ios - Pass a value from one ViewController to another in Objective-C

Question

I'm fairly new to Objective-C, programming. The one thing, I'm currently putting up with is passing a value from the first ViewController to the next one.

I've read this entry here and it didn't help me. Which is funny, since his answer has nearly 500 votes, so I must be the problem. What I did was the following.

I opened my PreviewViewController.m and added the following line #import "MainViewController.h" since I wanted to pass a value from the PreviewViewController to the `MainViewController. Then, when I switch the layouts ( which successfully works ) I want to pass a value.

MainViewController  *mainViewController     = [[MainViewController alloc] initWithNibName:@"MainViewController" bundle:nil];
mainViewController.userId                   = @"539897197";

As you can see, I want to pass the userId. For that, I also created a property in the MainViewController.h

@property ( nonatomic, strong ) NSString *userId;

Now, In my MainViewController.m I want to access the userId. But when I log it, the console tells me it is null. However, when I set the variable right before the NSLog it works, so it seems like the passing is the problem.

Additionally in the MainViewController.m I have the following line

@synthesize userId = _userId; 

but even when I removed that line and changed the NSLog to NSLog(@"%@",self.userId); the same problem occurred.

How can I successfully pass the variables? Which step am I doing wrong?

EDIT This is how I switch the layouts

UIViewController    *viewController         = [[MainViewController alloc]init];
MainViewController  *mainViewController     = [[MainViewController alloc] initWithNibName:@"MainViewController" bundle:nil];
mainViewController.userId                   = @"539897197";
[self presentViewController:viewController animated:YES completion:NULL];

Answer 1

Why not create a custom initializer and pass it in that way? Something like

- (id)initWithUserId:(NSString *)aUserId {
    self = [super initWithNibName:@"MainViewController" bundle:nil];
    if (self) {
        self.userId = aUserId
    }
    return self;
}

Then you can just do:

MainViewController *mvc = [[MainViewController alloc] initWithUserId:@"1234"]
[self presentViewController:mvc animated:YES completion:nil];