OC中Runtime super关键字

在平时的开发过程中，我们经常会执行[super xxx]来调用父类的方法，但是我们很少会去关心super关键字的底层是如何实现的，接下来我们来看下super关键字底层实现原理，我们新建一个工程，然后创建Person类和Student类，Student类继承自Person类，示例代码如下：

Person类

@interface Person : NSObject

- (void)run;
@end


@implementation Person

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

Student类

@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函数：

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

接下来我们执行命令xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc -fobjc-arc -fobjc-runtime=ios-8.0.0 Student.m将Student.m文件转换为底层c++文件，转换为底层的run方法代码如下：

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__);
}

我们将Studnnt类的run方法进行简化如下：

- (void)run {
    [super run];
    
    /**
     结构体objc_super包含两个成员：
     self：消息接收者(receiver)
     class_getSuperclass(objc_getClass("Student"))：消息接收者的父类(super_class)
     */
    struct objc_super superStruct = {
        self,
        class_getSuperclass(objc_getClass("Student"))
    };
    
    objc_msgSendSuper(superStruct, sel_registerName("run"));
    
    NSLog(@"%s", __func__);
}

我们发现当我们在run函数中执行[super run]后，最终底层转换为objc_msgSendSuper()消息发送，我们通过底层源码看下objc_msgSendSuper函数的定义，查找路径objc4 -> message.h -> objc_msgSendSuper，具体底层函数定义如下：

/**
 * 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);

我们从源码objc_msgSendSuper函数定义可以看到，函数接受两个参数

objc_super
SEL

我们再来看下objc_super结构体底层定义如下：

/// 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

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

简化objc_super结构体如下：


/**
 从`objc_msgSendSuper`函数的注释可以看到结构体的两个成员含义: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类 (官方解释：the superclass at which to start searching for the method implementation)
    __unsafe_unretained _Nonnull Class super_class;
};

通过objc_super底层定义，我们可以看到这个结构体也包含两个成员

receiver
super_class

我们通过objc_msgSendSuper函数底层定义的注释中可以查看到参数objc_super结构体中两个成员的具体含义和作用

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

从上面注释我们了解到receiver就是消息的接受者(也就是方法调用者)，superclass就是指从父类开始查找方法

因此在Student的init初始化函数中打印结果如下：

- (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函数的底层源码实现：

- (Class)class {
    // self为方法调用者，也就是消息接收者(receiver)，返回当前方法调用者的类对象
    return object_getClass(self);
}

- (Class)superclass {
    // self为方法调用者，也就是消息接收者(receiver)，返回当前方法调用者的父类对象
    return return self->superclass;
}

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

OC中Runtime super关键字

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