在MVVMLight框架的ViewModel中实现NavigationService

网上已经有很多方法了，比如通过Messenger来实现等等。这里我只讲述一种我比较喜欢的方法，因为它很方便

首先定义一个ViewModel基类，将所有ViewModel子类继承这个基类。在基类中定义

protected bool RemoveBackEntry { get; set; }
public NavigationService NavigationService { get; set; }
public NavigationContext NavigationContext { get; set; }
public virtual void OnNavigatedTo(NavigationEventArgs e) { }
public virtual void OnNavigatingFrom(NavigatingCancelEventArgs e) { }
public virtual void OnNavigatedFrom(NavigationEventArgs e)
{
if (RemoveBackEntry)
{
RemoveBackEntry = false;
NavigationService.RemoveBackEntry();
}
}

因为在MVVM中，我们将会在所有页面上都加上DataContext，只要拿到DataContext就能拿到ViewModel，所以我们就可以定义子类WPhoneApplicationPage来继承PhoneApplicationPage这个类。

public class WPhoneApplicationPage : PhoneApplicationPage
{
protected WPhoneApplicationPage()
{
Loaded += PageBaseLoaded;
}

private void PageBaseLoaded(object sender, RoutedEventArgs e)
{
var viewModel = DataContext as ViewModel.ViewModel;
if (viewModel != null)
{
viewModel.NavigationService = NavigationService;
}
}

protected override void OnNavigatedTo(NavigationEventArgs e)
{
base.OnNavigatedTo(e);

var viewModel = DataContext as ViewModel.ViewModel;
if (viewModel != null)
{
viewModel.NavigationContext = NavigationContext;
viewModel.OnNavigatedTo(e);
}
}

protected override void OnNavigatingFrom(NavigatingCancelEventArgs e)
{
base.OnNavigatingFrom(e);
var viewModel = DataContext as ViewModel.ViewModel;
if (viewModel != null)
{
viewModel.NavigationContext = NavigationContext;
viewModel.OnNavigatingFrom(e);
}
}

protected override void OnNavigatedFrom(NavigationEventArgs e)
{
base.OnNavigatedFrom(e);
var viewModel = DataContext as ViewModel.ViewModel;
if (viewModel != null)
{
viewModel.NavigationContext = NavigationContext;
viewModel.OnNavigatedFrom(e);
}
}
}

　　这样在ViewModel中就可以调用NavigationService了。

　　此外，还需要修改View，让View继承WPhoneApplicationPage

　　xaml布局文件中

　<phone:WPhoneApplicationPage //修改PhoneApplicationPage 为 WPhoneApplicationPage
　　xmlns:phone="clr-namespace:redrock.Core" //这里是你的WPhoneApplicationPage类所在的命名空间

　　cs类文件中

public partial class MainPage : WPhoneApplicationPage // 让MainPage继承WPhoneApplicationPage
{
　　public MainPage()
　　{
　　　　InitializeComponent();
　　}
}

　　这样你可以在你的ViewModel中使用NavigationService以及重载 ViewModel基类中提供的导航方法