iOS中的crash防护（四）NSNotificationCenter指定线程接收通知

   这篇文章本来是要写NSNotificationCenter造成的 crash的防护方案的，但是分析总结出来发现有两点：(1) NSNotificationCenter添加或者移除通知的时候不存在像KVO出现的重复添加或者重复移除的情况（苹果内部已经进行了处理，不需要开发者进行处理了）。(2) NSNotificationCenter造成的crash主要是由于开发者忘记移除通知造成的，这是开发者不好的开发习惯造成的，不在JKCrashProtect的优化方向（引导用户养成良好的编码习惯），而且这个问题在ios9以后已经被苹果优化掉了。
   但是使用过NSNotificationCenter的小伙伴都应该知道，通常情况下，是无法确定接收通知的的函数在指定的线程内执行的。在某些时候需要在接收通知函数内刷新UI，而如果此时我们无法保证刷新代码是在主线程内执行，那么就有可能会产生crash。我们虽然可可以主动的将线程切换到主线程来避免crash，但是这种做法费时费力，还容易遗漏。虽然后来苹果推出了新的API如下：

- (id <NSObject>)addObserverForName:(nullable NSNotificationName)name object:(nullable id)obj queue:(nullable NSOperationQueue *)queue usingBlock:(void (^)(NSNotification *note))block NS_AVAILABLE(10_6, 4_0);    // The return value is retained by the system, and should be held onto by the caller in    // order to remove the observer with removeObserver: later, to stop observation.

但是这个方法是通过block的形式来实现的，无法发送通知，和接收通知相分离，无法满足在不同页面间实现指定线程接收通知的需求。
  为了实现NSNotificationCenter指定线程接收通知，而且满足在不同页面间实现指定线程接收通知的需求。我这边写了一个NSNotificationCenter的catergory,新定义了几个方法。

- (void)postNotification:(nonnull NSNotification *)notification handleThread:(nullable NSThread *)thread;- (void)postNotificationName:(nonnull NSNotificationName)aName object:(nullable id)anObject handleThread:(nullable NSThread *)thread;- (void)postNotificationName:(nonnull NSNotificationName)aName object:(nullable id)anObject userInfo:(nullable NSDictionary *)aUserInfo handleThread:(nullable NSThread *)thread;

通过发送通知的时候指定接收通知的线程，我这边主要是将handleThread,保存到了notification的userInfo中，具体实现如下：

- (void)postNotification:(nonnull NSNotification *)notification handleThread:(nullable NSThread *)thread{    NSNotificationName aName =notification.name;    NSDictionary *userInfo = notification.userInfo;    id object = notification.object;    if (userInfo) {        NSMutableDictionary  *dic = [NSMutableDictionary dictionaryWithDictionary:userInfo];        [dic setObject:thread?:@"" forKey:@"thread"];        userInfo = [dic copy];    }else{        userInfo =@{@"thread":thread?:@""};    }     NSNotification *notif = [[NSNotification alloc] initWithName:aName object:object userInfo:userInfo];    [[NSNotificationCenter defaultCenter] postNotification:notif];}- (void)postNotificationName:(nonnull NSNotificationName)aName object:(nullable id)anObject handleThread:(nullable NSThread *)thread{    NSDictionary *userInfo=nil;    [self postNotificationName:aName object:anObject userInfo:userInfo handleThread:thread];}- (void)postNotificationName:(nonnull NSNotificationName)aName object:(nullable id)anObject userInfo:(nullable NSDictionary *)aUserInfo handleThread:(nullable NSThread *)thread{    if (aUserInfo) {        NSMutableDictionary  *dic = [NSMutableDictionary dictionaryWithDictionary:aUserInfo];        [dic setObject:thread?:@"" forKey:@"thread"];        aUserInfo = [dic copy];    }else{            aUserInfo =@{@"thread":thread?:@""};    }    [[NSNotificationCenter defaultCenter] postNotificationName:aName object:anObject userInfo:aUserInfo];}

如何将将notification中的thread信息读取出来，并根据thread信息将接收通知的函数切换到指定的线程去执行呢，我这边主要method swizzle对- (void)addObserver:(id)observer selector:(SEL)aSelector name:(nullable NSNotificationName)aName object:(nullable id)anObject 这个方法进行了方法交换，实现代码如下：

+ (void)load{    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        [NSNotificationCenter defaultCenter].isAspected =YES;        [self JKCrashProtectswizzleMethod:@selector(addObserver:selector:name:object:) withMethod:@selector(JKCrashProtectaddObserver:selector:name:object:) withClass:[NSNotificationCenter class]];    });}

然后在- (void)JKCrashProtectaddObserver:(id)observer selector:(SEL)aSelector name:(nullable NSNotificationName)aName object:(nullable id)anObject 这个函数中我这边通过JKUBSAspects对接收通知的函数进行hook操作，并且在相关的block内进行线程切换的处理，具体实现如下：

