bitcode

bitcode
1.原由
错误：An App ID with Identifier ‘com.domainName.AppName’ is not available. Please enter a different string.
解决：

原理：”Xcode 7 has a ENABLE_BITCODE option to embed bitcode in apps, app extensions, and frameworks. The option is turned on by default for iOS and is mandatory for watchOS projects submitted to the store. When bitcode is enabled for a target, all the objects, static libraries and user frameworks used when linking that target must contain bitcode. Otherwise, an error or a warning will be issued by the linker. (Note: missing bitcode is currently a warning for iOS, but it will become an error in an upcoming beta release of Xcode 7.) ENABLE_BITCODE should be consistently turned on for all the targets. If you use a library or framework provided by a third party, please contact the vendor for an updated version which contains bitcode.” Dear God, do we need to wait for all libs to update? :S
2.bitcode
Bitcode is an intermediate representation of a compiled program. Apps you upload to iTunes Connect that contain bitcode will be compiled and linked on the App Store. Including bitcode will allow Apple to re-optimize your app binary in the future without the need to submit a new version of your app to the store.
说的是bitcode是被编译程序的一种中间形式的代码。包含bitcode配置的程序将会在App store上被编译和链接。bitcode允许苹果在后期重新优化我们程序的二进制文件，而不需要我们重新提交一个新的版本到App store上。
继续看，在What’s New in Xcode-New Features in Xcode 7中，还有一段如下的描述
Bitcode. When you archive for submission to the App Store, Xcode will compile your app into an intermediate representation. The App Store will then compile the bitcode down into the 64 or 32 bit executables as necessary.
当我们提交程序到App store上时，Xcode会将程序编译为一个中间表现形式(bitcode)。然后App store会再将这个botcode编译为可执行的64位或32位程序。
3.Bitcode配置
在上面的错误提示中，提到了如何处理我们遇到的问题：
You must rebuild it with bitcode enabled (Xcode setting ENABLE_BITCODE), obtain an updated library from the vendor, or disable bitcode for this target. for architecture arm64
要么让第三方库支持，要么关闭target的bitcode选项。
实际上在Xcode 7中，我们新建一个iOS程序时，bitcode选项默认是设置为YES的。我们可以在”Build Settings”->”Enable Bitcode”选项中看到这个设置。
不过，我们现在需要考虑的是三个平台：iOS，Mac OS，watchOS。
* 对应iOS，bitcode是可选的。
* 对于watchOS，bitcode是必须的。
* Mac OS不支持bitcode。
4.引申－－－LLVM 命名最早源自于底层虚拟机（Low Level Virtual Machine）。LLVM是构架编译器(compiler)的框架系统，以C++编写而成，用于优化以任意程序语言编写的程序的编译时间(compile-time)、链接时间(link-time)、运行时间(run-time)以及空闲时间(idle-time)，对开发者保持开放，并兼容已有脚本