Runtime 是 iOS编程人员的核心基础知识，Objc Runtime使得C具有了面向对象能力，在程序运行时创建，检查，修改类、对象和它们的方法。
runtime是开源的，GitHub 有可调式的源码 objc4_debug(待工程配置过程👍)，也可以去官网找 objc4.
学习参考 苹果官方的Runtime编程指南
isa & id
第一印象指针，在<objc.h>中可以看到：
/// An opaque type that represents an Objective-C class.
typedef struct objc_class *Class;
/// Represents an instance of a class.
struct objc_object {
Class _Nonnull isa OBJC_ISA_AVAILABILITY;
};
/// A pointer to an instance of a class.
typedef struct objc_object *id;
};
-
isa是objc_object一个属性，类型是Class -
Class是指向objc_class的一个指针，存放着objc_class的地址 -
id是一个objc_object结构类型的指针，这个类型的对象能够转换成任何一种对象
Class
objc_class 是什么呢？让我们看到<objc/runtime.h>
struct objc_class {
Class _Nonnull isa OBJC_ISA_AVAILABILITY;
#if !__OBJC2__
Class _Nullable super_class OBJC2_UNAVAILABLE; // 父类
const char * _Nonnull name OBJC2_UNAVAILABLE; // 类名
long version OBJC2_UNAVAILABLE; // 类的版本信息，默认为0
long info OBJC2_UNAVAILABLE; // 类信息，供运行期间使用的一些位标识
long instance_size OBJC2_UNAVAILABLE; // 该类的实例变量大小
struct objc_ivar_list * _Nullable ivars OBJC2_UNAVAILABLE; // 该类的成员变量链表
struct objc_method_list * _Nullable * _Nullable methodLists OBJC2_UNAVAILABLE; // 方法定义的链表
struct objc_cache * _Nonnull cache OBJC2_UNAVAILABLE; // 方法缓存
struct objc_protocol_list * _Nullable protocols OBJC2_UNAVAILABLE;
#endif
} OBJC2_UNAVAILABLE;
/* Use `Class` instead of `struct objc_class *` */
-
objc_class中也有一个isa指针，指向Meta Class。作用：因为 Objc 的类本身也是一个对象，为了处理这个关系，runtime就创造了Meta Class，当给类发送 [NSObject alloc] 的消息时，实际上是把这个消息发给 NSObject Class Object。 -
cache作用：对象接受到一个消息，首次会根据isa指针查找消息对象，然后在methodLists遍历，调用后加入cache，下次直接从缓存获取，提高调用效率。
objc_ivar_list
objc_ivar_list 结构体存储objc_ivar(成员变量)数组列表
struct objc_ivar {
char * _Nullable ivar_name OBJC2_UNAVAILABLE; // 命名
char * _Nullable ivar_type OBJC2_UNAVAILABLE; // 类型
int ivar_offset OBJC2_UNAVAILABLE;
#ifdef __LP64__
int space OBJC2_UNAVAILABLE;
#endif
}
struct objc_ivar_list {
int ivar_count OBJC2_UNAVAILABLE;
#ifdef __LP64__
int space OBJC2_UNAVAILABLE;
#endif
/* variable length structure */
struct objc_ivar ivar_list[1] OBJC2_UNAVAILABLE;
} OBJC2_UNAVAILABLE;
objc_method_list
objc_method_list结构体存储objc_method(方法)数组列表
struct objc_method {
SEL _Nonnull method_name OBJC2_UNAVAILABLE;
char * _Nullable method_types OBJC2_UNAVAILABLE;
IMP _Nonnull method_imp OBJC2_UNAVAILABLE;
} OBJC2_UNAVAILABLE;
struct objc_method_list {
struct objc_method_list * _Nullable obsolete OBJC2_UNAVAILABLE;
int method_count OBJC2_UNAVAILABLE;
#ifdef __LP64__
int space OBJC2_UNAVAILABLE;
#endif
/* variable length structure */
struct objc_method method_list[1] OBJC2_UNAVAILABLE;
}
objc_cache
struct objc_cache {
unsigned int mask /* total = mask + 1 */ OBJC2_UNAVAILABLE;
unsigned int occupied OBJC2_UNAVAILABLE;
Method _Nullable buckets[1] OBJC2_UNAVAILABLE;
};
-
mask: 指定分配缓存bucket的总数。runtime 使用这个字段确定线性查找的索引位置 -
occupied: 实际占用缓存bucket的总数 - 指向
Method数据结构指针的数组，总数不能超过mask+1；指针可能为空，这标识缓存bucket没有被占用，数组随着时间增长
Meta Class & Super Class
提供一个实例，来体感metaclass和superclass
#import <Foundation/Foundation.h>
#import <objc/runtime.h>
typedef struct mtm_objc_class *mClass;
struct mtm_objc_class {
mClass ISA;
mClass superclass;
};
void testMetaClass(id self, SEL _cmd) {
NSLog(@"This object is %p", self);
