【xcconfig】
1.When you can use a .xcconfig file?
Use .xcconfig files if you find yourself changing the same build settings for each project you create. Place the build settings you're changing in the .xcconfig file. By using a .xcconfig file you can avoid editing those build settings every time you create a project.
Having the option to use different .xcconfig files for each build configuration allows you to place the debug build settings you're constantly changing in one .xcconfig file and place the release build settings you're changing in another xcconfig file. Use multiple .xcconfig files if the build settings you're changing have different values for debug and release builds or if you change some build settings only for debug builds and other build settings only for release builds.
2.How to create a .xcconfig file?
New File----->iOS Other------>Configuration Settings File
3.How to add the .xcconfig file to the project and do some settings?
Go to your project's info screen, under the configurations section expand the list until you find the target you want your scheme to apply to, then select it from the drop down box.
4. How to write the .xcconfig file?
Look for a build setting you want to add to your .xcconfig file in the build settings list, set it to the value you want it to be, then select that row, hit Command+C to copy, then push delete to get rid of your setting. And then, go into your .xcconfig file and paste the contents of your clipboard. You will notice that you copied the key and value.
参考：
1、http://blog.163.com/chenchen..1986/blog/static/7606314620125541841397
