iOS: 當app從background切換到foreground，如何通知uiviewcontroller

當app從background切換到foreground，會trigger AppDelegate.m的2個方法： applicationWillEnterForeground and  applicationDidBecomeActive

但卻不會trigger current view controller的 viewWillAppear and viewDidAppear 方法，那麼怎麼通知current view呢？

答案是通過在current view controller裡把self註冊為該event的observer！

[cpp] view
plaincopy

1. - (void) viewWillAppear:(BOOL)animated{  
2.     [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appWillEnterForegroundNotification) name:UIApplicationWillEnterForegroundNotification object:nil];  
3. }  
4.   
5.   
6. - (void) appWillEnterForegroundNotification{  
7.     NSLog(@"trigger event when will enter foreground.");  
8. }  

不過還有3個重要的issues：

1. 必須在view controller的 viewWillDisappear/viewDidDisappear 其中一個方法中remove this observer。這是因為假設從當前view切換到另一個view再返回到該view，又會註冊一次為observer，這樣當從bgground到foreground時就會trigger 2次observer的方法！

remove observer example:

[cpp] view
plaincopy

1. -(void) viewDidDisappear:(BOOL)animated{  
2.     [[NSNotificationCenter defaultCenter] removeObserver:self];      
3. }  

2. 從view A切換到view B，2個view都註冊了from bg to fg的observer，那麼當event發生時，2個view的trigger method會都被調用。而當view B close (即被destroy）後，則只會trigger view A的。

3. 為什麼不在didLoad裡add observer？其實是可以的，但有時不適合。例如，view A是root view，在didLoad method裡add observer，由於切換到其他view會remove observer，那麼當再返回view A時則不會再調用didLoad來add observer。

例子參考WillEnterForegroundNotificationDemo project.

rel links:

http://stackoverflow.com/questions/3771015/what-method-is-called-when-application-appears-from-background-on-iphone

http://stackoverflow.com/questions/3445268/respond-to-applicationwillenterforeground-in-a-uiview

http://stackoverflow.com/questions/6103616/alter-view-before-applicationwillenterforeground

http://stackoverflow.com/questions/7569187/changing-uiview-when-applicationwillenterforeground-fires

但卻不會trigger current view controller的 viewWillAppear and viewDidAppear 方法，那麼怎麼通知current view呢？

答案是通過在current view controller裡把self註冊為該event的observer！

[cpp] view
plaincopy

1. - (void) viewWillAppear:(BOOL)animated{  
2.     [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appWillEnterForegroundNotification) name:UIApplicationWillEnterForegroundNotification object:nil];  
3. }  
4.   
5.   
6. - (void) appWillEnterForegroundNotification{  
7.     NSLog(@"trigger event when will enter foreground.");  
8. }  

不過還有3個重要的issues：

1. 必須在view controller的 viewWillDisappear/viewDidDisappear 其中一個方法中remove this observer。這是因為假設從當前view切換到另一個view再返回到該view，又會註冊一次為observer，這樣當從bgground到foreground時就會trigger 2次observer的方法！

remove observer example:

[cpp] view
plaincopy

1. -(void) viewDidDisappear:(BOOL)animated{  
2.     [[NSNotificationCenter defaultCenter] removeObserver:self];      
3. }  

2. 從view A切換到view B，2個view都註冊了from bg to fg的observer，那麼當event發生時，2個view的trigger method會都被調用。而當view B close (即被destroy）後，則只會trigger view A的。

3. 為什麼不在didLoad裡add observer？其實是可以的，但有時不適合。例如，view A是root view，在didLoad method裡add observer，由於切換到其他view會remove observer，那麼當再返回view A時則不會再調用didLoad來add observer。

例子參考WillEnterForegroundNotificationDemo project.

rel links:

http://stackoverflow.com/questions/3771015/what-method-is-called-when-application-appears-from-background-on-iphone

http://stackoverflow.com/questions/3445268/respond-to-applicationwillenterforeground-in-a-uiview

http://stackoverflow.com/questions/6103616/alter-view-before-applicationwillenterforeground

http://stackoverflow.com/questions/7569187/changing-uiview-when-applicationwillenterforeground-fires