为了保证兼容性，我们常常需要将app在各种iOS版本下测试。但是通常，xcode最新版会将旧的sdk覆盖。拿最新的xcode4.2来说，升级安装后，只有iOS4.3和iOS5的模拟器。解决办法是我们手工来增加旧版的模拟器。具体如下：
首先下载旧版的SDK，对应的下载链接如下（需要先登陆开发者账号）:
- iPhone SDK 2.2.1
- iPhone SDK 3.0 with Xcode 3.1.3 – Leopard
- iPhone SDK 3.0 with Xcode 3.2 – Snow Leopard
- iPhone SDK 3.1 with Xcode 3.1.4 – Leopard
- iPhone SDK 3.1 with Xcode 3.2.1 – Snow Leopard
- iPhone SDK 3.1.2 with Xcode 3.1.4 – Leopard
- iPhone SDK 3.1.2 with Xcode 3.2.1 – Snow Leopard
- iPhone SDK 3.1.3 with Xcode 3.1.4 – Leopard
- iPhone SDK 3.1.3 with Xcode 3.2.1 – Snow Leopard
- iPhone SDK 3.2 Beta 4 with Xcode 3.2.2 – Snow Leopard
- iPhone SDK 3.2 Final with Xcode 3.2.2 – Snow Leopard
- iPhone SDK 4 Final with Xcode 3.2.3 – Snow Leopard
- iPhone SDK 4.0.1 with Xcode 3.2.3 – Snow Leopard
- iPhone SDK 4.0.2 with Xcode 3.2.3 – Snow Leopard
- iPhone SDK 4.1 with Xcode 3.2.4 – Snow Leopard
然后将旧版安装在不同目录。之后用ln将这些模拟器链到当前的xcode下面，示例代码如下：
cd /Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs
sudo ln -s /Developer-3.2.5/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator4.0.sdk .
sudo ln -s /Developer-3.2.5/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator4.1.sdk .
之后启动xcode，就可以看到相应的模拟器选项了。From: http://tangqiaoboy.blog.163.com/blog/static/116114258201191323919494/