今天通过源码的学习对类进行分析，从isa 的走位和继承链来进行展开。
我们先来看一下isa经典的流程图：
isa流程图.png
1 isa 的走位
先创建一个LGPerson类然后用lldb打印验证：
int main(int argc, const char * argv[]) {
@autoreleasepool {
// 0x00007ffffffffff8
LGPerson *p = [LGPerson alloc];
NSLog(@"%@",p);
lgTestClassNum();
lgTestNSObject();
lgTypes();
NSLog(@"isa 我来了!");
}
return 0;
}
断点位置NSLog(@"%@",p);
(lldb) x/4gx person
0x103237ca0: 0x001d800100008365 0x0000000000000000
0x103237cb0: 0x0000000000000000 0x0000000000000000
(lldb) x person
0x103237ca0: 65 83 00 00 01 80 1d 00 00 00 00 00 00 00 00 00 e...............
0x103237cb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
(lldb) p/x person
(LGPerson *) $2 = 0x0000000103237ca0
(lldb) x/4gx 0x0000000103237ca0
0x103237ca0: 0x001d800100008365 0x0000000000000000
0x103237cb0: 0x0000000000000000 0x0000000000000000
(lldb) 0x001d800100008365 通过几种方法可以拿到我们对象的 isa
error: '0x001d800100008365' is not a valid command.
(lldb) p/x 0x001d800100008365 & 0x00007ffffffffff8
(long) $3 = 0x0000000100008360
(lldb) p 0x0000000100008360
(long) $4 = 4295000928
(lldb) po 0x0000000100008360
LGPerson
(lldb) x/4gx 0x0000000100008360
0x100008360: 0x0000000100008338 0x00007fff8e7a2118
0x100008370: 0x00007fff67082140 0x0000802c00000000
(lldb) po 0x0000000100008338 & 0x00007ffffffffff8
LGPerson
(lldb) 竟然也得到LGPerson，表示不可思议
error: '竟然也得到LGPerson，表示不可思议' is not a valid command.
(lldb) p/x 0x0000000100008338 & 0x00007ffffffffff8
(long) $7 = 0x0000000100008338
(lldb)
通过lldb打印我们发现竟然可以获取到两个LGPerson
此时猜想0x0000000100008360 VS 0x0000000100008338
哪一个才是我们开辟的类?
分析类对象内存存在个数
void lgTestClassNum(void){
Class class1 = [LGPerson class];
Class class2 = [LGPerson alloc].class;
Class class3 = object_getClass([LGPerson alloc]);
Class class4 = [LGPerson alloc].class;
NSLog(@"\n%p-\n%p-\n%p-\n%p",class1,class2,class3,class4);
}
断点位位置 lgTestClassNum();
2021-06-17 22:59:44.232703+0800 002-isa分析[75019:5584135]
0x100008360-
0x100008360-
0x100008360-
0x100008360
打印结构发现：0x0000000100008360 才是我们的类，0x0000000100008338不是我们的类，那是是什么？一个新的东西，元类？
我可以得到暂时的结论：对象 isa -> 类 isa -> 元类
接下来我们通过MachOView来分析下，新的东西是什么：
截屏2021-06-17 下午11.06.50.png
LGPerson.png
截屏2021-06-17 下午11.15.26.png
可以看到新的东西多了个：
OBJC_METACLASS$_LGPerson ,也就我们平常听说的元类，由系统自动生成，证明我们的结论是正确的对象 isa -> 类 isa -> 元类
重新运行项目，我们继续探索：
(lldb) x/4gx person
0x103265100: 0x001d800100008365 0x0000000000000000
0x103265110: 0x0000000000000000 0x0000000000000000
(lldb) x person
0x103265100: 65 83 00 00 01 80 1d 00 00 00 00 00 00 00 00 00 e...............
