如何在內容視圖中為選取器設定ItemDisplayBinding

我有一個 ContentPage，它參考了一個 ContentView。在這個內容視圖中，我有一個自定義的 Picker 控制元件，我系結了其他屬性，例如ItemsSource，ItemSelected但 ItemDisplayBinding 很痛苦。我可以在 Xamarin Forms Picker 類的代碼中看到，它不像其他的那樣是一個簡單的可系結屬性欄位。目前，當我單擊 Picker 時出現此錯誤。我知道特定的顯示系結屬性，因為選擇器無需我初始化就可以正常作業。

錯誤

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

內容頁

<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);
    }
}

uj5u.com熱心網友回復：

確實，這個屬性很痛苦。我用一個小技巧解決了它：

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>

內容頁

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

標籤：xml xamarin xamarin.forms xamarin.android xamarin.ios

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