每个iOS程序都有一个独立的文件系统（存储空间），而且只能在对应的文件系统中进行操作，此区域被称为沙盒。应用必须待在自己的沙盒里，其他应用不能访问该沙盒。所有的非代码文件都要保存在此，例如属性文件plist、文本文件、图像、图标、媒体资源等。
iOS app 沙盒目录结构
沙盒中相关路径
AppName.app应用程序的程序包目录，包含应用程序的本身。由于应用程序必须经过签名，所以不能在运行时对这个目录中的内容进行修改，否则会导致应用程序无法启动。Documents/保存应用程序的重要数据文件和用户数据文件等。用户数据基本上都放在这个位置(例如从网上下载的图片或音乐文件)，该文件夹在应用程序更新时会自动备份，在连接iTunes时也可以自动同步备份其中的数据。-
Library：这个目录下有两个子目录,可创建子文件夹。可以用来放置您希望被备份但不希望被用户看到的数据。该路径下的文件夹，除Caches以外，都会被iTunes备份.Library/Caches: 保存应用程序使用时产生的支持文件和缓存文件(保存应用程序再次启动过程中需要的信息)，还有日志文件最好也放在这个目录。iTunes 同步时不会备份该目录并且可能被其他工具清理掉其中的数据。
Library/Preferences: 保存应用程序的偏好设置文件。NSUserDefaults类创建的数据和plist文件都放在这里。会被iTunes备份。 tmp/: 保存应用运行时所需要的临时数据。不会被iTunes备份。iPhone重启时，会被清空。
// 获取沙盒根目录路径
NSString *homeDir = NSHomeDirectory();
// 获取Documents目录路径
NSString *docDir = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserDomainMask,YES) firstObject];
//获取Library的目录路径
NSString *libDir = [NSSearchPathForDirectoriesInDomains(NSLibraryDirectory,NSUserDomainMask,YES) lastObject];
// 获取cache目录路径
NSString *cachesDir = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory,NSUserDomainMask,YES) firstObject];
// 获取tmp目录路径
NSString *tmpDir =NSTemporaryDirectory();
// 获取应用程序程序包中资源文件路径的方法：
NSString *bundle = [[NSBundle mainBundle] bundlePath];
NSLog(@"homeDir=%@ \n docDir=%@ \n libDir=%@ \n cachesDir=%@ \n tmpDir=%@ \n bundle=%@", homeDir,docDir, libDir, cachesDir, tmpDir, bundle);
结果为：
homeDir=/var/mobile/Containers/Data/Application/FBE41071-F5DD-4F02-9DFC-E9E473D3C917
docDir=/var/mobile/Containers/Data/Application/FBE41071-F5DD-4F02-9DFC-E9E473D3C917/Documents
libDir=/var/mobile/Containers/Data/Application/FBE41071-F5DD-4F02-9DFC-E9E473D3C917/Library
cachesDir=/var/mobile/Containers/Data/Application/FBE41071-F5DD-4F02-9DFC-E9E473D3C917/Library/Caches
tmpDir=/private/var/mobile/Containers/Data/Application/FBE41071-F5DD-4F02-9DFC-E9E473D3C917/tmp/
bundle=/var/containers/Bundle/Application/4A42EF08-30BA-4C19-B0C9-62903300C2BC/SandboxDemo.app
关于NSSearchPathForDirectoriesInDomains函数
FOUNDATION_EXPORT NSArray<NSString *> *NSSearchPathForDirectoriesInDomains(NSSearchPathDirectory directory, NSSearchPathDomainMask domainMask, BOOL expandTilde); 用于查找目录，返回指定范围内的指定名称的目录的路径集合。有三个参数：
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NSSearchPathDirectory directory想要查找的目录，是个枚举值，有很多值，有关于iOS的，有关于macOS，也有关于watchOS的。
typedef NS_ENUM(NSUInteger, NSSearchPathDirectory) {
NSApplicationDirectory = 1, // supported applications (Applications)
NSDemoApplicationDirectory, // unsupported applications, demonstration versions (Demos)
NSDeveloperApplicationDirectory, // developer applications (Developer/Applications). DEPRECATED - there is no one single Developer directory.
