iOS App 升級時文件的保留狀況

有這樣一個問題，用戶用iOS設備下載了大量的數據，保存在設備本地，若是用戶升級了應用自己，這些文件是否仍然存在。

找了一些官方解釋和Stackflow上的說明

Where You Should Put Your App’s Files

To prevent the syncing and backup processes on iOS devices from taking a long time, be selective about where you place files inside your app’s home directory. Apps that store large files can slow down the process of backing up to iTunes or iCloud. These apps can also consume a large amount of a user's available storage, which may encourage the user to delete the app or disable backup of that app's data to iCloud. With this in mind, you should store app data according to the following guidelines:

• Put user data in the <Application_Home>/Documents/. User data is any data that cannot be recreated by your app, such as user documents and other user-generated content.

• Handle support files—files your application downloads or generates and can recreate as needed—in one of two ways:

  • In iOS 5.0 and earlier, put support files in the <Application_Home>/Library/Caches directory to prevent them from being backed up

  • In iOS 5.0.1 and later, put support files in the <Application_Home>/Library/Application Support directory and apply thecom.apple.MobileBackup extended attribute to them. This attribute prevents the files from being backed up to iTunes or iCloud. If you have a large number of support files, you may store them in a custom subdirectory and apply the extended attribute to just the directory.

• Put data cache files in the <Application_Home>/Library/Caches directory. Examples of files you should put in this directory include (but are not limited to) database cache files and downloadable content, such as that used by magazine, newspaper, and map apps. Your app should be able to gracefully handle situations where cached data is deleted by the system to free up disk space.

• Put temporary data in the <Application_Home>/tmp directory. Temporary data comprises any data that you do not need to persist for an extended period of time. Remember to delete those files when you are done with them so that they do not continue to consume space on the user’s device.

也就是說，通常從網上下載的那些內容，包括雜誌、報紙、視頻什麼的，都要放在 <Application_Home> /Library/Caches  目錄下面

<Application_Home>/Library/

This directory is the top-level directory for files that are not user data files. You typically put files in one of several standard subdirectories but you can also create custom subdirectories for files you want backed up but not exposed to the user. You should not use this directory for user data files. The contents of this directory (with the exception of the Caches subdirectory) are backed up by iTunes. For additional information about the Library directory, see 「The Library Directory Stores App-Specific Files.」

Library目錄下面的內容，也會被被iTunes備份，可是Caches目錄除外，也就是說這些網絡下載內容文件放在這裏是不會被備份的，

Caches

Use this directory to write any app-specific support files that your app can re-create easily. Your app is generally responsible for managing the contents of this directory and for adding and deleting files as needed. In iOS 2.2 and later, the contents of this directory are not backed up by iTunes. In addition, iTunes removes files in this directory during a full restoration of the device. On iOS 5.0 and later, the system may delete the Caches directory on rare occasions when the system is very low on disk space. This will never occur while an app is running. However, you should be aware that iTunes restore is not necessarily the only condition under which the Caches directory can be erased.

上面寫的的比較清楚，Library/Caches目錄下，通常狀況下是不會被刪除的，我理解，也包括stackoverflow網友的證實，應用升級的時候也不會被刪除，實際上，應用升級的具體系統操做是這樣的：

Files Saved During Application Updates When a user downloads an application update, iTunes installs the update in a new application directory. It then moves the user’s data files from the old installation over to the new application directory before deleting the old installation. Files in the following directories are guaranteed to be preserved during the update process:

• Application_Home/Documents
• Application_Home/Library

Although files in other user directories may also be moved over, you should not rely on them being present after an update.

系統先新建一個應用目錄，而後安裝一個新版本應用，而後再把舊版本的應用數據拷貝過來，而後再刪除舊版本的應用安裝目錄。

最後注意一下，Library/Caches目錄下的數據雖然在通常狀況下不會被刪除，可是在系統恢復和其餘一些罕見的狀況下（好比很是底的磁盤空間條件下有可能）會被清除，因此你的程序每次都要檢查一些，這些文件都還在不在了，不在的話只能從新下了。

Application_Home/Library/Caches Use this directory to write any application-specific support files that you want to persist between launches of the application or during application updates. Your application is generally responsible for adding and removing these files. It should also be able to re-create these files as needed because iTunes removes them during a full restoration of the device. In iOS 2.2 and later, the contents of this directory are not backed up by iTunes.

參考鏈接：http://stackoverflow.com/questions/7155964/will-application-home-library-caches-be-clear-on-app-update/7277797#7277797

File System Programming Guide

https://developer.apple.com/library/ios/documentation/FileManagement/Conceptual/FileSystemProgrammingGuide/FileSystemOverview/FileSystemOverview.html#//apple_ref/doc/uid/TP40010672-CH2-SW1