectedItem, _secondSelectedItem;
}
这是一种稍微不同的方法,使用VM上的PropertyChange事件处理程序,这只是更改列表更新发生的位置。这可以说是比第一个样本更好的方法,因为它意味着属性设置者没有副作用:
public class MyViewModel : INotifyPropertyChanged
{
public MyViewModel()
{
this.PropertyChanged += new PropertyChangedEventHandler(MyViewModel_PropertyChanged);
}
private void MyViewModel_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
switch (e.PropertyName)
{
case "FirstSelectedItem":
RefreshListForComboB();
break;
case "SecondSelectedItem":
RefreshListForDataGrid();
break;
}
}
//ItemsSource of ComboA is bound to this list
public List<SomeObject> ComboAList
{
get { return _comboAList; }
set { _comboAList = value; }
}
//ItemsSource of ComboB is bound to this list
public List<SomeObject> ComboBList
{
get { return _comboBList; }
set
{
_comboBList = value;
OnPropertyChanged("ComboBList");
}
}
//ItemsSource of the dataGrid is bound to this list
public List<SomeObject> DataGridList
{
get { return _datagridList; }
set
{
_datagridList = value;
OnPropertyChanged("DataGridList");
}
}
//SelectedItem of ComboA is bound to this property
public SomeObject FirstSelectedItem
{
get { return _firstSelectedItem; }
set
{
_firstSelectedItem = value;
OnPropertyChanged("FirstSelectedItem");
}
}
//SelectedItem of ComboB is bound to this property
public SomeObject SecondSelectedItem
{
get { return _secondSelectedItem; }
set
{
_secondSelectedItem = value;
OnPropertyChanged("SecondSelectedItem");
}
}
private void RefreshListForComboB()
{
//do whatever is necessary to filter or create a list for comboB
ComboBList = doSomethingThatReturnsAListForComboB();
}
private void RefreshListForDataGrid()
{
//do whatever is necessary to filter or create the list for the DataGrid
DataGridList = doSomethingThatReturnsAListForDataGrid();
}
protected void OnPropertyChanged(string propertyName)
{
if (this.PropertyChanged != null)
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
#endregion
private List<SomeObject> _comboAList, _comboBList, _datagridList;
private SomeObject _firstSelectedItem, _secondSelectedItem;
}