运行时消息转发

原理

流程图

301129-a1159ef51f453da8.png

代码实现

进行一次发送消息会在相关的类对象中搜索方法列表，如果找不到则会沿着继承树一直搜索直到继承树的根部(通常为NSObject)，如果还是找不到并且消息转发都失败了就会执行doesNotRecognizeSelector:方法报 unrecongnized selector

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    //执行foo函数
    [self performSelector:@selector(test)];
}

+ (BOOL)resolveInstanceMethod:(SEL)sel {
    return YES;//返回YES，进入下一步转发
}

- (id)forwardingTargetForSelector:(SEL)aSelector {
    return nil;//返回nil，进入下一步转发
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    if ([NSStringFromSelector(aSelector) isEqualToString:@"test"]) {
        return [NSMethodSignature signatureWithObjCTypes:"v@:"];
    }
    
    return [super methodSignatureForSelector:aSelector];
}

- (void)forwardInvocation:(NSInvocation *)anInvocation {
    SEL sel = anInvocation.selector;

    Person *p = [Person new];
    if([p respondsToSelector:sel]) {
        [anInvocation invokeWithTarget:p];
    }
    else {
        [self doesNotRecognizeSelector:sel];
    }

}

@end

Runtime常见的应用场景

• 关联对象(OC Associated Objects)给分类增加属性
• 方法魔法(Method Swizzling)方法添加和替换
• KVO实现
• 消息转发(热更新)解决Bug(JSPatch)
• 实现NSCoding的自动归档和自动解档
• 实现字典和模型的自动转换