ExtendedDatePicker with Nullable values for Xamarin.Forms

CheckColorExtensions.cs

using UIKit;
using Xamarin.Forms;

namespace Check.iOS.Extensions
{
    /// <summary>
    /// Color extensions
    /// </summary>
    public static class CheckColorExtensions
    {
        internal static readonly UIColor SeventyPercentGrey = new UIColor(0.7f, 0.7f, 0.7f, 1);

        public static bool IsDefault(this Color color) => Color.Default == color;
    }
}

ExampleUsage.xaml

// Example usage
 <controls:ExtendedDatePicker
                   x:Name="ContactBirthDate"
                   Placeholder="{i18n:Translate Contact_BirthdayLabel}"
                   NullableDate="{Binding Contact.BirthDate}"
                   Format="d MMM yyyy"
                   XAlign="Start"
                   HasBorder="false"
                   HorizontalOptions="FillAndExpand"
                   StyleId="BirthDateFieldId"
                   VerticalOptions="Center"
                   IsEnabled="{Binding IsNotBusy}" >
</controls:ExtendedDatePicker>

ExtendedDatePicker.cs

using System;
using Xamarin.Forms;

namespace Check.Client.Core.Controls
{
    /// <summary>
    ///  Extended DatePicker for Nullable Values
    ///  Via: https://forums.xamarin.com/discussion/20028/datepicker-possible-to-bind-to-nullable-date-value
    ///  Via: https://github.com/XLabs/Xamarin-Forms-Labs/wiki/ExtendedEntry
    /// </summary>
    public class ExtendedDatePicker : DatePicker
    {
    	/// <summary>
	/// The font property
	/// </summary>
	public static readonly BindableProperty FontProperty =
	    BindableProperty.Create("Font", typeof(Font), typeof(ExtendedDatePicker), new Font());
            
        /// <summary>
        /// The NullableDate property
        /// </summary>
        public static readonly BindableProperty NullableDateProperty =
            BindableProperty.Create("NullableDate", typeof (DateTime?), typeof (ExtendedDatePicker), null, BindingMode.TwoWay);

        /// <summary>
        /// The XAlign property
        /// </summary>
        public static readonly BindableProperty XAlignProperty =
            BindableProperty.Create("XAlign", typeof(TextAlignment), typeof(ExtendedDatePicker),
            TextAlignment.Start);

        /// <summary>
        /// The HasBorder property
        /// </summary>
        public static readonly BindableProperty HasBorderProperty =
            BindableProperty.Create("HasBorder", typeof(bool), typeof(ExtendedDatePicker), true);

        /// <summary>
        /// The Placeholder property
        /// </summary>
        public static readonly BindableProperty PlaceholderProperty =
            BindableProperty.Create("Placeholder", typeof (string), typeof (ExtendedDatePicker), string.Empty, BindingMode.OneWay);

        /// <summary>
        /// The PlaceholderTextColor property
        /// </summary>
        public static readonly BindableProperty PlaceholderTextColorProperty =
            BindableProperty.Create("PlaceholderTextColor", typeof(Color), typeof(ExtendedDatePicker), Color.Default);


	/// <summary>
	/// Gets or sets the Font
	/// </summary>
	public Font Font
	{
             get { return (Font)GetValue(FontProperty); }
	     set { SetValue(FontProperty, value); }
	}
	
        /// <summary>
        /// Get or sets the NullableDate
        /// </summary>
        public DateTime? NullableDate
        {
            get { return (DateTime?) GetValue(NullableDateProperty); }
            set
            {
                if (value != NullableDate)
                {
                    SetValue(NullableDateProperty, value);
                    UpdateDate();
                }
            }
        }

        /// <summary>
        /// Gets or sets the X alignment of the text
        /// </summary>
        public TextAlignment XAlign
        {
            get { return (TextAlignment)GetValue(XAlignProperty); }
            set { SetValue(XAlignProperty, value); }
        }


