问题描述
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;
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。