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Platform Dependent Compilation

Unity includes a feature named "Platform Dependent Compilation". This consists of some preprocessor directives that let you partition your scripts to compile and execute a section of code exclusively for one of the supported
platforms.

Furthermore, you can run this code within the Editor, so you can compile the code specifically for your mobile/console and test it in the Editor!

Platform Defines

The platform defines that Unity supports for your scripts are:

UNITY_EDITOR	Define for calling Unity Editor scripts from your game code.
UNITY_STANDALONE_OSX	Platform define for compiling/executing code specifically for Mac OS (This includes Universal, PPC and Intel architectures).
UNITY_DASHBOARD_WIDGET	Platform define when creating code for Mac OS dashboard widgets.
UNITY_STANDALONE_WIN	Use this when you want to compile/execute code for Windows stand alone applications.
UNITY_STANDALONE_LINUX	Use this when you want to compile/execute code for Linux stand alone applications.
UNITY_STANDALONE	Use this to compile/execute code for any standalone platform (Mac, Windows or Linux).
UNITY_WEBPLAYER	Platform define for web player content (this includes Windows and Mac Web player executables).
UNITY_WII	Platform define for compiling/executing code for the Wii console.
UNITY_IPHONE	Platform define for compiling/executing code for the iPhone platform.
UNITY_ANDROID	Platform define for the Android platform.
UNITY_PS3	Platform define for running PlayStation 3 code.
UNITY_XBOX360	Platform define for executing Xbox 360 code.
UNITY_NACL	Platform define when compiling code for Google native client (this will be set additionally to UNITY_WEBPLAYER).
UNITY_FLASH	Platform define when compiling code for Adobe Flash.
UNITY_BLACKBERRY	Platform define for a Blackberry10 device.
UNITY_WP8	Platform define for Windows Phone 8.
UNITY_METRO	Platform define for Windows Store Apps (additionally NETFX_CORE is defined when compiling C# files against .NET Core).

详见http://docs.unity3d.com/Documentation/Manual/PlatformDependentCompilation.html