0x103265110: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
(lldb) p/x person
(LGPerson *) $2 = 0x0000000103265100
(lldb) x/4gx 0x0000000103265100
0x103265100: 0x001d800100008365 0x0000000000000000
0x103265110: 0x0000000000000000 0x0000000000000000
(lldb) p/x 0x001d800100008365 & 0x00007ffffffffff8
(long) $3 = 0x0000000100008360
(lldb) p 0x0000000100008360
(long) $4 = 4295000928
(lldb) po 0x0000000100008360
LGPerson
(lldb) x/4gx 0x0000000100008360
0x100008360: 0x0000000100008338 0x00007fff8e7a2118
0x100008370: 0x00007fff67082140 0x0000802c00000000
(lldb) po 0x0000000100008338 & 0x00007ffffffffff8
LGPerson
(lldb) p/x 0x0000000100008338 & 0x00007ffffffffff8
(long) $7 = 0x0000000100008338
(lldb) x/4gx 0x0000000100008338
0x100008338: 0x00007fff8e7a20f0 0x00007fff8e7a20f0
0x100008348: 0x0000000100405cc0 0x0003e03500000007
(lldb) po 0x00007fff8e7a20f0 & 0x00007ffffffffff8
NSObject
(lldb) po NSObject.class
NSObject
(lldb) p/x NSObject.class
(Class) $10 = 0x00007fff8e7a2118 NSObject
2021-06-17 23:40:25.795492+0800 002-isa分析[76268:5634302]
(lldb) x/4gx 0x00007fff8e7a2118
0x7fff8e7a2118: 0x00007fff8e7a20f0 0x0000000000000000
0x7fff8e7a2128: 0x00000001032654a0 0x0002801000000003
(lldb) p/x 0x00007fff8e7a20f0 & 0x00007ffffffffff8
(long) $11 = 0x00007fff8e7a20f0
(lldb) po 0x00007fff8e7a20f0 & 0x00007ffffffffff8
NSObject
(lldb)
截屏2021-06-17 下午11.48.14.png
由上图代码可以总结 isa 的走位大致如下：
isa走向.png
1.对象 isa -> 类 isa -> 元类 isa -> 根元类 isa -> 根元类
2.根类 isa -> 根元类 isa
2 继承链
2.1.继承链
void lgTestSuperClass(void){
LGTeacher *t = [LGTeacher alloc];
LGPerson *p = [LGPerson alloc];
NSLog(@"%@-%@",t,p);
NSLog(@"%@",class_getSuperclass(LGTeacher.class));
NSLog(@"%@",class_getSuperclass(LGPerson.class));
NSLog(@"%@",class_getSuperclass(NSObject.class));
}
2021-06-18 00:49:01.936188+0800 002-isa分析[78024:5697290] -
2021-06-18 00:49:01.936282+0800 002-isa分析[78024:5697290] LGPerson
2021-06-18 00:49:01.936325+0800 002-isa分析[78024:5697290] NSObject
2021-06-18 00:49:09.086433+0800 002-isa分析[78024:5697290] (null)
类的继承总结：
类 继承于 父类
父类 继承于 根类
根类 继承于 nil
2.1.NSObject 元类链
void lgTestNSObject(void){
// NSObject实例对象
NSObject *object1 = [NSObject alloc];
// NSObject类
Class class = object_getClass(object1);
// NSObject元类
Class metaClass = object_getClass(class);
// NSObject根元类
Class rootMetaClass = object_getClass(metaClass);
// NSObject根根元类
Class rootRootMetaClass = object_getClass(rootMetaClass);
NSLog(@"\n%p 实例对象\n%p 类\n%p 元类\n%p 根元类\n%p 根根元类",object1,class,metaClass,rootMetaClass,rootRootMetaClass);
// LGPerson元类
Class pMetaClass = object_getClass(LGPerson.class);
Class psuperClass = class_getSuperclass(pMetaClass);
NSLog(@"%@ - %p",psuperClass,psuperClass);
// LGTeacher -> LGPerson -> NSObject
// 元类也有一条继承链
Class tMetaClass = object_getClass(LGTeacher.class);
Class tsuperClass = class_getSuperclass(tMetaClass);
NSLog(@"%@ - %p",tsuperClass,tsuperClass);
// NSObject 根类特殊情况
Class nsuperClass = class_getSuperclass(NSObject.class);
NSLog(@"%@ - %p",nsuperClass,nsuperClass);
// 根元类 -> NSObject
Class rnsuperClass = class_getSuperclass(metaClass);
NSLog(@"%@ - %p",rnsuperClass,rnsuperClass);
}
2021-06-18 00:39:02.147072+0800 002-isa分析[76268:5634302]
0x1006040e0 实例对象
0x7fff8e7a2118 类
0x7fff8e7a20f0 元类
0x7fff8e7a20f0 根元类
0x7fff8e7a20f0 根根元类
2021-06-18 00:39:21.070988+0800 002-isa分析[76268:5634302] NSObject - 0x7fff8e7a20f0
2021-06-18 00:39:21.071134+0800 002-isa分析[76268:5634302] LGPerson - 0x100008338
2021-06-18 00:39:21.071260+0800 002-isa分析[76268:5634302] (null) - 0x0
2021-06-18 00:39:21.071362+0800 002-isa分析[76268:5634302] NSObject - 0x7fff8e7a2118
元类的继承总结：
元类 继承于 父元类
父元类 继承于 根元类
根元类 继承于 根类
根类 继承于 nil
继承链.png
通过上面初步从 isa 的走位和继承类两个地方入手对类初步的探究与分析；
3.源码分析类的结构
接下来我们继续来分析类的结构 查看Class 源码
typedef struct objc_class *Class;
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
...
