Fix DateTime slow first render (#24929)

Author: OvrBtn

src/Controls/src/Core/DatePicker/DatePicker.cs

@@ -1,5 +1,7 @@
 #nullable disable
 using System;
+using System.Threading.Tasks;
+using Microsoft.Maui.ApplicationModel;
 using Microsoft.Maui.Controls.Internals;
 using Microsoft.Maui.Graphics;
 
@@ -15,7 +17,7 @@ public partial class DatePicker : View, IFontElement, ITextElement, IElementConf
 		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);
+			defaultValueCreator: (bindable) => SetDefaultDate(bindable));
 
 		/// <summary>Bindable property for <see cref="MinimumDate"/>.</summary>
 		public static readonly BindableProperty MinimumDateProperty = BindableProperty.Create(nameof(MinimumDate), typeof(DateTime), typeof(DatePicker), new DateTime(1900, 1, 1),
@@ -49,6 +51,22 @@ public partial class DatePicker : View, IFontElement, ITextElement, IElementConf
 		public DatePicker()
 		{
 			_platformConfigurationRegistry = new Lazy<PlatformConfigurationRegistry<DatePicker>>(() => new PlatformConfigurationRegistry<DatePicker>(this));
+			InitializeDateTimeNowAndSetDate();
+		}
+
+		//TODO: if no better solution is found, move this DateTime initialization somewhere else
+		/// <summary>
+		/// Initialize <see cref="DateTime"/> on startup since first call without initializing beforehand takes too much time
+		/// See: https://github.com/dotnet/maui/issues/24929
+		/// </summary>
+		static DatePicker()
+		{
+			Task.Run(() =>
+			{
+				DateTime dateNow = DateTime.Now;
+				string dateString = dateNow.ToString();
+				_isDateTimeInitialized = true;
+			});
 		}
 
 		/// <include file="../../docs/Microsoft.Maui.Controls/DatePicker.xml" path="//Member[@MemberName='Date']/Docs/*" />
@@ -239,5 +257,41 @@ string IDatePicker.Format
 			get => Format;
 			set => SetValue(FormatProperty, value, SetterSpecificity.FromHandler);
 		}
+
+		static bool _isDateTimeInitialized = false;
+		static DateTime SetDefaultDate(BindableObject bindable)
+		{
+			if (_isDateTimeInitialized)
+			{
+				return DateTime.Now;
+			}
+			return (DateTime)MinimumDateProperty.DefaultValue;
+		}
+
+		/// <summary>
+		/// If <see cref="DateTime.Now"/> hasn't been initialized, initialize it on worker thread to avoid slowing down first render
+		/// See: https://github.com/dotnet/maui/issues/24929
+		/// </summary>
+		void InitializeDateTimeNowAndSetDate()
+		{
+			if (_isDateTimeInitialized)
+			{
+				return;
+			}
+			Task.Run(() =>
+			{
+				DateTime now = DateTime.Now;
+				_isDateTimeInitialized = true;
+				//Date might have been changed by client before initialization had a chance to finish
+				if (Date != MinimumDate)
+				{
+					return;
+				}
+				MainThread.BeginInvokeOnMainThread(() =>
+				{
+					Date = now;
+				});
+			});
+		}
 	}
 }

src/Core/src/Handlers/DatePicker/DatePickerHandler.Android.cs

@@ -19,9 +19,6 @@ protected override MauiDatePicker CreatePlatformView()
 
 			var date = VirtualView?.Date;
 
-			if (date != null)
-				_dialog = CreateDatePickerDialog(date.Value.Year, date.Value.Month, date.Value.Day);
-
 			return mauiDatePicker;
 		}
 

src/Core/src/Platform/Android/DatePickerExtensions.cs

@@ -1,6 +1,8 @@
 using System;
+using System.Threading.Tasks;
 using Android.App;
 using Android.Content.Res;
+using Microsoft.Maui.ApplicationModel;
 
 namespace Microsoft.Maui.Platform
 {
@@ -55,7 +57,32 @@ public static void UpdateMaximumDate(this MauiDatePicker platformDatePicker, IDa
 
 		internal static void SetText(this MauiDatePicker platformDatePicker, IDatePicker datePicker)
 		{
-			platformDatePicker.Text = datePicker.Date.ToString(datePicker.Format);
+			if (_isDateTimeToStringInitialized)
+			{
+				platformDatePicker.Text = datePicker.Date.ToString(datePicker.Format);
+			}
+			else
+			{
+				InitializeDateTimeToStringAndSetText(platformDatePicker, datePicker);
+			}
+		}
+
+		static bool _isDateTimeToStringInitialized;
+		/// <summary>
+		/// If <see cref="DateTime.ToString()"/> hasn't been initialized, initialize it on worker thread to avoid slowing down first render
+		/// See: https://github.com/dotnet/maui/issues/24929
+		/// </summary>
+		static void InitializeDateTimeToStringAndSetText(MauiDatePicker platformDatePicker, IDatePicker datePicker)
+		{
+			Task.Run(() =>
+			{
+				string dateString = datePicker.Date.ToString(datePicker.Format);
+				_isDateTimeToStringInitialized = true;
+				MainThread.BeginInvokeOnMainThread(() =>
+				{
+					platformDatePicker.Text = dateString;
+				});
+			});
 		}
 	}
 }