我对ListView的DataBinding有一点问题。
因为我想要一个包含MultiSelection的Listview,我需要实现一个名为 GenericSelectableItem 的自定义类,它存储数据,如果单元格 IsSelected
首先,这是MainPage的视图模型:
public class MainPageViewModel : BaseViewModel, INotifyPropertyChanged
{
private ObservableCollection<GenericSelectableItem<AudioFile>> _audiofiles = new ObservableCollection<GenericSelectableItem<AudioFile>>();
public ObservableCollection<GenericSelectableItem<AudioFile>> AudioFiles
{
get => _audiofiles ?? new ObservableCollection<GenericSelectableItem<AudioFile>>();
set
{
_audiofiles = value;
PropertyChanged(this, new PropertyChangedEventArgs(nameof(AudioFiles)));
}
}
}
MainPage的Xaml:
<!-- The Content -->
<ListView x:Name="listView" Grid.Row="1" HasUnevenRows="true" RowHeight="-1" ItemsSource="{Binding AudioFiles}" ItemSelected="ListView_OnItemSelected">
<ListView.ItemTemplate>
<DataTemplate>
<local:AudioViewCell Audiofile="{Binding Data}"/>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
2个用于制作多选ListView的助手类:
public class GenericSelectableItem<T> : SelectableItem
{
public GenericSelectableItem(T data)
: base(data)
{
}
public GenericSelectableItem(T data, bool isSelected)
: base(data, isSelected)
{
}
// this is safe as we are just returning the base value
public new T Data
{
get => (T)base.Data;
set => base.Data = value;
}
}
public class SelectableItem : BindableObject
{
public static readonly BindableProperty DataProperty =
BindableProperty.Create(
nameof(Data),
typeof(object),
typeof(SelectableItem),
(object) null);
public static readonly BindableProperty IsSelectedProperty =
BindableProperty.Create(
nameof(IsSelected),
typeof(object),
typeof(SelectableItem),
(object)false);
public SelectableItem(object data)
{
Data = data;
IsSelected = false;
}
public SelectableItem(object data, bool isSelected)
{
Data = data;
IsSelected = isSelected;
}
public object Data
{
get => (object)GetValue(DataProperty);
set => SetValue(DataProperty, value);
}
public bool IsSelected
{
get => (bool)GetValue(IsSelectedProperty);
set => SetValue(IsSelectedProperty, value);
}
}
AudioViewCell.Xaml中的绑定示例:
<Label x:Name="LblFilename" Text="{Binding Filename}"
VerticalTextAlignment="Center"
Style="{StaticResource CellLabel}"/>
AudioViewCell.cs
public partial class AudioViewCell : ViewCell
{
public static BindableProperty AudiofileProperty = BindableProperty.Create(
propertyName: nameof(Audiofile),
returnType: typeof(AudioFile),
declaringType: typeof(AudioViewCell),
defaultValue: null,
defaultBindingMode: BindingMode.OneWay);
public AudioFile Audiofile
{
get => (AudioFile) GetValue(AudiofileProperty);
set
{
Debug.WriteLine("Audiofile changed");
SetValue(AudiofileProperty, value);
((MenuItemViewModel) BindingContext).Audiofile = value;
}
}
public AudioViewCell()
{
InitializeComponent();
this.BindingContext = new MenuItemViewModel(SlAdditionalData, AwvWaveView);
}
}
最后是MenuItemViewModel:
public class MenuItemViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged = delegate { };
private AudioFile _audioFile;
public AudioFile Audiofile
{
get => _audioFile;
set
{
Debug.WriteLine("Setting Audiofile");
_audioFile = value;
PropertyChanged(this, new PropertyChangedEventArgs(nameof(Audiofile)));
PropertyChanged(this, new PropertyChangedEventArgs(nameof(Filename)));
}
}
public string Filename => Audiofile?.Filename;
}
似乎 GenericSelectableItem 中的Field Data 从未设置过,所以我认为绑定有问题
有谁知道更好的方法或为什么这不起作用? 非常感谢你的帮助!!
答案 0 :(得分:1)
TL; DR版本:深入了解您的cell和'cellViewModel'源代码我注意到您的绑定句柄存在混淆码。您正在处理在AudioViewCell的构造函数中设置的一个BindingContext,但它被ListView自动覆盖(在构造函数之后运行)。所以你站在没有数据的情况下渲染的ViewCell。
在此图片上,我试图展示您的模型正在发生什么:
请注意左边的黄色圆形箭头,它是您在构造函数中定义绑定上下文。之后被红色箭头覆盖(在listview渲染后设置)。
要使其按照您编码的方式运行,请按以下步骤操作:
MenuItemViewModel
的构造函数创建一个重载以接收“AudioFile”(MenuItemViewModel类是您真正的BindingContext); OnBindingContextChanged
方法以提取新的Data
字段,并将其作为参数发送到MenuItemViewModel
的新实例的构造函数; MenuItemViewModel
实例设置为内部视图的BindingContext(根据您的源代码,它是一个名为slRoot
的StackLayout)以下是步骤代码:
<强> 2 强>
public class MenuItemViewModel : INotifyPropertyChanged
{
// ...
public void SetAudiofile(AudioFile data)
{
Audiofile = data;
}
// ...
}
3和4:
public partial class AudioViewCell : ViewCell
{
// ...
protected override void OnBindingContextChanged()
{
base.OnBindingContextChanged();
// * I'm not sure if it's ok create a new instance to your binding context here, the old one ca be kept on memory due it's subscription. Think about create a method to set just the Audiofile property
slRoot.BindingContext = new MenuItemViewModel( thing, thing, ((GenericSelectableItem<AudioFile>)BindingContext).Data);
}
// ...
}
我已经测试过并且有效,但它远非理想的清洁解决方案。
如果您的意图是重用此单元格,我认为您应该公开可以绑定或不绑定的属性,让它需要显示将要显示的内容。视图单元应该只处理可视布局/行为,无论数据是什么都无关紧要。
P.S。:对不起我的英语不好,我希望这是可以理解的。