IOS开发知识点

iOS之类的结构分析

2020-09-13  本文已影响0人  过客Zhaopy

万物皆对象

我们知道在iOS中,id可以指向所有的实例对象,Class可以指向所有的类,我们来看一下他们的声明:

/// An opaque type that represents an Objective-C class.
typedef struct objc_class *Class;

/// A pointer to an instance of a class.
typedef struct objc_object *id;

从声明中可以看出,OC的所有实例对象都是由objc_object结构体扩展而来,我们知道objc_object结构体中有一个isa指针,所以所有的对象都有isa指针。
我们再来看一下objc_class的声明:

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。。。
}

objc_class的结构可以得出,OC中所有的其实也是对象,也存在isa指针,所以这就是我们常说的万物皆对象

对象、类、元类、根元类

在前一篇iOS之isa文章的最后,插入了一张图,如下:

类的继承.png
很多iOS开发者,肯定不止一次见到过这张图,我想肯定有人会有疑问,真的像图中这样吗?下面我们验证一下。
@interface PYTeacher : NSObject

@end

@implementation PYTeacher

@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        PYTeacher *teacher = [PYTeacher alloc];
        NSLog(@"%@", teacher);
    }
    return 0;
}

依然是上面这段代码,我们可以通过打印对象的内存分布来验证,依然以x86_64为例,ISA_MASK0x00007ffffffffff8ULL

对象的 isa 指向类

(lldb) x/2gx teacher
0x100712980: 0x001d800100002359 0x0000000000000000
// isa & ISA_MASK
(lldb) po 0x001d800100002359 & 0x00007ffffffffff8ULL
PYTeacher

从上面的打印结果,我们得出 teacher对象的 isa 指向 PYTeacher 类:

image.png

类的 isa 指向元类

由第一节我们得出,类也是对象,也存在isa指针,那类的isa指针指向哪里呢?我们打印一下类的isa

(lldb) x/2gx PYTeacher.class
0x100002358: 0x0000000100002330 0x00007fff940b3118

// 打印 PYTeacher 的元类
(lldb) po 0x0000000100002330 & 0x00007ffffffffff8ULL
PYTeacher

从打印结果,我们看到,PYTeacherisa也指向PYTeacher,那这两个PYTeacher类是同一个吗?我们打印一下他们各自的地址:

(lldb) p/x 0x0000000100002330 & 0x00007ffffffffff8ULL
// PYTeacher元类的地址
(unsigned long long) $4 = 0x0000000100002330

(lldb) p/x PYTeacher.class
// PYTeacher类的地址
(Class) $5 = 0x0000000100002358 PYTeacher

我们看到这两个PYTeacher的地址并不相同,实际上PYTeacherisa指向了他的元类 PYTeacher

image.png

元类的 isa 指向根元类

得到上面isa的指向流程以后,我们肯定会想,元类的 isa 指向谁呢?我们还是通过打印来看一下:

(lldb) x/2gx 0x0000000100002330 // PYTeacher元类的地址
0x100002330: 0x00007fff940b30f0 0x00007fff940b30f0

// 打印根元类
(lldb) po 0x00007fff940b30f0 & 0x00007ffffffffff8ULL
NSObject

从打印结果看,元类的isa指向NSObject,这时,我们会想这个NSObject是我们常用的那个NSObject吗?我们来验证一下:

(lldb) p/x 0x00007fff940b30f0 & 0x00007ffffffffff8ULL
(unsigned long long) $7 = 0x00007fff940b30f0 // 刚才得出的NSObject的地址

(lldb) p/x NSObject.class
(Class) $8 = 0x00007fff940b3118 NSObject // 我们常用的NSObject的地址

(lldb) x/2gx NSObject.class
0x7fff940b3118: 0x00007fff940b30f0 0x0000000000000000

(lldb) p/x 0x00007fff940b30f0 & 0x00007ffffffffff8ULL
(unsigned long long) $10 = 0x00007fff940b30f0 // NSObject的元类的地址