}
我们在源码中搜索objc_class，发现objc_class继承于objc_object,而objc_object里面包含一个默认的isa,
继续查看bits是存储类的信息的，而bits是有class_data_bits_t定义的，那我们就看下class_data_bits_t的源码：
struct class_data_bits_t {
friend objc_class;
// Values are the FAST_ flags above.
uintptr_t bits;
private:
.... 省略代码....
public:
class_rw_t* data() const {
return (class_rw_t *)(bits & FAST_DATA_MASK);
}
.... 省略代码....
// Get the class's ro data, even in the presence of concurrent realization.
// fixme this isn't really safe without a compiler barrier at least
// and probably a memory barrier when realizeClass changes the data field
const class_ro_t *safe_ro() const {
class_rw_t *maybe_rw = data();
if (maybe_rw->flags & RW_REALIZED) {
// maybe_rw is rw
return maybe_rw->ro();
} else {
// maybe_rw is actually ro
return (class_ro_t *)maybe_rw;
}
}
.... 省略代码....
};
主要看下class_rw_t和class_ro_t
struct class_rw_t {
.... 省略代码....
const method_array_t methods() const {
auto v = get_ro_or_rwe();
if (v.is()) {
return v.get(&ro_or_rw_ext)->methods;
} else {
return method_array_t{v.get(&ro_or_rw_ext)->baseMethods()};
}
}
const property_array_t properties() const {
auto v = get_ro_or_rwe();
if (v.is()) {
return v.get(&ro_or_rw_ext)->properties;
} else {
return property_array_t{v.get(&ro_or_rw_ext)->baseProperties};
}
}
const protocol_array_t protocols() const {
auto v = get_ro_or_rwe();
if (v.is()) {
return v.get(&ro_or_rw_ext)->protocols;
} else {
return protocol_array_t{v.get(&ro_or_rw_ext)->baseProtocols};
}
}
};
继续通过lldb来打印bits里面的信息
#import
NS_ASSUME_NONNULL_BEGIN
@interface LGPerson : NSObject{
NSString *subject;
}
@property (nonatomic, copy) NSString *name;
@property (nonatomic, copy) NSString *hobby;
- (void)sayNB;
+ (void)say666;
@end
NS_ASSUME_NONNULL_END
#import "LGPerson.h"
@implementation LGPerson
- (instancetype)init{
if (self = [super init]) {
self.name = @"Cooci";
}
return self;
}
- (void)sayNB{
}
+ (void)say666{
}
@end
打印结构：
KCObjcBuild was compiled with optimization - stepping may behave oddly; variables may not be available.
