NSNull Crash处理 （NullSafe 的原理）

问题场景

后端返回的数据中总会出现一些NSNull类型，当我们一处理程序就会崩溃，因此想到把返回的数据中的NSNull类型全部转换成@""空字符串

（1）原始的json串：后端返回的json串里面包含类型NSString，NSArray，NSDictionary，NSNull类型。

{"status":1,"service_name":null,"service_id":null,"img_url":"http://api.jgfw.me/assets/uploads/files/","price":null,"num":3,"service_info":{"service_type":null,"service_time":null,"service_detail":null,"customer_name":null,"customer_phone":null,"customer_address":"","new_jishi":"","old_jishi":null,"lat":null,"lon":null},"order_info":{"order_no":"E15031267469289848688","pay_time":null,"order_time":null,"price":0,"order_state":null}}

（2）用SBJson库:json串转换成字典

NSDictionary *jsonDic = [retString JSONValue];

 

"<null>" 就是NSNull 类型，直接使用会Crash.

摘要

NullSafe is a simple category on NSNull that returns nil for unrecognised messages instead of throwing an exception.

//
//  NullSafe.m
//
//  Version 1.2.2
//
//  Created by Nick Lockwood on 19/12/2012.
//  Copyright 2012 Charcoal Design
//
//  Distributed under the permissive zlib License
//  Get the latest version from here:
//
//  https://github.com/nicklockwood/NullSafe
//
//  This software is provided 'as-is', without any express or implied
//  warranty.  In no event will the authors be held liable for any damages
//  arising from the use of this software.
//
//  Permission is granted to anyone to use this software for any purpose,
//  including commercial applications, and to alter it and redistribute it
//  freely, subject to the following restrictions:
//
//  1. The origin of this software must not be misrepresented; you must not
//  claim that you wrote the original software. If you use this software
//  in a product, an acknowledgment in the product documentation would be
//  appreciated but is not required.
//
//  2. Altered source versions must be plainly marked as such, and must not be
//  misrepresented as being the original software.
//
//  3. This notice may not be removed or altered from any source distribution.
//


//Fix  issue      desc = "<null>";icon = "<null>";


#import <objc/runtime.h>
#import <Foundation/Foundation.h>


#ifndef NULLSAFE_ENABLED
#define NULLSAFE_ENABLED 1
#endif


#pragma GCC diagnostic ignored "-Wgnu-conditional-omitted-operand"


@implementation NSNull (NullSafe)

#if NULLSAFE_ENABLED

- (NSMethodSignature *)methodSignatureForSelector:(SEL)selector
{
    @synchronized([self class])
    {
        //look up method signature
        NSMethodSignature *signature = [super methodSignatureForSelector:selector];
        if (!signature)
        {
            //not supported by NSNull, search other classes
            static NSMutableSet *classList = nil;
            static NSMutableDictionary *signatureCache = nil;
            if (signatureCache == nil)
            {
                classList = [[NSMutableSet alloc] init];
                signatureCache = [[NSMutableDictionary alloc] init];
                
                //get class list
                int numClasses = objc_getClassList(NULL, 0);
                Class *classes = (Class *)malloc(sizeof(Class) * (unsigned long)numClasses);
                numClasses = objc_getClassList(classes, numClasses);
                
                //add to list for checking
                NSMutableSet *excluded = [NSMutableSet set];
                for (int i = 0; i < numClasses; i++)
                {
                    //determine if class has a superclass
                    Class someClass = classes[i];
                    Class superclass = class_getSuperclass(someClass);
                    while (superclass)
                    {
                        if (superclass == [NSObject class])
                        {
                            [classList addObject:someClass];
                            break;
                        }
                        [excluded addObject:NSStringFromClass(superclass)];
                        superclass = class_getSuperclass(superclass);
                    }
                }

                //remove all classes that have subclasses
                for (Class someClass in excluded)
                {
                    [classList removeObject:someClass];
                }

                //free class list
                free(classes);
            }
            
            //check implementation cache first
            NSString *selectorString = NSStringFromSelector(selector);
            signature = signatureCache[selectorString];
            if (!signature)
            {
                //find implementation
                for (Class someClass in classList)
                {
                    if ([someClass instancesRespondToSelector:selector])
                    {
                        signature = [someClass instanceMethodSignatureForSelector:selector];
                        break;
                    }
                }
                
                //cache for next time
                signatureCache[selectorString] = signature ?: [NSNull null];
            }
            else if ([signature isKindOfClass:[NSNull class]])
            {
                signature = nil;
            }
        }
        return signature;
    }
}

- (void)forwardInvocation:(NSInvocation *)invocation
{
    invocation.target = nil;
    [invocation invoke];
}

#endif

@end

当我们给一个NSNull对象发送消息的话，可能会崩溃（null是有内存的），而发送给nil的话，是不会崩溃的。
作者就是使用了这么一个原理，把发送给NSNull的而NSNull又无法处理的消息经过如下几步处理：

1. 创建一个方法缓存，这个缓存会缓存项目中类的所有类名。

2. 遍历缓存，寻找是否已经有可以执行此方法的类。

3. 如果有的话，返回这个NSMethodSignature。

4. 如果没有的话，返回nil,接下来会走forwardInvocation:方法。

5. [invocation invokeWithTarget:nil];将消息转发给nil。

那么，如何判断NSNull无法处理这个消息呢，在OC中，系统如果对某个实例发送消息之后，它（及其父类）无法处理（比如，没有这个方法等），系统就会发送methodSignatureForSelector消息，如果这个方法返回非空，那么就去执行返回的方法，如果为nil,则发送forwardInvocation消息。

这样就完成整个转发链了。

题外话：一般来说，我们不应该在我们的项目中使用NSNull类（大部分NSNull类的来源来自于接口的返回），而使用nil，在来源上，就应该堵上（要么你解析到null进行处理，要么和你的服务端说，不要给我返回null）。

reference：

1.https://segmentfault.com/q/1010000005064181

2.https://github.com/nicklockwood/NullSafe

3.http://blog.sina.com.cn/s/blog_5c91824f0102ve3c.html