我们看到刚才得出的NSObjectNSObject元类,也就是根元类

image.png

根元类的 isa 指向自己

验证到这里,肯定有人会想,根元类的 isa 又指向哪里呢?我们继续验证:

(lldb) x/2gx 0x00007fff940b30f0 // 根元类的地址
0x7fff940b30f0: 0x00007fff940b30f0 0x00007fff940b3118

(lldb) p/x 0x00007fff940b30f0 & 0x00007ffffffffff8ULL
(unsigned long long) $11 = 0x00007fff940b30f0 // 根元类isa指向的地址

(lldb) po 0x00007fff940b30f0 & 0x00007ffffffffff8ULL
NSObject

打印结果得出:根元类的 isa 指向了自己。

image.png

类的结构

我们在iOS的源码中看到objc_class的结构如下:

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。。。
}

objc_class的成员变量在内存中的分布如下:

image.png
从内存分布可以看出,我们可以通过移动pointer(类的地址指针),来读取不同地址的信息。我们知道 isa8 个字节,所以对 pointer移动8个字节,就可以读取superclass的内容,下面我们可以验证一下:
@interface PYEnglishTeacher : PYTeacher
@end

@implementation PYEnglishTeacher
@end

我们创建了一个PYEnglishTeacher类继承自PYTeacher类:

(lldb) p/x PYEnglishTeacher.class
(Class) $0 = 0x0000000100002438 PYEnglishTeacher

(lldb) po 0x0000000100002438 + 8
<PYTeacher: 0x100002440>

所以,对PYEnglishTeacher类的地址移动8个字节,就可以读取到superclass

Methods

我们发现在objc_class中,有isasuperclasscachebits等成员变量,但是我们自己声明的属性、方法、协议存储在哪呢?我们知道,在以前的objc_class定义中,是有objc_ivar_listobjc_method_listobjc_cacheobjc_protocol_list等成员变量的,那现在苹果是如何设计的呢?我们发现objc_class中有class_rw_t *data()的方法,class_rw_t定义如下:

struct class_rw_t {

   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 *>();
    }

    void set_ro(const class_ro_t *ro) {
        auto v = get_ro_or_rwe();
        if (v.is<class_rw_ext_t *>()) {
            v.get<class_rw_ext_t *>()->ro = ro;
        } else {
            set_ro_or_rwe(ro);
        }
    }

    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};
        }
    }
};

class_rw_t 的结构,我们得出可以通过 class_rw_t 来读取方法列表等。所以首先我们要知道,需要移动多少字节,才能读取objc_class中的bits
我们知道isa为8字节,superclass为8字节,那cache是多少字节呢?我们看一下cache_t的结构:

struct cache_t {

    explicit_atomic<struct bucket_t *> _buckets;  // 8字节
    explicit_atomic<mask_t> _mask;  // 4字节
    省略。。
#if __LP64__
    uint16_t _flags;  // 2字节
#endif
    uint16_t _occupied; // 2字节
}

所以 cache 是16字节。所以pointer移动32个字节就是bits的地址。我们打印一下PYTeacher

(lldb) p/x PYTeacher.class
(Class) $0 = 0x0000000100002208 PYTeacher

// 打印class_data_bits_t bits地址
(lldb) p (class_data_bits_t *)(0x0000000100002208 + 32)
(class_data_bits_t *) $1 = 0x0000000100002228

$1class_data_bits_t结构体的指针,通过调用data()方法可以得到class_rw_t的指针:

(lldb) p $1->data()
(class_rw_t *) $2 = 0x00000001021100b0

我们打印$2指向的class_rw_t结构体:

(lldb) p *$2
(class_rw_t) $3 = {
  flags = 2148007936
  witness = 0
  ro_or_rw_ext = {
    std::__1::atomic<unsigned long> = 4294975600
  }
  firstSubclass = PYEnglishTeacher
  nextSiblingClass = NSUUID
}

现在我们PYTeacher添加一些方法和属性:

@interface PYTeacher : NSObject {
    NSString *name;
}

@property (nonatomic, assign) int age;

- (void)teach;
+ (void)study;

