NSNotificationCenter

前几天面试富途证券，被问到添加通知的相关问题，当时有几个问题答错了，在此总结。

使用通知的要点

1.注册多少次，他的执行代码就会执行多少次

//1、注册多个通知
for (int i =0; i<3; i++) {
     [[NSNotificationCenter defaultCenter]addObserver:self selector:@selector(notifycationMehod:) name:@"testNotifycation" object:nil];
 }
//2、传递通知
[[NSNotificationCenter defaultCenter]postNotificationName:@"testNotifycation" object:nil];

输出结果

2016-12-01 17:06:23.868 NotifycationDemo[28703:10079806] receive notifycation object (null)
2016-12-01 17:06:23.868 NotifycationDemo[28703:10079806] receive notifycation object (null)
2016-12-01 17:06:23.869 NotifycationDemo[28703:10079806] receive notifycation object (null)

2.虽然注册多次通知，但是移除一次通知，同一个对象通知就会全部移除

3.add和Remove相关方法成对出现我们平时在使用通知的时候可能会在viewWillAppear方法里面add观察者，viewWillDisappear里面remove，但是我们如果忘记remove，那么如果我们导航到当前页面再导航到另外一个页面，然后再退回当前页面，这时候我们会得到2个输出结果。

- (void)viewWillAppear:(BOOL)animated{
    [super viewWillAppear:animated];
    //注册通知
    [[NSNotificationCenter defaultCenter]addObserver:self selector:@selector(notifycationMehod:) name:@"testNotifycation" object:nil];
}

- (void)viewWillDisappear:(BOOL)animated{
    [super viewWillDisappear:animated];
    //没有remove通知
}

2016-12-01 17:24:13.722 NotifycationDemo[28820:10087367] receive notifycation object (null)

2016-12-01 17:24:13.723 NotifycationDemo[28820:10087367] receive notifycation object (null)


4.移除一个name没有add的通知，不会崩溃

[[NSNotificationCenter defaultCenter]removeObserver:self name:@"unknownName" object:nil];

5 . 一个对象添加了观察者，但是没有移除，程序不会崩溃，这是为啥

@implementation NotifyTestClass

- (void)registerMyNotifycation{
    [[NSNotificationCenter defaultCenter]addObserver:self selector:@selector(notifycationMehod) name:@"testNotifycation" object:nil];
}

- (void)notifycationMehod{
    NSLog(@"NotifyTestClass receive notify");
}

- (void)dealloc{
    NSLog(@"NotifyTestClass dealloc");
}
@end

//局部变量 立刻释放
 NotifyTestClass *nt = [[NotifyTestClass alloc]init];
 [nt registerMyNotifycation];
//发送通知
[[NSNotificationCenter defaultCenter]postNotificationName:@"testNotifycation" object:nil];

输出结果

2016-12-01 19:29:15.039 NotifycationDemo[29035:10119206] NotifyTestClass dealloc

苹果官网文档有说明,iOS 9.0之后NSNotificationCenter不会对一个dealloc的观察者发送消息所以这样就不会崩溃了。果真我换了一个8.1的手机测试，程序崩溃了，原来是做了优化。

6 . ViewController对象在销毁的时候，系统会对他监听的通知进行清除

@implementation TwoViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    [[NSNotificationCenter defaultCenter]addObserver:self selector:@selector(notifycationMehod) name:@"testNotifycation" object:nil];
    // Do any additional setup after loading the view from its nib.
}
- (void)notifycationMehod{
    NSLog(@"TwoViewController receive notify");
}
- (void)dealloc{
    NSLog(@"TwoViewController dealloc");
}
@end

输出结果

2016-12-02 16:36:42.175 NotifycationDemo[31359:10305477] TwoViewController dealloc
2016-12-02 16:36:42.176 NotifycationDemo[31359:10305477] removeObserver = <TwoViewController: 0x7fa42381d720>

7 . 处理方法是和post通知方法在同一线程同步执行的

因此我们如果要接受一些通知更新UI的时候，我们需要回到主线程来处理。

 dispatch_async(dispatch_get_main_queue(), ^{
        //do your UI
    });

Reference:

NSNotificationCenter/NSNotification 使用注意点全面解析

深入思考NSNotification