got basic screen transitions working

Source/Demos/Pong/Screens/GameScreen.cs

@@ -169,7 +169,7 @@ namespace Pong.Screens
                 _ball.Position = new Vector2(ScreenWidth / 2f, ScreenHeight / 2f);
                 _ball.Velocity = new Vector2(_random.Next(2, 5) * -100, 100);
                 _leftScore++;
-                ScreenManager.LoadScreen(new MagentaScreen(_game));
+                ScreenManager.LoadScreen(new MagentaScreen(_game), new Transition(GraphicsDevice, Color.Black));
             }
 
             if (_ball.Position.X < -halfWidth && _ball.Velocity.X < 0)
@@ -177,7 +177,7 @@ namespace Pong.Screens
                 _ball.Position = new Vector2(ScreenWidth / 2f, ScreenHeight / 2f);
                 _ball.Velocity = new Vector2(_random.Next(2, 5) * 100, 100);
                 _rightScore++;
-                ScreenManager.LoadScreen(new MagentaScreen(_game));
+                ScreenManager.LoadScreen(new MagentaScreen(_game), new Transition(GraphicsDevice, Color.Black));
 
             }
         }

Source/Demos/Pong/Screens/MagentaScreen.cs

@@ -1,7 +1,6 @@
 using Microsoft.Xna.Framework;
 using Microsoft.Xna.Framework.Graphics;
 using Microsoft.Xna.Framework.Input;
-using MonoGame.Extended;
 using MonoGame.Extended.Screens;
 
 namespace Pong.Screens
@@ -27,16 +26,12 @@ namespace Pong.Screens
             var mouseState = Mouse.GetState();
 
             if (mouseState.LeftButton == ButtonState.Pressed)
-                ScreenManager.LoadScreen(new GameScreen(_game));
+                ScreenManager.LoadScreen(new GameScreen(_game), new Transition(_game.GraphicsDevice, Color.Black));
         }
 
         public override void Draw(GameTime gameTime)
         {
-            _game.GraphicsDevice.Clear(Color.Black);
-
-            _spriteBatch.Begin();
-            _spriteBatch.FillRectangle(new RectangleF(100, 100, 100, 100), Color.Magenta);
-            _spriteBatch.End();
+            _game.GraphicsDevice.Clear(Color.Magenta);
         }
     }
 }

Source/MonoGame.Extended/Screens/ScreenManager.cs

@@ -1,7 +1,69 @@
+using System;
 using Microsoft.Xna.Framework;
+using Microsoft.Xna.Framework.Graphics;
 
 namespace MonoGame.Extended.Screens
 {
+    public class Transition : IDisposable
+    {
+        private readonly GraphicsDevice _graphicsDevice;
+        private readonly SpriteBatch _spriteBatch;
+        private readonly Color _color;
+        private float _value;
+        private TransitionState _state = TransitionState.Out;
+
+        private enum TransitionState { In, Out }
+        
+        public Transition(GraphicsDevice graphicsDevice, Color color)
+        {
+            _graphicsDevice = graphicsDevice;
+            _color = color;
+            _spriteBatch = new SpriteBatch(graphicsDevice);
+        }
+
+        public event EventHandler StateChanged;
+
+        public void Dispose()
+        {
+        }
+
+        public bool Update(GameTime gameTime)
+        {
+            var elapsedSeconds = gameTime.GetElapsedSeconds();
+
+            switch (_state)
+            {
+                case TransitionState.Out:
+                    _value += elapsedSeconds;
+
+                    if (_value >= 1.0f)
+                    {
+                        _state = TransitionState.In;
+                        StateChanged?.Invoke(this, EventArgs.Empty);
+                    }
+                    break;
+
+                case TransitionState.In:
+                    _value -= elapsedSeconds;
+
+                    if (_value <= 0.0f)
+                        return false;
+                    break;
+                default:
+                    throw new ArgumentOutOfRangeException();
+            }
+
+            return true;
+        }
+
+        public void Draw(GameTime gameTime)
+        {
+            _spriteBatch.Begin(samplerState: SamplerState.PointClamp);
+            _spriteBatch.FillRectangle(_graphicsDevice.Viewport.Bounds, _color * MathHelper.Clamp(_value, 0, 1));
+            _spriteBatch.End();
+        }
+    }
+
     public class ScreenManager : SimpleDrawableGameComponent
     {
         public ScreenManager()
@@ -11,6 +73,13 @@ namespace MonoGame.Extended.Screens
         private Screen _activeScreen;
         private bool _isInitialized;
         private bool _isLoaded;
+        private Transition _activeTransition;
+
+        public void LoadScreen(Screen screen, Transition transition)
+        {
+            _activeTransition = transition;
+            _activeTransition.StateChanged += (sender, args) => LoadScreen(screen);
+        }
 
         public void LoadScreen(Screen screen)
         {
@@ -51,12 +120,24 @@ namespace MonoGame.Extended.Screens
 
         public override void Update(GameTime gameTime)
         {
-            _activeScreen?.Update(gameTime);
+            if (_activeTransition != null)
+            {
+                if (!_activeTransition.Update(gameTime))
+                {
+                    _activeTransition.Dispose();
+                    _activeTransition = null;
+                }
+            }
+            else
+            {
+                _activeScreen?.Update(gameTime);
+            }
         }
 
         public override void Draw(GameTime gameTime)
         {
-            _activeScreen.Draw(gameTime);
+            _activeScreen?.Draw(gameTime);
+            _activeTransition?.Draw(gameTime);
         }
     }
 }