从苹果开源源码库objc/libsystem/dyld/coreFoundation(CF) 下一份源码
可以看到runtime的两个版本：
objc-runtime-old; objc-2（objc-runtime-new）NSObject.mm 混编
alloc 调用流程
不管是直接看alloc，还是 objc_alloc
// Calls [cls alloc].
id
objc_alloc(Class cls)
{
return callAlloc(cls, true/*checkNil*/, false/*allocWithZone*/);
}
+ (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*/);
}
最后都是调用
callAlloc
// Call [cls alloc] or [cls allocWithZone:nil], with appropriate
// shortcutting optimizations.
static ALWAYS_INLINE id
callAlloc(Class cls, bool checkNil, bool allocWithZone=false)
{
if (slowpath(checkNil && !cls)) return nil;
#if __OBJC2__
if (fastpath(!cls->ISA()->hasCustomAWZ())) {
// No alloc/allocWithZone implementation. Go straight to the allocator.
// fixme store hasCustomAWZ in the non-meta class and
// add it to canAllocFast's summary
if (fastpath(cls->canAllocFast())) {
// No ctors, raw isa, etc. Go straight to the metal.
bool dtor = cls->hasCxxDtor();
id obj = (id)calloc(1, cls->bits.fastInstanceSize());
if (slowpath(!obj)) return callBadAllocHandler(cls);
obj->initInstanceIsa(cls, dtor);
return obj;
}
else {
// Has ctor or raw isa or something. Use the slower path.
id obj = class_createInstance(cls, 0);
if (slowpath(!obj)) return callBadAllocHandler(cls);
return obj;
}
}
#endif
// No shortcuts available.
if (allocWithZone) return [cls allocWithZone:nil];
return [cls alloc];
}
然后真正创建的方法
class_createInstance
id
class_createInstance(Class cls, size_t extraBytes)
{
return _class_createInstanceFromZone(cls, extraBytes, nil);
}
static __attribute__((always_inline))
id
_class_createInstanceFromZone(Class cls, size_t extraBytes, void *zone,
bool cxxConstruct = true,
size_t *outAllocatedSize = nil)
{
if (!cls) return nil;
assert(cls->isRealized());
// Read class's info bits all at once for performance
bool hasCxxCtor = cls->hasCxxCtor();
bool hasCxxDtor = cls->hasCxxDtor();
bool fast = cls->canAllocNonpointer();
size_t size = cls->instanceSize(extraBytes);
if (outAllocatedSize) *outAllocatedSize = size;
id obj;
if (!zone && fast) {
obj = (id)calloc(1, size);
if (!obj) return nil;
obj->initInstanceIsa(cls, hasCxxDtor);
}
else {
if (zone) {
obj = (id)malloc_zone_calloc ((malloc_zone_t *)zone, 1, size);
} else {
obj = (id)calloc(1, size);
}
if (!obj) return nil;
// Use raw pointer isa on the assumption that they might be
// doing something weird with the zone or RR.
obj->initIsa(cls);
}
if (cxxConstruct && hasCxxCtor) {
obj = _objc_constructOrFree(obj, cls);
}
return obj;
}
核心：
cls->instanceSize；calloc；initInstanceIsa
calloc 在 malloc中无法查看更多细节
截屏2019-12-24下午5.07.30.png
cls->instanceSize :计算对象需要占用的大小（系统分配），8字节对齐，最少16字节。
size_t instanceSize(size_t extraBytes) {
size_t size = alignedInstanceSize() + extraBytes;
// CF requires all objects be at least 16 bytes.
if (size < 16) size = 16;
return size;
}
// May be unaligned depending on class's ivars.
uint32_t unalignedInstanceSize() {
assert(isRealized());
return data()->ro->instanceSize;
}
// Class's ivar size rounded up to a pointer-size boundary.
uint32_t alignedInstanceSize() {
return word_align(unalignedInstanceSize());
}
//define WORD_MASK 7UL
static inline uint32_t word_align(uint32_t x) {
return (x + WORD_MASK) & ~WORD_MASK;
}
空对象默认有isa属性，isa 是个 typedef struct objc_class *Class（结构体指针），需要：8字节;
内存对齐截图
size：
- 对象需要的内存空间，8的倍数-8字节对齐
- 最少16字节，预留，防止意外
需要说明一下，我们常用的获取对象size的两个方法： malloc_size(object)返回是系统分配的内存大小，16字节对齐；而class_getInstanceSize([object class])是对象内存占用的大小，8字节对齐，并且没有最少16字节的逻辑，代码如下：
size_t class_getInstanceSize(Class cls)
{
if (!cls) return 0;
return cls->alignedInstanceSize();
}
// Class's ivar size rounded up to a pointer-size boundary.
uint32_t alignedInstanceSize() {
return word_align(unalignedInstanceSize());
}
代码跟踪
alloc 创建对象跟踪
calloc：申请内存空间
initInstanceIsa：关联class(isa)
initInstanceIsa 详解
