IOS5 的5个问题

1.缓存和临时文件夹的语义问题

虽然苹果应用使用的临时文件夹同跟目录下的 /tmp 并非同一个文件夹，但是开发者不应该认为它们有什么显著的差别，他们都是临时文件夹，所以性质相同。如果你有过 OS X 、Unix 或 Linux 背景，你应该能理解，苹果其实没有义务向用户保证缓存和临时文件到底能保存多久。

即使你以前从来没有听说过临时文件夹和缓存文件夹里面的东西会被系统删除这种情况，也不代表这个情况将来会有所改善，特别是苹果已经在文件中申明这些。

2.苹果相关文件的变化

最晚到2011年6月29日的时候，iOS发布之前，苹果的文件(web.archive.org存档，被墙)中涉及到<Application_Home>/Documents 的部分是这样描述的：

Use this directory to store user documents and application data files.

使用这个文件夹来存储用户文件和应用数据文件。

这个说明相当容易理解，这也很清楚的说明了为什么开发者对 iOS 5 中文件目录规则的改变不满。

关于<Application_Home>/Library/Caches 是这样说明的：

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.

使用这个文件夹来写应用在运行期间和更新期间需要用到的任何文件。你的应用必须负责好添加和删除这些文件。而且这些文件应该是可以被重建的，因为 iTunes 会在还原设备备份的时候删除这些文件。

这段说明中的措词，不可否认是比较模糊的。读这段说明的时候我的第一印象就是苹果在 iOS 5 中实施了一些与本条款相矛盾的根本性改变。但换个角度来说，通过”generally” 和 “able to re-create” 这些字眼，可以明显看出苹果基本上就是在警告开发者系统将怎么对待这些文件了。

iCloud 数据存储准则(需登录查看) 这样说明：

To ensure that backups are as efficient as possible, be sure to store your app’s data according to the following guidelines:

1. Only documents and other data that is user-generated, or that cannot otherwise be recreated by your application, should be stored in the <Application_Home>/Documents directory and will be automatically backed up by iCloud.

2. Data that can be downloaded again or regenerated should be stored in the <Application_Home>/Library/Caches directory. Examples of files you should put in the Caches directory include database cache files and downloadable content, such as that used by magazine, newspaper, and map applications.

3. Data that is used only temporarily should be stored in the <Application_Home>/tmp directory. Although these files are not backed up to iCloud, 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.