OC中Runtime super關(guān)鍵字

在平時(shí)的開發(fā)過程中敏晤，我們經(jīng)常會(huì)執(zhí)行[super xxx]來調(diào)用父類的方法遵岩，但是我們很少會(huì)去關(guān)心super關(guān)鍵字的底層是如何實(shí)現(xiàn)的集绰，接下來我們來看下super關(guān)鍵字底層實(shí)現(xiàn)原理菠劝，我們新建一個(gè)工程赛糟，然后創(chuàng)建Person類和Student類，Student類繼承自Person類翠拣，示例代碼如下：

Person類

@interface Person : NSObject

- (void)run;
@end


@implementation Person

- (void)run {
    NSLog(@"%s", __func__);
}
@end

Student類

@interface Student : Person

@end


@implementation Student

- (instancetype)init {
    if (self = [super init]) {
        NSLog(@"%@", [self class]); // Student
        NSLog(@"%@", [self superclass]); // Person
        
        NSLog(@"%@", [super class]); // Student
        NSLog(@"%@", [super superclass]); // Person
    }
    return self;
}

- (void)run {
    [super run];
    
    NSLog(@"%s", __func__);
}
@end

main函數(shù)：

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // insert code here...
        Student *stu = [[Student alloc] init];
        [stu run];
    }
    return 0;
}

接下來我們執(zhí)行命令xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc -fobjc-arc -fobjc-runtime=ios-8.0.0 Student.m將Student.m文件轉(zhuǎn)換為底層c++文件版仔，轉(zhuǎn)換為底層的run方法代碼如下：

static void _I_Student_run(Student * self, SEL _cmd) {
    ((void (*)(__rw_objc_super *, SEL))(void *)objc_msgSendSuper)((__rw_objc_super){(id)self, (id)class_getSuperclass(objc_getClass("Student"))}, sel_registerName("run"));

    NSLog((NSString *)&__NSConstantStringImpl__var_folders_lr_81gwkh751xzddx_ffhhb5_0m0000gn_T_Student_18b3ae_mi_0, __func__);
}

我們將Studnnt類的run方法進(jìn)行簡化如下：

- (void)run {
    [super run];
    
    /**
     結(jié)構(gòu)體objc_super包含兩個(gè)成員：
     self：消息接收者(receiver)
     class_getSuperclass(objc_getClass("Student"))：消息接收者的父類(super_class)
     */
    struct objc_super superStruct = {
        self,
        class_getSuperclass(objc_getClass("Student"))
    };
    
    objc_msgSendSuper(superStruct, sel_registerName("run"));
    
    NSLog(@"%s", __func__);
}

我們發(fā)現(xiàn)當(dāng)我們在run函數(shù)中執(zhí)行[super run]后，最終底層轉(zhuǎn)換為objc_msgSendSuper()消息發(fā)送误墓，我們通過底層源碼看下objc_msgSendSuper函數(shù)的定義蛮粮，查找路徑objc4 -> message.h -> objc_msgSendSuper，具體底層函數(shù)定義如下：

/**
 * Sends a message with a simple return value to the superclass of an instance of a class.
 *
 * @param super A pointer to an \c objc_super data structure. Pass values identifying the
 *  context the message was sent to, including the instance of the class that is to receive the
 *  message and the superclass at which to start searching for the method implementation.
 * @param op A pointer of type SEL. Pass the selector of the method that will handle the message.
 * @param ...
 *   A variable argument list containing the arguments to the method.
 *
 * @return The return value of the method identified by \e op.
 *
 * @see objc_msgSend
 */
 
OBJC_EXPORT id _Nullable
objc_msgSendSuper(struct objc_super * _Nonnull super, SEL _Nonnull op, ...)
    OBJC_AVAILABLE(10.0, 2.0, 9.0, 1.0, 2.0);

我們從源碼objc_msgSendSuper函數(shù)定義可以看到谜慌，函數(shù)接受兩個(gè)參數(shù)

objc_super
SEL

我們再來看下objc_super結(jié)構(gòu)體底層定義如下：

/// Specifies the superclass of an instance. 

struct objc_super {
    /// Specifies an instance of a class.
    
    // 消息接收者
    __unsafe_unretained _Nonnull id receiver;

    /// 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

    // 消息接收者的父類
    __unsafe_unretained _Nonnull Class super_class;
#endif
    /* super_class is the first class to search */
};

簡化objc_super結(jié)構(gòu)體如下：


/**
 從`objc_msgSendSuper`函數(shù)的注釋可以看到結(jié)構(gòu)體的兩個(gè)成員含義:the instance of the class that is to receive the message and the superclass at which to start searching for the method implementation.
 */
 
struct objc_super {
    // receiver就是消息接收者 (the instance of the class that is to receive the message)
    __unsafe_unretained _Nonnull id receiver;

    // super_class就是消息接收者的父類 (官方解釋：the superclass at which to start searching for the method implementation)
    __unsafe_unretained _Nonnull Class super_class;
};

通過objc_super底層定義然想，我們可以看到這個(gè)結(jié)構(gòu)體也包含兩個(gè)成員

receiver
super_class

我們通過objc_msgSendSuper函數(shù)底層定義的注釋中可以查看到參數(shù)objc_super結(jié)構(gòu)體中兩個(gè)成員的具體含義和作用

instance of the class that is to receive the message and the superclass at which to start searching for the method implementation

從上面注釋我們了解到receiver就是消息的接受者(也就是方法調(diào)用者)，superclass就是指從父類開始查找方法

因此在Student的init初始化函數(shù)中打印結(jié)果如下：

- (instancetype)init {
    if (self = [super init]) {
        NSLog(@"%@", [self class]); // Student
        NSLog(@"%@", [self superclass]); // Person
        
        NSLog(@"%@", [super class]); // Student
        NSLog(@"%@", [super superclass]); // Person
    }
    return self;
}

class和superclass函數(shù)的底層源碼實(shí)現(xiàn)：

- (Class)class {
    // self為方法調(diào)用者欣范，也就是消息接收者(receiver)变泄，返回當(dāng)前方法調(diào)用者的類對象
    return object_getClass(self);
}

- (Class)superclass {
    // self為方法調(diào)用者，也就是消息接收者(receiver)恼琼，返回當(dāng)前方法調(diào)用者的父類對象
    return return self->superclass;
}

講解示例Demo地址：https://github.com/guangqiang-liu/07.2-RunTimeSuper

OC中Runtime super關(guān)鍵字

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