编译：$clang - rewrite -objc main.m
消息发送机制 Person *p = [[Person alloc] init];

import <objc/message.h>

//iOS5苹果不建议使用底层方法，需要在Build Phases搜索msg设置

Person *p = [Person new];
objc_msgsend(p,@selector(run));
objc_msgsend(p,@selecor(eatFood:),@"fruit");

Class PersonClass = [Person class];
类方法：objc_msgsend(PersonClass, @selector(类方法),参数);
类也是特殊对象，等同[PersonClass performSelector:@selector(run)];

Runtime应用场景

• 归档/解档：序列化/反序列化 object以键值对的形式保存成二进制文件

for(int i = 0, i < 属性的个数， i++) {
  [Coder encodeObject:属性值 forKey:key];
} 

//Ivar:runtime里面Ivar代表属性成员变量 unsigned int count = 0;
 //im 中成员变量也可以拿到
//书 Objective_C 2.0 运行时系统编程指南
Ivar *ivars = Class_copyIvarList([Person class], & count); =
Ivar ivar = ivars[0];  const char *name = ivar_getName(ivar);

#import<objc/message.h>
//归档
- (void)encodeWithCoder:(NSCoder *)coder {
    unsigned int count = 0;
    Ivar  *ivars = Class_copyIvarList([Person class], &count);
    for (int i = 0; i < count; i++) {
        Ivar *ivars = class_copyIvarList([Person Class], &count);
        for (int i = 0; i < count; i++) {
            Ivar ivar = ivars[i];
            const char *name = ivar_getName(ivar);
            NSString *key = [NSString stringWithUTF8String:name];
            [coder encodeObject:[self valueForKey:key] forKey:key];
        }
        free(ivars);
    }
}

//解档
- (instancetype)initWithCoder:(NSCoder *)coder {
    if (self = [super init]) {
        unsigned int count = 0;
        Ivar  *ivars = Class_copyIvarList([Person class], &count);
        for (int i = 0; i < count; i++) {
            Ivar *ivars = class_copyIvarList([Person Class], &count);
            for (int i = 0; i < count; i++) {
                Ivar ivar = ivars[i];
                const char *name = ivar_getName(ivar);
                NSString *key = [NSString stringWithUTF8String:name];
                id value =  [coder decodeObjectForKey:key];
                [self setValue:value forKey:key];
            }
            free(ivars);
         }
    }
}