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iOS Build a Photo Browser iPhone App Gestures Creating a Detail View Controller

Tommy Choe
Tommy Choe
38,156 Points
Posted by Tommy Choe
Tommy Choe
38,156 Points

@interface in implementation file

I get that the interface is used as a class extension in the implementation file to declare private instance variables, but why can't the variables be declared in the viewDidLoad method?

1 Answer

Michael Hulet
Michael Hulet
47,912 Points
Michael Hulet
Michael Hulet
47,912 Points

In Objective-C, variables are only valid in the scope in which they are declared. Basically, if a variable is declared in the viewDidLoad method, they will only be available in the viewDidLoad method, and nowhere else. For example:

ViewController.h 
@import UIKit;
@interface MyVC : UIViewController
// This is a public variable. It is accessible anywhere inside this class, as well as to any file that #imports this header
@property (strong, nonatomic) NSString *publicString;
@end
ViewController.m 
#import "ViewController.h"

@interface MyVC ()
// This is a private variable. It is only accessible to methods inside this class
@property (strong, nonatomic) NSString *privateString;
@end

@implementation MyVC

-(void)viewDidLoad{
    NSString *localString = @"This is only accessible inside of viewDidLoad";
}

-(void)viewWillAppearAnimated:(BOOL)animated{
    // This is perfectly valid
    self.publicString = @"public";
    // This is also just fine
    self.privateString = @"private";
    // This is not
    localString = @"This code will not compile";
}
Tommy Choe
Tommy Choe
38,156 Points
Tommy Choe
Tommy Choe
38,156 Points

Thank you for the awesome explanation!