iOS通知和线程

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    // 注册通知
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(action:) name:@"notificationName" object:nil];
}

// 主线程发送通知
- (IBAction)actionA:(UIButton *)sender {
    // 发送通知
    NSNotificationCenter *center=[NSNotificationCenter defaultCenter];
    [center postNotificationName:@"notificationName" object:self userInfo:@{@"key":@"notificationName"}];
    
    NSLog(@"发送通知线程 = %@",[NSThread currentThread]);
}

// 子线程发送通知
- (IBAction)actionB:(UIButton *)sender {
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        // 发送通知
        NSNotificationCenter *center=[NSNotificationCenter defaultCenter];
        [center postNotificationName:@"notificationName" object:self userInfo:@{@"key":@"notificationName"}];
        NSLog(@"发送通知线程 = %@",[NSThread currentThread]);
    });
}

// 接收通知相应的方法
- (void)action:(NSNotification *)notice {
    NSLog(@"接收通知线程 = %@",[NSThread currentThread]);
    dispatch_async(dispatch_get_main_queue(), ^{
        NSLog(@"主线程更新 UI %@", [NSThread currentThread]);
    });
}

// 移除通知
- (void)dealloc {
    
    // 移除当前所有通知
    NSLog(@"移除了所有的通知");
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

@end

打印结果：
当在主线程发送通知时：

发送通知线程 = <NSThread: 0x600002c74780>{number = 1, name = main}
接收通知线程 = <NSThread: 0x600002c74780>{number = 1, name = main}

当在子线程发送通知时：

发送通知线程 = <NSThread: 0x600002c57d40>{number = 5, name = (null)}
接收通知线程 = <NSThread: 0x600002c57d40>{number = 5, name = (null)}

结论：
通知在哪个线程发送，接收就在那个线程。