ReactiveCocoa之RAC宏展開解析【iOS源碼閱讀】

RAC() 可以將Signal發出事件的值賦值給某個對象的某個屬性，其參數爲對象名和屬性名，平時用到的宏如：

RAC(TARGET, [KEYPATH, [NIL_VALUE]])

版本：ReactiveCocoa-2.3.1

RAC宏展開解析

以下通過分析宏順序展開分析依次得到的結果，並得到最終代碼

【這個宏封裝的牛逼，愣是看了好久！！！】，詳細如下：

一. RAC(TARGET, 2)

**  PA: RAC_(TARGET,2,nil)

**  PB: RAC_(TARGET,2)

1. metamacro_if_eq(1,metamacro_argcount(TARGET, 2)) (PA) (PB)
2. metamacro_if_eq(1,metamacro_at(20,TARGET, 2 , 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)) (PA) (PB)
3. metamacro_if_eq(1,metamacro_concat(metamacro_at,20)(TARGET, 2 , 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)) (PA) (PB)
4. metamacro_if_eq(1,metamacro_at20(TARGET, 2 , 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)) (PA) (PB)
5. metamacro_if_eq(1,metamacro_head(2, 1)) (PA) (PB)
6. metamacro_if_eq(1,metamacro_head_(2, 1, 0)) (PA) (PB)
7. metamacro_if_eq(1,2) (PA) (PB)#所以metamacro_argcount就是參數的個數
8. metamacro_concat(metamacro_if_eq,1)(2) (PA) (PB)
9. metamacro_if_eq1(2) (PA) (PB)
10. metamacro_if_eq0(metamacro_dec(2) (PA) (PB)
11. metamacro_if_eq0(metamacro_at(2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19)) (PA) (PB)
12. metamacro_if_eq0(metamacro_concat(metamacro_at,2)(-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19)) (PA) (PB)
13. metamacro_if_eq0(metamacro_at2(-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19)) (PA) (PB)
14. metamacro_if_eq0(metamacro_head(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19)) (PA) (PB)
15. metamacro_if_eq0(1) (PA) (PB)
16. metamacro_concat(metamacro_if_eq0_,1) (PA) (PB)
17. metamacro_if_eq0_1 (PA) (PB)
18. metamacro_expand_ (PA) (PB)#PA被metamacro_expand_帶走了，就剩下PB
19. PB

=>    RAC_(TARGET,2)

=>[[RACSubscriptingAssignmentTrampoline alloc] initWithTarget:(TARGET) nilValue:(NILVALUE)][@keypath(TARGET, 2)]

二. RAC(TARGET)

**  PA: (RAC_(TARGET, __VA_ARGS__, nil))

**  PB: (RAC_(TARGET, __VA_ARGS__))

1. metamacro_if_eq(1,metamacro_argcount(TARGET)) (PA) (PB)
2. metamacro_if_eq(1,metamacro_at20(TARGET, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)) (PA) (PB)
3. metamacro_if_eq(1,metamacro_head(1)) (PA) (PB)
4. metamacro_if_eq1(1) (PA) (PB)
5. metamacro_if_eq0(metamacro_dec(1)) (PA) (PB)
6. metamacro_if_eq0(metamacro_at1(-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19)) (PA) (PB)
7. metamacro_if_eq0(metamacro_head(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19)) (PA) (PB)
8. metamacro_if_eq0(0) (PA) (PB)
9. metamacro_if_eq0_0 (PA) (PB)
10. (PA) metamacro_consume_ (PB)
11. PA

=>RAC_(TARGET, __VA_ARGS__, nil)

三. RAC(TARGET, 2,3,4)

有了以上基礎，這裏簡要中介展開過程，直接講重點！

**  PA: RAC_(TARGET,__VA_ARGS__,nil)

**  PB: RAC_(TARGET,__VA_ARGS__)

1. metamacro_if_eq(1,4) (PA) (PB)#所以參數的個數4
2. metamacro_if_eq0_3 (PA) (PB)#metamacro_if_eq0_1~metamacro_if_eq0_20定義一樣，所以還是PB
3.         PB

=>RAC_(TARGET,__VA_ARGS__)

總結

• 如果只有1個參數的，就調用RAC_(TARGET, __VA_ARGS__, nil)
• 如果有多個參數的，就調用RAC_(TARGET, __VA_ARGS__)

具體如下：

#define RAC_(TARGET, KEYPATH, NILVALUE) \
    [[RACSubscriptingAssignmentTrampoline alloc] initWithTarget:(TARGET) nilValue:(NILVALUE)][@keypath(TARGET, KEYPATH)]

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以上分析過程中參考的宏定義

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#define metamacro_concat_(A, B) A ## B  
#define metamacro_concat(A, B) \
        metamacro_concat_(A, B)
#define metamacro_if_eq(A, B) \
        metamacro_concat(metamacro_if_eq, A)(B)
#define metamacro_at(N, ...) \
        metamacro_concat(metamacro_at, N)(__VA_ARGS__)
#define metamacro_argcount(...) \
        metamacro_at(20, __VA_ARGS__, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)
#define RAC_(TARGET, KEYPATH, NILVALUE) \
    [[RACSubscriptingAssignmentTrampoline alloc] initWithTarget:(TARGET) nilValue:(NILVALUE)][@keypath(TARGET, KEYPATH)]
#define RAC(TARGET, ...) \
    metamacro_if_eq(1, metamacro_argcount(__VA_ARGS__)) \
        (RAC_(TARGET, __VA_ARGS__, nil)) \
        (RAC_(TARGET, __VA_ARGS__))
#define metamacro_head_(FIRST, ...) FIRST
#define metamacro_head(...) \
        metamacro_head_(__VA_ARGS__, 0)
#define metamacro_at1(_0, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at2(_0, _1, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at20(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17, _18, _19, ...) metamacro_head(__VA_ARGS__)
#define metamacro_if_eq0(VALUE) \
    metamacro_concat(metamacro_if_eq0_, VALUE)
#define metamacro_dec(VAL) \
        metamacro_at(VAL, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19)
#define metamacro_if_eq1(VALUE) metamacro_if_eq0(metamacro_dec(VALUE))
#define metamacro_expand_(...) __VA_ARGS__
#define metamacro_if_eq0_1(...) metamacro_expand_
#define metamacro_consume_(...)
#define metamacro_if_eq0_0(...) __VA_ARGS__ metamacro_consume_