我NSCachesDirectory在我的 iPad 应用程序中使用过存储图像和 PDF。现在，我的客户抱怨他在下载了几个小时后就看不到下载的内容。iPad 有iOS 11.2.1.
在乞求中，我的想法是，可能存在内存不足的情况，因此，iOS 系统正在删除这些文件，因为这些文件存储在NSCachesDirectory. 但是，令人惊讶的是，iPad 中有足够的空间，即 4 GB 的可用空间和下载的内容不超过 500 MB 的大小。
所以，我有几个与此相关的问题：
(1)即使 iPad 有足够的空间，为什么系统会删除我的文件？
(2)是否有其他原因导致文件被删除
NSCachesDirectory？(3)如果 iOS 系统正在删除这些文件，那么在 App 内存中下载这些文件最安全的地方是什么？我发现那
NSCachesDirectory是可丢弃的内存，所以那不是最安全的地方。(4)将
NSDocumentDirectory是最适合我的情况的选择吗？或者可能是Apple提供的其他选择？
以下是 App 目录列表：
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
};