Gav*_*ard 3 .net c# events mvvm maui
我正在尝试将 ListView 的 ItemSelected 绑定到视图模型，但遇到了一些问题（由于我自己对其工作原理的误解）。
我有观点：
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:Local="clr-namespace:FireLearn.ViewModels"
x:Class="FireLearn.MainPage"
Title="Categories">
<ContentPage.BindingContext>
<Local:CategoryViewModel/>
</ContentPage.BindingContext>
<NavigationPage.TitleView>
<Label Text="Home"/>
</NavigationPage.TitleView>
<ListView
ItemsSource="{Binding Categories}"
HasUnevenRows="True"
IsPullToRefreshEnabled="True"
IsRefreshing="{Binding ListRefreshing, Mode=OneWay}"
RefreshCommand="{Binding RefreshCommand}"
ItemSelected="{Binding OnItemTappedChanged}"
SelectionMode="Single"
SelectedItem="{Binding SelectedCategory}">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<HorizontalStackLayout
Padding="8"
VerticalOptions="Fill"
HorizontalOptions="Fill">
<Image Source="cafs_bubbles.png"
HeightRequest="64"
MaximumWidthRequest="64"
HorizontalOptions="CenterAndExpand"
VerticalOptions="CenterAndExpand"/>
<VerticalStackLayout
Padding="8"
VerticalOptions="FillAndExpand"
HorizontalOptions="FillAndExpand">
<Label Text="{Binding FormattedName}"
SemanticProperties.HeadingLevel="Level1"
FontSize="Title"
HorizontalOptions="Start"/>
<Label Text="{Binding ItemCount}"
FontSize="Subtitle"/>
<Label Text="{Binding Description}"
HorizontalOptions="Center"
LineBreakMode="WordWrap"
FontSize="Caption"
VerticalOptions="CenterAndExpand"
MaxLines="0"/>
</VerticalStackLayout>
</HorizontalStackLayout>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</ContentPage>
这链接到视图模型：
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Diagnostics;
using System.Reflection.Emit;
using System.Runtime.CompilerServices;
using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;
using FireLearn.Models;
namespace FireLearn.ViewModels
{
public partial class CategoryViewModel : ObservableObject
{
public ObservableCollection<CategoryModel> categories = new ObservableCollection<CategoryModel>();
public ObservableCollection<CategoryModel> Categories
{
get => categories;
set => SetProperty(ref categories, value);
}
public bool listRefreshing = false;
public bool ListRefreshing
{
get => listRefreshing;
set => SetProperty(ref listRefreshing, value);
}
public CategoryModel selectedCategory = new CategoryModel();
public CategoryModel SelectedCategory
{
get => selectedCategory;
set
{
SetProperty(ref selectedCategory, value);
// Tap(value);
}
}
public RelayCommand RefreshCommand { get; set; }
//public RelayCommand TapCellCommand { get; set; }
public CategoryViewModel()
{
loadFromSource();
RefreshCommand = new RelayCommand(async () =>
{
Debug.WriteLine($"STARTED::{ListRefreshing}");
if (!ListRefreshing)
{
ListRefreshing = true;
try
{
await loadFromSource();
}
finally
{
ListRefreshing = false;
Debug.WriteLine($"DONE::{ListRefreshing}");
}
}
});
}
public async Task loadFromSource()
{
HttpClient httpClient = new()
{
Timeout = new TimeSpan(0, 0, 10)
};
Uri uri = new Uri("https://somewebsite.co.uk/wp-json/wp/v2/categories");
HttpResponseMessage msg = await httpClient.GetAsync(uri);
if (msg.IsSuccessStatusCode)
{
var result = CategoryModel.FromJson(await msg.Content.ReadAsStringAsync());
Categories = new ObservableCollection<CategoryModel>(result);
}
Debug.WriteLine("List Refreshed");
}
public void OnItemTappedChanged(System.Object sender, Microsoft.Maui.Controls.SelectedItemChangedEventArgs e)
{
var x = new ShellNavigationState();
Shell.Current.GoToAsync(nameof(NewPage1),
new Dictionary<string, object>
{
{
nameof(NewPage1),
SelectedCategory
}
});
}
}
}
我收到编译器错误“没有找到“ItemSelected”的属性、BindableProperty 或事件，或者值和属性之间的类型不匹配”，并且我真的不确定如何解决。如果我让 XAML 为我创建一个新事件，它会将其添加到 MainPage.Xaml.Cs 而不是 VM 中
ItemSelected需要一个事件处理程序，该事件处理程序通常只存在于视图的代码后面。由于 ViewModel 不应该了解有关 View 的任何信息，因此最好不要混合概念。您有几个选项可以在不破坏MVVM 模式的情况下解决这个问题。
首先，通过构造函数传入 ViewModel 来设置背后的代码，并添加事件处理程序，例如：
public partial class MainPage : ContentPage
{
private CategoryViewModel _viewModel;
public MainPage(CategoryViewModel viewModel)
{
_viewModel = viewModel;
}
public void OnItemSelectedChanged(object sender, SelectedItemChangedEventArgs e)
{
//call a method from the ViewModel, e.g.
