在ViewController之间共享变量

Question

I have an iPhone app with three pages, each of which allows the user to enter some text. On the final page I want to concatenate all three strings and print it out. I have a UIViewController (named PageXController ) for each page and I am trying to pass variables along to the final page. The method I currently try doesn't quite work. Here is an example:

I begin by declaring a string as an instance variable in PageThreeController.h

@interface PageThreeController : UIViewController{
    NSMutableString *string;
}

@property (nonatomic) NSMutableString *string;

Next I add the following to PageOneController.h ,

#import "PageThreeController.h"

@interface PageOneController : UIViewController

@property (nonatomic) PageThreeController *pageThree; 

In PageOneController I then attempt to set the string instance variable on page three;

- (IBAction)handleButton:(id)sender {
    _pageThree = [[PageThreeController alloc] init];
    _pageThree.from = [[NSMutableString alloc]init];
    [_pageThree.from appendString:@"Hello World"];
        NSLog(@"My string is %@ on page one.", _pageThree.from);
}

The NSLog prints out My string is 'Hello World' on page one. but when I add the same NSLog on PageThreeController.m before concatenating, 'string' is NULL .

It seems that I am making a separate copy of the pageThreeViewController . What do I need to do to change the actual value of string on page three? I am really new at this

Answer 1

Customize your prepareForSegue method in order to pass necessary vars through public properties of these controllers. For example:

#pragma mark - PreparaForSegue
-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
   if ([segue.identifier isEqualToString:@"goToDetail"]) {
           YourDestinationController *destinationController = segue.destinationViewController;
            destinationController.myString = self.myStringToPass;
    }
}

Where myString is a public property in YourDestinationController and myStringToPass is a property in your source controller (it could be in private scope)

Answer 2

If you need to access it from anywhere, you could store the variable in the AppDelegate. So if you had a variable like this in your AppDelegate:

@property (nonatomic) NSString *currentStringToPass;

Then you could access it from your ViewControllers by using the following code:

- (IBAction)handleButton:(id)sender {
    AppDelegate *app = (AppDelegate *)[[UIApplication sharedApplication] delegate];

    _pageThree = [[PageThreeController alloc] init];
    _pageThree.from = [[NSMutableString alloc]init];
    [app setCurrentStringToPass:@"Hello World"];
        NSLog(@"My string is %@ on page one.", [app currentStringToPass]);
}

Answer 3

The easiest way to pass data between ViewControllers is using the AppDelegate, even though there is other methods .

Method 1 - using AppDelegate. Add the following line in your Appdelegate.

@property (strong,nonatomic) NSMutableString *str;

To access the variable from any view controller,

 MyAppdeleagte appDelegate=[[UIApplication sharedApplication]delegate]; 

 NSMutableString *Stringdata=appDelegate.str;

Method 2 -Specifying objects. in this method , you can proceed as you are now doing and, just need to specify the view controller instance.

let you have are navigating from one controller to another , say FirstController to Second. Firstcontroller.h

@interface FirstController : UIViewController{
    NSMutableString *string;
}

@property (nonatomic) NSMutableString *string;

SecondController.h

@interface SecondController : UIViewController{

}

@property (strong,nonatomic) FirstController *firstScreen;

Within your implementation of the FirstController, before you navigate to the SecondController ,you have to specify the instance in SecondController.

in FirstController.m

SecondController *nextScreen=[self.storyboard instantiateViewControllerWithIdentifier:@"SecondView"];

nextScreen.firstScreen=self;

Then in your SecondController.m , you can simply get the String as

_firstScreen.string;