OC底层4-类的探究分析

作者: JEFF009 | 来源:发表于2021-06-18 01:08 被阅读0次

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[]) {![继承链.png](https://img.haomeiwen.com/i6347155/74efcdccb400046b.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

    @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] <LGPerson: 0x103265100>
(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] <LGTeacher: 0x103107950>-<LGPerson: 0x103107970>
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<class_rw_ext_t *>()) {
            return v.get<class_rw_ext_t *>(&ro_or_rw_ext)->methods;
        } else {
            return method_array_t{v.get<const class_ro_t *>(&ro_or_rw_ext)->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 *>(&ro_or_rw_ext)->properties;
        } else {
            return property_array_t{v.get<const class_ro_t *>(&ro_or_rw_ext)->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 *>(&ro_or_rw_ext)->protocols;
        } else {
            return protocol_array_t{v.get<const class_ro_t *>(&ro_or_rw_ext)->baseProtocols};
        }
    }
};

继续通过lldb来打印bits里面的信息

#import <Foundation/Foundation.h>

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<unsigned long> = {
      Value = 4294984104
    }
  }
  firstSubclass = nil
  nextSiblingClass = NSUUID
}
(lldb) p $3.methods()
(const method_array_t) $4 = {
  list_array_tt<method_t, method_list_t, method_list_t_authed_ptr> = {
     = {
      list = {
        ptr = 0x00000001000041f0
      }
      arrayAndFlag = 4294984176
    }
  }
}
(lldb) p $4.list
(const method_list_t_authed_ptr<method_list_t>) $5 = {
  ptr = 0x00000001000041f0
}
(lldb) p $5.ptr
(method_list_t *const) $6 = 0x00000001000041f0
(lldb) p *$6
(method_list_t) $7 = {
  entsize_list_tt<method_t, method_list_t, 4294901763, method_t::pointer_modifier> = (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<property_t, property_list_t, RawPtr> = {
     = {
      list = {
        ptr = 0x00000001000042f0
      }
      arrayAndFlag = 4294984432
    }
  }
}
(lldb) p $14.list
(const RawPtr<property_list_t>) $15 = {
  ptr = 0x00000001000042f0
}
(lldb) p $15.ptr
(property_list_t *const) $16 = 0x00000001000042f0
(lldb) p *16
error: <user expression 18>:1:1: indirection requires pointer operand ('int' invalid)
*16
^~~
(lldb) p *$16
(property_list_t) $17 = {
  entsize_list_tt<property_t, property_list_t, 0, PointerModifierNop> = (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....

OC底层4-类的探究分析

1 isa 的走位

2 继承链

2.1.继承链

2.1.NSObject 元类链

3.源码分析类的结构

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