OC底层探索 - 类 & 元类

Misaka10032
297 阅读5分钟

获取类

@interface TestClass : NSObject

@end

#import <objc/runtime.h>
TestClass *test = [TestClass new];
// 通过实例方法获取 
Class c1 = test.class;
// 通过类方法获取
Class c2 = [TestClass class];
// 通过runtime获取
Class c3 = object_getClass([TestClass new]);
NSLog(@"\n%p\n%p\n%p\n", c1, c2, c3);

打印得到的类都是同一地址，可见类在内存中只有一份。

image.png

我们到源码中看一下上面几个获取类的方法

+ (Class)class {
    return self;
}

- (Class)class {
    return object_getClass(self);
}

Class object_getClass(id obj)
{
    if (obj) return obj->getIsa();
    else return Nil;
}

可见，类方法是直接返回自己，这说明类也是一个对象。实例方法实际是调用runtime方法，runtime方法是通过实例的 isa 获取到的类。

类对象的 isa 指向元类 MetaClass

struct objc_class : objc_object {
  objc_class(const objc_class&) = delete;
  objc_class(objc_class&&) = delete;
  void operator=(const objc_class&) = delete;
  void operator=(objc_class&&) = delete;
    // Class ISA;
    Class superclass; // 父类
    cache_t cache;             // formerly cache pointer and vtable
    class_data_bits_t bits;    // class_rw_t * plus custom rr/alloc flags

源码可见 类 objc_class 继承自 objc_object, 那么类对象的 isa 指向谁呢？

类对象的 isa 指向元类( MetaClass ), 我们可以结合 OC底层探索 - isa 章节里的方式，获取 isa 里存放的地址，然后与runtime方法获取到的元类地址比较。

#import <objc/runtime.h>

Class testClass = TestClass.class;
Class metaClass = objc_getMetaClass("TestClass");

image.png

通过结果可以证明，类对象的 isa 指针中存放的是元类的地址。

继承关系 & isa指向

苹果提供的资料中有这样一张图，说明了 superclass 和 isa 的指向。

image.png

从中我们可以总结：

实例对象的isa指向类对象，类对象的isa指向元类对象，元类对象的isa指向根元类对象，根元类对象的isa指向自身

父类的元类就是元类的父类，根元类的父类就是NSObject类对象，NSObject类对象的父类为nil

下面我们通过 runtime 和 lldb 分别验证一下。

先构建有继承关系的几个测试类

@interface TestClass : NSObject

@end

@interface FirstSubTestClass : TestClass

@end

@interface SecondSubTestClass : FirstSubTestClass

@end

runtime验证继承关系和部分isa关系

    SecondSubTestClass *sub2 = [SecondSubTestClass new];
    Class sub2Class = sub2.class;
    Class sub2Superclas = class_getSuperclass(sub2Class);
    Class sub2MetaClass = objc_getMetaClass("SecondSubTestClass");
    Class sub2MetaSuperClass = class_getSuperclass(sub2MetaClass);
    NSLog(@"SecondSubTestClass");
    NSLog(@"类: %p", sub2Class);
    NSLog(@"父类: %p", sub2Superclas);
    NSLog(@"元类: %p", sub2MetaClass);
    NSLog(@"元类的父类: %p \n", sub2MetaSuperClass);


    FirstSubTestClass *sub1 = [FirstSubTestClass new];
    Class sub1Class = sub1.class;
    Class sub1Superclas = class_getSuperclass(sub1Class);
    Class sub1MetaClass = objc_getMetaClass("FirstSubTestClass");
    Class sub1MetaSuperClass = class_getSuperclass(sub1MetaClass);
    NSLog(@"FirstSubTestClass");
    NSLog(@"类: %p", sub1Class);
    NSLog(@"父类: %p", sub1Superclas);
    NSLog(@"元类: %p", sub1MetaClass);
    NSLog(@"元类的父类: %p \n", sub1MetaSuperClass);

    TestClass *test = [TestClass new];
    Class testClass = test.class;
    Class testSuperclas = class_getSuperclass(testClass);
    Class testMetaClass = objc_getMetaClass("TestClass");
    Class testMetaSuperClass = class_getSuperclass(testMetaClass);
    NSLog(@"TestClass");
    NSLog(@"类: %p", testClass);
    NSLog(@"父类: %p", testSuperclas);
    NSLog(@"元类: %p", testMetaClass);
    NSLog(@"元类的父类: %p \n", testMetaSuperClass);

    NSObject *obj = [NSObject new];
    Class objClass = obj.class;
    Class objSuperclas = class_getSuperclass(objClass);
    Class objMetaClass = objc_getMetaClass("NSObject");
    Class objMetaSuperClass = class_getSuperclass(objMetaClass);
    Class objSuperMetaSuperClass = class_getSuperclass(objMetaSuperClass);
    NSLog(@"NSObject");
    NSLog(@"类: %p", objClass);
    NSLog(@"父类: %p", objSuperclas);
    NSLog(@"元类: %p", objMetaClass);
    NSLog(@"元类的父类: %p \n", objMetaSuperClass);
    NSLog(@"NSObject: %p \n", objSuperMetaSuperClass);

image.png

image.png

lldb验证

SecondSubTestClass *sub2 = [SecondSubTestClass new];
FirstSubTestClass *sub1 = [FirstSubTestClass new];
TestClass *test = [TestClass new];
NSObject *obj = [NSObject new];

