__has_feature总结

持续更新中.....

objc_arc

解释：ARC支持

#if __has_feature(objc_arc)
    //    否则执行MAC
    NSLog(@"ARC");
#else
    NSLog(@"MRC");
# endif

objc_class_property

解释：类属性
SDWebImage中有：

// class property is available in Xcode 8. We will drop the Xcode 7.3 support in 5.x
#if __has_feature(objc_class_property)
/// Fade transition.
@property (nonatomic, class, nonnull, readonly) SDWebImageTransition *fadeTransition;
/// Flip from left transition.
@property (nonatomic, class, nonnull, readonly) SDWebImageTransition *flipFromLeftTransition;
/// Flip from right transition.
@property (nonatomic, class, nonnull, readonly) SDWebImageTransition *flipFromRightTransition;
/// Flip from top transition.
@property (nonatomic, class, nonnull, readonly) SDWebImageTransition *flipFromTopTransition;
/// Flip from bottom transition.
@property (nonatomic, class, nonnull, readonly) SDWebImageTransition *flipFromBottomTransition;
/// Curl up transition.
@property (nonatomic, class, nonnull, readonly) SDWebImageTransition *curlUpTransition;
/// Curl down transition.
@property (nonatomic, class, nonnull, readonly) SDWebImageTransition *curlDownTransition;
#else
+ (nonnull instancetype)fadeTransition;
+ (nonnull instancetype)flipFromLeftTransition;
+ (nonnull instancetype)flipFromRightTransition;
+ (nonnull instancetype)flipFromTopTransition;
+ (nonnull instancetype)flipFromBottomTransition;
+ (nonnull instancetype)curlUpTransition;
+ (nonnull instancetype)curlDownTransition;
#endif