修改block的实现，先打印参数，再输出原有实现

main.m

#import <Foundation/Foundation.h>

#import "ffi.h"

NSMutableArray *g_allocations;
ffi_cif g_cif;
ffi_closure *g_closure;
void *g_replacement_invoke;
void *g_origin_invoke;
id g_block;

enum {
    BLOCK_HAS_COPY_DISPOSE =  (1 << 25),
    BLOCK_HAS_CTOR =          (1 << 26), // helpers have C++ code
    BLOCK_IS_GLOBAL =         (1 << 28),
    BLOCK_HAS_STRET =         (1 << 29), // IFF BLOCK_HAS_SIGNATURE
    BLOCK_HAS_SIGNATURE =     (1 << 30),
};

typedef struct __block_descriptor {
    size_t reserved;
    size_t Block_size;
    void *rest[1];
}__block_descriptor;

typedef struct __block_impl {
    void *isa;
    int Flags;
    int Reserved;
    void *FuncPtr;
    __block_descriptor *descriptor;
}__block_impl;

const char *block_type_encode_string(id blk) {
    __block_impl *block_impl = (__bridge void *)blk;
    __block_descriptor *descriptor = block_impl->descriptor;
    assert(block_impl->Flags & BLOCK_HAS_SIGNATURE);
    int index = 0;
    if(block_impl->Flags & BLOCK_HAS_COPY_DISPOSE)
        index += 2;
    return descriptor->rest[index];
}

void invoke_original_block(void* ret, void **args) {
    if (g_origin_invoke) {
        ffi_call(&g_cif, g_origin_invoke, ret, args);
    }
}

static void ffi_clousure_func(ffi_cif *cif, void *ret, void **args, void *user_data) {
    int i = *((int *)args[1]);
    NSString *str = (__bridge NSString *)(*((void **)args[2]));
    NSLog(@"%d,%@", i, str);
    invoke_original_block(ret, args);
}

const char * size_and_alignment(const char *str, NSUInteger *sizep, NSUInteger *alignp, long *len) {
    const char *out = NSGetSizeAndAlignment(str, sizep, alignp);
    if(len)
        *len = out - str;
    while(isdigit(*out))
        out++;
    return out;
}

static int arg_count(const char *str) {
    int argcount = -1; // return type is the first one
    while(str && *str) {
        str = size_and_alignment(str, NULL, NULL, NULL);
        argcount++;
    }
    return argcount;
}

void * allocate(size_t howmuch) {
    NSMutableData *data = [NSMutableData dataWithLength:howmuch];
    [g_allocations addObject: data];
    return [data mutableBytes];
}

ffi_type * ffi_arg_for_encode(const char *str) {
#define SINT(type) do { \
if(str[0] == @encode(type)[0]) \
{ \
if(sizeof(type) == 1) \
return &ffi_type_sint8; \
else if(sizeof(type) == 2) \
return &ffi_type_sint16; \
else if(sizeof(type) == 4) \
return &ffi_type_sint32; \
else if(sizeof(type) == 8) \
return &ffi_type_sint64; \
else \
{ \
NSLog(@"Unknown size for type %s", #type); \
abort(); \
} \
} \
} while(0)
    
#define UINT(type) do { \
if(str[0] == @encode(type)[0]) \
{ \
if(sizeof(type) == 1) \
return &ffi_type_uint8; \
else if(sizeof(type) == 2) \
return &ffi_type_uint16; \
else if(sizeof(type) == 4) \
return &ffi_type_uint32; \
else if(sizeof(type) == 8) \
return &ffi_type_uint64; \
else \
{ \
NSLog(@"Unknown size for type %s", #type); \
abort(); \
} \
} \
} while(0)
    
#define INT(type) do { \
SINT(type); \
UINT(unsigned type); \
} while(0)
    
#define COND(type, name) do { \
if(str[0] == @encode(type)[0]) \
return &ffi_type_ ## name; \
} while(0)
    
#define PTR(type) COND(type, pointer)
    
#define STRUCT(structType, ...) do { \
if(strncmp(str, @encode(structType), strlen(@encode(structType))) == 0) \
{ \
ffi_type *elementsLocal[] = { __VA_ARGS__, NULL }; \
ffi_type **elements = allocate(sizeof(elementsLocal)); \
memcpy(elements, elementsLocal, sizeof(elementsLocal)); \
\
ffi_type *structType = allocate(sizeof(*structType)); \
structType->type = FFI_TYPE_STRUCT; \
structType->elements = elements; \
return structType; \
} \
} while(0)
    
