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];
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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
];
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>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:
-
对象的地址是 0x100408400.
-
类的地址是 0x100001350.
-
元类的地址是 0x100001328.
-
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):
-
对于3、4次打印相同,就是因为NSObject元类的类是它本身.
-
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`
-
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
-
很好的诠释了上边那张精髓图isa的指向
-
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:
-
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:
-
`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
-
`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)