        /// <summary>
        /// Gets or sets if the border should be shown or not
        /// </summary>
        public bool HasBorder
        {
            get { return (bool)GetValue(HasBorderProperty); }
            set { SetValue(HasBorderProperty, value); }
        }

        /// <summary>
        /// Get or sets the PlaceHolder
        /// </summary>
        public string Placeholder
        {
            get { return (string) GetValue(PlaceholderProperty); }
            set { SetValue(PlaceholderProperty, value); }
        }

        /// <summary>
        /// Sets color for placeholder text
        /// </summary>
        public Color PlaceholderTextColor
        {
            get { return (Color)GetValue(PlaceholderTextColorProperty); }
            set { SetValue(PlaceholderTextColorProperty, value); }
        }

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

        protected override void OnPropertyChanged(string propertyName = null)
        {
            base.OnPropertyChanged(propertyName);

            //Device.OnPlatform(() =>
            //{
                if (propertyName == IsFocusedProperty.PropertyName)
                {
                    if (IsFocused)
                    {
                        if (!NullableDate.HasValue)
                        {
                            Date = (DateTime) DateProperty.DefaultValue;
                        }
                    }
                    else
                    {
                        OnPropertyChanged(DateProperty.PropertyName);
                    }
                }
            //});

            if (propertyName == DateProperty.PropertyName)
            {
                NullableDate = Date;
            }

            if (propertyName == NullableDateProperty.PropertyName)
            {
                if (NullableDate.HasValue)
                {
                    Date = NullableDate.Value;
                }
            }
        }

        private void UpdateDate()
        {
            if (NullableDate.HasValue)
            {
                Date = NullableDate.Value;
            }
            else
            {
                Date = (DateTime)DateProperty.DefaultValue;
            }
        }
    }
}

ExtendedDatePickerDroidRenderer.cs

using Android.Views;
using Check.Client.Core.Controls;
using Check.Droid.Renderers;
using System.ComponentModel;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;

[assembly: ExportRenderer(typeof(ExtendedDatePicker), typeof(ExtendedDatePickerRenderer))]
namespace Check.Droid.Renderers
{
    /// <summary>
    ///  Extended DatePicker Renderer for Nullable Values
    ///  Via: https://forums.xamarin.com/discussion/20028/datepicker-possible-to-bind-to-nullable-date-value
    ///  Via: https://github.com/XLabs/Xamarin-Forms-Labs/wiki/ExtendedEntry
    /// </summary>
    public class ExtendedDatePickerRenderer : DatePickerRenderer
    {
        protected override void OnElementChanged(ElementChangedEventArgs<DatePicker> e)
        {
            base.OnElementChanged(e);

            var view = Element as ExtendedDatePicker;

            if (view != null)
            {
                SetFont(view);
                SetTextAlignment(view);
                // SetBorder(view);
                SetNullableText(view);
                SetPlaceholder(view);
                SetPlaceholderTextColor(view);
            }
        }

        protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            base.OnElementPropertyChanged(sender, e);

            var view = (ExtendedDatePicker)Element;

	    if (e.PropertyName == ExtendedDatePicker.FontProperty.PropertyName)
		SetFont(view);		
            else if (e.PropertyName == ExtendedDatePicker.XAlignProperty.PropertyName)
                SetTextAlignment(view);
            // else if (e.PropertyName == ExtendedDatePicker.HasBorderProperty.PropertyName)
            //  SetBorder(view);
            else if (e.PropertyName == ExtendedDatePicker.NullableDateProperty.PropertyName)
                SetNullableText(view);
            else if (e.PropertyName == ExtendedDatePicker.PlaceholderProperty.PropertyName)
                SetPlaceholder(view);
            else if (e.PropertyName == ExtendedDatePicker.PlaceholderTextColorProperty.PropertyName)
                SetPlaceholderTextColor(view);