- (void)JKCrashProtectaddObserver:(id)observer selector:(SEL)aSelector name:(nullable NSNotificationName)aName object:(nullable id)anObject{    NSString *selectorStr = NSStringFromSelector(aSelector);    NSString *className = NSStringFromClass([observer class]);    if ([selectorStr hasPrefix:@"_"] || [className hasPrefix:@"_"] || [aName hasPrefix:@"UI"]) {        [self JKCrashProtectaddObserver:observer selector:aSelector name:aName object:anObject];        return;    } //过滤系统方法，如果是系统方法直接交换方法，不进行线程切换的处理    NSObject *target = (NSObject *)observer;    target.isAspected = YES;    [target aspect_hookSelector:aSelector withOptions:JKUBSAspectPositionInstead usingBlock:^(id<JKUBSAspectInfo> data){        NSArray *arguments = [data arguments];        NSNotification *notif =arguments[0];         NSThread *thread = notif.userInfo[@"thread"];        thread=[thread isKindOfClass:[NSThread class]]?thread:[NSThread currentThread];         NSInvocation *invocation = [data originalInvocation];        SEL selector =invocation.selector;        id currentTarget = invocation.target;        [currentTarget performSelector:selector onThread:thread withObject:notif waitUntilDone:NO];        NSRunLoop *runLoop = [NSRunLoop currentRunLoop];        [runLoop run];    } error:nil];    [self JKCrashProtectaddObserver:observer selector:aSelector name:aName object:anObject];}

而且我这边做了处理，如果没有指定接收通知函数执行的线程，那么我这边默认是currenthread。
好了说了这么多，具体能不能用还要看测试是否过关，我的测试代码如下：
1）指定主线程接收通知

- (void)clicked{    dispatch_async(dispatch_get_global_queue(0, 0), ^{         [[NSThread currentThread] setName:@"childthread"];        NSLog(@"thread name111 %@",[NSThread currentThread].name);       // NSNotification *notif = [[NSNotification alloc] initWithName:@"printLog" object:nil userInfo:nil];        //[[NSNotificationCenter defaultCenter] postNotification:notif handleThread:nil];        //[[NSNotificationCenter defaultCenter]  postNotificationName:@"printLog" object:@(123) handleThread:[NSThread mainThread]];        [[NSNotificationCenter defaultCenter] postNotificationName:@"printLog" object:@"123" userInfo:@{@"name":@"jack"} handleThread:[NSThread mainThread]];    });}- (void)printLog:(NSNotification *)notification{    NSLog(@"%@",notification);    NSLog(@"thread name222 %@",[NSThread currentThread].name);}

运行结果如下：

2017-08-07 16:02:59.148 JKCrashProtect_Example[54276:8537577] thread name000 MainThread2017-08-07 16:03:00.436 JKCrashProtect_Example[54276:8537655] thread name111 childthread2017-08-07 16:03:00.438 JKCrashProtect_Example[54276:8537577] NSConcreteNotification 0x608000243660 {name = printLog; object = 123; userInfo = {    name = jack;    thread = "<NSThread: 0x608000065440>{number = 1, name = MainThread}";}}2017-08-07 16:03:00.438 JKCrashProtect_Example[54276:8537577] thread name222 MainThread2017-08-07 16:03:01.172 JKCrashProtect_Example[54276:8537656] thread name111 childthread2017-08-07 16:03:01.173 JKCrashProtect_Example[54276:8537577] NSConcreteNotification 0x600000250f80 {name = printLog; object = 123; userInfo = {    name = jack;    thread = "<NSThread: 0x608000065440>{number = 1, name = MainThread}";}}2017-08-07 16:03:01.173 KCrashProtect_Example[54276:8537577] thread name222 MainThread

大家可以看到thread name222 MainThread 接收通知的函数在主线程内执行。
2）指定子线程接收通知

- (void)clicked{    dispatch_async(dispatch_get_global_queue(0, 0), ^{         [[NSThread currentThread] setName:@"childthread"];        NSLog(@"thread name111 %@",[NSThread currentThread].name);       // NSNotification *notif = [[NSNotification alloc] initWithName:@"printLog" object:nil userInfo:nil];        //[[NSNotificationCenter defaultCenter] postNotification:notif handleThread:nil];        //[[NSNotificationCenter defaultCenter]  postNotificationName:@"printLog" object:@(123) handleThread:[NSThread mainThread]];        [[NSNotificationCenter defaultCenter] postNotificationName:@"printLog" object:@"123" userInfo:@{@"name":@"jack"} handleThread:[NSThread currentThread]];    });}- (void)printLog:(NSNotification *)notification{    NSLog(@"%@",notification);    NSLog(@"thread name222 %@",[NSThread currentThread].name);}

运行结果如下：

2017-08-07 16:06:36.265 JKCrashProtect_Example[54368:8546384] thread name111 childthread2017-08-07 16:06:36.266 JKCrashProtect_Example[54368:8546384] NSConcreteNotification 0x60800024ea30 {name = printLog; object = 123; userInfo = {    name = jack;    thread = "<NSThread: 0x600000074f00>{number = 3, name = childthread}";}}2017-08-07 16:06:36.267 JKCrashProtect_Example[54368:8546384] thread name222 childthread

大家可以看到我们接受通知的线程是在thread name222 childthread 内执行的。是我们指定的子线程。

3）未指定某个线程接收通知的，我们默认当前线程接收通知。
关键代码如下：

[[NSNotificationCenter defaultCenter] postNotificationName:@"printLog" object:@"123" userInfo:@{@"name":@"jack"} handleThread:nil];

具体结果就不贴出来了，大家感兴趣的话可以下载demo运行下试试。
demo下载地址
如果使用cocoapod的小伙伴也可以直接使用：

pod "JKCrashProtect"

导入到项目中进行验证。