HeaderTemplate 绑定在 Xamarin 表单中引发异常

Question

我有一个 xaml 的列表视图，如下所示

<?xml version="1.0" encoding="utf-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:myControls="clr-namespace:MyControls;assembly=App.Visuals"
         x:Class="SearchResultPage">
    <ContentPage.Resources>
        <ResourceDictionary>
            <myControls:TravelCardItemTemplateSelector x:Key="TravelResultTemplate" />
            <myControls:TravelSearchHeaderTemplateSelector x:Key="HeaderTemplateSelector" />
        </ResourceDictionary>
    </ContentPage.Resources>
    <ContentPage.Content>        
            <ListView Grid.Row ="2" 
                      CachingStrategy="RecycleElement"
                      IsRefreshing="{Binding IsBusy}" 
                      IsPullToRefreshEnabled="true" 
                      RefreshCommand="{Binding RefreshCommand}"
                      ItemsSource="{Binding SearchResults}" 
                      Style="{DynamicResource ListStyle}"
                      HasUnevenRows="True" 
                      ItemTemplate="{StaticResource TravelResultTemplate}"
                      HeaderTemplate="{StaticResource HeaderTemplateSelector}"/>
    </ContentPage.Content>
</ContentPage>

模板选择器如下所示

public class TravelSearchHeaderTemplateSelector : DataTemplateSelector
{
    protected override DataTemplate OnSelectTemplate(object item, BindableObject container)
    {
        if (item== null)
        {
            return null;
        }

        var headerTempleTemplate= new DataTemplate(typeof(TransportSearchHeaderPanelCell));
        return headerTempleTemplate;
    }
}

模板的 xaml 是一个视图单元格，如下所示

<?xml version="1.0" encoding="UTF-8"?>
<ViewCell xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="MyControls.TransportSearchHeaderPanelCell">
<ViewCell.View> <Grid ColumnSpacing="0" RowSpacing="0"> <Grid.ColumnDefinitions> <ColumnDefinition Width="*" /> <ColumnDefinition Width="*" /><ColumnDefinition Width="*" /></Grid.ColumnDefinitions>
        <Button Text="Price" Grid.Column="0" />
        <Button Text="Departure" Grid.Column="1" BackgroundColor="Transparent"/>
        <Button Text="Fastest" Grid.Column="2" BackgroundColor="Transparent"/>
    </Grid>
</ViewCell.View>
</ViewCell>

在运行时，应用程序抛出异常

{System.InvalidOperationException: LoadTemplate should not be null
  at Xamarin.Forms.ElementTemplate.CreateContent () [0x00008] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\ElementTemplate.cs:74 
  at Xamarin.Forms.ListView.ValidateHeaderFooterTemplate (Xamarin.Forms.BindableObject bindable, System.Object value) [0x0000b] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\ListView.cs:587 
  at Xamarin.Forms.BindableObject.SetValueCore (Xamarin.Forms.BindableProperty property, System.Object value, Xamarin.Forms.Internals.SetValueFlags attributes, Xamarin.Forms.BindableObject+SetValuePrivateFlags privateAttributes) [0x00071] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\BindableObject.cs:359 
  at Xamarin.Forms.BindableObject.SetValue (Xamarin.Forms.BindableProperty property, System.Object value, System.Boolean fromStyle, System.Boolean checkAccess) [0x0005f] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\BindableObject.cs:542 
  at Xamarin.Forms.BindableObject.SetValue (Xamarin.Forms.BindableProperty property, System.Object value) [0x00000] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\BindableObject.cs:83 
  at Xamarin.Forms.ListView.set_HeaderTemplate (Xamarin.Forms.DataTemplate value) [0x00000] in C:\BuildAgent3\work\ca3766cfc22354a1\Xamarin.Forms.Core\ListView.cs:164 }

绑定应该很简单，为什么会出现异常。

Answer 1

搜索我遇到的错误消息 this bug report，它似乎已经是一个相当老的错误了。由于您已从代码隐藏中验证它确实有效，因此该错误似乎仍然存在。

现在唯一的方法是从代码隐藏中解决它并等待他们修复它。或者，如果你足够勇敢；自己修复它，因为 Xamarin.Forms 是开源的。

HeaderTemplate 绑定在 Xamarin 表单中引发异常

HeaderTemplate binding throws an exception in Xamarin forms

c#

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