如何将 ListView 的/CollectionView 的 DataTemplate 绑定到 ViewModel 属性

分类：编程问答

如何解决如何将 ListView 的/CollectionView 的 DataTemplate 绑定到 ViewModel 属性

我正在构建供内部使用的 Xamarin Forms 控件库，并且我试图将 DataTemplate 定义为可绑定属性，该属性通过 ViewModel 传递并设置为 CollectionView。当我这样做时，我在 Xamarin Forms 非托管代码的深处得到了一个 NRE，我没有得到任何关于在捕获的异常中导致 NRE 的细节。所以这让我想知道这是否有可能做到。我在网上找到的所有示例都直接设置了 ItemTemplate，从来都不是绑定属性。那么，这是可行的吗？

这是 Xamarin.Forms 代码中发生异常的地方

        // Set the binding context _before_ we create the renderer; that way,it's available during OnElementChanged
        view.BindingContext = bindingContext;

该属性似乎是一个可绑定的属性，所以我认为这应该没有问题。

public static readonly BindableProperty ItemTemplateProperty;

集合视图...

            <CollectionView Grid.Row="3"
                        Margin="16,16,16"
                        ItemTemplate="{Binding ItemTemplate}" <--- this is what I am trying to make work
                        ItemsSource="{Binding Items}"
                        SelectedItem="{Binding SelectedItem}"
                        SelectionChangedCommand="{Binding SelectedItemChanged}"
                        SelectionMode="Single"
                        VerticalOptions="Center"
                        VerticalScrollBarVisibility="Always" />

查看模型...

    private DataTemplate _itemTemplate;
    public DataTemplate ItemTemplate
    {
        get
        {
            return _itemTemplate;
        }
        set
        {
            SetProperty(ref _itemTemplate,value);
        }
    }

    ...

    ItemTemplate = new DataTemplate(() =>
        {
            var grid = new Grid();

            var nameLabel = new Label { FontAttributes = FontAttributes.Bold };
            var ageLabel = new Label();
            var locationLabel = new Label { HorizontalTextAlignment = TextAlignment.End };

            nameLabel.SetBinding(Label.TextProperty,"Title");
            ageLabel.SetBinding(Label.TextProperty,"Sybtitle");
            locationLabel.SetBinding(Label.TextProperty,"Title");

            grid.Children.Add(nameLabel);
            grid.Children.Add(ageLabel,1,0);
            grid.Children.Add(locationLabel,2,0);

            return new ViewCell { View = grid };

        });

列表项模型...

    public class ListPickerListItem {

    public ListPickerListItem(string title,string subtitle = "",string icon = "",string image = "")
    {
        Title = title;
        Subtitle = subtitle;
        Icon = icon;
        Image = image;
    }

    public string Title { get; set; }
    public string Subtitle { get; set; }
    public string Icon { get; set; }
    public string Image { get; set; }

    // prioritize the Image over the Icon since the Image has a default placeholder
    public bool IsIconVisible { get { return string.IsNullOrEmpty(Icon) == false && string.IsNullOrEmpty(Image); } }
    public bool IsImageVisible { get { return string.IsNullOrEmpty(Image) == false; } }

}

异常堆栈...

