NSSearchPathForDirectoriesInDomains已经过时了。推荐使用FileManager的URL相关接口

let folderName = "MyNewFolder"
let fileManager = FileManager.default
let documentsFolder = try! fileManager.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: false)
let folderURL = documentsFolder.appendingPathComponent(folderName)
let folderExists = (try? folderURL.checkResourceIsReachable()) ?? false
do {
    if !folderExists {
        try fileManager.createDirectory(at: folderURL, withIntermediateDirectories: false)
    }
    let fileURL = folderURL.appendingPathComponent("log.txt")
    let hello = Data("hello".utf8)
    try hello.write(to: fileURL)
    
} catch { print(error) }

强烈建议不要通过连接字符串来构建路径。

更新：

在iOS 16+，macOS 13+中，Apple移动了API以将标准文件夹的URL导入URL。appendingPathComponent也已更改，以便能够告诉编译器路径表示目录

let folderName = "MyNewFolder"
let folderURL = URL.documentsDirectory.appending(path: folderName, directoryHint: .isDirectory)
let folderExists = (try? folderURL.checkResourceIsReachable()) ?? false
do {
    if !folderExists {
        try fileManager.createDirectory(at: folderURL, withIntermediateDirectories: false)
    }
    let fileURL = folderURL.appendingPathComponent("log.txt")
    let hello = Data("hello".utf8)
    try hello.write(to: fileURL)
    
} catch { print(error) }

你可以尝试

try? FileManager.default.createDirectory(atPath: dataPath, withIntermediateDirectories: false, attributes: nil)

do { 

    let sto =  URL(fileURLWithPath: dataPath + "log.txt")  // or let sto =  URL(fileURLWithPath: dataPath + "/log.txt") 

    try Data("SomeValue".utf8).write(to: sto)

    let read = try Data(contentsOf: sto)

    print(String(data: read, encoding: .utf8)!)
}
catch {

    print(error)
}

let filePath = dataPath + "/log.txt"
FileManager.default.createFile(filePath, contents:dataWithFileContents, attributes:nil)