Runtime objc4-779.1 通过runtime源码对OC对象初始化过程解析
常用对象初始化代码
[[NSArray alloc] init]
[NSArray new]
我们先根据 alloc+init的方式来捋一遍runtime初始化对象的过程,看看有哪些值得我们学习的地方.
以下书写方式为 <步骤号><代码所在文件><代码行数>
所以阅读本文,最好是同步对照 objc4-779.1 源码一起
1 NSObject.mm line 2317
+ (id)alloc {
return _objc_rootAlloc(self);
}
2 NSObject.mm line 1717
id _objc_rootAlloc(Class cls)
{
return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/);
}
3 NSObject.mm line 1697
// Call [cls alloc] or [cls allocWithZone:nil], with appropriate
// shortcutting optimizations.
static ALWAYS_INLINE id
callAlloc(Class cls, bool checkNil, bool allocWithZone=false)
{
// 是否是Objective-C 2.0
#if __OBJC2__
// 3.1 判定是否检查cls是否为空,如果检查,为空则返回nil
if (slowpath(checkNil && !cls)) return nil;
// 3.2 检查本类、父类的继承链上是否有实现自己的allocWithZone方法,如果没有,就默认调用元类的
if (fastpath(!cls->ISA()->hasCustomAWZ())) {
return _objc_rootAllocWithZone(cls, nil);
}
#endif
// 3.3 判断是否要调用allocWithZone方法,使用runtime的方法调用常规操作.
// 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));
}
4 NSObject.mm line2321
// 替代 alloc方法
// Replaced by ObjectAlloc
+ (id)allocWithZone:(struct _NSZone *)zone {
return _objc_rootAllocWithZone(self, (malloc_zone_t *)zone);
}
5 NSObject.mm line2534
id
_objc_rootAllocWithZone(Class cls, malloc_zone_t *zone)
{
id obj;
if (fastpath(!zone)) {
// class_createInstance内部实质调用的是 _class_createInstanceFromZone 方法
obj = class_createInstance(cls, 0);
} else {
obj = class_createInstanceFromZone(cls, 0, zone);
}
if (slowpath(!obj)) obj = _objc_callBadAllocHandler(cls);
return obj;
}
所以在本步骤,obj初始化的过程成看似分叉,其实都集合到了调用_class_createInstanceFromZone
6 objc-runtime-new.mm line7385
/***********************************************************************
* 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)
{
// 6.1 如果类对象本身没有被实现,则发出警告,并终止程序
ASSERT(cls->isRealized());
// 6.2 通过位运算获取类的信息,以提高性能
// Read class's info bits all at once for performance
// 是否有C++ 构造函数
bool hasCxxCtor = cxxConstruct && cls->hasCxxCtor();
// 是否有C++ 析构函数
bool hasCxxDtor = cls->hasCxxDtor();
//如果isa_t指针可用,则为true,反之为false,2.0版本中绝大多数类都是支持isa_t的,如果对isa和isa_t不清楚的,可以看之前的博文 https://blog.csdn.net/wxs0124/article/details/84401718
bool fast = cls->canAllocNonpointer();
size_t size;
// 初始化的大小,instanceSize保证大小必须大于等于16bytes,内部总结就是用class_ro_t的isa_t的大小(8bytes)加上extraBytes,如果小于16bytes则补齐为16bytes, 这样做是为了内存对齐.
size = cls->instanceSize(extraBytes);
if (outAllocatedSize) *outAllocatedSize = size;
id obj;
// 6.3开辟内存空间
if (zone) {
// 空间大小为size大小
obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);
} else {
// 空间初始化默认为0或者nil
obj = (id)calloc(1, size);
}
// 这里判断是否有构造函数,如果没有终止,到这里我们遇到了fastpath和slowpath,稍后我们看一下他的源码和作用
if (slowpath(!obj)) {
if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
return _objc_callBadAllocHandler(cls);
}
return nil;
}
// 6.4 初始化isa_t
if (!zone && fast) {
//initInstanceIsa方法内部也是调用initIsa方法,只是因没有空间并且有isa_t指针而传参不同
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);
}
7 objc-object.h line214
inline void
objc_object::initInstanceIsa(Class cls, bool hasCxxDtor)
{
ASSERT(!cls->instancesRequireRawIsa());
ASSERT(hasCxxDtor == cls->hasCxxDtor());
// 没有zone的情况下,该方法才回被调用,对应的,空间是默认为0或者nil的,需要创建isa_t
initIsa(cls, true, hasCxxDtor);
}
inline void
objc_object::initIsa(Class cls, bool nonpointer, bool hasCxxDtor)
{
ASSERT(!isTaggedPointer());
// 7.1 如果有isa_t
if (!nonpointer) {
isa = isa_t((uintptr_t)cls);
} else {
// 7.2如果没有isa_t 则进行初始化isa_t
ASSERT(!DisableNonpointerIsa);
ASSERT(!cls->instancesRequireRawIsa());
isa_t newisa(0);
#if SUPPORT_INDEXED_ISA
ASSERT(cls->classArrayIndex() > 0);
newisa.bits = ISA_INDEX_MAGIC_VALUE;
// isa.magic is part of ISA_MAGIC_VALUE
// isa.nonpointer is part of ISA_MAGIC_VALUE
newisa.has_cxx_dtor = hasCxxDtor;
newisa.indexcls = (uintptr_t)cls->classArrayIndex();
#else
newisa.bits = ISA_MAGIC_VALUE;
// isa.magic is part of ISA_MAGIC_VALUE
// isa.nonpointer is part of ISA_MAGIC_VALUE
newisa.has_cxx_dtor = hasCxxDtor;
newisa.shiftcls = (uintptr_t)cls >> 3;
#endif
// This write must be performed in a single store in some cases
// (for example when realizing a class because other threads
// may simultaneously try to use the class).
// fixme use atomics here to guarantee single-store and to
// guarantee memory order w.r.t. the class index table
// ...but not too atomic because we don't want to hurt instantiation
isa = newisa;
}
}
8 NSObject.mm line2331
- (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;
}
至此,对象通过alloc初始化的过程就完成了,最终目标就是对isa_t联合体内容的赋值,对内存的分配,并返回对象指针
解释一下fastpath slowpath
#define fastpath(x) (__builtin_expect(bool(x), 1))
#define slowpath(x) (__builtin_expect(bool(x), 0))
这两个方法是宏定义,核心为__builtin_expect(EXP,N) , 这是gcc提供的指令,意思为EXP==N的概率很大
也就说fastpath中x的值大概率为真,slowpath中x的值大概率为假
new方法初始化过程
+ (id)new {
return [callAlloc(self, false/*checkNil*/) init];
}
这样看来new方式和alloc+init方式在本质上并没有什么不同
总结一下
- 对象初始化的核心过程为根据
objc_class的状态开辟空间,并初始化isa_t内容,随后为对象分配内存并返回指向此内存区域的指针. - OC中的对象大小因为CF系统的内存对齐限制,所有对象默认大于等于16bytes
- 在isa_t初始化过程中使用了位运算进行性能提升,可以在日常开发中应用
fastpathslowpath应用gcc指令__builtin_expect(EXP,N),通过此指令优化编译器在编译时的代码布局,减少指令跳转带来的性能消耗.