at Xamarin.Forms.Platform.iOS.TemplatedCell.Bind (Xamarin.Forms.DataTemplate template,System.Object bindingContext,Xamarin.Forms.ItemsView itemsView) [0x00068] in D:\a\1\s\Xamarin.Forms.Platform.iOS\CollectionView\TemplatedCell.cs:117 
  at Xamarin.Forms.Platform.iOS.ItemsViewController`1[TItemsView].UpdateTemplatedCell (Xamarin.Forms.Platform.iOS.TemplatedCell cell,Foundation.NSIndexPath indexPath) [0x0007b] in D:\a\1\s\Xamarin.Forms.Platform.iOS\CollectionView\ItemsViewController.cs:262 
  at Xamarin.Forms.Platform.iOS.ItemsViewController`1[TItemsView].CreateMeasurementCell (Foundation.NSIndexPath indexPath) [0x0007b] in D:\a\1\s\Xamarin.Forms.Platform.iOS\CollectionView\ItemsViewController.cs:587 
  at Xamarin.Forms.Platform.iOS.ItemsViewController`1[TItemsView].GetPrototype () [0x0005d] in D:\a\1\s\Xamarin.Forms.Platform.iOS\CollectionView\ItemsViewController.cs:362 
  at Xamarin.Forms.Platform.iOS.ItemsViewLayout.DetermineCellSize () [0x00060] in D:\a\1\s\Xamarin.Forms.Platform.iOS\CollectionView\ItemsViewLayout.cs:211 
  at Xamarin.Forms.Platform.iOS.ListViewLayout.ConstrainTo (CoreGraphics.CGSize size) [0x0001e] in D:\a\1\s\Xamarin.Forms.Platform.iOS\CollectionView\ListViewLayout.cs:16 
  at Xamarin.Forms.Platform.iOS.ItemsViewLayout.SetInitialConstraints (CoreGraphics.CGSize size) [0x00007] in D:\a\1\s\Xamarin.Forms.Platform.iOS\CollectionView\ItemsViewLayout.cs:111 
  at Xamarin.Forms.Platform.iOS.ItemsViewController`1[TItemsView].EnsureLayoutInitialized () [0x00044] in D:\a\1\s\Xamarin.Forms.Platform.iOS\CollectionView\ItemsViewController.cs:192 
  at Xamarin.Forms.Platform.iOS.ItemsViewController`1[TItemsView].ViewDidLoad () [0x00035] in D:\a\1\s\Xamarin.Forms.Platform.iOS\CollectionView\ItemsViewController.cs:148 
  at (wrapper managed-to-native) ObjCRuntime.Messaging.IntPtr_objc_msgSendSuper(intptr,intptr)
  at UIKit.UIViewController.get_View () [0x0002a] in /Users/builder/azdo/_work/1/s/xamarin-macios/src/build/ios/native/UIKit/UIViewController.g.cs:3265 
  at Xamarin.Forms.Platform.iOS.ItemsViewRenderer`2[TItemsView,TViewController].SetUpNewElement (TItemsView newElement) [0x00022] in D:\a\1\s\Xamarin.Forms.Platform.iOS\CollectionView\ItemsViewRenderer.cs:102 
  at Xamarin.Forms.Platform.iOS.StructuredItemsViewRenderer`2[TItemsView,TViewController].SetUpNewElement (TItemsView newElement) [0x00000] in D:\a\1\s\Xamarin.Forms.Platform.iOS\CollectionView\StructuredItemsViewRenderer.cs:41 
  at Xamarin.Forms.Platform.iOS.SelectableItemsViewRenderer`2[TItemsView,TViewController].SetUpNewElement (TItemsView newElement) [0x00000] in D:\a\1\s\Xamarin.Forms.Platform.iOS\CollectionView\SelectableItemsViewRenderer.cs:34 
  at Xamarin.Forms.Platform.iOS.ItemsViewRenderer`2[TItemsView,TViewController].OnElementChanged (Xamarin.Forms.Platform.iOS.ElementChangedEventArgs`1[TElement] e) [0x0000c] in D:\a\1\s\Xamarin.Forms.Platform.iOS\CollectionView\ItemsViewRenderer.cs:36 
  at Xamarin.Forms.Platform.iOS.VisualElementRenderer`1[TElement].SetElement (TElement element) [0x00172] in D:\a\1\s\Xamarin.Forms.Platform.iOS\VisualElementRenderer.cs:296 
  at Xamarin.Forms.Platform.iOS.VisualElementRenderer`1[TElement].Xamarin.Forms.Platform.iOS.IVisualElementRenderer.SetElement (Xamarin.Forms.VisualElement element) [0x00000] in D:\a\1\s\Xamarin.Forms.Platform.iOS\VisualElementRenderer.cs:158 
  at Xamarin.Forms.Platform.iOS.Platform.CreateRenderer (Xamarin.Forms.VisualElement element) [0x00032] in D:\a\1\s\Xamarin.Forms.Platform.iOS\Platform.cs:240 
  at Xamarin.Forms.Platform.iOS.VisualElementPackager.OnChildAdded (Xamarin.Forms.VisualElement view) [0x0003e] in D:\a\1\s\Xamarin.Forms.Platform.iOS\VisualElementPackager.cs:115 
  at Xamarin.Forms.Platform.iOS.VisualElementPackager.Load () [0x0001e] in D:\a\1\s\Xamarin.Forms.Platform.iOS\VisualElementPackager.cs:49 