(lldb) x/1gx sub2
0x283d34ad0: 0x000021a10064d549
(lldb) p/x 0x000021a10064d549 & 0x0000000ffffffff8ULL
(unsigned long long) $1 = 0x000000010064d548 // SecondSubTestClass 类地址
(lldb) x/2gx 0x000000010064d548
// 根据objc_class结构体可知 第1个8字节是 isa， 第2个8字节是 superclass
0x10064d548: 0x000000010064d520 0x000000010064d4f8 
(lldb) p/x  0x000000010064d4f8 & 0x0000000ffffffff8ULL
(unsigned long long) $8 = 0x000000010064d4f8 // SecondSubTestClass 父类地址
(lldb) p/x 0x000000010064d520 & 0x0000000ffffffff8ULL
(unsigned long long) $12 = 0x000000010064d520 // SecondSubTestClass 元类地址
(lldb) x/2gx 0x000000010064d520
0x10064d520: 0x00000001f66a31e0 0x000000010064d4d0
(lldb) p/x 0x000000010064d4d0 & 0x0000000ffffffff8ULL
(unsigned long long) $13 = 0x000000010064d4d0 // SecondSubTestClass 元类的父类地址

(lldb) x/1gx sub1
0x283d34ae0: 0x000021a10064d4f9
(lldb) p/x 0x000021a10064d4f9 & 0x0000000ffffffff8ULL
(unsigned long long) $3 = 0x000000010064d4f8 // FirstSubTestClass 类地址
(lldb) x/2gx 0x000000010064d4f8
0x10064d4f8: 0x000000010064d4d0 0x000000010064d4a8
(lldb) p/x 0x000000010064d4a8 & 0x0000000ffffffff8ULL
(unsigned long long) $9 = 0x000000010064d4a8 // FirstSubTestClass 父类地址
(lldb) p/x 0x000000010064d4d0 & 0x0000000ffffffff8ULL
(unsigned long long) $14 = 0x000000010064d4d0 // FirstSubTestClass 元类地址
(lldb) x/2gx 0x000000010064d4d0 
0x10064d4d0: 0x00000001f66a31e0 0x000000010064d480
(lldb) p/x 0x000000010064d480 & 0x0000000ffffffff8ULL
(unsigned long long) $17 = 0x000000010064d480 // FirstSubTestClass 元类的父类地址
(lldb) p/x 0x00000001f66a31e0 & 0x0000000ffffffff8ULL
(unsigned long long) $20 = 0x00000001f66a31e0 // FirstSubTestClass 元类的isa指向

(lldb) x/1gx test
0x283d34af0: 0x000021a10064d4a9
(lldb) p/x 0x000021a10064d4a9 & 0x0000000ffffffff8ULL
(unsigned long long) $5 = 0x000000010064d4a8 // TestClass 类地址
(lldb) x/2gx 0x000000010064d4a8
0x10064d4a8: 0x000000010064d480 0x00000001f66a3208
(lldb) p/x 0x00000001f66a3208 & 0x0000000ffffffff8ULL
(unsigned long long) $10 = 0x00000001f66a3208 // TestClass 父类地址
(lldb) p/x 0x000000010064d480 & 0x0000000ffffffff8ULL
(unsigned long long) $15 = 0x000000010064d480 // TestClass 元类地址
(lldb) x/2gx 0x000000010064d480
0x10064d480: 0x00000001f66a31e0 0x00000001f66a31e0
(lldb) p/x 0x00000001f66a31e0 & 0x0000000ffffffff8ULL
(unsigned long long) $18 = 0x00000001f66a31e0 // TestClass 元类的父类地址
(lldb) p/x 0x00000001f66a31e0 & 0x0000000ffffffff8ULL
(unsigned long long) $20 = 0x00000001f66a31e0 // TestClass 元类的isa指向

(lldb) x/1gx obj
0x283d34b00: 0x000021a1f66a3209
(lldb) p/x 0x000021a1f66a3209 & 0x0000000ffffffff8ULL
(unsigned long long) $7 = 0x00000001f66a3208 // NSObject 类地址
(lldb) x/2gx 0x00000001f66a3208
0x1f66a3208: 0x00000001f66a31e0 0x0000000000000000
(lldb) p/x 0x0000000000000000 & 0x0000000ffffffff8ULL
(unsigned long long) $11 = 0x0000000000000000 // NSObject 父类地址
(lldb) p/x 0x00000001f66a31e0 & 0x0000000ffffffff8ULL
(unsigned long long) $16 = 0x00000001f66a31e0 // NSObject 元类地址
(lldb) x/2gx 0x00000001f66a31e0
0x1f66a31e0: 0x00000001f66a31e0 0x00000001f66a3208
(lldb) p/x 0x00000001f66a3208 & 0x0000000ffffffff8ULL
(unsigned long long) $19 = 0x00000001f66a3208 // NSObject 元类的父类地址
(lldb) p/x 0x00000001f66a31e0 & 0x0000000ffffffff8ULL
(unsigned long long) $20 = 0x00000001f66a31e0 // NSObject 元类的isa指向

image.png

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