IOS uses NSUserDefault to implement interface value transmission and data storage

SecondViewController *secondvc = [SecondViewController new];
//Create system singleton NSUserDefaults instance object NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
//Save value[defaults setObject:_firsttextfield.text forKey:@"firstTGP"];
  NSLog(@"%@",defaults);

  [ pushViewController:secondvc animated:YES];

//Create an object to get stored dataNSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
//Receive dataNSString *str = [defaults objectForKey:@"firstTGP"];
// Assign to the control_secondlabel.text = str;

#pragma mark---The first method://The reason why the singleton method uses the plus sign: There cannot be an instance object to call before creation+(instancetype)sharedDatahandle{
//For thread safety (add thread lock)  @synchronized(self) {
    if (!dataHandle) {
      dataHandle = [[DataHandle alloc]init];
    }
  }

  return dataHandle;
}

#pragma mark---The second method:
+(instancetype)sharedDatahandle{
  static DataHandle *dataHandle = nil;
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    dataHandle = [[DataHandle alloc]init];
  });

  return dataHandle;
}

  = [[UIBarButtonItem alloc]initWithTitle:@"back" style:UIBarButtonItemStylePlain target:self action:@selector(backAC)];
  DataHandle *handle = [DataHandle sharedDatahandle];
   = @"Annual salary";
- (void)backAC{
  [ popViewControllerAnimated:YES];

}

- (void)viewWillAppear:(BOOL)animated{// Value operation from back to front  //Get the value (it cannot be written in ViewDidLoad)  DataHandle *handle = [DataHandle sharedDatahandle];
   = ;
}

  //Use NSUserDefault for storage  //1: Create an array to be stored  NSArray *array = @[@"11", @"22",@"33"];
  //Create NSUserDefault object  NSUserDefaults *user = [NSUserDefaults standardUserDefaults];
  [user setObject:array forKey:@"userArray"];
  NSLog(@"%@",user);

  //Read  NSArray *newarr = [user objectForKey:@"userArray"];
  NSLog(@"%@",newarr);

#warning Note: Assigning the same Key is approximately equal to one overwrite, and the uniqueness of each Key must be ensured.// All objects stored in NSUserDefaults are immutable (this is very critical, if you make a mistake, the program will have bugs). For example, if I want to store an NSMutableArray object, I must first create an immutable array (NSArray) and then store it in NSUserDefaults.NSMutableArray *mutableArray = [NSMutableArray arrayWithObjects:@"123",@"234",@"456", nil];
NSArray *array1 = [NSArray arrayWithArray:mutableArray];
NSUserDefaults *user2 = [NSUserDefaults standardUserDefaults];
[user2 setObject:array1 forKey:@"What we store here must be immutable."];

// It is the same to retrieve the data. You want to use the data in NSUserDefaults to assign values ​​to the variable array./*-----------------------------------------------------------------------------------------------------------------------------
// NSMutableArray *mutablearray = [user objectForKey:@"What we store here must be immutable"];// // After writing this, mutableArray becomes an immutable array. If you want to add or delete data in the array, there will be a bug.//  NSLog(@"%@",mutablearray);

/*------  correct --------*/
NSMutableArray *mutablearray = [NSMutableArray arrayWithArray:[user2 objectForKey:@"What we store here must be immutable."]];
NSLog(@"%@",mutablearray);