通知传值

-> 大多反向传值（必须先注册观察者，再发送通知，才能收到通知的原因）

////  ViewController.m//  -//#import "ViewController.h"#import "SecondVC.h"@interface ViewController ()@end@implementation ViewController- (void)viewDidLoad {    [super viewDidLoad];    [_nextButton addTarget:self action:@selector(nextPage) forControlEvents:UIControlEventTouchUpInside];    // 1.注册观察者    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(observerDidReceived:) name:@"PassToPreviousNotification" object:nil];}// 2.观察到，执行方法- (void)observerDidReceived:(NSNotification*)notification{    _receiveLabel.text = [notification.userInfo objectForKey:@"valueString"];}-(void)nextPage{    SecondVC * secondVC = [[UIStoryboard storyboardWithName:@"Main" bundle:nil] instantiateViewControllerWithIdentifier:@"SecondVC"];    [self presentViewController:secondVC animated:true completion:nil];}// 3.释放通知- (void)dealloc{    [[NSNotificationCenter defaultCenter] removeObserver:self];}@end

////  SecondVC.m////#import "SecondVC.h"@interface SecondVC ()@end@implementation SecondVC- (void)viewDidLoad {    [super viewDidLoad];    [_backButton addTarget:self action:@selector(backToPreviousPage) forControlEvents:UIControlEventTouchUpInside];}- (void)backToPreviousPage{    // 发送通知    [[NSNotificationCenter defaultCenter] postNotificationName:@"PassToPreviousNotification" object:self userInfo:@{@"valueString":_contentTextF.text}];    [self dismissViewControllerAnimated:true completion:nil];}@end