  at Xamarin.Forms.Platform.iOS.VisualElementRenderer`1[TElement].SetElement (TElement element) [0x0012e] in D:\a\1\s\Xamarin.Forms.Platform.iOS\VisualElementRenderer.cs:283 
  at Xamarin.Forms.Platform.iOS.VisualElementRenderer`1[TElement].Xamarin.Forms.Platform.iOS.IVisualElementRenderer.SetElement (Xamarin.Forms.VisualElement element) [0x00000] in D:\a\1\s\Xamarin.Forms.Platform.iOS\VisualElementRenderer.cs:158 
  at Xamarin.Forms.Platform.iOS.Platform.CreateRenderer (Xamarin.Forms.VisualElement element) [0x00032] in D:\a\1\s\Xamarin.Forms.Platform.iOS\Platform.cs:240 
  at Xamarin.Forms.Platform.iOS.VisualElementPackager.OnChildAdded (Xamarin.Forms.VisualElement view) [0x0003e] in D:\a\1\s\Xamarin.Forms.Platform.iOS\VisualElementPackager.cs:115 
  at Xamarin.Forms.Platform.iOS.VisualElementPackager.Load () [0x0001e] in D:\a\1\s\Xamarin.Forms.Platform.iOS\VisualElementPackager.cs:49 
  at Xamarin.Forms.Platform.iOS.VisualElementRenderer`1[TElement].SetElement (TElement element) [0x0012e] in D:\a\1\s\Xamarin.Forms.Platform.iOS\VisualElementRenderer.cs:283 
  at Xamarin.Forms.Platform.iOS.VisualElementRenderer`1[TElement].Xamarin.Forms.Platform.iOS.IVisualElementRenderer.SetElement (Xamarin.Forms.VisualElement element) [0x00000] in D:\a\1\s\Xamarin.Forms.Platform.iOS\VisualElementRenderer.cs:158 
  at Xamarin.Forms.Platform.iOS.Platform.CreateRenderer (Xamarin.Forms.VisualElement element) [0x00032] in D:\a\1\s\Xamarin.Forms.Platform.iOS\Platform.cs:240 
  at Xamarin.Forms.Platform.iOS.VisualElementPackager.OnChildAdded (Xamarin.Forms.VisualElement view) [0x0003e] in D:\a\1\s\Xamarin.Forms.Platform.iOS\VisualElementPackager.cs:115 
  at Xamarin.Forms.Platform.iOS.VisualElementPackager.Load () [0x0001e] in D:\a\1\s\Xamarin.Forms.Platform.iOS\VisualElementPackager.cs:49 
  at Xamarin.Forms.Platform.iOS.PageRenderer.ViewDidLoad () [0x0008f] in D:\a\1\s\Xamarin.Forms.Platform.iOS\Renderers\PageRenderer.cs:251 
  at Rg.Plugins.Popup.IOS.Renderers.PopupPageRenderer.ViewDidLoad () [0x00000] in Z:\Documents\OpenSource\Rg.Plugins.Popup\Rg.Plugins.Popup\Platforms\Ios\Renderers\PopupPageRenderer.cs:74 
  at (wrapper managed-to-native) ObjCRuntime.Messaging.IntPtr_objc_msgSendSuper(intptr,intptr)
  at UIKit.UIViewController.get_View () [0x0002a] in /Users/builder/azdo/_work/1/s/xamarin-macios/src/build/ios/native/UIKit/UIViewController.g.cs:3265 
  at Xamarin.Forms.Platform.iOS.PageRenderer.get_NativeView () [0x00000] in D:\a\1\s\Xamarin.Forms.Platform.iOS\Renderers\PageRenderer.cs:103 
  at Xamarin.Forms.Platform.iOS.PageRenderer.SetElement (Xamarin.Forms.VisualElement element) [0x0003d] in D:\a\1\s\Xamarin.Forms.Platform.iOS\Renderers\PageRenderer.cs:119 
  at Xamarin.Forms.Platform.iOS.Platform.CreateRenderer (Xamarin.Forms.VisualElement element) [0x00032] in D:\a\1\s\Xamarin.Forms.Platform.iOS\Platform.cs:240 
  at Rg.Plugins.Popup.IOS.Extensions.PlatformExtension.GetOrCreateRenderer (Xamarin.Forms.VisualElement bindable) [0x0000a] in Z:\Documents\OpenSource\Rg.Plugins.Popup\Rg.Plugins.Popup\Platforms\Ios\Extensions\PlatformExtension.cs:21 
  at Rg.Plugins.Popup.IOS.Impl.PopupPlatformIos.AddAsync (Rg.Plugins.Popup.Pages.PopupPage page) [0x00052] in Z:\Documents\OpenSource\Rg.Plugins.Popup\Rg.Plugins.Popup\Platforms\Ios\Impl\PopupPlatformIos.cs:55 
  at Rg.Plugins.Popup.Services.PopupNavigationImpl.AddAsync (Rg.Plugins.Popup.Pages.PopupPage page) [0x00000] in Z:\Documents\OpenSource\Rg.Plugins.Popup\Rg.Plugins.Popup\Services\PopupNavigationImpl.cs:175 
  at Rg.Plugins.Popup.Services.PopupNavigationImpl+<>c__DisplayClass20_0.<PushAsync>b__0 () [0x00047] in Z:\Documents\OpenSource\Rg.Plugins.Popup\Rg.Plugins.Popup\Services\PopupNavigationImpl.cs:73

解决方法

感谢@FabriBertani 为the link 指出了异常的原因。 “使用 CollectionView 时，切勿将 DataTemplate 对象的根元素设置为 ViewCell。这将导致抛出异常，因为 CollectionView 没有单元格的概念。”将模板的外部视图设为 Grid 而不是不受支持的 Cell 可以解决问题。

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