While reading XXX pngcrush caught libpng error: N

While reading /XXX/XXX/XXX/img1.png pngcrush caught libpng error: Not a PNG filCould not find file: /Users/XXX/Library/Developer/Xcode/DerivedData/CookBookDemo-cnttcobrbmvbddeiesgsxnfwihfe/Build/Products/Debug-iphoneos/CookBookDemo.app/img1.pngCommand /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/copypng emitted errors but did not return a nonzero exit code to indicate failure

原因，就是引入的png圖片，圖片內部編碼不符合真正的png格式。（很可能就是jpg圖片修改了一下後綴名）。