TLDR：
let desktopDir = try? FileManager.default.url(for: .desktopDirectory,in: .userDomainMask,appropriateFor: nil,create: false)
let documentDir = try? FileManager.default.url(for: .documentDirectory,create: false)
let downloadsDir = try? FileManager.default.url(for: .downloadsDirectory,create: false)
从FileManager类获取此信息有两种方法：
这个：
func urls(for directory: FileManager.SearchPathDirectory,in domainMask: FileManager.SearchPathDomainMask) -> [URL]
Returns an array of URLs for the specified common directory in the requested domains. […] The directories are ordered according to the order of the domain mask constants,with items in the user domain first and items in the system domain last.
文档：https://developer.apple.com/documentation/foundation/filemanager/1407726-urls
(在@ leo-dabus的回答中已经提到了)
还有这个：
(我更喜欢,因为它更通用并返回单个URL)
func url(for directory: FileManager.SearchPathDirectory,in domain: FileManager.SearchPathDomainMask,appropriateFor url: URL?,create shouldCreate: Bool) throws -> URL
Locates and optionally creates the specified common directory in a domain.
文件：https://developer.apple.com/documentation/foundation/filemanager/1407693-url
任何一个：
https://developer.apple.com/documentation/foundation/filemanager.searchpathdomainmask中定义的有效域是：
.userDomainMask: The user’s home directory—the place to install user’s personal items (~).
.localDomainMask: Local to the
current machine—the place to install items available to everyone on
this machine.
.networkDomainMask: Publicly available location in the
local area network—the place to install items available on the network
(/Network).
.systemDomainMask: Provided by Apple—can’t be modified (/System) .
.allDomainsMask: All domains.
目录参数有很多可能性,在https://developer.apple.com/documentation/foundation/filemanager.searchpathdirectory中定义：
.applicationDirectory: Supported applications (/Applications).
.demoApplicationDirectory: Unsupported applications and demonstration
versions.
.developerApplicationDirectory: Developer applications
(/Developer/Applications).
.adminApplicationDirectory System and
network administration applications.
.libraryDirectory: Various
user-visible documentation,support,and configuration files
(/Library).
.developerDirectory: Developer resources (/Developer).
.userDirectory: User home directories (/Users).
.documentationDirectory: Documentation.
.documentDirectory: Document directory.
.coreServiceDirectory: Location of core services
(System/Library/CoreServices).
.autosavedInformationDirectory:
Location of user’s autosaved documents (Library/Autosave Information).
.desktopDirectory: Location of user’s desktop directory.
.cachesDirectory: Location of discardable cache files (Library/Caches).
.applicationSupportDirectory: Location of application support files
(Library/Application Support).
.downloadsDirectory: Location of the
user’s downloads directory.
.inputMethodsDirectory: Location of
Input Methods (Library/Input Methods).
.moviesDirectory: Location of
user’s Movies directory (~/Movies).
.musicDirectory: Location of
user’s Music directory (~/Music).
.picturesDirectory: Location of
user’s Pictures directory (~/Pictures).
.printerDescriptionDirectory: Location of system’s PPDs directory
(Library/Printers/PPDs).
.sharedPublicDirectory: Location of user’s
Public sharing directory (~/Public).
.preferencePanesDirectory:
Location of the PreferencePanes directory for use with System
Preferences (Library/PreferencePanes).
.applicationScriptsDirectory:
Location of the user scripts folder for the calling application
(~/Library/Application Scripts/),
.itemReplacementDirectory: Passed to the FileManager method
url(for:in:appropriateFor:create:) in order to create a temporary
directory.
.allApplicationsDirectory: All directories where
applications can occur.
.allLibrariesDirectory: All directories
where resources can occur.
.trashDirectory: Location of the trash directory.
