SDWebImage 使用 @weakify 和 @strongify 来防止block循环引用
用法
@weakify(self);
block {
@strongify(self);
}
通过 XCode-Editor-Assistant-Preprocess
查看展开后的结果
@autoreleasepool {} __attribute__((objc_ownership(weak))) __typeof__(self) self_weak_ = (self);
block {
__attribute__((objc_ownership(strong))) __typeof__(self) self = self_weak_; ...
}
探究@weakify(self)实现原理
weakify
#ifndef weakify
#define weakify(...) \
sd_keywordify \
metamacro_foreach_cxt(sd_weakify_,, __weak, __VA_ARGS__)
#endif
weakify 可以传入多个参数,对多个参数添加weak修饰
sd_keywordify
#if DEBUG
#define sd_keywordify autoreleasepool {}
#else
#define sd_keywordify try {} @catch (...) {}
#endif
因此@weakify(...)
展开
debug 下是
@autoreleasepool {} metamacro_foreach_cxt(sd_weakify_,, __weak, __VA_ARGS__)
release 下是
@try {} @catch (...) {} metamacro_foreach_cxt(sd_weakify_,, __weak, __VA_ARGS__)
我们传入X,Y 两个参数,看看 @weakify(X, Y)
是怎么一步步展开的
@weakify(X, Y)
-> @autoreleasepool {} metamacro_foreach_cxt(sd_weakify_,, __weak, X, Y)
sd_weakify_
定义
#define sd_weakify_(INDEX, CONTEXT, VAR) \
CONTEXT __typeof__(VAR) metamacro_concat(VAR, _weak_) = (VAR);
metamacro_foreach_cxt
定义
#define metamacro_foreach_cxt(MACRO, SEP, CONTEXT, ...) \
metamacro_concat(metamacro_foreach_cxt, metamacro_argcount(__VA_ARGS__))(MACRO, SEP, CONTEXT, __VA_ARGS__)
继续
@weakify(X, Y)
-> @autoreleasepool {} metamacro_foreach_cxt(sd_weakify_,, __weak, X, Y)
-> @autoreleasepool {} metamacro_concat(metamacro_foreach_cxt, 2)(sd_weakify_, , __weak, X, Y)
metamacro_concat
连接两个参数
#define metamacro_concat(A, B) \
metamacro_concat_(A, B)
#define metamacro_concat_(A, B) A ## B
metamacro_argcount 求参数的个数, 最多支持20个参数
#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 metamacro_at(N, ...) \
metamacro_concat(metamacro_at, N)(__VA_ARGS__)
// metamacro_at expansions
#define metamacro_at0(...) metamacro_head(__VA_ARGS__)
#define metamacro_at1(_0, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at2(_0, _1, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at3(_0, _1, _2, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at4(_0, _1, _2, _3, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at5(_0, _1, _2, _3, _4, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at6(_0, _1, _2, _3, _4, _5, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at7(_0, _1, _2, _3, _4, _5, _6, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at8(_0, _1, _2, _3, _4, _5, _6, _7, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at9(_0, _1, _2, _3, _4, _5, _6, _7, _8, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at10(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at11(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at12(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at13(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at14(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at15(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at16(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at17(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at18(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17, ...) metamacro_head(__VA_ARGS__)
#define metamacro_at19(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17, _18, ...) 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_head(...) \
metamacro_head_(__VA_ARGS__, 0)
#define metamacro_head_(FIRST, ...) FIRST
求 metamacro_argcount(X,Y)
metamacro_argcount(X,Y)
-> metamacro_at(20, X, Y, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)
-> metamacro_concat(metamacro_at, 20)(X, Y, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)
-> metamacro_at20(X, Y, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)
-> metamacro_head(2,1)
-> metamacro_head_(2,1,0)
-> 2
继续
@weakify(X, Y)
@weakify(X, Y)
-> @autoreleasepool {} metamacro_foreach_cxt(sd_weakify_,, __weak, X, Y)
-> @autoreleasepool {} metamacro_concat(metamacro_foreach_cxt, 2)(sd_weakify_, , __weak, X, Y)
-> @autoreleasepool {} metamacro_foreach_cxt2(sd_weakify_, , __weak, X, Y)
找到metamacro_foreach_cxt2
的定义
// metamacro_foreach_cxt expansions
#define metamacro_foreach_cxt0(MACRO, SEP, CONTEXT)
#define metamacro_foreach_cxt1(MACRO, SEP, CONTEXT, _0) MACRO(0, CONTEXT, _0)
#define metamacro_foreach_cxt2(MACRO, SEP, CONTEXT, _0, _1) \
