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)
[[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
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