通过重写 NSNull 的消息转发方法，来避免给 [NSNull null] 发消息时的闪退问题

NSNull+LCFMessageForwarding.h

//
//  NSNull+LCFMessageForwarding.h
//  NSNull
//
//  Created by leichunfeng on 15/12/22.
//  Copyright © 2015年 leichunfeng. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface NSNull (LCFMessageForwarding)

@end

NSNull+LCFMessageForwarding.m

//
//  NSNull+LCFMessageForwarding.m
//  NSNull
//
//  Created by leichunfeng on 15/12/22.
//  Copyright © 2015年 leichunfeng. All rights reserved.
//

#import "NSNull+LCFMessageForwarding.h"
#import <objc/runtime.h>

@implementation NSNull (LCFMessageForwarding)

+ (void)load {
    static dispatch_once_t onceToken1;
    dispatch_once(&onceToken1, ^{
        Class class = [self class];
        
        SEL originalSelector = @selector(forwardInvocation:);
        SEL swizzledSelector = @selector(lcf_forwardInvocation:);
        
        Method originalMethod = class_getInstanceMethod(class, originalSelector);
        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
        
        BOOL success = class_addMethod(class, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod));
        if (success) {
            class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
        } else {
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });
    
    static dispatch_once_t onceToken2;
    dispatch_once(&onceToken2, ^{
        Class class = [self class];
        
        SEL originalSelector = @selector(methodSignatureForSelector:);
        SEL swizzledSelector = @selector(lcf_methodSignatureForSelector:);
        
        Method originalMethod = class_getInstanceMethod(class, originalSelector);
        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
        
        BOOL success = class_addMethod(class, originalSelector, method_getImplementation(swizzledMethod), method_getTypeEncoding(swizzledMethod));
        if (success) {
            class_replaceMethod(class, swizzledSelector, method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
        } else {
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }
    });
}

#pragma mark - Method Swizzling

- (void)lcf_forwardInvocation:(NSInvocation *)anInvocation {
    if ([self respondsToSelector:anInvocation.selector]) {
        [anInvocation invokeWithTarget:self];
    }
}

- (NSMethodSignature *)lcf_methodSignatureForSelector:(SEL)aSelector {
    NSMethodSignature *methodSignature = [NSNull instanceMethodSignatureForSelector:aSelector];
    if (methodSignature == nil) {
        methodSignature = [NSMethodSignature signatureWithObjCTypes:"@^v^c"];
    }
    return methodSignature;
}

@end