@synthesize practice objective-c with user-interface

I blocked on an issue since 1 hours about Setter/Getter on objective-c

I implement the code below to do some action depending on user choose.

In my view controller interface :

@property  NSString* view_test;

In my implementation

@synthesize view_test;

The user acces to an other view after a click on a button with this method

- (IBAction)PPBarButtonClick:(id)sender {
    [self setView_test:@"test"];
    NSLog(@"view_test %@", [self view_test]); // Print test
}

And after my viewDidLoad method:

- (void)viewDidLoad
{
    [super viewDidLoad];
    NSLog(@"view_test %@", [self view_test]); // print always null

    if ([[self view_test] isEqualToString:(@"test")]) {

      //some action

    } else {

      // other action

    }

}

And my trace

2016-04-04 07:44:43.266 MyApp[19926:675795] view_test (null)
2016-04-04 07:45:11.817 MyApp[19926:675795] view_test test
2016-04-04 07:45:11.925 MyApp[19926:675795] view_test (null)

I don't understand why after setting "test" to view_test, the print retu (null)? I'm lost

Thanks