如何在内容视图中为选取器设置 ItemDisplayBinding

Question

我有一个 ContentPage，它引用了一个 ContentView。在此内容视图中，我有一个自定义 Picker 控件，我绑定了其他属性，例如ItemsSource、ItemSelected 但 ItemDisplayBinding 很痛苦。我可以在 Xamarin Forms Picker class 的代码中看到它不像其他字段那样是一个直接可绑定的属性字段。目前，当我单击选择器时出现此错误。我知道特定的显示绑定属性因为选择器在我没有初始化的情况下工作正常。

错误

Java.Lang.NullPointerException
  Message=Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference

内容页

<controls:MyPickerView PickerTitle="Select time period"
                                       PickerItemDisplayBinding="{Binding Display}" 
                                       PickerItemsSource="{Binding AppointmentRangeOptions}"/>

MyPickerView.xaml

<ContentView.Content>
        <Frame >
            <Picker Title = "{Binding PickerTitle}"
                                   HorizontalOptions="FillAndExpand"
                                   IsEnabled="{Binding PickerIsEnabled}"
                                   ItemDisplayBinding="{Binding PickerItemDisplayBinding}" 
                                   SelectedItem="{Binding SelectedItem, Mode=TwoWay}" 
                                   ItemsSource="{Binding PickerItemsSource, Mode=OneWay}"/>
        </Frame>
    </ContentView.Content>

MyPickerView.xaml.cs

public partial class MyPickerView : ContentView
{
    public MyPickerView()
    {
        InitializeComponent();
        Content.BindingContext = this;
    }

    public static readonly BindableProperty PickerIsEnabledProperty = BindableProperty.Create(
        nameof(PickerIsEnabled),
        typeof(bool),
        typeof(MyPickerView),
        null);

    public bool PickerIsEnabled
    {
        get => (bool)GetValue(PickerIsEnabledProperty);
        set => SetValue(PickerIsEnabledProperty, value);
    }

    public static readonly BindableProperty SelectedItemProperty = BindableProperty.Create(
        nameof(SelectedItem),
        typeof(object),
        typeof(MyPickerView),
        null);

    public object SelectedItem
    {
        get => GetValue(SelectedItemProperty);
        set => SetValue(SelectedItemProperty, value);
    }

    public static readonly BindableProperty PickerTitleProperty = BindableProperty.Create(
        nameof(PickerTitle),
        typeof(string),
        typeof(MyPickerView),
        null);

    public string PickerTitle
    {
        get => (string)GetValue(PickerTitleProperty);
        set => SetValue(PickerTitleProperty, value);
    }

    public static readonly BindableProperty PickerItemDisplayBindingProperty = BindableProperty.Create(
        nameof(PickerItemDisplayBinding),
        typeof(string),
        typeof(MyPicker)
        );

    public string PickerItemDisplayBinding
    {
        get => (string)GetValue(PickerItemDisplayBindingProperty);
        set => SetValue(PickerItemDisplayBindingProperty, value);
    }

    public static readonly BindableProperty PickerItemsSourceProperty = BindableProperty.Create(
        nameof(PickerItemsSource),
        typeof(IList),
        typeof(MyPickerView));

    public IList PickerItemsSource
    {
        get => (IList)GetValue(PickerItemsSourceProperty);
        set => SetValue(PickerItemsSourceProperty, value);
    }
}

Answer 1

的确，这属性是一种痛苦。我用一个小技巧解决了它：

MyPickerView.xaml.cs

public BindingBase ItemDisplayBinding
{
    get => MyPicker?.ItemDisplayBinding;
    set => MyPicker.ItemDisplayBinding = value;
} // no need to define BindableProperty

MyPickerView.xaml

<ContentView.Content>
    <Frame>
        <Picker x:Name="MyPicker" />
    </Frame>
</ContentView.Content>

内容页

<controls:MyPickerView 
    PickerTitle="Select time period"
    ItemDisplayBinding="{Binding Display}" 
    PickerItemsSource="{Binding AppointmentRangeOptions}"/>

如何在内容视图中为选取器设置 ItemDisplayBinding

How do I set ItemDisplayBinding for a Picker inside a Content View

xaml

xamarin.ios

xamarin.android

xamarin

xamarin.forms

错误

内容页

MyPickerView.xaml

MyPickerView.xaml.cs