iOS6 中的 iOS7 特定代码

Question

我需要在某些地方使用iOS7特定的代码，通常直到现在这还没有引起太大问题。对于第一个 if 语句，我尝试了一些不同的方法，接缝下方的语句是推荐的方法。没有一个有效。我得到的错误是这样的：

dyld: Symbol not found: _UITransitionContextToViewControllerKey
  Referenced from: /Users/pese/Library/Application Support/iPhone Simulator/6.1/Applications/0A4B5156-84D8-41DE-C9D1-2E4C9DB38983/aaaa.app/aaaa
  Expected in: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator6.1.sdk/System/Library/Frameworks/UIKit.framework/UIKit
 in /Users/pese/Library/Application Support/iPhone Simulator/6.1/Applications/0A4B5156-84D8-41DE-C9D1-2E4C9DB38983/aaaa.app/aaaa
Program ended with exit code: 0

和我的代码：

if ( &UITransitionContextToViewControllerKey != nil )
{
    id<UIViewControllerTransitionCoordinator> tc = self.topViewController.transitionCoordinator;
    [tc animateAlongsideTransition:^(id<UIViewControllerTransitionCoordinatorContext> context) {
        CGRect newRect = _inRect;
        if ([context viewControllerForKey:UITransitionContextToViewControllerKey] == [self.viewControllers objectAtIndex:0])
        {
            newRect = _outRect;
        }
        _backButton.frame = newRect;
    } completion:^(id<UIViewControllerTransitionCoordinatorContext> context) {
        BOOL enableSwipeToGoBack = YES;
        if ([context viewControllerForKey:UITransitionContextToViewControllerKey] == [self.viewControllers objectAtIndex:0] && ![context isCancelled])
        {
            enableSwipeToGoBack = NO;
        }
        self.interactivePopGestureRecognizer.enabled = enableSwipeToGoBack;
    }];
}

如果我只是在 if 语句中输入 NO，它可以工作，但我想编译器会在编译期间删除代码。如果我用nil替换两个UITransitionContextToViewControllerKey，它也可以工作。此外，导致错误的符号在UIKit/UIViewControllerTransitioning.h中定义，如下所示：

UIKIT_EXTERN NSString *const UITransitionContextToViewControllerKey NS_AVAILABLE_IOS(7_0);

任何帮助将不胜感激。

溶液：

使 UIKit 框架可选，并在测试时更改为：

NSString * const *exists = &UITransitionContextToViewControllerKey;
if ( exists != NULL )
    ....

Answer 1

正如其他人所提到的，测试功能通常更好，但您可以使用编译器指令根据操作系统版本有条件地进行编译。

#ifdef __IPHONE_OS_VERSION_MIN_REQUIRED
    // target is iOS
    #if __IPHONE_OS_VERSION_MIN_REQUIRED < 70000
    // target is lower than iOS 7.0
    NSLog(@"This message should only appear if iOS version is 6.x or lower");
    #else
    // target is at least iOS 7.0
    #endif
#endif