DatePicker.cs

using System;
using Xamarin.Forms.Internals;
using Xamarin.Forms.Platform;

namespace Xamarin.Forms
{
	[RenderWith(typeof(_DatePickerRenderer))]
	public class DatePicker : View, IFontElement, ITextElement, IElementConfiguration<DatePicker>
	{
		public static readonly BindableProperty FormatProperty = BindableProperty.Create(nameof(Format), typeof(string), typeof(DatePicker), "d");

		public static readonly BindableProperty DateProperty = BindableProperty.Create(nameof(Date), typeof(DateTime), typeof(DatePicker), default(DateTime), BindingMode.TwoWay,
			coerceValue: CoerceDate,
			propertyChanged: DatePropertyChanged,
			defaultValueCreator: (bindable) => DateTime.Today);

		public static readonly BindableProperty MinimumDateProperty = BindableProperty.Create(nameof(MinimumDate), typeof(DateTime), typeof(DatePicker), new DateTime(1900, 1, 1),
			validateValue: ValidateMinimumDate, coerceValue: CoerceMinimumDate);

		public static readonly BindableProperty MaximumDateProperty = BindableProperty.Create(nameof(MaximumDate), typeof(DateTime), typeof(DatePicker), new DateTime(2100, 12, 31),
			validateValue: ValidateMaximumDate, coerceValue: CoerceMaximumDate);

		public static readonly BindableProperty TextColorProperty = TextElement.TextColorProperty;

		public static readonly BindableProperty CharacterSpacingProperty = TextElement.CharacterSpacingProperty;

		public static readonly BindableProperty FontFamilyProperty = FontElement.FontFamilyProperty;

		public static readonly BindableProperty FontSizeProperty = FontElement.FontSizeProperty;

		public static readonly BindableProperty FontAttributesProperty = FontElement.FontAttributesProperty;

		public static readonly BindableProperty TextTransformProperty = TextElement.TextTransformProperty;

		readonly Lazy<PlatformConfigurationRegistry<DatePicker>> _platformConfigurationRegistry;

		public DatePicker()
		{
			_platformConfigurationRegistry = new Lazy<PlatformConfigurationRegistry<DatePicker>>(() => new PlatformConfigurationRegistry<DatePicker>(this));
		}

		public DateTime Date
		{
			get { return (DateTime)GetValue(DateProperty); }
			set { SetValue(DateProperty, value); }
		}

		public string Format
		{
			get { return (string)GetValue(FormatProperty); }
			set { SetValue(FormatProperty, value); }
		}

		public TextTransform TextTransform
		{
			get { return (TextTransform)GetValue(TextTransformProperty); }
			set { SetValue(TextTransformProperty, value); }
		}

		public DateTime MaximumDate
		{
			get { return (DateTime)GetValue(MaximumDateProperty); }
			set { SetValue(MaximumDateProperty, value); }
		}

		public DateTime MinimumDate
		{
			get { return (DateTime)GetValue(MinimumDateProperty); }
			set { SetValue(MinimumDateProperty, value); }
		}

		public Color TextColor
		{
			get { return (Color)GetValue(TextElement.TextColorProperty); }
			set { SetValue(TextElement.TextColorProperty, value); }
		}

		public double CharacterSpacing
		{
			get { return (double)GetValue(TextElement.CharacterSpacingProperty); }
			set { SetValue(TextElement.CharacterSpacingProperty, value); }
		}

		public FontAttributes FontAttributes
		{
			get { return (FontAttributes)GetValue(FontAttributesProperty); }
			set { SetValue(FontAttributesProperty, value); }
		}

		public string FontFamily
		{
			get { return (string)GetValue(FontFamilyProperty); }
			set { SetValue(FontFamilyProperty, value); }
		}

		[TypeConverter(typeof(FontSizeConverter))]
		public double FontSize
		{
			get { return (double)GetValue(FontSizeProperty); }
			set { SetValue(FontSizeProperty, value); }
		}

		void IFontElement.OnFontFamilyChanged(string oldValue, string newValue) =>
			InvalidateMeasureInternal(InvalidationTrigger.MeasureChanged);

		void IFontElement.OnFontSizeChanged(double oldValue, double newValue) =>
			InvalidateMeasureInternal(InvalidationTrigger.MeasureChanged);

		void IFontElement.OnFontChanged(Font oldValue, Font newValue) =>
			InvalidateMeasureInternal(InvalidationTrigger.MeasureChanged);

		double IFontElement.FontSizeDefaultValueCreator() =>
			Device.GetNamedSize(NamedSize.Default, (DatePicker)this);

		void IFontElement.OnFontAttributesChanged(FontAttributes oldValue, FontAttributes newValue) =>
			InvalidateMeasureInternal(InvalidationTrigger.MeasureChanged);

		void ITextElement.OnTextTransformChanged(TextTransform oldValue, TextTransform newValue)
		{
		}

		public virtual string UpdateFormsText(string source, TextTransform textTransform)
			=> TextTransformUtilites.GetTransformedText(source, textTransform);

		public event EventHandler<DateChangedEventArgs> DateSelected;

		static object CoerceDate(BindableObject bindable, object value)
		{
			var picker = (DatePicker)bindable;
			DateTime dateValue = ((DateTime)value).Date;

			if (dateValue > picker.MaximumDate)
				dateValue = picker.MaximumDate;

			if (dateValue < picker.MinimumDate)
				dateValue = picker.MinimumDate;

			return dateValue;
		}

		static object CoerceMaximumDate(BindableObject bindable, object value)
		{
			DateTime dateValue = ((DateTime)value).Date;
			var picker = (DatePicker)bindable;
			if (picker.Date > dateValue)
				picker.Date = dateValue;

			return dateValue;
		}

		static object CoerceMinimumDate(BindableObject bindable, object value)
		{
			DateTime dateValue = ((DateTime)value).Date;
			var picker = (DatePicker)bindable;
			if (picker.Date < dateValue)
				picker.Date = dateValue;

			return dateValue;
		}

		static void DatePropertyChanged(BindableObject bindable, object oldValue, object newValue)
		{
			var datePicker = (DatePicker)bindable;
			EventHandler<DateChangedEventArgs> selected = datePicker.DateSelected;

			if (selected != null)
				selected(datePicker, new DateChangedEventArgs((DateTime)oldValue, (DateTime)newValue));
		}

		static bool ValidateMaximumDate(BindableObject bindable, object value)
		{
			return ((DateTime)value).Date >= ((DatePicker)bindable).MinimumDate.Date;
		}

		static bool ValidateMinimumDate(BindableObject bindable, object value)
		{
			return ((DateTime)value).Date <= ((DatePicker)bindable).MaximumDate.Date;
		}

		public IPlatformElementConfiguration<T, DatePicker> On<T>() where T : IConfigPlatform
		{
			return _platformConfigurationRegistry.Value.On<T>();
		}

		void ITextElement.OnTextColorPropertyChanged(Color oldValue, Color newValue)
		{
		}

		void ITextElement.OnCharacterSpacingPropertyChanged(double oldValue, double newValue)
		{
			InvalidateMeasure();
		}

	}
}