    SINT(_Bool);
    SINT(signed char);
    UINT(unsigned char);
    INT(short);
    INT(int);
    INT(long);
    INT(long long);
    
    PTR(id);
    PTR(Class);
    PTR(SEL);
    PTR(void *);
    PTR(char *);
    PTR(void (*)(void));
    
    COND(float, float);
    COND(double, double);
    
    COND(void, void);
    
    ffi_type *CGFloatFFI = sizeof(CGFloat) == sizeof(float) ? &ffi_type_float : &ffi_type_double;
    STRUCT(CGRect, CGFloatFFI, CGFloatFFI, CGFloatFFI, CGFloatFFI);
    STRUCT(CGPoint, CGFloatFFI, CGFloatFFI);
    STRUCT(CGSize, CGFloatFFI, CGFloatFFI);
    
#if !TARGET_OS_IPHONE
    STRUCT(NSRect, CGFloatFFI, CGFloatFFI, CGFloatFFI, CGFloatFFI);
    STRUCT(NSPoint, CGFloatFFI, CGFloatFFI);
    STRUCT(NSSize, CGFloatFFI, CGFloatFFI);
#endif
    
    NSLog(@"Unknown encode string %s", str);
    abort();
}

ffi_type ** args_with_encode_string(const char *str, int *out_count) {
    int count = arg_count(str);
    ffi_type **arg_types = allocate(count * sizeof(*arg_types));
    
    int i = -1; // 第一个是返回值，需要排除
    while(str && *str) {
        const char *next = size_and_alignment(str, NULL, NULL, NULL);
        if(i >= 0)
            arg_types[i] = ffi_arg_for_encode(str);
        i++;
        str = next;
    }
    
    *out_count = count;
    return arg_types;
}

void prep_cif() {
    int args_count;
    const char *str = block_type_encode_string(g_block);
    ffi_type **arg_types = args_with_encode_string(str, &args_count);
    
    ffi_status status = ffi_prep_cif(&g_cif, FFI_DEFAULT_ABI, args_count, ffi_arg_for_encode(str), arg_types);
    if(status != FFI_OK) {
        NSLog(@"Got result %ld from ffi_prep_cif", (long)status);
        abort();
    }
}

void prep_closure() {
    
    prep_cif();
    
    g_closure = ffi_closure_alloc(sizeof(ffi_closure), &g_replacement_invoke);
    ffi_status status = ffi_prep_closure_loc(g_closure, &g_cif, ffi_clousure_func, NULL, g_replacement_invoke);
    if(status != FFI_OK) {
        NSLog(@"ffi_prep_closure returned %d", (int)status);
        abort();
    }
    
    g_origin_invoke = ((__bridge __block_impl *)g_block)->FuncPtr;
   ((__bridge __block_impl *)g_block)->FuncPtr = g_replacement_invoke;
}

void HookBlockToPrintArguments(id blk) {
    g_block = blk;
    prep_closure();
}

#pragma mark - 第二种实现方式
//static void (*orig_func)(void *v ,int i, NSString *str);

//void hookFunc(void *v ,int i, NSString *str) {
//    NSLog(@"%d,%@", i, str);
//    orig_func(v,i,str);
//}

//void HookBlockToPrintArguments(id blk) {
//    __block_impl *ptr = (__bridge __block_impl *)blk;
//    orig_func = ptr->FuncPtr;
//    ptr->FuncPtr = &hookFunc;
//}

int main(int argc, const char * argv[]) {
    
    void (^blk)(int, NSString *) = ^void(int i, NSString *str) {
        NSLog(@"original invoke");
    };

    HookBlockToPrintArguments(blk);
    blk(1,@"aaa");

    return 0;
}

太牛比了，膜拜，已粉。

前排膜拜大佬。

膜拜 不得不服

膜拜大神！！!

新增第二种实现方式

前排膜拜大佬。

前排膜拜大佬。

前排膜拜大佬。

前排膜拜大佬。

后排膜拜大佬。

再次膜拜大佬。

但其实这只实现了特定的block替换，如果block的参数不确定，或者把int跟String一换，这个程序就崩了。我感觉提问者的意思应该是写一个通用的方法。而不是只能换这一种block。