@end

@implementation PYTeacher

- (void)teach {
    
}

+ (void)study {
    
}

@end

这时,我们再打印class_rw_t的内容:

(lldb) p/x PYTeacher.class
(Class) $0 = 0x0000000100002300 PYTeacher

// 打印bits的地址
(lldb) p (class_data_bits_t *)(0x0000000100002300 + 32)
(class_data_bits_t *) $1 = 0x0000000100002320

// 获取class_rw_t的指针
(lldb) p $1->data()
(class_rw_t *) $2 = 0x0000000100677d80

// 打印 class_rw_t 内容
(lldb) p *$2
(class_rw_t) $3 = {
  flags = 2148007936
  witness = 0
  ro_or_rw_ext = {
    std::__1::atomic<unsigned long> = 4294975632
  }
  firstSubclass = PYEnglishTeacher
  nextSiblingClass = NSUUID
}

// 获取class_rw_t中的方法列表
(lldb) p $3.methods()
(const method_array_t) $4 = {
  list_array_tt<method_t, method_list_t> = {
     = {
      list = 0x00000001000020d8
      arrayAndFlag = 4294975704
    }
  }
}

// 获取method_list_t的数组指针
(lldb) p $4.list
(method_list_t *const) $5 = 0x00000001000020d8

// 打印method_list_t数组,其实就是数组的第0位
(lldb) p *$5
(method_list_t) $6 = {
  entsize_list_tt<method_t, method_list_t, 3> = {
    entsizeAndFlags = 26
    count = 4
    first = {
      name = "teach"
      types = 0x0000000100000f74 "v16@0:8"
      imp = 0x0000000100000de0 (KCObjc`-[PYTeacher teach] at main.m:24)
    }
  }
}

// 打印数组中的内容
(lldb) p $6.get(0)
(method_t) $7 = {
  name = "teach"
  types = 0x0000000100000f74 "v16@0:8"
  imp = 0x0000000100000de0 (KCObjc`-[PYTeacher teach] at main.m:24)
}

(lldb) p $6.get(1)
(method_t) $8 = {
  name = ".cxx_destruct"
  types = 0x0000000100000f74 "v16@0:8"
  imp = 0x0000000100000e30 (KCObjc`-[PYTeacher .cxx_destruct] at main.m:22)
}

(lldb) p $6.get(2)
(method_t) $9 = {
  name = "age"
  types = 0x0000000100000f8a "i16@0:8"
  imp = 0x0000000100000df0 (KCObjc`-[PYTeacher age] at main.m:15)
}

(lldb) p $6.get(3)
(method_t) $10 = {
  name = "setAge:"
  types = 0x0000000100000f92 "v20@0:8i16"
  imp = 0x0000000100000e10 (KCObjc`-[PYTeacher setAge:] at main.m:15)
}

我们看到方法列表中有4个方法,但是没有
+ (void)study方法,为什么呢?
仔细观察我们会发现,这4个方法,除了1个C++的方法外,其他3个都是对象方法,也就是对象方法存在类的方法列表里,我们说万物皆对象元类对象,难道说类对象”对象方法“存在元类的方法列表里?下面我们验证一下:

(lldb) x/2gx PYTeacher.class
0x100002300: 0x00000001000022d8 0x0000000100334140
(lldb) po 0x00000001000022d8 & 0x00007ffffffffff8ULL
PYTeacher

// 打印 PYTeacher元类 的地址
(lldb) p/x 0x00000001000022d8 & 0x00007ffffffffff8ULL
(unsigned long long) $13 = 0x00000001000022d8

// 打印元类中的 bits
(lldb) p (class_data_bits_t *)(0x00000001000022d8 + 32)
(class_data_bits_t *) $14 = 0x00000001000022f8

// 获取 class_rw_t 的地址
(lldb) p $14->data()
(class_rw_t *) $15 = 0x0000000100677d40

// 打印 class_rw_t 的内容
(lldb) p *$15
(class_rw_t) $16 = {
  flags = 2684878849
  witness = 1
  ro_or_rw_ext = {
    std::__1::atomic<unsigned long> = 4294975528
  }
  firstSubclass = 0x0000000100002328
  nextSiblingClass = 0x00007fff8b968cd8
}

