Xcode is an integrated development environment (IDE) containing a suite of software development tools developed by Apple for developing software for OS X and iOS. First released in 2003, the latest stable release is version 6.4 and is available via the Mac App Store free of charge for OS X Yosemite users.[1] Registered developers can download preview releases and previous versions of the suite through the Apple Developerwebsite.[2] However, Apple recently made a beta version of version 7.x of the software available to those of the public with Apple Developer accounts.[3]
1.在Xcode下运行不同的sdk 可以打开 Xcode 显示包内容 找到Contents－》Developer－》Platforms－》iPhoneOS.platform－》Developer－》SDKs 文件下增加对应的 sdk文件夹 （例如 增加 iPhoneOS6.1.sdk 和 iPhoneOS7.0.sdk）这样 Xcode 同时拥有2个 sdk
2.存在多个Xcode,下载其他版本.dmg进行安装，选择保留两者即可
当系统中有多个版本的 Xcode 时，可以使用‘xcode-select’命令选择默认版本
sudo xcode-select -switch /Applications/Xcode.app/Contents/Developer
Ref:https://en.m.wikipedia.org/wiki/Xcode
Ref:http://pan.baidu.com/disk/home#path=%252FXcode
Ref:https://developer.apple.com/downloads/
Ref:http://adcdownload.apple.com/Developer_Tools/Xcode_[6.3.2]/Xcode_[6.3.2].dmg