NSLog(@"Class is %@, super class is %@", [self class], [self superclass]);
Class currentClass = [self class];
for (int i = 0; i < 4; i++) {
NSLog(@"Following the [isa] pointer %d timer gives %p, class name is %@", i, currentClass, NSStringFromClass(currentClass));
currentClass = object_getClass(currentClass);
}
mClass testClass = (__bridge mClass)[self superclass];
for (int i = 0; i < 4; i++) {
NSLog(@"Following the [super] pointer %d timer gives %p", i, testClass);
testClass = testClass ? testClass->superclass : nil;
}
NSLog(@"NSObject's class is %p", [NSObject class]);
Class meta_NSObject = object_getClass([NSObject class]);
NSLog(@"NSobject's meta class is %p, meta super class is %p", meta_NSObject, ((__bridge mClass)meta_NSObject)->superclass);
}
int main(int argc, const char * argv[]) {
@autoreleasepool {
// insert code here...
Class newClass = objc_allocateClassPair([NSURL class], "TestClass", 0);
class_addMethod(newClass, @selector(testHandler), (IMP)testMetaClass, "v@:");
objc_registerClassPair(newClass);
id instance = [[newClass alloc] initWithString:@"http://www.baidu.com"];
[instance performSelector:@selector(testHandler)];
}
return 0;
}
结果如下
ObjcTest[74791:1048449] This object is 0x100784c00
ObjcTest[74791:1048449] Class is TestClass, super class is NSURL
ObjcTest[74791:1048449] Following the [isa] pointer 0 timer gives 0x100784420, class name is TestClass
ObjcTest[74791:1048449] Following the [isa] pointer 1 timer gives 0x100784450, class name is TestClass
ObjcTest[74791:1048449] Following the [isa] pointer 2 timer gives 0x1003310f0, class name is NSObject
ObjcTest[74791:1048449] Following the [isa] pointer 3 timer gives 0x1003310f0, class name is NSObject
ObjcTest[74791:1048449] Following the [super] pointer 0 timer gives 0x7fff859d7230
ObjcTest[74791:1048449] Following the [super] pointer 1 timer gives 0x100331140
ObjcTest[74791:1048449] Following the [super] pointer 2 timer gives 0x0
ObjcTest[74791:1048449] Following the [super] pointer 3 timer gives 0x0
ObjcTest[74791:1048449] NSObject's class is 0x100331140
ObjcTest[74791:1048449] NSobject's meta class is 0x1003310f0, meta super class is 0x100331140
把日志中的地址放到图里，应该就更好理解 isa 链表 和 super 链表
class_isa_super_class.png
小测试
来看看能答对几个？
BOOL res1 = [(id)[NSObject class] isKindOfClass:[NSObject class]];
BOOL res2 = [(id)[NSObject class] isMemberOfClass:[NSObject class]];
BOOL res3 = [(id)[NSURL class] isKindOfClass:[NSURL class]];
BOOL res4 = [(id)[NSURL class] isMemberOfClass:[NSURL class]];
解密开始
- 误区：
isKindOfClass有类方法和对象方法，实现有区别 - res1：第一次，Class 和 MetaClass(Class->ISA)对比，失败；再跟NSObject's meta class 的 superclass 即 NSObject class，结果为true
- res2：Class->ISA == Class 为false，下面依此类推
- res3 = false
- res4 = false
+ (BOOL)isKindOfClass:(Class)cls {
for (Class tcls = self->ISA(); tcls; tcls = tcls->superclass) {
if (tcls == cls) return YES;
}
return NO;
}
- (BOOL)isKindOfClass:(Class)cls {
for (Class tcls = [self class]; tcls; tcls = tcls->superclass) {
if (tcls == cls) return YES;
}
return NO;
}
+ (BOOL)isMemberOfClass:(Class)cls {
return self->ISA() == cls;
}
- (BOOL)isMemberOfClass:(Class)cls {
return [self class] == cls;
}
