Runtime-内存模型

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;
};

1. isa 是objc_object一个属性，类型是Class
2. Class 是指向 objc_class的一个指针，存放着objc_class的地址
3. 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 *` */

1. 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。
2. 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;
};

1. 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
2. occupied: 实际占用缓存 bucket的总数
3. 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

1. 误区：isKindOfClass有类方法和对象方法，实现有区别
2. 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
3. res2：Class->ISA == Class 为false，下面依此类推
4. res3 = false
5. 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;
}