[Resizetizer] Do not update images if they have not been updated.

src/SingleProject/Resizetizer/src/AndroidAdaptiveIconGenerator.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
 
@@ -52,7 +53,7 @@ public IEnumerable<ResizedImageInfo> Generate()
 		void ProcessBackground(List<ResizedImageInfo> results, DirectoryInfo fullIntermediateOutputPath)
 		{
 			var backgroundFile = Info.Filename;
-			var backgroundExists = File.Exists(backgroundFile);
+			var (backgroundExists, backgroundModified) = Utils.FileExists(backgroundFile);
 			var backgroundDestFilename = AppIconName + "_background.png";
 
 			if (backgroundExists)
@@ -64,8 +65,16 @@ void ProcessBackground(List<ResizedImageInfo> results, DirectoryInfo fullInterme
 			{
 				var dir = Path.Combine(fullIntermediateOutputPath.FullName, dpi.Path);
 				var destination = Path.Combine(dir, backgroundDestFilename);
+				var (destinationExists, destinationModified) = Utils.FileExists(destination);
 				Directory.CreateDirectory(dir);
 
+				if (destinationModified > backgroundModified)
+				{
+					Logger.Log($"Skipping `{backgroundFile}` => `{destination}` file is up to date.");
+					results.Add(new ResizedImageInfo { Dpi = dpi, Filename = destination });
+					continue;
+				}
+
 				Logger.Log($"App Icon Background Part: " + destination);
 
 				if (backgroundExists)
@@ -88,7 +97,7 @@ void ProcessBackground(List<ResizedImageInfo> results, DirectoryInfo fullInterme
 		void ProcessForeground(List<ResizedImageInfo> results, DirectoryInfo fullIntermediateOutputPath)
 		{
 			var foregroundFile = Info.ForegroundFilename;
-			var foregroundExists = File.Exists(foregroundFile);
+			var (foregroundExists, foregroundModified) = Utils.FileExists(foregroundFile);
 			var foregroundDestFilename = AppIconName + "_foreground.png";
 
 			if (foregroundExists)
@@ -100,8 +109,16 @@ void ProcessForeground(List<ResizedImageInfo> results, DirectoryInfo fullInterme
 			{
 				var dir = Path.Combine(fullIntermediateOutputPath.FullName, dpi.Path);
 				var destination = Path.Combine(dir, foregroundDestFilename);
+				var (destinationExists, destinationModified) = Utils.FileExists(destination);
 				Directory.CreateDirectory(dir);
 
+				if (destinationModified > foregroundModified)
+				{
+					Logger.Log($"Skipping `{foregroundFile}` => `{destination}` file is up to date.");
+					results.Add(new ResizedImageInfo { Dpi = dpi, Filename = destination });
+					continue;
+				}
+
 				Logger.Log($"App Icon Foreground Part: " + destination);
 
 				if (foregroundExists)
@@ -123,14 +140,20 @@ void ProcessForeground(List<ResizedImageInfo> results, DirectoryInfo fullInterme
 
 		void ProcessAdaptiveIcon(List<ResizedImageInfo> results, DirectoryInfo fullIntermediateOutputPath)
 		{
-			var adaptiveIconXmlStr = AdaptiveIconDrawableXml
-				.Replace("{name}", AppIconName);
-
 			var dir = Path.Combine(fullIntermediateOutputPath.FullName, "mipmap-anydpi-v26");
 			var adaptiveIconDestination = Path.Combine(dir, AppIconName + ".xml");
 			var adaptiveIconRoundDestination = Path.Combine(dir, AppIconName + "_round.xml");
 			Directory.CreateDirectory(dir);
 
+			if (File.Exists(adaptiveIconDestination) && File.Exists(adaptiveIconRoundDestination)) {
+				results.Add(new ResizedImageInfo { Dpi = new DpiPath("mipmap-anydpi-v26", 1), Filename = adaptiveIconDestination });
+				results.Add(new ResizedImageInfo { Dpi = new DpiPath("mipmap-anydpi-v26", 1, "_round"), Filename = adaptiveIconRoundDestination });
+				return;
+			}
+
+			var adaptiveIconXmlStr = AdaptiveIconDrawableXml
+				.Replace("{name}", AppIconName);
+
 			// Write out the adaptive icon xml drawables
 			File.WriteAllText(adaptiveIconDestination, adaptiveIconXmlStr);
 			File.WriteAllText(adaptiveIconRoundDestination, adaptiveIconXmlStr);

src/SingleProject/Resizetizer/src/AppleIconAssetsGenerator.cs

@@ -41,6 +41,17 @@ public IEnumerable<ResizedImageInfo> Generate()
 			var assetContentsFile = Path.Combine(outputAssetsDir, "Contents.json");
 			var appIconSetContentsFile = Path.Combine(outputAppIconSetDir, "Contents.json");
 
