app store 增量更新

https://developer.apple.com/library/ios/qa/qa1779/_index.html

从iOS6.x到iOS7.0支持文件级别的增量更新，即用新文件替换掉修改过的文件。

从7.1开始支持文件内容级别的增量更新，即只更新修改过的内容。可是相对的，安装时间会增长。

官方建议：

To optimize the size of your app updates, you should consider two tips:

• Do not make unnecessary modifications to files. Compare the contents of the prior and new versions of your app with diff or another directory comparison tool and verify that you've only changed what you expect within your app bundle.

• Content that you expect to change in an update should be stored in separate files from content that you don't expect to change. This reduces the size of the update package and increases its install speed.