OC类原理探索之成员变量

前言

在上一篇文章中我们探索到类的属性、方法、协议存储的位置，今天来探索一些类的成员变量

1.探究类的成员变量

在2020 WWDC讲解的时候，有下面一张类的结构图

从图中我们很明显可以看到类的成员变量存储在class_ro_t结构体中。那么下面我们来获取一下class_ro_t结构

1.1 获取类的class_ro_t结构

1.2 为什么在class_ro_t与class_rw_ext_t都有方法、属性、协议呢？

struct class_rw_ext_t {
    DECLARE_AUTHED_PTR_TEMPLATE(class_ro_t)
    class_ro_t_authed_ptr<const class_ro_t> ro;
    method_array_t methods;
    property_array_t properties;
    protocol_array_t protocols;
    char  *demangledName;
    uint32_t version;
};

1.2.1 clean memory与dirty memory

回答这个问题首先先了解一些什么是dirty memory与clean memory

clean memory:是指加载后不会发生更改的内存
dirty memory:是指在进程进行时会发生更改的内存

类结构一经使用就会变成dirty memory，因为运行时会向它写入新的数据. 比如我们可以通过苹果提供的api动态添加一个方法或者一个属性。

dirty memory比clean memory要比消耗的多，因为只要进程在运行，它就必须一直存在。

clean memory可以进程移除，从而节省更多的空间。因为如果你需要clean memory，系统可以从磁盘重新加载。

所以苹果为了节省内存就把class_rw_t分离为class_ro_t只读(clean memory)与class_rw_ext_t可读可写(dirty memory)两部分

class_rw_ext里面的方法、属性、协议是苹果在运行时的时候，从class_ro_t从拷贝的一份。底层源码的实现如下：

class_rw_ext_t *class_rw_t::extAlloc(const class_ro_t *ro, bool deepCopy)
{
    runtimeLock.assertLocked();
    auto rwe = objc::zalloc<class_rw_ext_t>();
    rwe->version = (ro->flags & RO_META) ? 7 : 0;
    method_list_t *list = ro->baseMethods();
    if (list) {
        if (deepCopy) list = list->duplicate();
        rwe->methods.attachLists(&list, 1);
    }
    // See comments in objc_duplicateClass
    // property lists and protocol lists historically
    // have not been deep-copied
    // This is probably wrong and ought to be fixed some day
    property_list_t *proplist = ro->baseProperties;
    if (proplist) {
        rwe->properties.attachLists(&proplist, 1);
    }
    protocol_list_t *protolist = ro->baseProtocols;
    if (protolist) {
        rwe->protocols.attachLists(&protolist, 1);
    }
    set_ro_or_rwe(rwe, ro);
    return rwe;
}

1.3 获取类的成员变量

成员变量在底层是ivar_t结构体

struct ivar_t {
#if __x86_64__
    // *offset was originally 64-bit on some x86_64 platforms.
    // We read and write only 32 bits of it.
    // Some metadata provides all 64 bits. This is harmless for unsigned 
    // little-endian values.
    // Some code uses all 64 bits. class_addIvar() over-allocates the 
    // offset for their benefit.
#endif
    int32_t *offset;
    const char *name;
    const char *type;
    // alignment is sometimes -1; use alignment() instead
    uint32_t alignment_raw;
    uint32_t size;
    uint32_t alignment() const {
         if  (alignment_raw == ~(uint32_t)0)  return  1U << WORD_SHIFT;
    return  1 << alignment_raw;
    }
};

1.3 类的属性与成员变量的区别

类的属性 = 带下划线的成员变量 + setter + getter方法

属性的setter方法与getter方法

static NSString * _I_ZFPerson_nickName(ZFPerson * self, SEL _cmd) { return (*(NSString **)((char *)self + OBJC_IVAR_$_ZFPerson$_nickName)); }
extern "C" __declspec(dllimport) void objc_setProperty (id, SEL, long, id, bool, bool);
static void _I_ZFPerson_setNickName_(ZFPerson * self, SEL _cmd, NSString *nickName) { objc_setProperty (self, _cmd, __OFFSETOFIVAR__(struct ZFPerson, _nickName), (id)nickName, 0, 1); }

static NSInteger _I_ZFPerson_age2(ZFPerson * self, SEL _cmd) { return (*(NSInteger *)((char *)self + OBJC_IVAR_$_ZFPerson$_age2)); }
static void _I_ZFPerson_setAge2_(ZFPerson * self, SEL _cmd, NSInteger age2) { (*(NSInteger *)((char *)self + OBJC_IVAR_$_ZFPerson$_age2)) = age2; }

static NSObject * _I_ZFPerson_obj(ZFPerson * self, SEL _cmd) { return (*(NSObject **)((char *)self + OBJC_IVAR_$_ZFPerson$_obj)); }
static void _I_ZFPerson_setObj_(ZFPerson * self, SEL _cmd, NSObject *obj) { (*(NSObject **)((char *)self + OBJC_IVAR_$_ZFPerson$_obj)) = obj; }

从上面编译的c++代码中我们可以看出，copy修饰的nickName的setter方法是调用了一个
objc_setProperty方法，strong修饰obj与assign修饰age2的setter方法是通过获取到成员变量直接赋新值的过程

1.4 探究copy修饰的属性setter方法(objc_setProperty)

void objc_setProperty(id self, SEL _cmd, ptrdiff_t offset, id newValue, BOOL atomic, signed char shouldCopy) 
{
    bool copy = (shouldCopy && shouldCopy != MUTABLE_COPY);
    bool mutableCopy = (shouldCopy == MUTABLE_COPY);
    reallySetProperty(self, _cmd, newValue, offset, atomic, copy, mutableCopy);
}
static inline void reallySetProperty(id self, SEL _cmd, id newValue, ptrdiff_t offset, bool atomic, bool copy, bool mutableCopy)
{
    if (offset == 0) {
        object_setClass(self, newValue);
        return;
    }
    id oldValue;
    id *slot = (id*) ((char*)self + offset);
    if (copy) {
        newValue = [newValue copyWithZone:nil];
    } else if (mutableCopy) {
        newValue = [newValue mutableCopyWithZone:nil];
    } else {
        if (*slot == newValue) return;
        newValue = objc_retain(newValue);
    }
    if (!atomic) {
        oldValue = *slot;
        *slot = newValue;
    } else {
        spinlock_t& slotlock = PropertyLocks[slot];
        slotlock.lock();
        oldValue = *slot;
        *slot = newValue;        
        slotlock.unlock();
    }
    objc_release(oldValue);
}

从源码中可以看出set_objc_setProperty就是一个拷贝变量并且赋值的过程。