iOS对象的底层探索(上)_app

在探索OC对象底层本质之前，先了解一下clang

Clang

clang是⼀个由Apple主导编写，基于LLVM的C/C++/Objective-C编译器主要用于底层编译，将oc文件转换成c++文件，方便理解底层原理

OC是面向对象的语言，开发中一切的基础，首先需要一个对象，如果没有的话，可以alloc一个。本篇文章主要探索alloc的内部执行流程。

首先自定义Person继承与NSObject

//创建对象
 Person *person = [[Person alloc] init];

断点在该行，control + step into进入断点即可看到调用objc_alloc函数，再源码如下

alloc调用一次向下执行

+ (id)alloc {
    return _objc_rootAlloc(self);
}

// Base class implementation of +alloc. cls is not nil.
// Calls [cls allocWithZone:nil].
id
_objc_rootAlloc(Class cls)
{
    return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/);
}

//alloc源码
// Call [cls alloc] or [cls allocWithZone:nil], with appropriate
// shortcutting optimizations.
static ALWAYS_INLINE id
callAlloc(Class cls, bool checkNil, bool allocWithZone=false)
{
#if __OBJC2__
    if (slowpath(checkNil && !cls)) return nil;
    if (fastpath(!cls->ISA()->hasCustomAWZ())) {
        return _objc_rootAllocWithZone(cls, nil);
    }
#endif

    // No shortcuts available.
    if (allocWithZone) {
        return ((id(*)(id, SEL, struct _NSZone *))objc_msgSend)(cls, @selector(allocWithZone:), nil);
    }
    return ((id(*)(id, SEL))objc_msgSend)(cls, @selector(alloc));
}

NEVER_INLINE
id
_objc_rootAllocWithZone(Class cls, malloc_zone_t *zone __unused)
{
    // allocWithZone under __OBJC2__ ignores the zone parameter
    return _class_createInstanceFromZone(cls, 0, nil,
                                         OBJECT_CONSTRUCT_CALL_BADALLOC);
}

//oc源码
/***********************************************************************
* class_createInstance
* fixme
* Locking: none
*
* Note: this function has been carefully written so that the fastpath
* takes no branch.
**********************************************************************/
static ALWAYS_INLINE id
_class_createInstanceFromZone(Class cls, size_t extraBytes, void *zone,
                              int construct_flags = OBJECT_CONSTRUCT_NONE,
                              bool cxxConstruct = true,
                              size_t *outAllocatedSize = nil)
{
    ASSERT(cls->isRealized());

    // Read class's info bits all at once for performance
    bool hasCxxCtor = cxxConstruct && cls->hasCxxCtor();
    bool hasCxxDtor = cls->hasCxxDtor();
    bool fast = cls->canAllocNonpointer();
    size_t size;
    //计算开辟内存空间大小
    size = cls->instanceSize(extraBytes);
    if (outAllocatedSize) *outAllocatedSize = size;
    //分配内存空间
    id obj;
    if (zone) {
        obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);
    } else {
        obj = (id)calloc(1, size);
    }
    if (slowpath(!obj)) {
        if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
            return _objc_callBadAllocHandler(cls);
        }
        return nil;
    }
    //关联内存和类
    if (!zone && fast) {
        obj->initInstanceIsa(cls, hasCxxDtor);
    } else {
        // Use raw pointer isa on the assumption that they might be
        // doing something weird with the zone or RR.
        obj->initIsa(cls);
    }

    if (fastpath(!hasCxxCtor)) {
        return obj;
    }

    construct_flags |= OBJECT_CONSTRUCT_FREE_ONFAILURE;
    return object_cxxConstructFromClass(obj, cls, construct_flags);
}

理论alloc执行流程：

alloc->_objc_rootAlloc->callAlloc->_objc_rootAllocWithZone->_class_createInstanceFromZone

但其实并非如此，貌似被欺骗了，接下来按debug一步一步跟踪调试进行探究。

实际debug后alloc的执行流程： 1、代码首先走到对象alloc的地方，然后起开所有断点进行一步一步调试

2.代码首先进入callAlloc并直接最后一步return消息发送((id(*)(id, SEL))objc_msgSend)(cls, @selector(alloc))

3、执行alloc *** 作，并调用_objc_rootAlloc 4、调用_objc_rootAlloc函数，_objc_rootAlloc内部再次调用callAlloc

5、callAlloc执行流程

6、调用_objc_rootAllocWithZone

7、调用_class_createInstanceFromZone,计算内存大小,calloc开辟内存空间,关联类和内存

callAlloc->objc_msgSend->alloc->_objc_rootAlloc->callAlloc->objc_rootAllocWithZone->_class_createInstanceFromZone->calloc

_class_createInstanceFromZone 中计算内存的方法 size= cls->instanceSize(extraBytes);具体是根据CPU64位还是32位进行字节对齐。

字节对照表