Xamarin - 带有构造函数的 ViewModel，其参数作为 Xaml 中的绑定上下文答案

【问题标题】：Xamarin - ViewModel with constructor with parameters as Binding Context in XamlXamarin - 带有构造函数的 ViewModel，其参数作为 Xaml 中的绑定上下文
【发布时间】：2019-12-20 01:54:36
【问题描述】：

我有一个简单的问题。我正在尝试在 xamarin 中创建我的跨平台应用程序，这是场景。

我有一个页面，上面有住宿清单。我的想法是，当我点击一个住宿时，它会打开一个包含更多详细信息的新页面。此事件涵盖了它：

private void ListView_ItemTapped(object sender, ItemTappedEventArgs e)
{
    var Odabrani = e.Item;

    PropertyInfo pi = Odabrani.GetType().GetProperty("SmjestajId");
    int id = Convert.ToInt32(pi.GetValue(Odabrani,null).ToString());
    Application.Current.MainPage = new SmjestajViewPage(id);
}

SmjestajViewPage 将是打开的页面，显然我需要该住宿的 ID 来获取有关它的相关信息。这很好用，我得到了正确的 ID。

这是我目前所拥有的 xaml.cs 文件：

[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class SmjestajViewPage : ContentPage
{
    private int _Id;
    SmjestajVM viewmodel = null;
    public SmjestajViewPage()
    {
        InitializeComponent();
    }

    public SmjestajViewPage(int id)
    {
        InitializeComponent();
        _Id = id;
        BindingContext = viewmodel = new SmjestajVM(id);           
    }
}

这是 SmjestajVM 到目前为止的样子：

public class SmjestajVM : BaseViewModel
{
    int _SmjestajId;
    public SmjestajVM(int Id)
    {
        _SmjestajId = Id;
    }

}

但是，除非我添加无参数构造函数，否则此选项不会作为 xaml 编辑过程中的选项之一出现。到目前为止，这就是我的 xaml 文件的样子：

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:d="http://xamarin.com/schemas/2014/forms/design"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:ViewModels="clr-namespace:RS2_Booking.MobileApp.ViewModels"
             mc:Ignorable="d"
             x:Class="RS2_Booking.MobileApp.Views.SmjestajViewPage"
            >
    <ContentPage.BindingContext>
        <ViewModels:SmjestajVM></ViewModels:SmjestajVM>
    </ContentPage.BindingContext>
    <ContentPage.Content>
        <StackLayout>
            <Label Text="Welcome to Xamarin.Forms!"
                VerticalOptions="CenterAndExpand" 
                HorizontalOptions="CenterAndExpand" />
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

问题在于这一行：

  <ContentPage.BindingContext>
        <ViewModels:SmjestajVM></ViewModels:SmjestajVM>
    </ContentPage.BindingContext>

如何编写它才能正常工作？我尝试了 SmjestajVM(Id) 和 SmjestajVM("Id") 但都没有成功。

【问题讨论】：

你不能在 XAML 中用 vanilla XF 做到这一点。您要么需要在代码隐藏中完成，要么使用 MVVM 框架
你能解释一下你所说的代码隐藏是什么意思吗？
xaml.cs 文件

标签： xaml xamarin viewmodel

【解决方案1】：

我认为你不能在xaml 中做到这一点，Xamarin.forms 中没有这样的语法。到目前为止，您所做的是正确的。

或者你可以在xaml中设置BindingContext：

  <ContentPage.BindingContext>
        <ViewModels:SmjestajVM></ViewModels:SmjestajVM>
  </ContentPage.BindingContext>

然后可以在后面的代码中获取bindingContext并设置id：

public partial class MainPage : ContentPage
{

    private int _Id;
    public MainPage()
    {
        InitializeComponent();

        SmjestajVM m = this.BindingContext as SmjestajVM;
        m.SmjestajId = _Id;
    }
}


public class SmjestajVM
{
    public int _SmjestajId { get; set; }

    public int SmjestajId
    {
        set
        {
            _SmjestajId = value;

            updateViewModel();
        }
        get
        {
            return _SmjestajId;
        }
    }

    public SmjestajVM()
    {
    }

    public void updateViewModel(){

    }
}

您也可以查看this thread 了解更多信息。

【讨论】：