metamacro_foreach_cxt1(MACRO, SEP, CONTEXT, _0) \
SEP \
MACRO(1, CONTEXT, _1)
#define metamacro_foreach_cxt3(MACRO, SEP, CONTEXT, _0, _1, _2) \
metamacro_foreach_cxt2(MACRO, SEP, CONTEXT, _0, _1) \
SEP \
MACRO(2, CONTEXT, _2)
#define metamacro_foreach_cxt4(MACRO, SEP, CONTEXT, _0, _1, _2, _3) \
metamacro_foreach_cxt3(MACRO, SEP, CONTEXT, _0, _1, _2) \
SEP \
MACRO(3, CONTEXT, _3)
#define metamacro_foreach_cxt5(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4) \
metamacro_foreach_cxt4(MACRO, SEP, CONTEXT, _0, _1, _2, _3) \
SEP \
MACRO(4, CONTEXT, _4)
#define metamacro_foreach_cxt6(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5) \
metamacro_foreach_cxt5(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4) \
SEP \
MACRO(5, CONTEXT, _5)
#define metamacro_foreach_cxt7(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6) \
metamacro_foreach_cxt6(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5) \
SEP \
MACRO(6, CONTEXT, _6)
#define metamacro_foreach_cxt8(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7) \
metamacro_foreach_cxt7(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6) \
SEP \
MACRO(7, CONTEXT, _7)
#define metamacro_foreach_cxt9(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8) \
metamacro_foreach_cxt8(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7) \
SEP \
MACRO(8, CONTEXT, _8)
#define metamacro_foreach_cxt10(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9) \
metamacro_foreach_cxt9(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8) \
SEP \
MACRO(9, CONTEXT, _9)
#define metamacro_foreach_cxt11(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10) \
metamacro_foreach_cxt10(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9) \
SEP \
MACRO(10, CONTEXT, _10)
#define metamacro_foreach_cxt12(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11) \
metamacro_foreach_cxt11(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10) \
SEP \
MACRO(11, CONTEXT, _11)
#define metamacro_foreach_cxt13(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12) \
metamacro_foreach_cxt12(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11) \
SEP \
MACRO(12, CONTEXT, _12)
#define metamacro_foreach_cxt14(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13) \
metamacro_foreach_cxt13(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12) \
SEP \
MACRO(13, CONTEXT, _13)
#define metamacro_foreach_cxt15(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14) \
metamacro_foreach_cxt14(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13) \
SEP \
MACRO(14, CONTEXT, _14)
#define metamacro_foreach_cxt16(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15) \
metamacro_foreach_cxt15(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14) \
SEP \
MACRO(15, CONTEXT, _15)
#define metamacro_foreach_cxt17(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16) \
metamacro_foreach_cxt16(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15) \
SEP \
MACRO(16, CONTEXT, _16)
#define metamacro_foreach_cxt18(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17) \
metamacro_foreach_cxt17(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16) \
SEP \
MACRO(17, CONTEXT, _17)
#define metamacro_foreach_cxt19(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17, _18) \
metamacro_foreach_cxt18(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17) \
SEP \
MACRO(18, CONTEXT, _18)
#define metamacro_foreach_cxt20(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17, _18, _19) \
metamacro_foreach_cxt19(MACRO, SEP, CONTEXT, _0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17, _18) \
SEP \
MACRO(19, CONTEXT, _19)
继续
@weakify(X, Y)
-> @autoreleasepool {} metamacro_foreach_cxt(sd_weakify_,, __weak, X, Y)
-> @autoreleasepool {} metamacro_concat(metamacro_foreach_cxt, 2)(sd_weakify_, , __weak, X, Y)
-> @autoreleasepool {} metamacro_foreach_cxt2(sd_weakify_, , __weak, X, Y)
-> @autoreleasepool {} metamacro_foreach_cxt1(sd_weakify_,, __weak, X) \ \ sd_weakify_(1, __weak, Y)
-> sd_weakify_(0, __weak, X) \ sd_weakify_(1, __weak, Y)
-> __weak __typdeof__(X) X__weak_ = X; __weak __typdeof__(Y) Y__weak_ = Y;
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