首先要说的是我在
WPF和MVVM模式的最开始.
在尝试一些自定义命令时,我想知道如何使用ICommand接口中的CanExecute Methode.
在我的例子中,我有一个SaveCommand,我只能在对象可以保存时启用.我的保存按钮的XAML代码如下所示：
<Button Content="Save" Command="{Binding SaveCommand,Mode=TwoWay}" />
这是我的保存类的代码：
class Save : ICommand
{
public MainWindowviewmodel viewmodel { get; set; }
public Save(MainWindowviewmodel viewmodel)
{
this.viewmodel = viewmodel;
}
public bool CanExecute(object parameter)
{
if (viewmodel.IsSaveable == false)
return false;
return true;
}
public event EventHandler CanExecuteChanged;
public void Execute(object parameter)
{
viewmodel.Save();
}
}
public ICommand SaveCommand
{
get
{
saveCommand = new Save(this);
return saveCommand;
}
set
{
saveCommand = value;
}
}
这个结构不起作用.当isSaveable为true时,该按钮不会启用其自身.
解决方法
不要定义自己的ICommand实现,而是使用
RelayCommand.
在下面的示例代码中,当用户在TextBox中键入内容时,将启用保存按钮.
XAML：
<Window x:Class="RelayCommandDemo.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">
<StackPanel HorizontalAlignment="Center">
<TextBox Text="{Binding Name,UpdateSourceTrigger=PropertyChanged}" Margin="5" Width="120"/>
<Button Content="Save" Command="{Binding SaveCommand}" Margin="3"/>
</StackPanel>
</Window>
代码背后：
using System;
using System.Windows;
using System.Windows.Input;
namespace RelayCommandDemo
{
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
DataContext = new VM();
}
}
public class VM
{
public String Name { get; set; }
private ICommand _SaveCommand;
public ICommand SaveCommand
{
get { return _SaveCommand; }
}
public VM()
{
_SaveCommand = new RelayCommand(SaveCommand_Execute,SaveCommand_CanExecute);
}
public void SaveCommand_Execute()
{
MessageBox.Show("Save Called");
}
public bool SaveCommand_CanExecute()
{
if (string.IsNullOrEmpty(Name))
return false;
else
return true;
}
}
public class RelayCommand : ICommand
{
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
private Action methodToExecute;
private Func<bool> canExecuteEvaluator;
public RelayCommand(Action methodToExecute,Func<bool> canExecuteEvaluator)
{
this.methodToExecute = methodToExecute;
this.canExecuteEvaluator = canExecuteEvaluator;
}
public RelayCommand(Action methodToExecute)
: this(methodToExecute,null)
{
}
public bool CanExecute(object parameter)
{
if (this.canExecuteEvaluator == null)
{
return true;
}
else
{
bool result = this.canExecuteEvaluator.Invoke();
return result;
}
}
public void Execute(object parameter)
{
this.methodToExecute.Invoke();
}
}
}