// 获取 元类 的方法列表
(lldb) p $16.methods()
(const method_array_t) $17 = {
  list_array_tt<method_t, method_list_t> = {
     = {
      list = 0x0000000100002070
      arrayAndFlag = 4294975600
    }
  }
}

// 打印方法列表的地址
(lldb) p $17.list
(method_list_t *const) $18 = 0x0000000100002070

// 打印方法列表信息
(lldb) p *$18
(method_list_t) $19 = {
  entsize_list_tt<method_t, method_list_t, 3> = {
    entsizeAndFlags = 26
    count = 1
    first = {
      name = "study"
      types = 0x0000000100000f74 "v16@0:8"
      imp = 0x0000000100000dd0 (KCObjc`+[PYTeacher study] at main.m:28)
    }
  }
}

正如我们猜测的那样,+ (void)study方法存在元类方法列表中。

Properties

我们再来看一下property

(lldb) p $3.properties()
(const property_array_t) $20 = {
  list_array_tt<property_t, property_list_t> = {
     = {
      list = 0x0000000100002188
      arrayAndFlag = 4294975880
    }
  }
}

// 打印属性列表地址
(lldb) p $20.list
(property_list_t *const) $21 = 0x0000000100002188

// 打印属性列表信息
(lldb) p *$21
(property_list_t) $22 = {
  entsize_list_tt<property_t, property_list_t, 0> = {
    entsizeAndFlags = 16
    count = 1
    first = (name = "age", attributes = "Ti,N,V_age")
  }
}

这里我们看到了age属性,在class_rw_t中,我们却没找到成员变量的存储位置,但是class_rw_t中,有获取和设置class_ro_t的方法,我们来看一下class_ro_t结构体。

class_ro_t

class_ro_t结构体如下:

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;

    省略。。。
};

我们看到class_ro_t中有const ivar_list_t * ivars,我们来打印一下:

(lldb) p $3.ro()
(const class_ro_t *) $23 = 0x0000000100002090

// 打印 class_ro_t 内容
(lldb) p *$23
(const class_ro_t) $24 = {
  flags = 388
  instanceStart = 8
  instanceSize = 20
  reserved = 0
  ivarLayout = 0x0000000100000f28 "\x01"
  name = 0x0000000100000f1e "PYTeacher"
  baseMethodList = 0x00000001000020d8
  baseProtocols = 0x0000000000000000
  ivars = 0x0000000100002140
  weakIvarLayout = 0x0000000000000000
  baseProperties = 0x0000000100002188
  _swiftMetadataInitializer_NEVER_USE = {}
}

// 打印 ivar_list_t 地址
(lldb) p $24->ivars
(const ivar_list_t *const) $25 = 0x0000000100002140
  Fix-it applied, fixed expression was: 
    $24.ivars

// 打印 ivar_list_t 内容
(lldb) p *$25
(const ivar_list_t) $26 = {
  entsize_list_tt<ivar_t, ivar_list_t, 0> = {
    entsizeAndFlags = 32
    count = 2
    first = {
      offset = 0x00000001000022c8
      name = 0x0000000100000f4a "name"
      type = 0x0000000100000f7c "@\"NSString\""
      alignment_raw = 3
      size = 8
    }
  }
}

// 获取成员变量
(lldb) p $26.get(0)
(ivar_t) $27 = {
  offset = 0x00000001000022c8
  name = 0x0000000100000f4a "name"
  type = 0x0000000100000f7c "@\"NSString\""
  alignment_raw = 3
  size = 8
}

(lldb) p $26.get(1)
(ivar_t) $28 = {
  offset = 0x00000001000022d0
  name = 0x0000000100000f4f "_age"
  type = 0x0000000100000f88 "i"
  alignment_raw = 2
  size = 4
}

ivar_list_t中存储这name_age成员变量。

小结

  1. 对象的成员变量存储在class_ro_t中;
  2. 对象方法属性存储在class_rw_t中;
  3. 类方法存储在元类class_rw_t中。
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