前言
本文是基于objc-781源碼進(jìn)行分析探索
實例對象與類對象底層結(jié)構(gòu)，isa信息分布
拓補(bǔ)圖.001.jpeg
在
objc-runtime-new.h中，
struct objc_class : objc_object {
// Class ISA;
Class superclass;
cache_t cache; // formerly cache pointer and vtable
class_data_bits_t bits; // class_rw_t * plus custom rr/alloc flags
class_rw_t *data() const {
return bits.data();
}
.........
}
objc.h中：
struct objc_object {
Class _Nonnull isa OBJC_ISA_AVAILABILITY;
};
這里的Class _Nonnull isa,已經(jīng)是通過isa_t isa->ISA(),讀取到shiftcls信息；
具體可以在objc-private.h看到：
struct objc_object {
private:
isa_t isa;
public:
// ISA() assumes this is NOT a tagged pointer object
Class ISA();
// rawISA() assumes this is NOT a tagged pointer object or a non pointer ISA
Class rawISA();
// getIsa() allows this to be a tagged pointer object
Class getIsa();
uintptr_t isaBits() const;
........
}
Class ISA()實現(xiàn)：
inline Class
objc_object::ISA()
{
ASSERT(!isTaggedPointer());
#if SUPPORT_INDEXED_ISA
if (isa.nonpointer) {
uintptr_t slot = isa.indexcls;
return classForIndex((unsigned)slot);
}
return (Class)isa.bits;
#else
return (Class)(isa.bits & ISA_MASK);
#endif
//# define ISA_MASK 0x00007ffffffffff8ULL
}
在objc-private.h中查看isa_t，一個聯(lián)合體位域:
union isa_t {
isa_t() { }
isa_t(uintptr_t value) : bits(value) { }
Class cls;
uintptr_t bits;
#if defined(ISA_BITFIELD)
struct {
ISA_BITFIELD; // defined in isa.h
};
#endif
};
在isa.h中，ISA_BITFIELD的位域分布：(在x86_64和arm64架構(gòu)中略有不同)
# 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
objc_class 結(jié)構(gòu)體內(nèi)部核心結(jié)構(gòu)
拓補(bǔ)圖.002.jpeg
在
objc-runtime-new.h中的objc_class結(jié)構(gòu)體
struct objc_class : objc_object {
// Class ISA;
Class superclass;
cache_t cache; // formerly cache pointer and vtable
class_data_bits_t bits; // class_rw_t * plus custom rr/alloc flags
class_rw_t *data() const {
return bits.data();
}
//..........省略...........
}
通過bits.data()獲取class_rw_t *:
struct class_rw_t {
// Be warned that Symbolication knows the layout of this structure.
uint32_t flags;
uint16_t witness;
#if SUPPORT_INDEXED_ISA
uint16_t index;
#endif
explicit_atomic<uintptr_t> ro_or_rw_ext;
Class firstSubclass;
Class nextSiblingClass;
private:
//.........省略........
public:
//.........省略........
const class_ro_t *ro() const {
auto v = get_ro_or_rwe();
if (slowpath(v.is<class_rw_ext_t *>())) {
return v.get<class_rw_ext_t *>()->ro;
}
return v.get<const class_ro_t *>();
}
const method_array_t methods() const {
auto v = get_ro_or_rwe();
if (v.is<class_rw_ext_t *>()) {
return v.get<class_rw_ext_t *>()->methods;
} else {
return method_array_t{v.get<const class_ro_t *>()->baseMethods()};
}
}
const property_array_t properties() const {
auto v = get_ro_or_rwe();
if (v.is<class_rw_ext_t *>()) {
return v.get<class_rw_ext_t *>()->properties;
} else {
return property_array_t{v.get<const class_ro_t *>()->baseProperties};
}
}
const protocol_array_t protocols() const {
auto v = get_ro_or_rwe();
if (v.is<class_rw_ext_t *>()) {
return v.get<class_rw_ext_t *>()->protocols;
} else {
return protocol_array_t{v.get<const class_ro_t *>()->baseProtocols};
}
}
繼續(xù)進(jìn)入class_ro_t結(jié)構(gòu)體：
struct class_ro_t {
uint32_t flags;
uint32_t instanceStart;
uint32_t instanceSize;
#ifdef __LP64__
uint32_t reserved;
#endif
const uint8_t * ivarLayout;
const char * name;
method_list_t * baseMethodList;
protocol_list_t * baseProtocols;
const ivar_list_t * ivars;
const uint8_t * weakIvarLayout;
property_list_t *baseProperties;
// This field exists only when RO_HAS_SWIFT_INITIALIZER is set.
_objc_swiftMetadataInitializer __ptrauth_objc_method_list_imp _swiftMetadataInitializer_NEVER_USE[0];
_objc_swiftMetadataInitializer swiftMetadataInitializer() const {
if (flags & RO_HAS_SWIFT_INITIALIZER) {
return _swiftMetadataInitializer_NEVER_USE[0];
} else {
return nil;
}
}
method_list_t *baseMethods() const {
return baseMethodList;
}
class_ro_t *duplicate() const {
if (flags & RO_HAS_SWIFT_INITIALIZER) {
size_t size = sizeof(*this) + sizeof(_swiftMetadataInitializer_NEVER_USE[0]);
class_ro_t *ro = (class_ro_t *)memdup(this, size);
ro->_swiftMetadataInitializer_NEVER_USE[0] = this->_swiftMetadataInitializer_NEVER_USE[0];
return ro;
} else {
size_t size = sizeof(*this);
class_ro_t *ro = (class_ro_t *)memdup(this, size);
return ro;
}
}
};
總結(jié)
這里并不需要糾結(jié)為什么的問題，因為蘋果就是這樣設(shè)計的。主要就是基于源碼對iOS底層有一個更加深入的理解。
源碼地址
