Question

简介：我有一个发送通知的队列。视图控制器订阅它们，当它接收到它们时，它会显示一个图像。

问题：第一次，一切顺利。当我稍后回到视图时，我会看到收到通知但未显示图像的日志。

注意：后台线程是一个队列：dispatch_queue_create("scan", DISPATCH_QUEUE_SCAN)，并从此队列发布通知。

@property(strong, nonatomic) id observer;

- (void)viewDidAppear:(BOOL) animated {
  [super viewDidAppear:animated];
  self.observer = [[NSNotificationCenter defaultCenter] addObserverForName:MY_NOTIF object:nil queue:nil usingBlock:^(NSNotification *note) {
    dispatch_async(dispatch_get_main_queue(), ^{
      NSLog(@"notification");
      [self.img setImage:[UIImage imageNamed:@"register-ok"]];
      [self.img setNeedsDisplay];  // useless
    });
  }
}

- (void)viewDidDisappear:(BOOL) animated {
  [super viewDidDisappear:animated];
  [[NSNotificationCenter defaultCenter] removeObserver:observer name:MY_NOTIF object:nil];
}

这真让我发疯。在此先感谢您的帮助

编辑：完成的更改：

NSLog(@"viewDidAppear. createObserver. self: %@");
[[NSNotificationCenter defaultCenter] addObserverForName:MY_NOTIF object:nil queue:nil usingBlock:^(NSNotification *note) {
  NSLog(@"will call redraww. self: %@");
  [self performSelectorOnMainThread:@selector(redraww:) withObject:nil waitUntilDone:YES];
}

-(void) redraww:(NSObject*)input {
    NSLog(@"redraww self : %@", self);
    [self.img setImage:[UIImage imageNamed:@"register-ok"]];
}

-(void) onRegistrationFinish {
  NSLog(@"remove observer. self %@", self);
  [[NSNotificationCenter defaultCenter] removeObserver:self name:(NSString *)NOTIF_SOLE_UUID object:nil];
}

控制台日志

viewDidAppear. createObserver self: <RegisterLeftViewController: 0x15e34550>
will call redraww. self:<RegisterLeftViewController: 0x15e34550>
redrawww self : <RegisterLeftViewController: 0x15e34550>
remove observer. self <RegisterLeftViewController: 0x15e34550>

************************* Second time

viewDidAppear. createObserver self: <RegisterLeftViewController: 0x15e98ba0>
will call redraww. self:<RegisterLeftViewController: 0x15e34550>
redrawww self : <RegisterLeftViewController: 0x15e34550>
remove observer. self <RegisterLeftViewController: 0x15e98ba0>

Answer 1

好像你正在做一些可能导致麻烦的事情。

问题可能来自许多地方。以下是一些可以解决问题的方法：

将观察者注册移至viewDidLoad
将观察员注销移至dealloc
使用__block __weak id observer确保在通知处理程序中正确捕获自我。请参阅iOS NSNotificationCenter Observer not being removed

希望有所帮助

有人可以在iOS中解释这种奇怪的行为（中央调度+通知+ UI刷新）吗？

1 个答案: