面試題
- (instancetype)init
{
if (self = [super init]) {
NSLog(@"[self class] = %@", [self class]);
NSLog(@"[self superclass] = %@", [self superclass]);
NSLog(@"[super class] = %@", [super class]);
NSLog(@"[super superclass] = %@", [super superclass]);
}
return self;
}
@end
2021-06-23 14:22:30.124579+0800 super[15478:176287] [self class] = Dog
2021-06-23 14:22:30.124700+0800 super[15478:176287] [self superclass] = Animal
2021-06-23 14:22:30.124805+0800 super[15478:176287] [super class] = Dog
2021-06-23 14:22:30.124903+0800 super[15478:176287] [super superclass] = Animal
上述代碼中可以發(fā)現(xiàn)無(wú)論是self還是super調(diào)用class或superclass的結(jié)果都是相同的。
super底層實(shí)現(xiàn)
使用下面命令將 Dog.m轉(zhuǎn)換成c++
xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc -fobjc-arc -fobjc-runtime=ios-8.0.0 Dog.m
實(shí)現(xiàn)如下
static instancetype _I_Dog_init(Dog * self, SEL _cmd) {
if (self = ((Dog *(*)(__rw_objc_super *, SEL))(void *)objc_msgSendSuper)((__rw_objc_super){(id)self, (id)class_getSuperclass(objc_getClass("Dog"))}, sel_registerName("init"))) {
NSLog((NSString *)&__NSConstantStringImpl__var_folders_dw_4ylsshdj70nf81ndx_765tfh0000gn_T_Dog_def1c4_mi_0, ((Class (*)(id, SEL))(void *)objc_msgSend)((id)self, sel_registerName("class")));
NSLog((NSString *)&__NSConstantStringImpl__var_folders_dw_4ylsshdj70nf81ndx_765tfh0000gn_T_Dog_def1c4_mi_1, ((Class (*)(id, SEL))(void *)objc_msgSend)((id)self, sel_registerName("superclass")));
NSLog((NSString *)&__NSConstantStringImpl__var_folders_dw_4ylsshdj70nf81ndx_765tfh0000gn_T_Dog_def1c4_mi_2, ((Class (*)(__rw_objc_super *, SEL))(void *)objc_msgSendSuper)((__rw_objc_super){(id)self, (id)class_getSuperclass(objc_getClass("Dog"))}, sel_registerName("class")));
NSLog((NSString *)&__NSConstantStringImpl__var_folders_dw_4ylsshdj70nf81ndx_765tfh0000gn_T_Dog_def1c4_mi_3, ((Class (*)(__rw_objc_super *, SEL))(void *)objc_msgSendSuper)((__rw_objc_super){(id)self, (id)class_getSuperclass(objc_getClass("Dog"))}, sel_registerName("superclass")));
}
return self;
}
objc_msgSendSuper函數(shù)內(nèi)傳遞了兩個(gè)參數(shù)汤踏。__rw_objc_super結(jié)構(gòu)體和sel_registerName("class")方法名舔腾。
__rw_objc_super結(jié)構(gòu)體內(nèi)傳入的參數(shù)是self和class_getSuperclass(objc_getClass("Dog"))
objc_msgSendSuper的底層結(jié)構(gòu)
/// Specifies the superclass of an instance.
struct objc_super {
/// Specifies an instance of a class.
__unsafe_unretained _Nonnull id receiver; // 消息接受者
/// Specifies the particular superclass of the instance to message.
#if !defined(__cplusplus) && !__OBJC2__
/* For compatibility with old objc-runtime.h header */
__unsafe_unretained _Nonnull Class class;
#else
__unsafe_unretained _Nonnull Class super_class; // 從父類開始搜索方法
#endif
/* super_class is the first class to search */
};
從objc_msgSendSuper的底層結(jié)構(gòu)可以看到
super調(diào)用方法的消息接受者receiver仍然是self，只是從父類的類對(duì)象開始去查找方法哗脖。
super匯編實(shí)現(xiàn)
上圖中可以發(fā)現(xiàn)super底層其實(shí)調(diào)用的是objc_msgSendSuper2函數(shù)常空，我們來到源碼中查找一下objc_msgSendSuper2函數(shù)的底層實(shí)現(xiàn)，我們可以在匯編文件中找到其相關(guān)底層實(shí)現(xiàn)啤它。
/********************************************************************
* id objc_msgSendSuper2(struct objc_super *super, SEL op, ...)
*
* struct objc_super {
* id receiver;
* Class cls; // SUBCLASS of the class to search
* }
********************************************************************/
ENTRY _objc_msgSendSuper2
ldr r9, [r0, #CLASS] // class = struct super->class
ldr r9, [r9, #SUPERCLASS] // class = class->superclass
CacheLookup NORMAL, _objc_msgSendSuper2
// cache hit, IMP in r12, eq already set for nonstret forwarding
ldr r0, [r0, #RECEIVER] // load real receiver
bx r12 // call imp
CacheLookup2 NORMAL, _objc_msgSendSuper2
// cache miss
ldr r9, [r0, #CLASS] // class = struct super->class
ldr r9, [r9, #SUPERCLASS] // class = class->superclass
ldr r0, [r0, #RECEIVER] // load real receiver
b __objc_msgSend_uncached
END_ENTRY _objc_msgSendSuper2
ENTRY _objc_msgLookupSuper2
ldr r9, [r0, #CLASS] // class = struct super->class
ldr r9, [r9, #SUPERCLASS] // class = class->superclass
CacheLookup NORMAL, _objc_msgLookupSuper2
// cache hit, IMP in r12, eq already set for nonstret forwarding
ldr r0, [r0, #RECEIVER] // load real receiver
bx lr
CacheLookup2 NORMAL, _objc_msgLookupSuper2
// cache miss
ldr r9, [r0, #CLASS]
ldr r9, [r9, #SUPERCLASS] // r9 = class to search
ldr r0, [r0, #RECEIVER] // load real receiver
b __objc_msgLookup_uncached
END_ENTRY _objc_msgLookupSuper2
struct objc_super {
id receiver;
Class cls; // SUBCLASS of the class to search
}
從匯編實(shí)現(xiàn)可以看到工扎，消息接受者是receiver衔蹲，另外一個(gè)參數(shù)是當(dāng)前類，然后通過class->superclass舆驶，去父類中cache中搜索方法，搜索的起始位置是父類
