Use file hash to check for file changes before transpiling (#299)

src/React.Core/Babel.cs

@@ -350,12 +350,36 @@ string filename
 		)
 		{
 			var outputPath = GetOutputPath(filename);
-			var sourceMapPath = GetSourceMapOutputPath(filename);
 			var contents = _fileSystem.ReadAsString(filename);
-			var result = TransformWithHeader(filename, contents, null);
-			_fileSystem.WriteAsString(outputPath, result.Code);
-			_fileSystem.WriteAsString(sourceMapPath, result.SourceMap == null ? string.Empty : result.SourceMap.ToJson());
-			return outputPath;
+            if (MustTranspileFile(contents, outputPath))
+            {
+                var result = TransformWithHeader(filename, contents, null);
+
+                var sourceMapPath = GetSourceMapOutputPath(filename);
+                _fileSystem.WriteAsString(outputPath, result.Code);
+                _fileSystem.WriteAsString(sourceMapPath, result.SourceMap == null ? string.Empty : result.SourceMap.ToJson());                
+            }
+		    return outputPath;
 		}
+
+        /// <summary>
+        /// Checks whether an input file (given as inputFileContents) should be transpiled
+        /// by calculating the hash and comparing it to the hash value stored
+        /// in the file given by outputPath. If the outputPath file does not
+        /// exist the input file should always be transpiled.
+        /// </summary>
+        /// <param name="inputFileContents">The contents of the input file.</param>
+        /// <param name="outputPath">The output path of the (possibly previously) generated file.</param>
+        /// <returns>Returns true if the file should be transpiled, false otherwise.</returns>
+	    public virtual bool MustTranspileFile(string inputFileContents, string outputPath)
+	    {
+	        if (!File.Exists(outputPath))
+	            return true;
+
+	        var hashForInputFile = _fileCacheHash.CalculateHash(inputFileContents);
+	        var existingOutputContents = _fileSystem.ReadAsString(outputPath);
+	        var fileHasNotChanged = _fileCacheHash.ValidateHash(existingOutputContents, hashForInputFile);
+	        return !fileHasNotChanged;
+	    }
 	}
 }