Prism.WPF -- Prism框架使用（下）

本文参考Prism官方示例

命令使用

Prism提供了两种命令：DelegateCommand和CompositeCommand。

DelegateCommand

DelegateCommand封装了两个委托：Execute和CanExecute，使用如下：

// view
<Button Command="{Binding ExecuteDelegateCommand}" Content="DelegateCommand"/>

// viewmodel
public DelegateCommand ExecuteDelegateCommand { get; private set; }

public MainWindowViewModel()
{
    ExecuteDelegateCommand = new DelegateCommand(Execute, CanExecute);
}

private void Execute()
{
    UpdateText = $"Updated: {DateTime.Now}";
}

private bool CanExecute()
{
    return IsEnabled;
}

CompositeCommand

CompositeCommand为复合命令，由多个子命令构成。当调用CompositeCommand时，将依次调用每个子命令。默认情况下，当所有子命令CanExecute均返回true时才会执行CompositeCommand。使用方法如下：

// Project.Core中定义接口及实现
public interface IApplicationCommands
{
    CompositeCommand SaveCommand { get; }
}

public class ApplicationCommands : IApplicationCommands
{
    private CompositeCommand _saveCommand = new CompositeCommand();
    public CompositeCommand SaveCommand
    {
        get { return _saveCommand; }
    }
}

// App.xaml.cs中注册单例对象
protected override void RegisterTypes(IContainerRegistry containerRegistry)
{
    containerRegistry.RegisterSingleton<
        IApplicationCommands, ApplicationCommands>();
}

// viewmodel中添加子命令
public TabViewModel(IApplicationCommands applicationCommands)
{
    _applicationCommands = applicationCommands;

    UpdateCommand = new DelegateCommand(Update).ObservesCanExecute(
        () => CanUpdate);

    _applicationCommands.SaveCommand.RegisterCommand(UpdateCommand);
}

// view中执行命令(需在对应的viewmodel的构造函数中传入IApplicationCommands实例)
<Button Content="Save" Command="{Binding ApplicationCommands.SaveCommand}"/>

EventAggregator

EventAggregator是一种事件机制，解决了松耦合模块间的通信问题。使用方法如下：

// Project.core中声明事件类型
public class MessageSentEvent : PubSubEvent<string>
{
}

// viewmodel中发布事件
IEventAggregator _ea;
public MessageViewModel(IEventAggregator ea)
{
    _ea = ea;
    // 发布事件的命令
    SendMessageCommand = new DelegateCommand(SendMessage);
}

private void SendMessage()
{
    _ea.GetEvent<MessageSentEvent>().Publish(Message);
}

// viewmodel中订阅事件
IEventAggregator _ea;
public MessageListViewModel(IEventAggregator ea)
{
    _ea = ea;
    _ea.GetEvent<MessageSentEvent>().Subscribe(MessageReceived);
    // 如下方式可以过滤事件，可通过第二个参数指定处理线程
    // _ea.GetEvent<MessageSentEvent>().Subscribe(MessageReceived,
        ThreadOption.PublisherThread, false,
        (filter) => filter.Contains("Brian"));
}

private void MessageReceived(string message)
{
    // hava a message
}

RegionNavigation

区别于View Discovery和View Injection，RegionNavigation可通过region名称与要导航的视图名称实现更通用的视图导航功能，使用如下：

// 模块类中注册导航视图
public void RegisterTypes(IContainerRegistry containerRegistry)
{
    containerRegistry.RegisterForNavigation<ViewA>();
}

// xaml导航命令
<Button Command="{Binding NavigateCommand}" CommandParameter="ViewA" >Navigate to View A</Button>

// viewmodel实现导航
public DelegateCommand<string> NavigateCommand { get; private set; }

public MainWindowViewModel(IRegionManager regionManager)
{
    _regionManager = regionManager;
    NavigateCommand = new DelegateCommand<string>(Navigate);
}

private void Navigate(string navigatePath)
{
    if (navigatePath != null)
        _regionManager.RequestNavigate("ContentRegion",
            navigatePath， NavigationCompleted);
}

// 可指定导航完成回调
private void NavigationCompleted(NavigationResult result)
{
    // ...
}

INavigationAware接口

INavigationAware接口包含三个方法：OnNavigatedFrom、OnNavigatedTo、IsNavigationTarge。当ViewAViewModel及ViewBViewModel均实现了INavigationAware接口，ViewA导航到ViewB时，先调用ViewA的OnNavigatedFrom方法，然后调用ViewB的IsNavigationTarge，当其返回true时，调用OnNavigatedTo方法，若IsNavigationTarge返回false，创建新ViewB。示例如下：

public class ViewAViewModel : BindableBase, INavigationAware
{
    public void OnNavigatedTo(NavigationContext navigationContext)
    {
        // ...
    }

    public bool IsNavigationTarget(NavigationContext navigationContext)
    {
        return true;
    }

    public void OnNavigatedFrom(NavigationContext navigationContext)
    {
        // ...
    }
}