(lldb) p/x LGPerson.class
(Class) $0 = 0x0000000100004410 LGPerson
(lldb) p (class_data_bits_t*) 0x0000000100004430
(class_data_bits_t *) $1 = 0x0000000100004430
(lldb) p $1->data()
(class_rw_t *) $2 = 0x0000000101e060f0
(lldb) p *$2
(class_rw_t) $3 = {
flags = 2148007936
witness = 0
ro_or_rw_ext = {
std::__1::atomic = {
Value = 4294984104
}
}
firstSubclass = nil
nextSiblingClass = NSUUID
}
(lldb) p $3.methods()
(const method_array_t) $4 = {
list_array_tt = {
= {
list = {
ptr = 0x00000001000041f0
}
arrayAndFlag = 4294984176
}
}
}
(lldb) p $4.list
(const method_list_t_authed_ptr) $5 = {
ptr = 0x00000001000041f0
}
(lldb) p $5.ptr
(method_list_t *const) $6 = 0x00000001000041f0
(lldb) p *$6
(method_list_t) $7 = {
entsize_list_tt = (entsizeAndFlags = 27, count = 6)
}
(lldb) p $7.get(0)
(method_t) $8 = {}
(lldb) p $7.get(1)
(method_t) $9 = {}
(lldb) p $7.get(2)
(method_t) $10 = {}
(lldb) p $7.get(3)
(method_t) $11 = {}
(lldb) p $7.get(4)
(method_t) $12 = {}
(lldb) p $7.get(5)
(method_t) $13 = {}
(lldb) p $7.get(6)
Assertion failed: (i < count), function get, file /Users/fengjiefeng/Desktop/逻辑教育V14--iOS底层开发课程/1.iOS底层大师班/20210616-大师班-第4节课-类的原理分析上/20210616-大师班第4天-类的原理分析上/01--课堂代码/004-类的结构分析/runtime/objc-runtime-new.h, line 624.
error: Execution was interrupted, reason: signal SIGABRT.
The process has been returned to the state before expression evaluation.
(lldb) p $7.get(0).big()
(method_t::big) $20 = {
name = "sayNB"
types = 0x0000000100003f77 "v16@0:8"
imp = 0x0000000100003d40 (KCObjcBuild`-[LGPerson sayNB])
}
(lldb) p $7.get(1).big()
(method_t::big) $21 = {
name = "hobby"
types = 0x0000000100003f6f "@16@0:8"
imp = 0x0000000100003db0 (KCObjcBuild`-[LGPerson hobby])
}
(lldb) p $7.get(2).big()
(method_t::big) $22 = {
name = "setHobby:"
types = 0x0000000100003f8b "v24@0:8@16"
imp = 0x0000000100003de0 (KCObjcBuild`-[LGPerson setHobby:])
}
(lldb) p $7.get(3).big()
(method_t::big) $23 = {
name = "init"
types = 0x0000000100003f6f "@16@0:8"
imp = 0x0000000100003ce0 (KCObjcBuild`-[LGPerson init])
}
(lldb) p $7.get(4).big()
(method_t::big) $24 = {
name = "name"
types = 0x0000000100003f6f "@16@0:8"
imp = 0x0000000100003d50 (KCObjcBuild`-[LGPerson name])
}
(lldb) p $7.get(5).big()
(method_t::big) $25 = {
name = "setName:"
types = 0x0000000100003f8b "v24@0:8@16"
imp = 0x0000000100003d80 (KCObjcBuild`-[LGPerson setName:])
}
(lldb) p $3.properties()
(const property_array_t) $14 = {
list_array_tt = {
= {
list = {
ptr = 0x00000001000042f0
}
arrayAndFlag = 4294984432
}
}
}
(lldb) p $14.list
(const RawPtr) $15 = {
ptr = 0x00000001000042f0
}
(lldb) p $15.ptr
(property_list_t *const) $16 = 0x00000001000042f0
(lldb) p *16
error: :1:1: indirection requires pointer operand ('int' invalid)
*16
^~~
(lldb) p *$16
(property_list_t) $17 = {
entsize_list_tt = (entsizeAndFlags = 16, count = 2)
}
(lldb) p $17.get(0)
(property_t) $18 = (name = "name", attributes = "T@\"NSString\",C,N,V_name")
(lldb) p $17.get(1)
(property_t) $19 = (name = "hobby", attributes = "T@\"NSString\",C,N,V_hobby")
(lldb) p $17.get(2)//越界了
Assertion failed: (i < count), function get, file /Users/fengjiefeng/Desktop/逻辑教育V14--iOS底层开发课程/1.iOS底层大师班/20210616-大师班-第4节课-类的原理分析上/20210616-大师班第4天-类的原理分析上/01--课堂代码/004-类的结构分析/runtime/objc-runtime-new.h, line 624.
error: Execution was interrupted, reason: signal SIGABRT.
The process has been returned to the state before expression evaluation.
(lldb)
还未完成....