常用的Objective-C特性检查

• 检查是否支持blocks

 __has_extension(blocks)

• 检查是否支持instancetye上下文关键词

__has_feature(objc_instancetype)

@interface A+ (instancetype)constructAnA;@end

• 检查是否支持arc

__has_feature(objc_arc)

__has_feature(objc_arc_weak) //同时检查是否支持__weak指针

• 检查是否支持固定基础类型的枚举

__has_feature(objc_fixed_enum)

typedef enum : unsigned char { Red, Green, Blue } Color;

• 检查是否支持对象字面值

__has_feature(objc_array_literals)

__has_feature(objc_dictionary_literals)

NSArray *array = @[ @"Hello", NSApp, [NSNumber numberWithInt:42] ];

NSDictionary *dictionary = @{    @"name" : NSUserName(),    @"date" : [NSDate date],    @"processInfo" : [NSProcessInfo processInfo]};

• 检查是否支持对象下标（OC的对象指针现在可以像C一样做下标操作）

__has_feature(objc_subscripting)

NSMutableArray *array = ...;NSUInteger idx = ...;id newObject = ...;id oldObject = array[idx];array[idx] = newObject;         // replace oldObject with newObjectNSMutableDictionary *dictionary = ...;NSString *key = ...;oldObject = dictionary[key];dictionary[key] = newObject;    // replace oldObject with newObject

• 检查是否支持属性的自动合成（不使用@dynamic的情况下，自动生成存取方法）

__has_feature(objc_default_synthesize_properties)

使用范例（来源：MBProgressHUD源码）：

#ifndef MB_INSTANCETYPE#if __has_feature(objc_instancetype)#define MB_INSTANCETYPE instancetype#else#define MB_INSTANCETYPE id#endif#endif#ifndef MB_STRONG#if __has_feature(objc_arc)#define MB_STRONG strong#else#define MB_STRONG retain#endif#endif#ifndef MB_WEAK#if __has_feature(objc_arc_weak)#define MB_WEAK weak#elif __has_feature(objc_arc)#define MB_WEAK unsafe_unretained#else#define MB_WEAK assign#endif#endif#if NS_BLOCKS_AVAILABLEtypedef void (^MBProgressHUDCompletionBlock)();#endif