Backwards or Forwards Compatibility mode, triggered by the version standard attribute.

version标准属性触发的向后或向前兼容模式。

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The use-when attribute can be used for forwards and backwards compatibility processing.

可使用 use-when 属性进行向前和向后兼容处理。

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Backwards compatibility (BC) and FC modes are always triggered by the version attribute, not by any processor switch or launch-time option.

向后兼容(BC)和向前兼容(FC)方式都是通过version属性触发的，而不是处理程序开关或者启动选项。

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Due to compatibility concerns, the NSBaselineOffsetAttributeName attribute is no longer supported in iOS 6.

ENGADGET: Apple seeds iOS 6 beta 4 to developers: here's the changelog