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)。__rw_objc_super結(jié)構(gòu)體和sel_registerName("class")方法名。

__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。

super匯編實(shí)現(xiàn)

image.png

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)，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)。

/********************************************************************
 * 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，消息接受者是receiver，另外一個(gè)參數(shù)是當(dāng)前類，然后通過class->superclass，去父類中cache中搜索方法，搜索的起始位置是父類