iOS 底层alloc init new 源码流程示例分析

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

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

static ALWAYS_INLINE id
callAlloc(Class cls, bool checkNil, bool allocWithZone=false)
{
#if __OBJC2__ //有可用的编译器优化
    if (slowpath(checkNil && !cls)) return nil;
    //判断是否自定义实现了 +allocWithZone 方法
    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));

#define fastpath(x) (__builtin_expect(bool(x), 1))
#define slowpath(x) (__builtin_expect(bool(x), 0))

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

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
    // 一次性读取累的的信息以提高性能
    bool hasCxxCtor = cxxConstruct && cls->hasCxxCtor();
    bool hasCxxDtor = cls->hasCxxDtor();
    bool fast = cls->canAllocNonpointer();
    size_t size;
    size = cls->instanceSize(extraBytes);
    if (outAllocatedSize) *outAllocatedSize = size;
    id obj;
#if SUPPORT_ZONES
    // 支持zone
    // 早期的内存是通过zone  申请的 ilo89i='
    if (zone) {
        obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);
    } else {
#endif
        obj = (id)calloc(1, size);
#if SUPPORT_ZONES
    }
#endif
    if (slowpath(!obj)) {
        if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
            return _objc_callBadAllocHandler(cls);
        }
        return nil;
    }
    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);
}

inline size_t instanceSize(size_t extraBytes) const {
// 快速计算内存大小
        if (fastpath(cache.hasFastInstanceSize(extraBytes))) {
            return cache.fastInstanceSize(extraBytes);
        }
        size_t size = alignedInstanceSize() + extraBytes;
        // CF requires all objects be at least 16 bytes.
        if (size < 16) size = 16;
        return size;
    }

size_t fastInstanceSize(size_t extra) const
    {
        ASSERT(hasFastInstanceSize(extra));
        if (__builtin_constant_p(extra) && extra == 0) {
            return _flags & FAST_CACHE_ALLOC_MASK16;
        } else {
            size_t size = _flags & FAST_CACHE_ALLOC_MASK;
            // remove the FAST_CACHE_ALLOC_DELTA16 that was added
            // by setFastInstanceSize
            return align16(size + extra - FAST_CACHE_ALLOC_DELTA16);
        }
    }

static inline size_t align16(size_t x) {
    return (x + size_t(15)) & ~size_t(15);
}

- (id)init {
    return _objc_rootInit(self);
}

id
_objc_rootInit(id obj)
{
    // In practice, it will be hard to rely on this function.
    // Many classes do not properly chain -init calls.
    return obj;
}

+ (id)new {
    return [callAlloc(self, false/*checkNil*/) init];
}