薄弱环节框架
由于 adMob 包的最新版本，我已将 MessageUI 框架添加到我的项目中。由于我希望将应用程序部署到 2.x 操作系统设备，因此我按照建议使用了弱链接 MessageUI。
如果我为 iPhone 设备 3.0 构建，它工作得很好。
如果我为 iPhone 设备 2.2.1 构建，我会收到链接错误：“ld：未找到框架 MessageUI”
我哪里错了？
Due to the last release of the adMob package, I have added the MessageUI framework to my project. As I wish to deploy my application to 2.x OS devices, I have weak linked MessageUI as advised.
If I build for iPhone device 3.0, it works fine.
If I build for iPhone device 2.2.1, I get a link error: "ld: framework not found MessageUI"
Where could I be wrong?
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这个答案缺少重要信息 -
您还必须手动设置特定框架以链接为“弱”而不是“必需”：
在 xcode 中 - 右键单击目标（!!!!） ->获取信息->在“常规”选项卡的链接库列表中，右侧有一个标题为“类型”的列。
对于以前版本的 IOS 中不存在的框架，需要在那里执行更改。
This answer is missing important info -
You also have to manually set the specific framework to link as "weak" instead of "required" :
in xcode - right click on the TARGET (!!!!) -> get info -> in the "General" tab, in the linked libraries list, you have a column on the right side titled "type".
The change needs to be performed there for frameworks that don't exist in previous versions of IOS.
您收到该错误是因为您正在针对未实现 MessageUI 框架的 SDK 版本进行构建。
您需要做的是针对 iPhone OS 3.0 进行构建，但在目标的构建设置中将 iPhone OS 部署目标设置为 iPhone OS 2.0（或您希望最终应用程序支持的任何最低版本）。这样，您就可以针对较新的框架进行弱链接，但仍然可以将应用程序部署到较旧的设备上。
You are getting that error because you are building against a version of the SDK that does not implemement the MessageUI framework.
What you need to do is to build for iPhone OS 3.0, but in the build settings for your target set the iPhone OS Deployment Target to iPhone OS 2.0 (or whatever minimum version you'd like to support with your final application). This way, you weak-link against the newer framework, but can still deploy the application to older devices.