我在iPad App中使用了NSCachesDirectory来存储图像和PDF。现在，我的客户的抱怨是他在下载几个小时后无法看到下载的内容。 iPad有iOS 11.2.1。
在乞讨时，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中。但是，令人惊讶的是，iPad有足够的空间，即4 GB的可用空间，下载的内容不超过500 MB。
所以，我几乎没有与此相关的问题：
（1）即使iPad中有足够的空间，为什么要删除系统 我的档案？
（2）是否可能存在删除文件的其他原因
NSCachesDirectory？（3）如果iOS系统正在删除这些文件，那么最安全的地方是什么 应用程序内存下载这些文件？我发现了
NSCachesDirectory是可丢弃的内存，因此不安全 的地方。（4）
NSDocumentDirectory将是我最合适的选择 案件 ？或者可能是Apple给出的其他选择？
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
};