IConfirmNavigationRequest接口

IConfirmNavigationRequest接口继承了INavigationAware接口，并添加了ConfirmNavigationRequest方法。若ViewAViewModel实现了IConfirmNavigationRequest接口，当ViewA导航到ViewB时，先调用ConfirmNavigationRequest方法，若continuationCallback()参数为true，将继续执行导航，执行OnNavigatedFrom方法；若continuationCallback()参数为false，停止导航。示例如下：

public class ViewAViewModel : BindableBase, IConfirmNavigationRequest
{
    public void ConfirmNavigationRequest(NavigationContext navigationContext,
        Action<bool> continuationCallback)
    {
        bool result = true;

        if (MessageBox.Show("Do you to navigate?", "Navigate?",
            MessageBoxButton.YesNo) == MessageBoxResult.No)
            result = false;

        continuationCallback(result);
    }

    public bool IsNavigationTarget(NavigationContext navigationContext)
    {
        return true;
    }

    public void OnNavigatedFrom(NavigationContext navigationContext)
    {
    }

    public void OnNavigatedTo(NavigationContext navigationContext)
    {
    }
}

IRegionMemberLifetime接口

IRegionMemberLifetime接口只包含一个KeepAlive只读属性。其默认值为true，若其为false，则当该region导航到其它视图时，实现了该接口的当前视图将从IRegion.Views集合中移除并回收。若为true，即使导航到其它视图，该视图依然存在于IRegion.Views集合。示例如下：

public class ViewAViewModel : BindableBase, IRegionMemberLifetime
{
    public bool KeepAlive
    {
        get
        {
            return false;
        }
    }
}

参数传递

可使用NavigationParameters实现导航时的参数传递，使用方法如下：

// 导航命令
private void PersonSelected(Person person)
{
    var parameters = new NavigationParameters();
    parameters.Add("person", person);

    if (person != null)
        _regionManager.RequestNavigate("PersonDetailsRegion",
            "PersonDetail", parameters);
}

// 参数处理
public void OnNavigatedTo(NavigationContext navigationContext)
{
    var person = navigationContext.Parameters["person"] as Person;
    // ...
}

public bool IsNavigationTarget(NavigationContext navigationContext)
{
    var person = navigationContext.Parameters["person"] as Person;
    // ...
}

Navigation Journal

Navigation Journal可以记录导航的过程，其通过IRegionNavigationJournal接口实现。通过Navigation Journal，可以实现向前/向后导航。示例如下：

// GoForward
public class PersonListViewModel : BindableBase, INavigationAware
{
    IRegionNavigationJournal _journal;
    public DelegateCommand GoForwardCommand { get; set; }

    public PersonListViewModel(IRegionManager regionManager)
    {
        ...
        GoForwardCommand = new DelegateCommand(GoForward, CanGoForward);
    }

    // IRegionNavigationJournal.GoBack到行至此
    public void OnNavigatedTo(NavigationContext navigationContext)
    {
        _journal = navigationContext.NavigationService.Journal;
        GoForwardCommand.RaiseCanExecuteChanged();
    }

    private void GoForward()
    {
        _journal.GoForward();
    }

    private bool CanGoForward()
    {
        return _journal != null && _journal.CanGoForward;
    }
}

// GoBack
public class PersonDetailViewModel : BindableBase, INavigationAware
{
    IRegionNavigationJournal _journal;
    public DelegateCommand GoBackCommand { get; set; }

    public PersonDetailViewModel()
    {
        GoBackCommand = new DelegateCommand(GoBack);
    }

    public void OnNavigatedTo(NavigationContext navigationContext)
    {
        _journal = navigationContext.NavigationService.Journal;
    }

    private void GoBack()
    {
        _journal.GoBack();
    }
}

InvokeCommandAction

Prism提供了InvokeCommandAction以使ViewModel处理View的事件，示例如下：

// view xaml
<ListBox ItemsSource="{Binding Items}" SelectionMode="Single">
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="SelectionChanged">
            <prism:InvokeCommandAction Command="{Binding SelectedCommand}"
                TriggerParameterPath="AddedItems" />
        </i:EventTrigger>
    </i:Interaction.Triggers>
</ListBox>

// viewmodel
public DelegateCommand<object[]> SelectedCommand { get; private set; }

public MainWindowViewModel()
{
    ...
    SelectedCommand = new DelegateCommand<object[]>(OnItemSelected);
}

private void OnItemSelected(object[] selectedItems)
{
    if (selectedItems != null && selectedItems.Count() > 0)
        SelectedItemText = selectedItems.FirstOrDefault().ToString();
}