iOS中类、元类、isa详解
类相信大家都知道是什么,如果看过runtime的源码或者看过相关的文章对isa肯定也不陌生,不过元类(meta class)大家可能就比较陌生了。不过大家也不要担心,我会细细道来,让大家明白它到底是个什么东西。
先看一段大家非常熟悉的代码:
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Person *person = [[Person alloc] init]; |
为什么Person类名就能调用到alloc方法吗?到底怎么找到了alloc的方法了呢?
1.首先,在相应操作的对象中的缓存方法列表中找调用的方法,如果找到,转向相应实现并执行。
2.如果没找到,在相应操作的对象中的方法列表中找调用的方法,如果找到,转向相应实现执行
3.如果没找到,去父类指针所指向的对象中执行1,2.
4.以此类推,如果一直到根类还没找到,转向拦截调用,走消息转发机制。
5.如果没有重写拦截调用的方法,程序报错。
上边是我从网上一篇文章摘录的查找alloc的方法的大体过程。如果是实例方法(声明以`-`开头)这个描述的换个过程还是可以的,不过如果是类方法(声明以`+`开头比如`alloc`方法)还是有所欠缺的!
元类
`元类`也是类,是描述`Class `类对象的类。
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Class aclass = [Person class ]; |
>一切皆对象。每一个对象都对应一个类。 `Person` 类就是`person`变量对象的类,换句话说就是`person`对象的isa指向`Person`对应的结构体的类;`aclass`也是对象,描述它的类就是元类,换句话说`aclass`对象的isa指向的就是`元类`。
**元类保存了类方法的列表**。当一个类方法被调用时,元类会首先查找它本身是否有该类方法的实现,如果没有则该元类会向它的父类查找该方法,直到一直找到继承链的头。(回答文章上边查找方法所欠缺的地方)
这张图是非常精髓的,直接诠释了元类和isa。大家可以一边阅读本文,一边回忆此图,多看几遍。
上边都是概念性质偏多,不知道大家理解的如何。现在看一个实例来具体介绍上边的内容。
代码示例
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// Created by FlyOceanFish on 2018/1/9. // Copyright © 2018年 FlyOceanFish. All rights reserved. // # import # import @ interface Person: NSObject @end @implementation Person + ( void )printStatic{ } - ( void )print{ NSLog(@ "This object is %p." , self); NSLog(@ "Class is %@, and super is %@." , [self class ], [self superclass]); const char *name = object_getClassName(self); Class metaClass = objc_getMetaClass(name); NSLog(@ "MetaClass is %p" ,metaClass); Class currentClass = [self class ]; for ( int i = 1 ; i < 5 ; i++) { NSLog(@ "Following the isa pointer %d times gives %p" , i, currentClass); unsigned int countMethod = 0 ; NSLog(@ "---------------**%d start**-----------------------" ,i); Method * methods = class_copyMethodList(currentClass, &countMethod); [self printMethod:countMethod methods:methods ]; NSLog(@ "---------------**%d end**-----------------------" ,i); currentClass = object_getClass(currentClass); } NSLog(@ "NSObject's class is %p" , [NSObject class ]); NSLog(@ "NSObject's meta class is %p" , object_getClass([NSObject class ])); } - ( void )printMethod:( int )count methods:(Method *) methods{ for ( int j = 0 ; j < count; j++) { Method method = methods[j]; SEL methodSEL = method_getName(method); const char * selName = sel_getName(methodSEL); if (methodSEL) { NSLog(@ "sel------%s" , selName); } } } @end @ interface Animal: NSObject @end @implementation Animal - ( void )print{ NSLog(@ "This object is %p." , self); NSLog(@ "Class is %@, and super is %@." , [self class ], [self superclass]); const char *name = object_getClassName(self); Class metaClass = objc_getMetaClass(name); NSLog(@ "MetaClass is %p" ,metaClass); Class currentClass = [self class ]; for ( int i = 1 ; i < 5 ; i++) { NSLog(@ "Following the isa pointer %d times gives %p" , i, currentClass); currentClass = object_getClass(currentClass); } NSLog(@ "NSObject's class is %p" , [NSObject class ]); NSLog(@ "NSObject's meta class is %p" , object_getClass([NSObject class ])); } @end int main( int argc, const char * argv[]) { @autoreleasepool { Person *person = [[Person alloc] init]; Class class = [Person class ]; [person print]; // printf("-------------------------------- "); // Animal *animal = [[Animal alloc] init]; // [animal print]; } return 0 ; } |
这个示例有两部分功能:
1. 大家只看`Person`的演示功能即可。
2. 观察Person和Animal两个对象的打印(打印方法名的可以注释掉,将main方法中的代码注释打开)
`Person`的演示功能(不打印方法名称)
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This object is 0x100408400 . Class is Person, and super is NSObject. MetaClass is 0x100001328 Following the isa pointer 1 times gives 0x100001350 Following the isa pointer 2 times gives 0x100001328 Following the isa pointer 3 times gives 0x7fffb9a4f0f0 Following the isa pointer 4 times gives 0x7fffb9a4f0f0 NSObject's class is 0x7fffb9a4f140 NSObject's meta class is 0x7fffb9a4f0f0 |
我们来观察isa到达过的地址的值:
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对象的地址是 0x100408400.
