c# - Xamarin 自定义 DatePicker 失去绑定(bind)

Question

我有这个自定义 View :

public class VolosDatePicker : Button {
    private DatePicker _Picker;
    private IViewContainer<View> _ParentLayout;

    public static readonly BindableProperty DateProperty = BindableProperty.Create(nameof(Date), typeof(DateTime?), typeof(VolosDatePicker));
    public DateTime? Date {
        get { return (DateTime?)GetValue(DateProperty); }
        set { SetValue(DateProperty, value); }
    }

    private string TextFormat = Utente.UteLoggato.formatoData;
    private string DefaultText = "click me";

    //public static readonly BindableProperty DefaultTextProperty = BindableProperty.Create<VolosDatePicker, string>(p => p.DefaultText, "Pick Date...");

    public VolosDatePicker() {
        //create the datepicker, make it invisible on the form.
        _Picker = new DatePicker {
            IsVisible = false
        };

        //handle the focus/unfocus or rather the showing and hiding of the dateipicker
        _Picker.Focused += _Picker_Focused;
        _Picker.Unfocused += _Picker_Unfocused;

        //command for the button
        Command = new Command((obj) => {
            //try to get the parent layout and add the datepicker
            if (_ParentLayout == null) {
                _ParentLayout = _GetParentLayout(Parent);
                if (_ParentLayout != null) {
                    //add the picker to the closest layout up the tree
                    _ParentLayout.Children.Add(_Picker);
                } else {
                    throw new InvalidOperationException("The DatePickerButton needs to be inside an Layout type control that can have other children");
                }
            }
            //show the picker modal
            _Picker.Focus();
        });
        _UpdateText();
    }

    private IViewContainer<View> _GetParentLayout(Element Parent) {
        //StackLayout, RelativeLayout, Grid, and AbsoluteLayout all implement IViewContainer,
        //it would be very rare that this method would return null.
        IViewContainer<View> parent = Parent as IViewContainer<View>;
        if (Parent == null) {
            return null;
        } else if (parent != null) {
            return parent;
        } else {
            return _GetParentLayout(Parent.Parent);
        }
    }

    void _Picker_Focused(object sender, FocusEventArgs e) {
        //default the date to now if Date is empty
        _Picker.Date = Date ?? DateTime.Now;
    }

    void _Picker_Unfocused(object sender, FocusEventArgs e) {
        //this always sets.. can't cancel the dialog.
        Date = _Picker.Date;
        _UpdateText();
    }

    protected override void OnBindingContextChanged() {
        base.OnBindingContextChanged();
        _UpdateText();
    }

    private void _UpdateText() {
        //the button has a default text, use that the first time.
        if (Date != null) {
            //default formatting is in the FormatProperty BindableProperty 
            base.Text = Date.Value.ToString(TextFormat);
        } else {
            base.Text = DefaultText;
        }
    }

    protected override void OnPropertyChanging(string propertyName = null) {
        //set this so there is an old date for the DateChangedEventArgs
        base.OnPropertyChanging(propertyName);
        if (propertyName == DateProperty.PropertyName) {
            //_OldDate = Date;
            _UpdateText();
            //if (_OldDate != null && Date != null) DateSelected(this, new DateChangedEventArgs((DateTime)_OldDate, (DateTime)Date));
        }
    }

    protected override void OnPropertyChanged(string propertyName = null) {
        base.OnPropertyChanged(propertyName);
        if (propertyName == DateProperty.PropertyName) {              
            _UpdateText();
        }
    }
}

在我的 XAML 页面中，我有这个绑定(bind):

<view:VolosDatePicker Date="{Binding WoSpesaDett.DataSpesa}" Grid.Row="0" Grid.Column="1" />

我是这样设置这个日期的:

WoSpesaDett.DataSpesa = new System.DateTime(2015, 5, 18);
RaisePropertyChanged(() => WoSpesaDett);

在我使用此方法之前，DatePicker 的日期值设置正确，但在我尝试使用控件选择器设置日期后，上述方法停止工作。

如果我从控件选取器中选择任何日期，然后尝试使用上述方法以编程方式设置日期，则日期不会更改。

它似乎失去了与属性 WoSpesaDett.DataSpesa 的绑定(bind)。

Answer 1

根据documentation :

Optionally, when creating a BindableProperty instance, the following parameters can be specified:

The binding mode. This is used to specify the direction in which property value changes will propagate. In the default binding mode, changes will propagate from the source to the target.

尝试将绑定(bind)的模式设置为 TwoWay，如下所示:

<view:VolosDatePicker Date="{Binding WoSpesaDett.DataSpesa, Mode=TwoWay}" Grid.Row="0" Grid.Column="1" />

或者编辑 BindableProperty 本身。

希望对您有所帮助!