NSInvocation 详解

@interface NSInvocation : NSObject {
@private
    __strong void *_frame;
    __strong void *_retdata;
    id _signature;
    id _container;
    uint8_t _retainedArgs;
    uint8_t _reserved[15];
}

// 通过NSMethodSignature对象创建NSInvocation对象，NSMethodSignature为方法签名类
+ (NSInvocation *)invocationWithMethodSignature:(NSMethodSignature *)sig;

// 获取NSMethodSignature对象
@property (readonly, retain) NSMethodSignature *methodSignature;

// 保留参数，它会将传入的所有参数以及target都retain一遍
- (void)retainArguments;

// 判断参数是否还存在
// 调用retainArguments之前，值为NO，调用之后值为YES
@property (readonly) BOOL argumentsRetained;

// 设置消息调用者，注意：target最好不要是局部变量
@property (nullable, assign) id target;

// 设置要调用的消息
@property SEL selector;

// 获取消息返回值
- (void)getReturnValue:(void *)retLoc;

// 设置消息返回值
- (void)setReturnValue:(void *)retLoc;

// 获取消息参数
- (void)getArgument:(void *)argumentLocation atIndex:(NSInteger)idx;

// 设置消息参数
- (void)setArgument:(void *)argumentLocation atIndex:(NSInteger)idx;

// 发送消息，即执行方法
- (void)invoke;

// target发送消息，即target执行方法
- (void)invokeWithTarget:(id)target;

@end

- (id)performSelector:(SEL)aSelector withArguments:(NSArray *)arguments {

    if (aSelector == nil) return nil;
    NSMethodSignature *signature = [[self class] instanceMethodSignatureForSelector:aSelector];
    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
    invocation.target = self;
    invocation.selector = aSelector;

    // invocation 有2个隐藏参数，所以 argument 从2开始
    if ([arguments isKindOfClass:[NSArray class]]) {
        NSInteger count = MIN(arguments.count, signature.numberOfArguments - 2);
        for (int i = 0; i < count; i++) {
            const char *type = [signature getArgumentTypeAtIndex:2 + i];

            // 需要做参数类型判断然后解析成对应类型，这里默认所有参数均为OC对象
            if (strcmp(type, "@") == 0) {
                id argument = arguments[i];
                [invocation setArgument:&argument atIndex:2 + i];
            }
        }
    }

    [invocation invoke];

    id returnVal;
    if (strcmp(signature.methodReturnType, "@") == 0) {
        [invocation getReturnValue:&returnVal];
    }
    // 需要做返回类型判断。比如返回值为常量需要包装成对象，这里仅以最简单的`@`为例
    return returnVal;
}

作者：01_Jack
链接：http://www.jianshu.com/p/177e44a411db

/**
 现在我们有了方法的签名，现在只需要将方法打包转发
 
 @param anInvocation  通过签名得到的方法转发对象
 */
-(void)forwardInvocation:(NSInvocation *)anInvocation{
    if ([NSStringFromSelector(anInvocation.selector) isEqualToString:@"R"]) {
        [anInvocation setTarget:self];
        [anInvocation setSelector:@selector(R:)];
        NSString * name = @"流浪法师";
        //第一个和第二个参数是target和sel
        [anInvocation setArgument:&name atIndex:2];
        [anInvocation retainArguments];
        [anInvocation invoke];
        
        NSString * returnStr = [NSString stringWithFormat:@"%@死于大招之下",name];
        [anInvocation setReturnValue:&returnStr];
        [anInvocation getReturnValue:&returnStr];
    }
    
}


-(NSString *)R:(NSString *)emery{
    return nil;
}