        }

        /// <summary>
        /// Sets the text alignment.
        /// </summary>
        /// <param name="view">The view.</param>
        private void SetTextAlignment(ExtendedDatePicker view)
        {
            switch (view.XAlign)
            {
                case Xamarin.Forms.TextAlignment.Center:
                    Control.Gravity = GravityFlags.CenterHorizontal;
                    break;
                case Xamarin.Forms.TextAlignment.End:
                    Control.Gravity = GravityFlags.End;
                    break;
                case Xamarin.Forms.TextAlignment.Start:
                    Control.Gravity = GravityFlags.Start;
                    break;
            }
        }
        
        		/// <summary>
		/// Sets the font.
		/// </summary>
		/// <param name="view">The view.</param>
		private void SetFont(ExtendedDatePicker view)
		{
			if (view.Font != Font.Default)
			{
				Control.TextSize = view.Font.ToScaledPixel();
				Control.Typeface = view.Font.ToExtendedTypeface(Context);
			}
		}

        /// <summary>
        /// Set text based on nullable value
        /// </summary>
        /// <param name="view"></param>
        private void SetNullableText(ExtendedDatePicker view)
        {
            if (view.NullableDate == null)
                Control.Text = string.Empty;
        }

        /// <summary>
        /// Set the placeholder
        /// </summary>
        /// <param name="view"></param>
        private void SetPlaceholder(ExtendedDatePicker view)
        {
            Control.Hint = view.Placeholder;
        }

        /// <summary>
        /// Sets the color of the placeholder text.
        /// </summary>
        /// <param name="view">The view.</param>
        private void SetPlaceholderTextColor(ExtendedDatePicker view)
        {
            if (view.PlaceholderTextColor != Color.Default)
            {
                Control.SetHintTextColor(view.PlaceholderTextColor.ToAndroid());
            }
        }
    }
}

ExtendedDatePickeriOSRenderer.cs

using Xamarin.Forms.Platform.iOS;
using Xamarin.Forms;
using Check.Client.Core.Controls;
using System.ComponentModel;
using UIKit;
using Check.iOS.Renderers;
using Check.iOS.Extensions;
using Foundation;
using System;
using CoreGraphics;

[assembly: ExportRenderer(typeof(ExtendedDatePicker), typeof(ExtendedDatePickerRenderer))]
namespace Check.iOS.Renderers
{
    /// <summary>
    ///  Extended DatePicker Renderer for Nullable Values
    ///  Via: https://forums.xamarin.com/discussion/20028/datepicker-possible-to-bind-to-nullable-date-value
    ///  Via: https://github.com/XLabs/Xamarin-Forms-Labs/wiki/ExtendedEntry
    /// </summary>
    public class ExtendedDatePickerRenderer : DatePickerRenderer
    {
        protected override void OnElementChanged(ElementChangedEventArgs<DatePicker> e)
        {
            base.OnElementChanged(e);

            var view = Element as ExtendedDatePicker;

            if (view != null)
            {
                SetFont(view);
                SetTextAlignment(view);
                SetBorder(view);
                SetNullableText(view);
                SetPlaceholderTextColor(view);

                ResizeHeight();
            }
        }

        protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            base.OnElementPropertyChanged(sender, e);

            var view = (ExtendedDatePicker)Element;

			if (e.PropertyName == ExtendedDatePicker.FontProperty.PropertyName)
				SetFont(view);
            else if (e.PropertyName == ExtendedDatePicker.XAlignProperty.PropertyName)
                SetTextAlignment(view);
            else if (e.PropertyName == ExtendedDatePicker.HasBorderProperty.PropertyName)
                SetBorder(view);
            else if (e.PropertyName == ExtendedDatePicker.NullableDateProperty.PropertyName)
                SetNullableText(view);
            else if (e.PropertyName == ExtendedDatePicker.PlaceholderTextColorProperty.PropertyName)
                SetPlaceholderTextColor(view);

