【iOS底层了解一点】-01 alloc流程

参考底层源码：objc4-818.2

流程：

示例代码：

    Person *per = [Person alloc];

-第1步:

_objc_rootAlloc(Class cls) {
    return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/);
}

-第2步：

_objc_rootAlloc(Class cls) { 
    return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/); 
}

-第3步：

// 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__
    //如果checkNil需要检验nil为yes && cls == nil => return 不处理
    if (slowpath(checkNil && !cls)) return nil;    
    //有没有实现Alloc 或者 AllocWithZone，如果没有实现实现就会进入实现部分，继承NSObject、NSProxy的对象都是false
    if (fastpath(!cls->ISA()->hasCustomAWZ())) {
        return _objc_rootAllocWithZone(cls, nil); //内部调用_class_createInstanceFromZone，接下来看它的实现
    }

#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));
}

-第4步：

_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);
}

-第五步：

/***********************************************************************
* 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());//如果已经实现过就会抛出异常
    
    // Read class's info bits all at once for performance

    bool hasCxxCtor = cxxConstruct && cls->hasCxxCtor();//是否有构造函数
    bool hasCxxDtor = cls->hasCxxDtor();//是否有栖构函数
    bool fast = cls->canAllocNonpointer();//是否无指针也就是优化的对象

    size_t size;
    size = cls->instanceSize(extraBytes);//根据默认内存0设置对象的内存
    if (outAllocatedSize) *outAllocatedSize = size;

    id obj;
    if (zone) {//根据型参数不会走
        obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);
    } else {//调用c语言的calloc,分配内存
        obj = (**id**)calloc(1, size);
    }


    //如果obj内存分配失败，抛出异常后者返回nil
    if (slowpath(!obj)) {
        if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
            return _objc_callBadAllocHandler(cls);
        }
        return nil;
    }

    if (!zone && fast) {/*如果不使用allocZone && 需要快速建立*/
        obj->initInstanceIsa(cls, hasCxxDtor);
    } else { /*直接初始化isa，一个ias默认是8字节，16位*/
        // 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);
}