Improved implementation of updateFileContents()

BXSwiftUtils/Files/URL+Contents.swift

@@ -27,6 +27,40 @@ public extension URL
 		return urls
 	}
 
+
+	/// Updates the contents of a file on disk using the supplied handler closure.
+	///
+	/// This method can be used to modify the content of a file on disk - withour the need to read the whole file
+	/// into memory. This is useful for very large file (e.g. image or video files) and loading just small chunks
+	/// at a time is feasible.
+	///
+	/// - parameter offset: The offset from the start of the file where updating should begin
+	/// - parameter length: The number of bytes to be updated. Pass 0 to update all bytes up to end of file.
+	/// - parameter transformDataHandler: This block receives the original Data from the file and modifies it as needed
+	/// - returns: The number of bytes that were written back to the file
+
+	@discardableResult public func updateFileContents(offset: UInt64, length: Int, using transformDataHandler: (inout Data)->() ) throws -> Int
+	{
+		// Open the file for reading & writing
+
+		let handle = try FileHandle(forUpdating: self)
+
+		// Read the data from the file and transform it with the transformDataHandler
+
+		handle.seek(toFileOffset: offset)
+		var data = length>0 ? handle.readData(ofLength:length) : handle.readDataToEndOfFile()
+		transformDataHandler(&data)
+
+		// Write the modified data back to file at correct offset
+
+		handle.seek(toFileOffset: offset)
+		handle.write(data)
+		handle.synchronizeFile()
+
+		// Return the number of bytes that were written
+
+		return data.count
+	}
 }