Block使有注意点

　　在ios推出block后，apple非常推荐开发者所有。但是可能出现内存问题。

可以添加如下宏：

#ifndef    weakify
#if __has_feature(objc_arc)

#define weakify( x ) \
_Pragma("clang diagnostic push") _Pragma("clang diagnostic ignored \"-Wshadow\"") autoreleasepool{} __weak __typeof__(x) __weak_##x##__ = x; _Pragma("clang diagnostic pop")

#else

#define weakify( x ) \
_Pragma("clang diagnostic push") _Pragma("clang diagnostic ignored \"-Wshadow\"") autoreleasepool{} __block __typeof__(x) __block_##x##__ = x; _Pragma("clang diagnostic pop")

#endif
#endif

#ifndef    strongify
#if __has_feature(objc_arc)

#define strongify( x ) \
_Pragma("clang diagnostic push") _Pragma("clang diagnostic ignored \"-Wshadow\"") try{} @finally{} __typeof__(x) x = __weak_##x##__; _Pragma("clang diagnostic pop")

#else

#define strongify( x ) \
_Pragma("clang diagnostic push") _Pragma("clang diagnostic ignored \"-Wshadow\"") try{} @finally{} __typeof__(x) x = __block_##x##__; _Pragma("clang diagnostic pop")

#endif
#endif

使用时：

    @weakify(self);
    self.block = ^{
        @strongify(self);
        NSLog(@"AViewControllerAViewController %@",self);
    };