问题:当我们构建自己的ios app时，Xcode会优化压缩我们的程序里的png图片，如果想再次查看正常图片的话，需要反压缩，怎么做呢？
答案：用Xcode自带的工具。

Q: When I build my iOS application, Xcode optimizes the PNG files within my application’s bundle, meaning that Preview can’t display them. How can I view these optimized files?

A: This optimization is done by the pngcrush tool, which you can find inside Xcode. The pngcrush tool supports a command line option, -revert-iphone-optimizations, that undoes the optimizations done during the Xcode build process. So, to view an optimized PNG file, you should first undo the optimization and then open it with Preview.

Listing 1 shows how you can use the pngcrush tool to convert an iOS-optimized PNG file (Local.png) to a standard PNG file (Local-standard.png).

Listing 1: Undoing iOS PNG optimization

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$ /Applications/Xcode.app/Contents/Developer\
/Platforms/iPhoneOS.platform/Developer/usr/bin/pngcrush \
-revert-iphone-optimizations -q Local.png Local-standard.png

If you have Xcode installed in a non-standard place, you can locate the tool using xcode-select, as shown in Listing 2.

Listing 2: Using xcode-select to locate pngcrush

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$ "`xcode-select -print-path`"\
/Platforms/iPhoneOS.platform/Developer/usr/bin/pngcrush \
-revert-iphone-optimizations -q Local.png Local-standard.png