            ResizeHeight();
        }

        /// <summary>
        /// Sets the text alignment.
        /// </summary>
        /// <param name="view">The view.</param>
        private void SetTextAlignment(ExtendedDatePicker view)
        {
            switch (view.XAlign)
            {
                case TextAlignment.Center:
                    Control.TextAlignment = UITextAlignment.Center;
                    break;
                case TextAlignment.End:
                    Control.TextAlignment = UITextAlignment.Right;
                    break;
                case TextAlignment.Start:
                    Control.TextAlignment = UITextAlignment.Left;
                    break;
            }
        }

		/// <summary>
		/// Sets the font.
		/// </summary>
		/// <param name="view">The view.</param>
		private void SetFont(ExtendedDatePicker view)
		{
			UIFont uiFont;
			if (view.Font != Font.Default && (uiFont = view.Font.ToUIFont()) != null)
				Control.Font = uiFont;
			else if (view.Font == Font.Default)
				Control.Font = UIFont.SystemFontOfSize(17f);
		}

        /// <summary>
        /// Sets the border.
        /// </summary>
        /// <param name="view">The view.</param>
        private void SetBorder(ExtendedDatePicker view)
        {
            Control.BorderStyle = view.HasBorder ? UITextBorderStyle.RoundedRect : UITextBorderStyle.None;
        }

        /// <summary>
        /// Set text based on nullable value
        /// </summary>
        /// <param name="view"></param>
        private void SetNullableText(ExtendedDatePicker view)
        {
            if (view.NullableDate == null)
                Control.Text = string.Empty;
        }

        /// <summary>
        /// Resizes the height.
        /// </summary>
        private void ResizeHeight()
        {
            if (Element.HeightRequest >= 0) return;

            var height = Math.Max(Bounds.Height,
                new UITextField { Font = Control.Font }.IntrinsicContentSize.Height) * 2;

            Control.Frame = new CGRect(0.0f, 0.0f, (nfloat)Element.Width, (nfloat)height);

            Element.HeightRequest = height;
        }

        /// <summary>
        /// Sets the color of the placeholder text.
        /// </summary>
        /// <param name="view">The view.</param>
        private void SetPlaceholderTextColor(ExtendedDatePicker view)
        {
            if (!string.IsNullOrEmpty(view.Placeholder))
            {
                var foregroundUIColor = view.PlaceholderTextColor.ToUIColor(CheckColorExtensions.SeventyPercentGrey);
                var backgroundUIColor = view.BackgroundColor.ToUIColor();
                var targetFont = Control.Font;
                Control.AttributedPlaceholder = new NSAttributedString(view.Placeholder, targetFont, foregroundUIColor, backgroundUIColor);
            }
        }
    }
}

Anyone interested in UWP could look at this issue on a project featuring iOS and Android renderers. Here I outline an approach for a 'right-click' to clear behaviour. Not perfect but it might help someone:

CrossGeeks/ClearableDatePickerSample#10

Does anyone ever use this Control? I have a fundamental issue with it and nobody else seems to have that problem.

In ExtendedDatePicker OnBindingContextChanged UpdateDate is called.

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

UpdateDate checks the NullableDate property and if that is Null it updates the underlying DatePicker's Date property to min date of 01/01/1900.

 private void UpdateDate()
        {
            if (NullableDate.HasValue)
            {
                Date = NullableDate.Value;
            }
            else
            {
                Date = (DateTime)DateProperty.DefaultValue;
            }
        }

Setting of the Date property triggers an OnPropertyChnaged in ExtendedDatePicker that then sets the NullableDate to the newly updated Date of 01/01/1900 therefore removing the null Date!

if (propertyName == DateProperty.PropertyName)
{
        NullableDate = Date;
}

This means you never have a NullableDate that is Null after the first pass of the Binding code. Surely other people have seen this issue too