_viewModel.DoSomething(e.SelectedItem);
}
//...
}
然后使用 XAML 中的事件处理程序：
<ListView
ItemsSource="{Binding Categories}"
HasUnevenRows="True"
IsPullToRefreshEnabled="True"
IsRefreshing="{Binding ListRefreshing, Mode=OneWay}"
RefreshCommand="{Binding RefreshCommand}"
ItemSelected="OnItemSelectedChanged"
SelectionMode="Single"
SelectedItem="{Binding SelectedCategory}">
<!-- skipping irrelevant stuff -->
</ListView>
请注意，这不使用绑定。
然后，您可以在您的代码中CategoryViewModel定义一个方法，该方法将所选项目作为参数：
public partial class CategoryViewModel : ObservableObject
{
//...
public void DoSomething(object item)
{
//do something with the item, e.g. cast it to Category
}
}
您还可以使用MAUI 社区工具包中的EventToCommandBehavior，而不是从后面的代码处理 ViewModel 方法的调用：
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:Local="clr-namespace:FireLearn.ViewModels"
xmlns:toolkit="http://schemas.microsoft.com/dotnet/2022/maui/toolkit"
x:Class="FireLearn.MainPage"
Title="Categories">
<ContentPage.Resources>
<ResourceDictionary>
<toolkit:SelectedItemEventArgsConverter x:Key="SelectedItemEventArgsConverter" />
</ResourceDictionary>
</ContentPage.Resources>
<ListView
ItemsSource="{Binding Categories}"
HasUnevenRows="True"
IsPullToRefreshEnabled="True"
IsRefreshing="{Binding ListRefreshing, Mode=OneWay}"
RefreshCommand="{Binding RefreshCommand}"
SelectionMode="Single"
SelectedItem="{Binding SelectedCategory}">
<ListView.Behaviors>
<toolkit:EventToCommandBehavior
EventName="ItemSelected"
Command="{Binding ItemSelectedCommand}"
EventArgsConverter="{StaticResource SelectedItemEventArgsConverter}" />
</ListView.Behaviors>
<!-- skipping irrelevant stuff -->
</ListView>
</ContentPage>
然后，在您的 ViewModel 中，您可以定义ItemSelectedCommand：
public partial class CategoryViewModel : ObservableObject
{
[RelayCommand]
private void ItemSelected(object item)
{
//do something with the item, e.g. cast it to Category
}
// ...
}
这是执行此操作的首选方法。选项 1 只是另一种可能性，但它EventToCommandBehavior是更好的选择。
请注意，这是一个使用 MVVM 源生成器的示例（因为您已经在使用 MVVM 社区工具包）。完整的Command通常会这样实现：
public partial class CategoryViewModel : ObservableObject
{
private IRelayCommand<object> _itemSelectedCommand;
public IRelayCommand<object> ItemSelectedCommand => _itemSelectedCommand ?? (_itemSelectedCommand = new RelayCommand<object>(ItemSelected));
private void ItemSelected(object item)
{
//do something with the item, e.g. cast it to Category
}
// ...
}