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类的地址是 0x100001350.
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元类的地址是 0x100001328.
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根元类(NSObject的元类)的地址是 0x7fffb9a4f0f0.
对于本次打印我们可以做出以下结论(可以再去看一遍上边那张精髓的图):
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对于3、4次打印相同,就是因为NSObject元类的类是它本身.
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我们在实例化对象的时候,其实是创建了许多对象,这就是我们说的类簇。也对应了我们在用runtime创建类的时候`objc_allocateClassPair(xx,xx)`中是`ClassPair`而不是`bjc_allocateClass`
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通过地址的大小也可以看出对象实例化先后,地址越小的越先实例化
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很好的诠释了上边那张精髓图isa的指向
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NSObject的两个地址都非常大(哈哈哈哈哈!为什么非常大啊??接下往下看)
`Person`的演示功能(打印方法名称)
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Class is Person, and super is NSObject. MetaClass is 0x100002378 Following the isa pointer 1 times gives 0x1000023a0 ---------------** 1 start**----------------------- sel------printMethod:methods: sel------print ---------------** 1 end**----------------------- Following the isa pointer 2 times gives 0x100002378 ---------------** 2 start**----------------------- sel------printStatic ---------------** 2 end**----------------------- Following the isa pointer 3 times gives 0x7fffb9a4f0f0 ---------------** 3 start**----------------------- |
我只把重要的复制出来了,`NSObject`的所有的方法名没有复制出来,在此处不是重要的。
此次打印结果的结论:
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类方法(静态方法)是存储在元类中的
观察Person和Animal两个对象的打印
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This object is 0x100508e70 . Class is Person, and super is NSObject. MetaClass is 0x100001338 Following the isa pointer 1 times gives 0x100001360 Following the isa pointer 2 times gives 0x100001338 Following the isa pointer 3 times gives 0x7fffb9a4f0f0 Following the isa pointer 4 times gives 0x7fffb9a4f0f0 NSObject's class is 0x7fffb9a4f140 NSObject's meta class is 0x7fffb9a4f0f0 -------------------------------- This object is 0x100675ed0 . Class is Animal, and super is NSObject. MetaClass is 0x100001388 Following the isa pointer 1 times gives 0x1000013b0 Following the isa pointer 2 times gives 0x100001388 Following the isa pointer 3 times gives 0x7fffb9a4f0f0 Following the isa pointer 4 times gives 0x7fffb9a4f0f0 NSObject's class is 0x7fffb9a4f140 NSObject's meta class is 0x7fffb9a4f0f0 Program ended with exit code: 0 |
此次打印的结论:
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`Animal`相关打印的地址都比`Person`的大。再次诠释了栈是由大往小排列的。栈口在最小的地方
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`Animal`和`Person`的`NSObject`的两个地址一样。(知道为什么大了吗?其实就是保证这两个地址足够大,以致于永远在栈中。这样整个程序中其实就是存在一个,有点像单例的意思)