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**
