OC创建对象alloc流程图
2021-09-24 本文已影响0人
竖着走的大闸蟹
开局先上alloc流程图
alloc流程图.png
1.在项目Demo中：创建对象 [HSPerson alloc]将调用底层的alloc函数创建对象
+ (id)alloc {
return_objc_rootAlloc(self);
}
- alloc创建对象的时候会调用内部 _objc_rootAlloc函数
// 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*/);
}
3. 从_objc_rootAlloc进一步调用callAlloc函数、当前对象没有自定义的allocWithZone方法的话、会走快速创建方法；有自定义的allocWithZone方法，调用之后也会走 _objc_rootAllocWithZone方法
// 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())) {//hasCustomAWZ :自定义的allocWithZone
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));
}
4.经过快速创建继续走 _objc_rootAllocWithZone方法、
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);
}
5.而最终创建对象落在了 _class_createInstanceFromZone方法上
cls->instanceSize 计算需要的内存空间大小
calloc 向系统申请开辟内存，返回地址指针
obj->initInstanceIsa 关联到相应的类
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;
//计算出需要的内存空间大小
size = cls->instanceSize(extraBytes);
if (outAllocatedSize) *outAllocatedSize = size;
id obj;
if (zone) {
obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);
} else {
//向系统申请开辟内存，返回地址指针
obj = (id)calloc(1, size);
}
if (slowpath(!obj)) {
if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
return _objc_callBadAllocHandler(cls);
}
return nil;
}
//关联到相应的类
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);
}
6、关联到相应的类过程
inline void
objc_object::initInstanceIsa(Class cls, bool hasCxxDtor)
{
ASSERT(!cls->instancesRequireRawIsa());
ASSERT(hasCxxDtor == cls->hasCxxDtor());
initIsa(cls, true, hasCxxDtor);
}
其他：zone存在时、走 obj->initIsa(cls) 方法
inline void
objc_object::initIsa(Class cls)
{
initIsa(cls, false, false);
}
7、最终initIsa方法做了些赋值操作
inline void
objc_object::initIsa(Class cls, bool nonpointer, bool hasCxxDtor)
{
ASSERT(!isTaggedPointer());
//nonpointer 表示是否对isa指针开启指针优化 0 纯isa指针、1:不止是类对象地址，isa包含了类对象，对象的引用等
if (!nonpointer) {
isa = isa_t((uintptr_t)cls);
} else {
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、在 initIsa方法中如果对指针开启指针优化、那么其中的涉及到联合体和位域、
union isa_t {//联合体
isa_t() { }
isa_t(uintptr_t value) : bits(value) { }
Class cls;
uintptr_t bits;
#if defined(ISA_BITFIELD)
struct {//位域：告诉位置区域
ISA_BITFIELD; // defined in isa.h
};
#endif
};
9、ISA_BITFIELD的定义在下方显示、uintptr_t定义为下
typedef unsigned long uintptr_t;
SUPPORT_PACKED_ISA 判断当前设备为arm64移动结构还是 x86_64电脑设备、这两类设备类型中无符号长整型 uintptr_t 对应的数据值相加均为 64
#if SUPPORT_PACKED_ISA
// extra_rc must be the MSB-most field (so it matches carry/overflow flags)
// nonpointer must be the LSB (fixme or get rid of it)
// shiftcls must occupy the same bits that a real class pointer would
// bits + RC_ONE is equivalent to extra_rc + 1
// RC_HALF is the high bit of extra_rc (i.e. half of its range)
// future expansion:
// uintptr_t fast_rr : 1; // no r/r overrides
// uintptr_t lock : 2; // lock for atomic property, @synch
// uintptr_t extraBytes : 1; // allocated with extra bytes
# if __arm64__
# define ISA_MASK 0x0000000ffffffff8ULL
# define ISA_MAGIC_MASK 0x000003f000000001ULL
# define ISA_MAGIC_VALUE 0x000001a000000001ULL
# define ISA_BITFIELD \
uintptr_t nonpointer : 1; \
uintptr_t has_assoc : 1; \
uintptr_t has_cxx_dtor : 1; \
uintptr_t shiftcls : 33; /*MACH_VM_MAX_ADDRESS 0x1000000000*/ \
uintptr_t magic : 6; \
uintptr_t weakly_referenced : 1; \
uintptr_t deallocating : 1; \
uintptr_t has_sidetable_rc : 1; \
uintptr_t extra_rc : 19
# define RC_ONE (1ULL<<45)
# define RC_HALF (1ULL<<18)
# elif __x86_64__
# define ISA_MASK 0x00007ffffffffff8ULL
# define ISA_MAGIC_MASK 0x001f800000000001ULL
# define ISA_MAGIC_VALUE 0x001d800000000001ULL
# define ISA_BITFIELD \
uintptr_t nonpointer : 1; \
uintptr_t has_assoc : 1; \
uintptr_t has_cxx_dtor : 1; \
uintptr_t shiftcls : 44; /*MACH_VM_MAX_ADDRESS 0x7fffffe00000*/ \
uintptr_t magic : 6; \
uintptr_t weakly_referenced : 1; \
uintptr_t deallocating : 1; \
uintptr_t has_sidetable_rc : 1; \
uintptr_t extra_rc : 8
# define RC_ONE (1ULL<<56)
# define RC_HALF (1ULL<<7)
# else
# error unknown architecture for packed isa
# endif
// SUPPORT_PACKED_ISA
#endif
10、ISA_BITFIELD涉及的变量含义
nonpointer: 表示是否对指针开启指针优化、0:纯isa指针，1:不止是类对象地址，isa中包含了类信息，对象的引用计数等。
has_assoc： 关联对象标志位，0没有，1存在
has_cxx_dtor： 该对象是否有C++或者Objc的析构器，如果有析构函数，则需要做析构逻辑，如果没有，则可以更快的释放对象。
shiftcls： 存储类指针的值。开启指针优化的情况下，在arm64架构中有33位用来存储类指针。
magic： 用于调试器判断当前对象是真的对象还是没有初始化的空间。
weakly_referenced:标志对象是否被指向或者曾经指向一个ARC的弱变量，没有弱引用的对象可以更快释放。
deallocating： 标志对象是否正在释放对象。
has_sidetable_rc：当对象引用计数大于10时，则需要借用该变量存储进位
extra_rc： 当表示该对象的引用计数值，实际上是引用计数减1。如果对象的引用计数为10，那么extra_rc为9。如果引用计数大于10，则需要使用has_sidetable_rc。
