iOS - superclass 底层分析

[toc]

面试题

先切入一个面试问题, 下面代码会输出什么?

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

        NSLog(@"--------------------------------");

        // objc_msgSendSuper({self, [Person class]}, @selector(class));
        // ({消息接收者, 消息接收者父类}, SEL)
        NSLog(@"[super class] = %@", [super class]); // Student
        NSLog(@"[super superclass] = %@", [super superclass]); // Person
    }
    return self;
}

底层分析

OC代码

// Student: Person: NSObject
- (void)run {
    [super run]; // super调用的消息接收者仍然是 self (Student实例)
}

转成C++代码

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"));
} 

// objc4/message.h
id _Nullable objc_msgSendSuper(struct objc_super * _Nonnull super, SEL _Nonnull op, ...)

上面代码拆解

struct __rw_objc_super rcv = {
    self,
    class_getSuperclass(objc_getClass("Student"))
}

objc_msgSendSuper(rcv, @selector(run))

objc_super

__rw_objc_super 等价于 objc_super

// objc4/message.h
/// Specifies the superclass of an instance. 
struct objc_super {
    /// Specifies an instance of a class.
    __unsafe_unretained _Nonnull id receiver; // 消息接收者
    
    __unsafe_unretained _Nonnull Class super_class; // 消息接收者的父类
    /* super_class is the first class to search */
};

class_getSuperclass()

// objc4/runtime.h
Class _Nullable class_getSuperclass(Class _Nullable cls)

// objc4/objc-class.mm
Class class_getSuperclass(Class cls) {
    if (!cls) return nil;
    return cls->superclass;
}

objc_msgSendSuper()

可以看到, 使用 super 调用方法, 底层被转换成 objc_msgSendSuper()

/** 
 * 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
 */
// super 里封装了消息接收者(方法调用者) 和 消息接收者的父类
id _Nullable objc_msgSendSuper(struct objc_super * _Nonnull super, SEL _Nonnull op, ...)

苹果注释说, superclass 的作用是从哪里开始查找方法实现; 即从 objc_super.super_class 开始查找,

所以, super 调用会直接从父类开始查找方法的实现

[super funcName];

而super调用的消息接收者仍然是子类对象, 只不过是从父类从开始查找方法的实现

相当于还是self在调用方法

objc_msgSendSuper2 ★

而打断点分析汇编, 可以看到, 实际真正调用的是 objc_msgSendSuper2

// objc4/objc-abi.h
// objc_msgSendSuper2() takes the current search class, not its superclass.
id _Nullable objc_msgSendSuper2(struct objc_super * _Nonnull super, SEL _Nonnull op, ...);

struct objc_super2 {
    id receiver; // 消息接收者
    Class current_class; // 当前类, 即receiver的Class对象
};

汇编实现

该方法会拿到当前类, 在内部根据class找到superclass, 然后开始查找方法实现, 实际效果和 objc_msgSendSuper 是一样的

ENTRY _objc_msgSendSuper2
UNWIND _objc_msgSendSuper2, NoFrame

ldp p0, p16, [x0]       // p0 = real receiver, p16 = class
ldr p16, [x16, #SUPERCLASS] // p16 = class->superclass
CacheLookup NORMAL, _objc_msgSendSuper2

END_ENTRY _objc_msgSendSuper2

class 和 superclass 方法

每个OC类都有class方法, 可想而知, class 的方法实现在 NSObject 上, 所以不管 self调用, 还是super调用, 最终都是在 NSObject 上找到

// objc4/NSObject.mm
+ (Class)class {
    return self;
}

- (Class)class {
    return object_getClass(self);
}

+ (Class)superclass {
    return self->superclass;
}

// class_getSuperclass(object_getClass(self));
- (Class)superclass {
    return [self class]->superclass;
}

superclass 指针

class 对象的 superclass 指针

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meta-class 对象的 superclass 指针

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