我有以下问题。
我有以下简单的xaml:
<TextBox Name="NameBox" Text ="{Binding Name}" />
<Button Content="Save" Command="{Binding SaveCommand}" CommandParameter="{Binding Entity}" />
我将此Window的DataContext绑定到以下View Model
public class MyViewModel
{
public SimpleModel Entity { get; set; }
private ICommand _saveCommand;
public ICommand SaveCommand { get { return _saveCommand ?? (_saveCommand = new MyCommand(OnSaveItem, parameter => CanSaveItem())); } }
public void OnSaveItem(object parameter)
{
// some code
}
public virtual bool CanSaveItem()
{
return !String.IsNullOrWhiteSpace(Entity.Name);
}
}
SimpleModel是
public class SimpleModel
{
public int Id { get; set; }
public string Name { get; set; }
}
这段代码大部分都是正确的,但我不能 make方法CanSaveItem正常工作。我不知道如何告诉SaveCommand ViewModel的属性已更改。我知道我必须使用CanExecuteChanged或CommandManager.InvalidateRequerySuggested,我试图使用他们的一些但我不知道如何正确地做它并没有生效。你能帮我解决这个问题吗?
UPD。
public class MyCommand : ICommand
{
public MyCommand(Action<object> execute, Predicate<object> canExecute)
{
_canExecute = canExecute;
_execute = execute;
}
public bool CanExecute(object parameter)
{
return _canExecute(parameter);
}
public void Execute(object parameter)
{
_execute(parameter);
}
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
private readonly Predicate<object> _canExecute;
private readonly Action<object> _execute;
}
答案 0 :(得分:3)
看起来你处于早期的学习曲线上,这可能令人困惑......有时候对我来说也是如此。
无论如何,我对你所拥有的东西做了一些细微的改动,并解释了我对他们的所作所为。
public class MyViewModel
{
public SimpleModel Entity { get; set; }
private MyCommand _saveCommand;
public MyCommand SaveCommand { get { return _saveCommand ?? (_saveCommand = new MyCommand(OnSaveItem, parameter => CanSaveItem())); } }
public MyViewModel()
{
//------ You need to create an instance of your entity to bind to
Entity = new SimpleModel();
//-- I added an event handler as your "Entity" object doesn't know
//-- about the button on the view model. So when it has something
//-- change, have it call anybody listening to its exposed event.
Entity.SomethingChanged += MyMVVM_SomethingChanged;
}
void MyMVVM_SomethingChanged(object sender, EventArgs e)
{
// Tell our mvvm command object to re-check its CanExecute
SaveCommand.RaiseCanExecuteChanged();
}
public void OnSaveItem(object parameter)
{
// some code
}
public virtual bool CanSaveItem()
{
//-- Checking directly to your Entity object
return !String.IsNullOrWhiteSpace(Entity.Name);
}
}
public class SimpleModel
{
//-- Simple constructor to default some values so when you run
//-- your form, you SHOULD see the values immediately to KNOW
//-- the bindings are correctly talking to this entity.
public SimpleModel()
{
_name = "test1";
_Id = 123;
}
//-- changed to public and private... and notice in the setter
//-- to call this class's "somethingChanged" method
private int _Id;
public int Id
{
get { return _Id; }
set
{
_Id = value;
somethingChanged("Id");
}
}
private string _name;
public string Name
{ get { return _name; }
set { _name = value;
somethingChanged( "Name" );
}
}
//-- Expose publicly for anything else to listen to (i.e. your view model)
public event EventHandler SomethingChanged;
//-- So, when any property above changes, it calls this method with whatever
//-- its property is just as a reference. Then checks. Is there anything
//-- listening to our exposed event handler? If so, pass the information on
private void somethingChanged( string whatProperty)
{
// if something is listening
if (SomethingChanged != null)
SomethingChanged(whatProperty, null);
}
}
public class MyCommand : ICommand
{
public MyCommand(Action<object> execute, Predicate<object> canExecute)
{
_canExecute = canExecute;
_execute = execute;
}
public bool CanExecute(object parameter)
{
return _canExecute(parameter);
}
public void Execute(object parameter)
{
_execute(parameter);
}
private readonly Predicate<object> _canExecute;
private readonly Action<object> _execute;
//-- Change to the event handler definition, just expose it
public event EventHandler CanExecuteChanged;
//-- Now expose this method so your mvvm can call it and it rechecks
//-- it's own CanExecute reference
public void RaiseCanExecuteChanged()
{
if (CanExecuteChanged != null)
CanExecuteChanged(this, new EventArgs());
}
}
最后,表单中的绑定。我不知道你是如何将视图的“DataContext”设置为视图模型的,但假设这一切都正确且没有问题,请将文本框和命令按钮调整为类似
<TextBox Name="NameBox" Text ="{Binding Entity.Name,
NotifyOnTargetUpdated=True, UpdateSourceTrigger=PropertyChanged}" />
<Button Content="Save" Command="{Binding SaveCommand}" CommandParameter="{Binding Entity}" />
请注意,文本绑定是MVVM上的“Entity”对象,然后是Entity对象的“.Name”属性。这里重要的是UpdateSourceTrigger。这会强制更新每个字符更改的数据绑定,因此只要删除最后一个字符,或者开始键入第一个字符,就会分别刷新“保存”按钮。
答案 1 :(得分:1)
我会尝试调用 CommandManager.InvalidateRequerySuggested 。