app store 增量更新

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

從iOS6.x到iOS7.0支持文件級別的增量更新，即用新文件替換掉修改過的文件。

從7.1開始支持文件內容級別的增量更新，即只更新修改過的內容。可是相對的，安裝時間會增長。

官方建議：

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.