Xamarin Prism 无法使用参数导航

Question

对于我的应用程序，我使用框架创建自己的按钮并向其添加轻触手势。在这里，我使用 prism 的导航到带有参数的特定页面。然而。我要使用的视图模型不会触发导航到方法。这是一些代码。

在调试过程中，添加参数似乎没有问题。然而，视图模型的构造函数被调用。

按钮

public class FolderButton : Frame
{
    public FolderButton(Folder folder, INavigationService navigationService)
    {
        var navParams = new NavigationParameters();
        navParams.Add("folder", folder);
        GestureRecognizers.Add(new TapGestureRecognizer()
        {
            Command = new Command(async () => { await navigationService.NavigateAsync("FolderInventory", navParams); }),
        });

        BackgroundColor = Color.CornflowerBlue;
        var thickness = new Thickness();
        thickness.Bottom = 10;
        thickness.Left = 10;
        thickness.Right = 10;
        thickness.Top = 10;
        Margin = thickness;
        CornerRadius = 5;

        var completeStack = new StackLayout();
        var imgStack = new StackLayout();
        imgStack.Padding = thickness;
        imgStack.Children.Add(new Image { Source = "folder.png" });
        completeStack.Children.Add(imgStack);
        var lblStack = new StackLayout();
        lblStack.Padding = thickness;
        lblStack.Children.Add(new Label
        {
            Text = folder.Name,
            HorizontalTextAlignment = TextAlignment.Center,
            VerticalTextAlignment = TextAlignment.Start
        });
        completeStack.Children.Add(lblStack);

        Content = completeStack;
    }
}

称为视图模型

public class FolderInventoryViewModel : BindableBase, INavigatedAware
{
    public Folder Folder => _folder;

    private readonly INavigationService _navigationService;
    private Folder _folder;
    private readonly ISQLiteService _sqlService;
    private List<Frame> _buttons;

    public List<Frame> Buttons
    {
        get => _buttons;
        set => _buttons = value;
    }

    public FolderInventoryViewModel(Folder folder, INavigationService navigationService, ISQLiteService sqlService)
    {
        _folder = folder;
        _sqlService = sqlService;
        _navigationService = navigationService;
        GetItemsForFolder();
    }

    private void GetItemsForFolder()
    {
        var itemList = _sqlService.GetAllFolderItems(Folder.Name);

        foreach (var item in itemList)
        {
            var itemButton = new ItemButton(_navigationService, item);
            _buttons.Add(itemButton);
        }
    }

    public void OnNavigatedFrom(NavigationParameters parameters)
    {
        if (parameters["folder"] is Folder folder)
        {
            _folder = folder;
        }

    }

    public void OnNavigatedTo(NavigationParameters parameters)
    {
        if (parameters["folder"] is Folder folder)
        {
            _folder = folder;
        }
    }
}

Answer 1

这不是使用框架的本质。要正确使用带有NavigationParameters 的 Prism，您首先要正确维护其背后的 MVVM 理念。

例如

<Button Command="{Binding testCommand}" text="TestButton"/>

您的 ViewModel（请原谅，您需要将 NavigationService 注入 ViewModel 的构造函数）

private DelegateCommand _testCommand;
public DelegateCommand testCommand =>
        _testCommand?? (_testCommand= new DelegateCommand(ExecuteTest));
private void ExecuteTest()
{
    NavigationParameters navigationParameters = new NavigationParameters();
    navigationParameters.Add("yourParameterId", value);
    NavigationService.NavigateAsync("YourPage", navigationParameters);
}

然后进入下一页

将 INavigationAware 继承到您的 NextPage：YourNextPage: BaseViewModel, INavigationAware

INavigationAware 有 3 个方法 NavigatingTo、NavigatedTo、NavigatedFrom

在 OnNavigatedTo 里面可以调用你传递的参数

public void OnNavigatedTo(NavigationParameters parameters)
{
    //You can check if parameters has value before passing it inside

    if(parameters.ContainsKey("YourParameterId")
    {
         yourItem = (dataType)parameters[YourParameterId];
    }
}

另请注意：构造函数总是在导航方法之前被调用