从＆amp;读取和写入数据使用Swift逐步增加文档目录中的文本文件

Question

Swift 4：如您所知，以下是如何从＆amp ;;读取或写入数据。使用Swift文档目录中的文本文件：

        let DocumentDirURL = try! FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: true)

        let fileURL = DocumentDirURL.appendingPathComponent("Test").appendingPathExtension("txt")
        print("FilePath: \(fileURL.path)")

        let writeString = "Add string"

        do {
            // Write to the file

            try writeString.write(to: fileURL, atomically: true, encoding: String.Encoding.utf8)

        } catch let error as NSError {

            print("Failed writing to URL: \(fileURL), Error: " + error.localizedDescription)

        }

        var readString = "" // Used to store the file contents

        do {

            // Read the file contents

            readString = try String(contentsOf: fileURL)

        } catch let error as NSError {

            print("Failed reading from URL: \(fileURL), Error: " + error.localizedDescription)

        }

        print("File Text: \(readString)")

它会将Add string写入Test.txt文件。

下次将"Add string"更改为"Add some other string."时，Test.txt文件将更改为Add some other string.

这意味着＆＃34;添加字符串＆＃34;将替换为＆＃34;添加其他字符串。＆＃34;。但我想逐步将这两个字符串添加到Test.file中，以便Test.txt文件包含：添加字符串，添加其他字符串。

有没有办法让它成真？感谢。