+			var (sourceExists, sourceModified) = Utils.FileExists(Info.Filename);
+			var (destinationExists, destinationModified) = Utils.FileExists(appIconSetContentsFile);
+
+			if (destinationModified > sourceModified)
+			{
+				Logger.Log($"Skipping `{Info.Filename}` => `{appIconSetContentsFile}` file is up to date.");
+				return new List<ResizedImageInfo> {
+					new ResizedImageInfo { Dpi = new DpiPath("", 1), Filename = appIconSetContentsFile }
+				};
+			}
+
 			var infoJsonProp = new JsonObject
 			{
 				["info"] = new JsonObject

src/SingleProject/Resizetizer/src/ResizetizeImages.cs

@@ -170,9 +170,17 @@ void ProcessAppIcon(ResizeImageInfo img, ConcurrentBag<ResizedImageInfo> resized
 
 				var destination = Resizer.GetRasterFileDestination(img, dpi, IntermediateOutputPath)
 					.Replace("{name}", appIconName);
+				var (sourceExists, sourceModified) = Utils.FileExists(img.Filename);
+				var (destinationExists, destinationModified) = Utils.FileExists(destination);
 
 				LogDebugMessage($"App Icon Destination: " + destination);
 
+				if (destinationModified > sourceModified)
+				{
+					Logger.Log($"Skipping `{img.Filename}` => `{destination}` file is up to date.");
+					continue;
+				}
+
 				appTool.Resize(dpi, destination);
 			}
 		}

src/SingleProject/Resizetizer/src/Utils.cs

@@ -1,4 +1,5 @@
-using System.IO;
+using System;
+using System.IO;
 using System.Text.RegularExpressions;
 using SkiaSharp;
 
@@ -49,5 +50,12 @@ public static bool IsValidResourceFilename(string filename)
 
 			return null;
 		}
+
+		public static (bool Exists, DateTime Modified) FileExists(string path)
+		{
+			var exists = File.Exists(path);
+			var modified = exists ? File.GetLastWriteTimeUtc(path) : System.DateTime.MinValue;
+			return (exists, modified);
+		}
 	}
 }

src/SingleProject/Resizetizer/src/WindowsIconGenerator.cs

@@ -1,4 +1,5 @@
-using System.IO;
+using System;
+using System.IO;
 using SkiaSharp;
 
 namespace Microsoft.Maui.Resizetizer
@@ -27,11 +28,20 @@ public ResizedImageInfo Generate()
 			string destination = Path.Combine(destinationFolder, $"{fileName}.ico");
 			Directory.CreateDirectory(destinationFolder);
 
+			var (sourceExists, sourceModified) = Utils.FileExists(Info.Filename);
+			var (destinationExists, destinationModified) = Utils.FileExists(destination);
+
 			Logger.Log($"Generating ICO: {destination}");
 
 			var tools = new SkiaSharpAppIconTools(Info, Logger);
 			var dpi = new DpiPath(fileName, 1.0m, size: new SKSize(64, 64));
 
+			if (destinationModified > sourceModified)
+			{
+				Logger.Log($"Skipping `{Info.Filename}` => `{destination}` file is up to date.");
+				return new ResizedImageInfo { Dpi = dpi, Filename = destination };
+			}
+
 			MemoryStream memoryStream = new MemoryStream();
 			tools.Resize(dpi, destination, () => memoryStream);
 			memoryStream.Position = 0;

src/SingleProject/Resizetizer/test/UnitTests/ResizetizeImagesTests.cs

@@ -1362,6 +1362,36 @@ public void ShouldResize(string filename, string baseSize, bool resize)
 				var size = ResizeImageInfo.Parse(item);
 				Assert.Equal(resize, size.Resize);
 			}
+
+			[Theory]
+			[InlineData("android")]
+			[InlineData("uwp")]
+			[InlineData("ios")]
+			public void GenerationSkippedOnIncrementalBuild(string platform)
+			{
+				var items = new[]
+				{
+					new TaskItem("images/dotnet_logo.svg", new Dictionary<string, string>
+					{
+						["IsAppIcon"] = bool.TrueString,
+						["ForegroundFile"] = $"images/dotnet_foreground.svg",
+						["Link"] = "appicon",
+						["BackgroundFile"] = $"images/dotnet_background.svg",
+					}),
+				};
+
+				var task = GetNewTask(platform, items);
+				var success = task.Execute();
+				Assert.True(success, LogErrorEvents.FirstOrDefault()?.Message);
+
+				LogErrorEvents.Clear();
+				LogMessageEvents.Clear();
+				task = GetNewTask(platform, items);
+				success = task.Execute();
+				Assert.True(success, LogErrorEvents.FirstOrDefault()?.Message);
+
+				Assert.True(LogMessageEvents.Any(x => x.Message.Contains("Skipping ", StringComparison.OrdinalIgnoreCase)), $"Image generation should have been skipped.");
+			}
 		}
 	}
 }