ios NSMethodSignature and NSInvocation 消息转发

1.首先获取消息转发时连个函数内部具体内容 MARK：这里是拿[@"xxxxx" length]调用拿来举例说明

(lldb) po signature
<NSMethodSignature: 0x170076f80>
    number of arguments =
    frame size =
    is special struct return? NO
    return value: -------- -------- -------- --------
        type encoding (Q) 'Q'
        flags {}
        modifiers {}
        frame {offset = , offset adjust = , size = , size adjust = }
        memory {offset = , size = }
    argument : -------- -------- -------- --------
        type encoding (@) '@'
        flags {isObject}
        modifiers {}
        frame {offset = , offset adjust = , size = , size adjust = }
        memory {offset = , size = }
    argument : -------- -------- -------- --------
        type encoding (:) ':'
        flags {}
        modifiers {}
        frame {offset = , offset adjust = , size = , size adjust = }
        memory {offset = , size = }

(lldb) po anInvocation
<NSInvocation: 0x178075d80>
return value: {Q}
target: {@} 0x17801e350
selector: {:} length

apple doc : https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtTypeEncodings.html

讲解type encoding具体用法:http://nshipster.com/type-encodings/

2. 具体案例：json解析数据时 NSJSONSerialization 会自动把他们换成 NSNull。当我们再去用dict[@“hello”]的时候，就会出触发exception，导致程序崩溃（ios6会出现）

两种实现：

第一种：
#import "NSNull+OVNatural.h" 

@implementation NSNull (OVNatural)
- (void)forwardInvocation:(NSInvocation *)invocation
{
    if ([self respondsToSelector:[invocation selector]]) {
        [invocation invokeWithTarget:self];
    }
} 

- (NSMethodSignature *)methodSignatureForSelector:(SEL)selector
{
    NSMethodSignature *sig = [[NSNull class] instanceMethodSignatureForSelector:selector];
    if(sig == nil) {
        sig = [NSMethodSignature signatureWithObjCTypes:"@^v^c"];
    }
    return sig;
} 

@end 

第二种：
#define NSNullObjects @[@"",@0,@{},@[]] 

@interface NSNull (InternalNullExtention)
@end 

@implementation NSNull (InternalNullExtention) 

- (NSMethodSignature*)methodSignatureForSelector:(SEL)selector
{
    NSMethodSignature* signature = [super methodSignatureForSelector:selector];
    if (!signature) {
        for (NSObject *object in NSNullObjects) {
            signature = [object methodSignatureForSelector:selector];
            if (signature) {
                break;
            }
        } 

    }
    return signature;
} 

- (void)forwardInvocation:(NSInvocation *)anInvocation
{
    SEL aSelector = [anInvocation selector]; 

    for (NSObject *object in NSNullObjects) {
        if ([object respondsToSelector:aSelector]) {
            [anInvocation invokeWithTarget:object];
            return;
        }
    } 

    [self doesNotRecognizeSelector:aSelector];
}
@end 

- (void)forwardInvocation:(NSInvocation *)anInvocation  只需改变调用目标，使消息在新目标上得以调用即可。然而这样实现出来的方法与“备援接收者”方案所实现的方法等效，所以很少有人采用这么简单的实现方式。比较有用的实现方式为：在触发消息前，先以某种方式改变消息内容，比如追加另外一个参数。

实现此方法时，若发现某调用操作不应由本类处理，则需调用超类的同名方法。这样的话，继承体系中的每个类都有机会处理此调用请求，直至NSObject。如果最后调用了NSObject类的方法，那么该方法还会继而调用“doesNotRecognizeSelector:”以抛出异常，此异常表明选择子最终未能得到处理。

未完，待续....