NSAdminApplicationDirectory, // system and network administration applications (Administration)
NSLibraryDirectory, // various documentation, support, and configuration files, resources (Library)
NSDeveloperDirectory, // developer resources (Developer) DEPRECATED - there is no one single Developer directory.
NSUserDirectory, // user home directories (Users)
NSDocumentationDirectory, // documentation (Documentation)
NSDocumentDirectory, // documents (Documents)
NSCoreServiceDirectory, // location of CoreServices directory (System/Library/CoreServices)
NSAutosavedInformationDirectory API_AVAILABLE(macos(10.6), ios(4.0), watchos(2.0), tvos(9.0)) = 11, // location of autosaved documents (Documents/Autosaved)
NSDesktopDirectory = 12, // location of user's desktop
NSCachesDirectory = 13, // location of discardable cache files (Library/Caches)
NSApplicationSupportDirectory = 14, // location of application support files (plug-ins, etc) (Library/Application Support)
NSDownloadsDirectory API_AVAILABLE(macos(10.5), ios(2.0), watchos(2.0), tvos(9.0)) = 15, // location of the user's "Downloads" directory
NSInputMethodsDirectory API_AVAILABLE(macos(10.6), ios(4.0), watchos(2.0), tvos(9.0)) = 16, // input methods (Library/Input Methods)
NSMoviesDirectory API_AVAILABLE(macos(10.6), ios(4.0), watchos(2.0), tvos(9.0)) = 17, // location of user's Movies directory (~/Movies)
NSMusicDirectory API_AVAILABLE(macos(10.6), ios(4.0), watchos(2.0), tvos(9.0)) = 18, // location of user's Music directory (~/Music)
NSPicturesDirectory API_AVAILABLE(macos(10.6), ios(4.0), watchos(2.0), tvos(9.0)) = 19, // location of user's Pictures directory (~/Pictures)
NSPrinterDescriptionDirectory API_AVAILABLE(macos(10.6), ios(4.0), watchos(2.0), tvos(9.0)) = 20, // location of system's PPDs directory (Library/Printers/PPDs)
NSSharedPublicDirectory API_AVAILABLE(macos(10.6), ios(4.0), watchos(2.0), tvos(9.0)) = 21, // location of user's Public sharing directory (~/Public)
NSPreferencePanesDirectory API_AVAILABLE(macos(10.6), ios(4.0), watchos(2.0), tvos(9.0)) = 22, // location of the PreferencePanes directory for use with System Preferences (Library/PreferencePanes)
NSApplicationScriptsDirectory NS_ENUM_AVAILABLE(10_8, NA) = 23, // location of the user scripts folder for the calling application (~/Library/Application Scripts/code-signing-id)
NSItemReplacementDirectory API_AVAILABLE(macos(10.6), ios(4.0), watchos(2.0), tvos(9.0)) = 99, // For use with NSFileManager's URLForDirectory:inDomain:appropriateForURL:create:error:
NSAllApplicationsDirectory = 100, // all directories where applications can occur
NSAllLibrariesDirectory = 101, // all directories where resources can occur
NSTrashDirectory API_AVAILABLE(macos(10.8), ios(11.0)) API_UNAVAILABLE(watchos, tvos) = 102 // location of Trash directory
};
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NSSearchPathDomainMask domainMask表示“想要从哪个路径区域保护区查找”。
typedef NS_OPTIONS(NSUInteger, NSSearchPathDomainMask) {
NSUserDomainMask =1, // 用户的主目录
NSLocalDomainMask =2, // 当前机器的本地目录
NSNetworkDomainMask =4, //在网络中公开可用的位置
NSSystemDomainMask =8, // 被苹果系统提供的，不可更改的位置 (/System)
NSAllDomainsMask = 0x0ffff // 上述所有及未来的位置
};
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BOOL expandTilde表示是否用波浪线显示部分目录路径。~在*nix系统表示当前用户的Home目录。列如上面获取cache目录路径如果使用NO，那么结果就是cachesDir=~/Library/Caches
参考：
沙盒和NSBundle
NSSearchPathForDirectoriesInDomains方法使用
沙盒和NSBundle
文件系统官方手册
