iOS-分类重写原类方法时,如何调用原类方法

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原文链接: www.jianshu.com
目录
  • 原理
  • 原理验证一(表面验证)
  • 原理验证二 (使用 runtime)
  • 原理验证三(底层验证)
  • 如何调用原类方法
一 原理

平常开发中,我们经常给某些类添加分类,新增新的方法。

原理

  • Category编译之后的底层结构是struct category_t,里面存储着分类的对象方法、类方法、属性、协议信息
  • 在程序运行的时候,runtime会将Category的数据,合并到类信息中(类对象、元类对象中)

Category的底层结构

struct _category_t {
    const char *name;
    struct _class_t *cls;
    const struct _method_list_t *instance_methods;  // 对象方法列表
    const struct _method_list_t *class_methods;  // 类方法列表
    const struct _protocol_list_t *protocols;  // 协议列表
    const struct _prop_list_t *properties;  // 属性列表
};

Category的加载处理过程

  • 1.通过Runtime加载某个类的所有Category数据
  • 2.把所有Category的方法、属性、协议数据,合并到一个大数组中,后面参与编译的Category数据,会在数组的前面
  • 3.将合并后的分类数据(方法、属性、协议),插入到类原来数据的前面
合并方法.png
二 代码佐证
2.1 从表层验证
  • 代码如下
@interface CSPerson : NSObject {
- (void)run;
@end

@implementation CSPerson
- (void)run {
    NSLog(@"%s",__FUNCTION__);
}
@end

#pragma mark - CSPerson (Extension)

@interface CSPerson (Extension)
- (void)run;
@end

@implementation CSPerson (Extension)
- (void)run {
    NSLog(@"%s",__FUNCTION__);
}
@end
  • 调用
CSStudent *stu = [[CSStudent alloc] init];
[stu run];

运行结果如下

-[CSPerson(Extension) run]

总结:当分类重写原类方法时,调用分类方法

2.2 借助 Runtime 验证
  • Student类和Student (Extension)
@interface Student : NSObject
- (void)run;
@end

@implementation Student
- (void)run {
    NSLog(@"%s",__FUNCTION__);
}
@end

@interface Student (Extension)
- (void)run;
@end

@implementation Student (Extension)
- (void)run {
    NSLog(@"%s",__FUNCTION__);
}
@end
  • runtime获取底层信息
- (void)getAllClassMethod {
    u_int count;
    Method *methods = class_copyMethodList([Student class], &count);
    NSInteger index = 0;
    
    for (int i = 0; i < count; i++) {
        SEL name = method_getName(methods[i]);
        IMP imp = method_getImplementation(methods[i]);
        const char *encode = method_getTypeEncoding(methods[i]);
        NSString *strName = [NSString stringWithCString:sel_getName(name) encoding:NSUTF8StringEncoding];
        
        NSLog(@"%@",NSStringFromSelector(name));
        NSLog(@"%s",encode);
        NSLog(@"%@",strName);
        NSLog(@"----------------------");
    }
}
  • 打印结果如下
2019-06-26 09:34:49.135221+0800 CallClassMethodNoCategory[50897:14264151] v16@0:8
2019-06-26 09:34:49.135759+0800 CallClassMethodNoCategory[50897:14264151] run
(lldb) po imp
(CallClassMethodNoCategory`-[Student(Extension) run] at Student+Extension.m:13)

2019-06-26 09:35:33.648576+0800 CallClassMethodNoCategory[50897:14264151] ----------------------
2019-06-26 09:35:33.648773+0800 CallClassMethodNoCategory[50897:14264151] v16@0:8
2019-06-26 09:35:33.648873+0800 CallClassMethodNoCategory[50897:14264151] run
(lldb) po imp
(CallClassMethodNoCategory`-[Student run] at Student.m:13)

通过runtime,我们可以知道,分类同名方法位于方法列表methods的前面。

2.3 从代码底层验证
  • 首先需要 MJ 写的一个类,用于窥探底层结构,MJClassInfo.h
#import <Foundation/Foundation.h>

#ifndef MJClassInfo_h
#define MJClassInfo_h

# if __arm64__
#   define ISA_MASK        0x0000000ffffffff8ULL
# elif __x86_64__
#   define ISA_MASK        0x00007ffffffffff8ULL
# endif

