相信大家多NullSafe并不陌生,但是为什么就一个文件,还不用导入就能解决NULL导致的崩溃呢.
首先.不用导入是用到了runtime黑魔法.写的NULL的分类.自动生效.
具体的就看我给大家的代码解析吧.
就164行,不多.
//
// 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.
//
#import <objc/runtime.h>
#import <Foundation/Foundation.h>
#ifndef NULLSAFE_ENABLED
#define NULLSAFE_ENABLED 1
#endif
#ifdef DEBUG
#define NULLSAFE_ENABLED 0
#endif
//屏蔽编译器警告
#pragma GCC diagnostic ignored "-Wgnu-conditional-omitted-operand"
//分类
@implementation NSNull (NullSafe)
//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
//
//创建一个方法缓存，这个缓存会缓存项目中类的所有类名。
//遍历缓存，寻找是否已经有可以执行此方法的类。
//
//如果有的话，返回这个NSMethodSignature。
//
//如果没有的话，返回nil,接下来会走forwardInvocation:方法。
//
//[invocation invokeWithTarget:nil];将消息转发给nil。
/*
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。
*/
//如果NULLSAFE_ENABLED 为真,执行下面的代码块.
#if NULLSAFE_ENABLED
- (NSMethodSignature *)methodSignatureForSelector:(SEL)selector
{
@synchronized([self class])//锁住,防止并发.
{
//look up method signature
NSMethodSignature *signature = [super methodSignatureForSelector:selector];//调用父类方法.
if (!signature)//如果为空
{
//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);//获取所有注册类的数量
Class *classes = (Class *)malloc(sizeof(Class) * (unsigned long)numClasses);//根据类的数目调整类的空间
numClasses = objc_getClassList(classes, numClasses);//已分配好内存空间的数组 classes 中存放元素
//add to list for checking
NSMutableSet *excluded = [NSMutableSet set];//选用集合是怕重复.
for (int i = 0; i < numClasses; i++)//遍历
{
//determine if class has a superclass
Class someClass = classes[i];//类
Class superclass = class_getSuperclass(someClass);//父类
while (superclass)//父类不为空的话
{
if (superclass == [NSObject class])//到了根类
{
[classList addObject:someClass];
break;
}
[excluded addObject:NSStringFromClass(superclass)];//添加不是根类的类.
superclass = class_getSuperclass(superclass);//父类递归
}
}
//remove all classes that have subclasses
for (Class someClass in excluded)
{
[classList removeObject:someClass];//移除掉所有有子类的类.
//这里解释一下,因为子类会继承父类,所以只要子类就好了,避免冗余.
}
//free class list
free(classes);//手动释放类空间
}
//check implementation cache first
NSString *selectorString = NSStringFromSelector(selector);
signature = signatureCache[selectorString];
if (!signature)
{
//find implementation
for (Class someClass in classList)
{
if ([someClass instancesRespondToSelector:selector])//该类的实例是否响应这个方法
{
signature = [someClass instanceMethodSignatureForSelector:selector];
break;
}
}
//再缓存以备下次使用.
//cache for next time
signatureCache[selectorString] = signature ?: [NSNull null];
}
else if ([signature isKindOfClass:[NSNull class]])//属于NULL就改为nil.
{
signature = nil;
}
}
return signature;
}
}
//把目标对象置于nil.
- (void)forwardInvocation:(NSInvocation *)invocation
{
invocation.target = nil;
[invocation invoke];
}
#endif
@end
