一丶使用场景

修改第三方库的代码,hook太复杂,直接写个B类;重定向,修改源码;
完美

二丶具体实施

A类

@interface ZBAAAA : NSObject
@end
@implementation ZBAAAA
@end

B类

@interface ZBBBBB : NSObject
- (void)bbbb;
@end

@implementation ZBBBBB
//必须实现
- (instancetype)alloc
{
    return [[self class] alloc];
}
- (void)bbbb
{
    NSLog(@"%s",__func__);
}
@end

结果:

图片.png

三丶解释

知识点:
isa机制图

图片.png

- (NSArray *)NSObjectMetaMethodList {
    u_int count;
    NSMutableArray *methodList = [NSMutableArray array];
    Class metaClass = objc_getMetaClass([@"NSObject" UTF8String]);
    Method *methods = class_copyMethodList(metaClass, &count);
    for (int i = 0; i < count; i++) {
        SEL name = method_getName(methods[i]);
        NSString *strName = [NSString stringWithCString:sel_getName(name)
                                               encoding:NSUTF8StringEncoding];
        [methodList addObject:strName];
    }
    free(methods);
    return methodList;
}

- (NSArray *)methodList {
    u_int count;
    NSMutableArray *methodList = [NSMutableArray array];
    Method *methods = class_copyMethodList([self class], &count);
    for (int i = 0; i < count; i++) {
        SEL name = method_getName(methods[i]);
        NSString *strName = [NSString stringWithCString:sel_getName(name)
                                               encoding:NSUTF8StringEncoding];
        [methodList addObject:strName];
    }
    free(methods);
    return methodList;
}

首先,alloc方法是在NSObject的meta类里;
[ZBAAAA alloc]

图片.png

通过object_setClass
[ZBAAAA alloc]变成

图片.png

本来Aclass 的isa 会到 AMeta里面去找,最终到root meta class 里面找到alloc;
重定向B之后
Aclass 的isa 会到 Bclass里面去找,最终到B root class (NSObject);NSObject里面压根没有alloc;
所以需要

@implementation ZBBBBB
//必须实现
- (instancetype)alloc
{
    return [[self class] alloc];
}
- (void)bbbb
{
    NSLog(@"%s",__func__);
}
@end