使用 RelayCommand 时，ActivityIndicator 在某些情况下未运行

Question

我正在尝试实现 RelayCommand 的内置 IsRunning 属性，如 Julian 所描述，但由于某种原因，唯一有效的场景是使用“保存”按钮命令时。

这是我迄今为止尝试过的：

XAML 结构

<Grid>
    <VerticalStackLayout Padding="20" Spacing="20" VerticalOptions="Fill">
        <Picker Title="Pick Book" 
            ItemsSource="{Binding Books}"
            SelectedItem="{Binding SelectedBook}"
            ItemDisplayBinding="{Binding BookName}">
        </Picker>

        <Picker Title="Pick Chapter"
            ItemsSource="{Binding Chapters}"
            SelectedItem="{Binding SelectedChapter}"
            ItemDisplayBinding="{Binding ChapterName}" />

        <Button Text="Save" 
            Command="{Binding SaveButtonCommand}" />
    </VerticalStackLayout>

    <ActivityIndicator x:Name="booksActivity"
      IsVisible="True"
      IsRunning="{Binding BooksServiceCommand.IsRunning}"/>

    <ActivityIndicator x:Name="chaptersActivity"
      IsVisible="True"
      IsRunning="{Binding ChaptersServiceCommand.IsRunning}"/>

    <ActivityIndicator x:Name="buttonActivity"
      IsVisible="True"
      IsRunning="{Binding SaveButtonCommand.IsRunning}"/>
</Grid>

ViewModel 结构

public partial class BooksViewModel : ObservableObject
{
    public class Book { public int BookId { get; set; } public string BookName { get; set; } }
    public class Chapter { public string ChapterName { get; set; } }

    [ObservableProperty]
    private Book _selectedBook;

    [ObservableProperty]
    Chapter _selectedChapter;

    [ObservableProperty]
    ObservableCollection<Book> _books;

    [ObservableProperty]
    ObservableCollection<Chapter> _chapters;

从构造函数调用 BooksService 不会触发 ActivityIndicator 环

    public BooksViewModel()
    {
       Task.Run(() => BooksServiceAsync());
    }

    [RelayCommand]
    public async Task BooksServiceAsync()
    {
        // Run ActivityIndicator ring here
        await Task.Delay(TimeSpan.FromSeconds(5));
        Books = await Task.Run(() => FetchBooks());
    }

    private ObservableCollection<Book> FetchBooks()
    {
        return  [ 
            new Book { BookName = "Book1"}, 
            new Book { BookName = "Book2" }, 
            new Book { BookName = "Book3" }];
    }

OnSelectedBookChanged 不会触发 ActivityIndicator 环

    partial void OnSelectedBookChanged(Book value)
    {

        Task.Run(() => ChaptersServiceAsync(value.BookId));
    }

    [RelayCommand]
    public async Task ChaptersServiceAsync(int bookId)
    {
        // Run ActivityIndicator ring here
        await Task.Delay(TimeSpan.FromSeconds(5));
        Chapters = await Task.Run(() => FetchChapters());
    }

    private ObservableCollection<Chapter> FetchChapters()
    {
        return [
            new Chapter { ChapterName = "Chapter1"},
            new Chapter { ChapterName = "Chapter2" },
            new Chapter { ChapterName = "Chapter3" }];
    }

    

按钮场景正在工作并成功触发 ActivityIndicator 响铃

    [RelayCommand]
    public async Task SaveButton()
    {
        // Run ActivityIndicator ring here
        await Task.Delay(TimeSpan.FromSeconds(5));
    }
}

这里可能出了什么问题？

Answer 1

我正在尝试实现 Julian 所描述的 RelayCommand 的内置 IsRunning 属性，但由于某种原因，唯一有效的方案是使用“保存”按钮命令时。

我测试了您提供的代码并重现了问题。

BooksService 和 ChaptersServiceAsync 没有触发 ActivityIndicator 环的原因是您没有使用

BooksServiceCommand

或

ChaptersServiceAsyncCommand

。当你在构造函数中输入“BooksSer”时，IDE的智能关联将显示

BooksServiceCommand

：

这也说明Button Scenario正在工作并成功触发了ActivityIndicator环。

所以，如果添加一个按钮来绑定（使用）

BooksServiceCommand

，就可以触发ActivityIndicator环。

<VerticalStackLayout Padding="20" Spacing="20">
    <Picker Title="Pick Book"
    ItemsSource="{Binding Books}"
    SelectedItem="{Binding SelectedBook}"
    ItemDisplayBinding="{Binding BookName}">
    </Picker>

   <Picker Title="Pick Chapter"
    ItemsSource="{Binding Chapters}"
    SelectedItem="{Binding SelectedChapter}"
    ItemDisplayBinding="{Binding ChapterName}" />

   <Button Text="Save"
    Command="{Binding SaveButtonCommand}" />

    <Button Text="LoadBooks"
    Command="{Binding BooksServiceCommand}" />
</VerticalStackLayout>