#if __LP64__
typedef uint32_t mask_t;
#else
typedef uint16_t mask_t;
#endif
typedef uintptr_t cache_key_t;

struct bucket_t {
    cache_key_t _key;
    IMP _imp;
};

struct cache_t {
    bucket_t *_buckets;
    mask_t _mask;
    mask_t _occupied;
};

struct entsize_list_tt {
    uint32_t entsizeAndFlags;
    uint32_t count;
};

struct method_t {
    SEL name;
    const char *types;
    IMP imp;
};

struct method_list_t : entsize_list_tt {
    method_t first;
    method_t second;
    method_t third;
    method_t fourth;
    method_t last;
};

struct ivar_t {
    int32_t *offset;
    const char *name;
    const char *type;
    uint32_t alignment_raw;
    uint32_t size;
};

struct ivar_list_t : entsize_list_tt {
    ivar_t first;
};

struct property_t {
    const char *name;
    const char *attributes;
};

struct property_list_t : entsize_list_tt {
    property_t first;
};

struct chained_property_list {
    chained_property_list *next;
    uint32_t count;
    property_t list[0];
};

typedef uintptr_t protocol_ref_t;
struct protocol_list_t {
    uintptr_t count;
    protocol_ref_t list[0];
};

struct class_ro_t {
    uint32_t flags;
    uint32_t instanceStart;
    uint32_t instanceSize;  // instance对象占用的内存空间
#ifdef __LP64__
    uint32_t reserved;
#endif
    const uint8_t * ivarLayout;
    const char * name;  // 类名
    method_list_t * baseMethodList;
    protocol_list_t * baseProtocols;
    const ivar_list_t * ivars;  // 成员变量列表
    const uint8_t * weakIvarLayout;
    property_list_t *baseProperties;
};

struct class_rw_t {
    uint32_t flags;
    uint32_t version;
    const class_ro_t *ro;
    method_list_t * methods;    // 方法列表
    property_list_t *properties;    // 属性列表
    const protocol_list_t * protocols;  // 协议列表
    Class firstSubclass;
    Class nextSiblingClass;
    char *demangledName;
};

#define FAST_DATA_MASK          0x00007ffffffffff8UL
struct class_data_bits_t {
    uintptr_t bits;
public:
    class_rw_t* data() {
        return (class_rw_t *)(bits & FAST_DATA_MASK);
    }
};

/* OC对象 */
struct mj_objc_object {
    void *isa;
};

/* 类对象 */
struct mj_objc_class : mj_objc_object {
    Class superclass;
    cache_t cache;
    class_data_bits_t bits;
public:
    class_rw_t* data() {
        return bits.data();
    }
    
    mj_objc_class* metaClass() {
        return (mj_objc_class *)((long long)isa & ISA_MASK);
    }
};

#endif /* MJClassInfo_h */

其中需要特别注意的是,获取方法列表

struct method_list_t : entsize_list_tt {
    method_t first;
    method_t second;
    method_t third;
    method_t fourth;
    method_t last;
};
  • 测试代码
@interface CSPerson : NSObject {
@public
    int _age;
}
@property(nonatomic, assign)int no;
- (void)personInstanceMethod;
+ (void)personClassMethod;
- (void)run;
@end

@implementation CSPerson
- (void)run {
    NSLog(@"%s",__FUNCTION__);
}
- (void)personInstanceMethod {
}
+ (void)personClassMethod {
}
@end

#pragma mark - CSPerson (Extension)

@interface CSPerson (Extension)
- (void)run;
@end

@implementation CSPerson (Extension)
- (void)run {
    NSLog(@"%s",__FUNCTION__);
}
@end
  • 调用
mj_objc_class *personClass = (__bridge mj_objc_class *)([CSPerson class]);
        
class_rw_t *personClassData = personClass->data();
        
class_rw_t *personMetaClassData = personClass->metaClass()->data();
  • 运行结果如下(打断点)
    image.png

总结:通过打印底层方法列表结构,我们可以知道,分类同名方法位于原类方法后面,所以分类方法优先调用。

推荐参考文章 iOS-底层原理(14)isa-Class的结构详解

三 如何调用原类方法
  • Student类和Student (Extension)
@interface CSPerson : NSObject {
- (void)run;
@end

@implementation CSPerson
- (void)run {
    NSLog(@"%s",__FUNCTION__);
}
@end

#pragma mark - CSPerson (Extension)

@interface CSPerson (Extension)
- (void)run;
@end

@implementation CSPerson (Extension)
- (void)run {
    NSLog(@"%s",__FUNCTION__);
}
@end
  • 使用 runtime调用
- (void)callClassMethod {
    u_int count;
    Method *methods = class_copyMethodList([Student class], &count);
    NSInteger index = 0;
    
    for (int i = 0; i < count; i++) {
        SEL name = method_getName(methods[i]);
        NSString *strName = [NSString stringWithCString:sel_getName(name) encoding:NSUTF8StringEncoding];

        if ([strName isEqualToString:@"run"]) {
            index = i;  // 先获取原类方法在方法列表中的索引
        }
    }
    
    // 调用方法
    Student *stu = [[Student alloc] init];
    SEL sel = method_getName(methods[index]);
    IMP imp = method_getImplementation(methods[index]);
    ((void (*)(id, SEL))imp)(stu,sel);
}
  • 运行结果
2019-06-26 09:39:20.744704+0800 CallClassMethodNoCategory[51029:14270433] -[Student run]

总结:通过遍历类Student的方法列表,或许run方法在方法列表methods的索引,然后调用即可。


项目连接地址
isa-class-struct
CallClassMethodNoCategory