OC底层原理

MethodSwizzling

2020-03-04  本文已影响0人  只写Bug程序猿

方法交换

坑点1

例子:

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.dataArray = @[@"Jason",@"Lee",@"LI",@"Hello"];
    
    NSLog(@"%@",[self.dataArray objectAtIndex:4]);
}

很明数组越界报异常,那么就可以利用runtime,进行方法hook

+(void)load{
    Method oriMethod = class_getInstanceMethod([self class], @selector(objectAtIndex:));
    Method swiMethod = class_getInstanceMethod([self class], @selector(js_objectAtIndex:));
    method_exchangeImplementations(oriMethod, swiMethod);
}
-(id)js_objectAtIndex:(NSUInteger)index{
    if (index > self.count - 1) {
        NSLog(@"数组越界了兄嘚");
        return nil;
    }
    return [self js_objectAtIndex:index];
}

发现还是报错


报错信息

-[__NSArrayI objectAtIndex:]怎么会是他呢,不是NSArray么?因为是类簇的设计,我们将代码进行修改

+(void)load{
    Method oriMethod = class_getInstanceMethod(objc_getClass("__NSArrayI"), @selector(objectAtIndex:));
    Method swiMethod = class_getInstanceMethod(objc_getClass("__NSArrayI"), @selector(js_objectAtIndex:));
    method_exchangeImplementations(oriMethod, swiMethod);
}
-(id)js_objectAtIndex:(NSUInteger)index{
    if (index > self.count - 1) {
        NSLog(@"数组越界了兄嘚");
        return nil;
    }
    return [self js_objectAtIndex:index];
}

这时候发现程序已经不崩溃

交换NSArray、NSMutableArray、NSDictionary、NSMutableDictionary的时候注意类簇的问题:

坑点2

按照以上的写法,如果我们手动调用一下load方法会怎么样呢

- (void)viewDidLoad {
    [super viewDidLoad];
    [NSArray load];
    self.dataArray = @[@"Jason",@"Lee",@"LI",@"Hello"];
    
    NSLog(@"%@",[self.dataArray objectAtIndex:4]);
}

发先也会崩溃,因为手动调用load方法又将已经交换过的方法交换了回去,等于啥也没做,调用的还是系统的方法
所以hook的时候要这样写

+(void)load{
    //防止一些骚操作,比如手动调用hook方法
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Method oriMethod = class_getInstanceMethod(objc_getClass("__NSArrayI"), @selector(objectAtIndex:));
        Method swiMethod = class_getInstanceMethod(objc_getClass("__NSArrayI"), @selector(js_objectAtIndex:));
        method_exchangeImplementations(oriMethod, swiMethod);
    });
}
坑点3

子类没有实现,但是父类实现了

@interface Person : NSObject
-(void)method1;
-(void)method2;
@end
@implementation Person
-(void)method1{
    NSLog(@"method1");
}
-(void)method2{
    NSLog(@"method2");
}
@end

============
student 没有实现任何东西
@interface Student : Person

@end
@implementation Student

@end

现在我们建一个student的分类,hook,交换继承自父类的method1

@implementation Student (JS)
+(void)load{
    Method oriMethod = class_getInstanceMethod([self class], @selector(method1));
    Method swiMethod = class_getInstanceMethod([self class], @selector(js_method1));
    method_exchangeImplementations(oriMethod, swiMethod);
}
-(id)js_method1{
    
    return  [self js_method1];
}
@end

然后调用

- (void)viewDidLoad {
    [super viewDidLoad];
    Student *student = [[Student alloc] init];
    [student method1];
    Person *p = [Person alloc];
    [p method1];
    // Do any additional setup after loading the view.
}

发现报错

报错信息
发现把父类的方法也交换了,而父类没有实现js_method1,所以报错
怎么解决呢
  1. 在子类重写method1发现一切正常
  2. 根据第一点的思想,方法交换的时候能不能先尝试给他加一个方法,交换自己的方法不要交换父类的,如果添加成功证明自己没有这个方法,那么直接替换,如果添加不成功证明已经有了该方法可以直接交换
+(void)load{
    Method oriMethod = class_getInstanceMethod([self class], @selector(method1));
    Method swiMethod = class_getInstanceMethod([self class], @selector(js_method1));
    BOOL sucess = class_addMethod([self class], @selector(method1), method_getImplementation(swiMethod), method_getTypeEncoding(oriMethod));
    if (sucess) {
        class_replaceMethod([self class],@selector(js_method1) ,method_getImplementation(oriMethod) , method_getTypeEncoding(oriMethod));
    }else{
        method_exchangeImplementations(oriMethod, swiMethod);
    }
    
}
坑点4

父类子类都没有实现

@interface Person : NSObject

@end
@implementation Person

@end
=========
@interface Student : Person
-(void)sayHello;
@end
@implementation Student

@end

这时候我们添加一个判断,判断oriMethod是否存在,如果不存在替换后将swizzledSel赋值一个不做任何事情的空实现,

+(void)load{
    Method oriMethod = class_getInstanceMethod([self class], @selector(sayHello));
    Method swiMethod = class_getInstanceMethod([self class], @selector(js_method1));
    if (!oriMethod) {
        class_addMethod([self class], @selector(sayHello), method_getImplementation(swiMethod), method_getTypeEncoding(swiMethod));
        method_setImplementation(swiMethod, imp_implementationWithBlock(^(id self,SEL _cmd){}));
    }
    BOOL sucess = class_addMethod([self class], @selector(sayHello), method_getImplementation(swiMethod), method_getTypeEncoding(swiMethod));
    if (sucess) {
        class_replaceMethod([self class],@selector(js_method1) ,method_getImplementation(oriMethod) , method_getTypeEncoding(oriMethod));
    }else{
        method_exchangeImplementations(oriMethod, swiMethod);
    }
    
}
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