系統自帶的通知，例如：UITextField

- (void)viewDidLoad {
    [super viewDidLoad];
    
        //UIApplicationWillResignActiveNotification將要失去活動狀態的通知
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(receiveWillResignActiveNotification:) name:UIApplicationWillResignActiveNotification object:nil];
    
        //UIKeyboardWillChangeFrameNotification鍵盤的frame將要發生變化時的通知
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(receiveKeyboardFrameChangeNotification:) name:UIKeyboardWillChangeFrameNotification object:nil];
        //設定清除按鈕的現實方法，預設是不顯示。
    _textField.clearButtonMode = UITextFieldViewModeWhileEditing;
    
        //UITextFieldTextDidChangeNotification textField內容發生編輯時的通知。
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(receiveTextFieldChangeNotification:) name:UITextFieldTextDidChangeNotification object:nil];
}
 

- (void)receiveWillResignActiveNotification:(NSNotification *)noti
{
    _textField.text = @"";
}

- (void)receiveKeyboardFrameChangeNotification:(NSNotification *)noti
{
    NSLog(@"鍵盤noti.object   %@",noti.object);
    //noti.userInfo傳過來的引數輸出了鍵盤的frame變化座標
    NSLog(@"鍵盤noti.userInfo %@",noti.userInfo);
}

- (void)receiveTextFieldChangeNotification:(NSNotification *)noti
{
    NSLog(@"文字框[noti.object text] %@",[noti.object text]);
}