add FPS counter

GitOrigin-RevId: 7186a6f3cd9ba35ff51b2232ebef67cbe3ad544c

Jumpy.Shared/FPSCounter.cs

@@ -0,0 +1,26 @@
+using System;
+
+namespace Jumpy {
+  class FpsCounter {
+    private double fps = 0;
+    private int[] frameTimes = new int[60];
+    private int idx = 0;
+
+    public double Fps {
+      get => fps;
+    }
+
+    public void Update() {
+      var now = Environment.TickCount;  // ms
+      if (frameTimes[idx] != 0) {
+        var timeElapsed = now - frameTimes[idx];
+        fps = 1000.0 * frameTimes.Length / timeElapsed;
+      }
+      frameTimes[idx] = now;
+      idx++;
+      if (idx == frameTimes.Length) {
+        idx = 0;
+      }
+    }
+  }
+}

Jumpy.Shared/Jumpy.Shared.projitems

@@ -10,6 +10,7 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="$(MSBuildThisFileDirectory)Camera.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)FpsCounter.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)IDisplay.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)KeyboardInput.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)JumpyGame.cs" />

Jumpy.Shared/JumpyGame.cs

@@ -2,6 +2,7 @@
 using Microsoft.Xna.Framework.Graphics;
 using Microsoft.Xna.Framework.Input;
 
+using System;
 using System.Collections.Generic;
 
 namespace Jumpy {
@@ -14,6 +15,7 @@ namespace Jumpy {
     bool fullScreen = false;
     IDisplay display;
 
+    FpsCounter fpsCounter = new FpsCounter();
     Player player;
 
     public JumpyGame() {
@@ -70,6 +72,10 @@ namespace Jumpy {
 
     // Called when the game should draw itself.
     protected override void Draw(GameTime gameTime) {
+      // We need to update the FPS counter in Draw() since Update() might get called more
+      // frequently, especially when gameTime.IsRunningSlowly.
+      fpsCounter.Update();
+
       // Draw scene to RenderTarget.
       GraphicsDevice.SetRenderTarget(renderTarget);
       // GraphicsDevice.DepthStencilState = new DepthStencilState() { DepthBufferEnable = true };
@@ -87,7 +93,8 @@ namespace Jumpy {
       Rectangle drawRect = new Rectangle(
         0, 0, GraphicsDevice.Viewport.Width, GraphicsDevice.Viewport.Height);
       spriteBatch.Draw(renderTarget, drawRect, Color.White);
-      spriteBatch.DrawString(font, "FPS: 60", new Vector2(10, 10), Color.White);
+      string fpsText = $"FPS: {fpsCounter.Fps:F1}";
+      spriteBatch.DrawString(font, fpsText, new Vector2(10, 10), Color.White);
       spriteBatch.End();
 
       base.Draw(gameTime);