这篇文章是参考了欧阳大哥的实现并在此基础上进行了一些扩展，原文链接在此一种查看Block中引用的所有外部对象的实现方法。想要详细了解如何解析block中引用的对象直接查看原文链接即可。

扩展说明：在原作者的实现中，没有实现打印__block类型的引用对象，我在此进行了补充。下图是打印出block中所有的引用对象

打印block引用的外部对象

核心代码如下：

static NSString *block_description(id block, SEL _cmd) {
    struct Block_layout *blockLayout = (__bridge struct Block_layout *)block;
    struct Block_descriptor_1 *desc1 = blockLayout->descriptor;
    struct Block_descriptor_2 *desc2 = NULL;
    struct Block_descriptor_3 *desc3 = NULL;
    
    NSMutableString *printStr = [NSMutableString new];
    
    if (blockLayout->flags & BLOCK_HAS_COPY_DISPOSE) {
        desc2 = (struct Block_descriptor_2 *)(desc1 + 1);
    } else {
        return printStr;
    }
    
    if (blockLayout->flags & BLOCK_HAS_EXTENDED_LAYOUT) {
        desc3 = (struct Block_descriptor_3 *)(desc2 + 1);
    } else {
        return printStr;
    }
    
    if (desc3->layout == 0) {
        return printStr;
    }
    
    
    const char *extendLayout = desc3->layout;
    
    //如果layout小于0x1000，则将layout本身当成一个12bit的数据，此时是compact encoding
    //压缩编码方式0xXYZ: X表示强引用数量，Y表示__block引用数量，Z表示弱引用数量，对该编码解码，从而统一下面的处理逻辑
    if (extendLayout < (const char *)0x1000) {
        char compactEncoding[4] = {0,0,0,0};
        unsigned short xyz = (unsigned short)extendLayout;
        unsigned char x = (xyz >> 8) & 0xf;
        unsigned char y = (xyz >> 4) & 0xf;
        unsigned char z = (xyz & 0xf);
        
        int idx = 0;
        if (x) {
            x--;
            compactEncoding[idx++] = (BLOCK_LAYOUT_STRONG << 4) | x;
        }
        if (y) {
            y--;
            compactEncoding[idx++] = (BLOCK_LAYOUT_BYREF << 4) | y;
        }
        if (z) {
            z--;
            compactEncoding[idx++] = (BLOCK_LAYOUT_WEAK << 4) | z;
        }
        extendLayout = compactEncoding;
    }
    
    //上面的代码解码了compact encoding情况
    //下面进行layout的解析,block内部会优先把对象类型排在前面
    
    
    //当layout大于0x1000,layout就是指向了一个字符串的指针
    int index = 0;
    int objOffest = sizeof(struct Block_layout);//block内部对象的偏移
    
    char *typeArr[4] = {"strong  ","byref   ","weak    ","unretain"};
    char *byrefTypeArr[3] = {"__strong __block","__weak __block","__unsafe_unretained __block"};
    
    
    while (extendLayout[index] != '\0') {
        
        unsigned char PN = extendLayout[index];
        unsigned char P = (PN >> 4) & 0xf; // 类型描述，strong，byref，weak等等
        unsigned char N = (PN & 0xf) + 1; //对应类型的个数
        
        //只针对对象类型进行判断
        if (P >= BLOCK_LAYOUT_STRONG && P <= BLOCK_LAYOUT_UNRETAINED) {
            
            int typeIndex = P - BLOCK_LAYOUT_STRONG;
            
            if (P != BLOCK_LAYOUT_BYREF) {
                [printStr appendFormat:@"\n引用类型：%s, 引用对象：",typeArr[typeIndex]];
                for (int i = 0; i < N; i++) {
                    
                    void *objPointer = *(void **)((void *)blockLayout + objOffest);
                    id obj = (__bridge id)(objPointer);
                    
                    [printStr appendFormat:@"%@",obj];
                    if (i != N - 1) {
                        [printStr appendString:@"，"];
                    }
                    
                    objOffest += sizeof(void *);//因为都是对象类型，每次偏移一个指针的大小
                }
                
            } else {
                //额外处理__block封装的类型，严格来说byref不能算是对象类型，虽然它有isa，但是该值被设为了0
                [printStr appendFormat:@"\n引用类型：%s, 引用对象：",typeArr[typeIndex]];
                
                NSInteger byrefObjCount = 0;//计数byref封装obj的数量
                
                
                for (int i = 0; i < N; i++) {
                    
                    struct Block_byref *byref1 = *(struct Block_byref **)((void *)blockLayout + objOffest);
                    byref1 = byref1->forwarding;
                    if ((byref1->flags & BLOCK_BYREF_LAYOUT_MASK) != BLOCK_BYREF_LAYOUT_NON_OBJECT) {//说明是obj对象
                        int byrefTypeIndex = (((byref1->flags - BLOCK_BYREF_LAYOUT_STRONG) >> 28) & 0xf);
                        
                        void *objPointer = *(void **)((void *)byref1 + byref1->size - sizeof(void *));
                        id obj = (__bridge id)(objPointer);
                        
                        if (byrefObjCount > 0) {
                            [printStr appendString:@"，"];
                        }
                        [printStr appendFormat:@"%@(%s)",obj, byrefTypeArr[byrefTypeIndex]];
                        
                        byrefObjCount++;
                    }
                    
                    objOffest += sizeof(void *);//因为都是对象类型，每次偏移一个指针的大小
                }
                
                if (byrefObjCount == 0) {
                    [printStr appendString:@"无__block修饰的对象"];
                }
                
            }
            
        }
        
        index++;
    }
    
    return printStr;
}

完整代码链接BlockExtendLayout