Notification的发送与接收处理都是在同一个线程中,如果我们希望一个Notification的post线程与转发线程不是同一个线程的话就要对通知进行重定向，“重定向”就是我们在Notification所在的默认线程中捕获这些分发的通知，然后将其重定向到指定的线程中

一种重定向的实现思路是自定义一个通知队列(注意，不是NSNotificationQueue对象，而是一个数组)，让这个队列去维护那些我们需要重定向的Notification。我们仍然是像平常一样去注册一个通知的观察者，当Notification来了时，先看看post这个Notification的线程是不是我们所期望的线程，如果不是，则将这个Notification存储到我们的队列中，并发送一个信号(signal)到期望的线程中，来告诉这个线程需要处理一个Notification。指定的线程在收到信号后，将Notification从队列中移除，并进行处理。官方代码如下:#####

@interface ViewController () 
@property (nonatomic) NSMutableArray    *notifications;         // 通知队列
@property (nonatomic) NSThread          *notificationThread;    // 期望线程
@property (nonatomic) NSLock            *notificationLock;      // 用于对通知队列加锁的锁对象，避免线程冲突
@property (nonatomic) NSMachPort        *notificationPort;      // 用于向期望线程发送信号的通信端口
 
@end
 
@implementation ViewController
 
- (void)viewDidLoad {
    [super viewDidLoad];
 
    NSLog(@"current thread = %@", [NSThread currentThread]);
 
    // 初始化
    self.notifications = [[NSMutableArray alloc] init];
    self.notificationLock = [[NSLock alloc] init];
 
    self.notificationThread = [NSThread currentThread];
    self.notificationPort = [[NSMachPort alloc] init];
    self.notificationPort.delegate = self;
 
    // 往当前线程的run loop添加端口源
    // 当Mach消息到达而接收线程的run loop没有运行时，则内核会保存这条消息，直到下一次进入run loop
    [[NSRunLoop currentRunLoop] addPort:self.notificationPort
                                forMode:(__bridge NSString *)kCFRunLoopCommonModes];
 
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(processNotification:) name:@"TestNotification" object:nil];
 
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
 
        [[NSNotificationCenter defaultCenter] postNotificationName:TEST_NOTIFICATION object:nil userInfo:nil];
 
    });
}
 
- (void)handleMachMessage:(void *)msg {
 
    [self.notificationLock lock];
 
    **while** ([self.notifications count]) {
        NSNotification *notification = [self.notifications objectAtIndex:0];
        [self.notifications removeObjectAtIndex:0];
        [self.notificationLock unlock];
        [self processNotification:notification];
        [self.notificationLock lock];
    };
 
    [self.notificationLock unlock];
}
 
- (void)processNotification:(NSNotification *)notification {
 
    if ([NSThread currentThread] != _notificationThread) {
        // Forward the notification to the correct thread.
        [self.notificationLock lock];
        [self.notifications addObject:notification];
        [self.notificationLock unlock];
        [self.notificationPort sendBeforeDate:[NSDate date]
                                   components:nil
                                         from:nil
                                     reserved:0];
    }
    else {
        // Process the notification here;
        NSLog(@"current thread = %@", [NSThread currentThread]);
        NSLog(@"process notification");
    }
}
 
@end

运行后，其输出如下：

p.p1 {margin: 0.0px 0.0px 0.0px 0.0px; font: 11.0px Menlo; color: #ffffff}span.s1 {font-variant-ligatures: no-common-ligatures}

**current thread = <NSThread: 0x7fc8f9f003f0>{number = 1, name = main}**
**current thread =<NSThread: 0x7fc8f9f003f0>{number = 1, name = main}**
**process notification**