To understand Strong and Weak reference consider below example, suppose we have method named as displayLocalVariable.
-(void)displayLocalVariable
{
NSString myName = @"ABC";
NSLog(@"My name is = %@", myName);
}
In above method scope of myName variable is limited to displayLocalVariable method, once the method gets finished myName variable which is holding the string "ABC" will get deallocated from the memory.
Now what if we want to hold the myName variable value throughout our view controller life cycle. For this we can create the property named as username which will have Strong reference to the variable myName(see self.username = myName; in below code), as below,
@interface LoginViewController ()
@property(nonatomic,strong) NSString* username;
@property(nonatomic,weak) NSString* dummyName;
- (void)displayLocalVariable;
@end
@implementation LoginViewController
- (void)viewDidLoad
{
[super viewDidLoad];
}
-(void)viewWillAppear:(BOOL)animated
{
[self displayLocalVariable];
}
- (void)displayLocalVariable
{
NSString myName = @"ABC";
NSLog(@"My name is = %@", myName);
self.username = myName;
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
}
@end
Now in above code you can see myName has been assigned to self.username and self.username is having a strong reference(as we declared in interface using @property) to myName(indirectly it's having Strong reference to "ABC" string). Hence String myName will not get deallocated from memory till self.username is alive.
Now consider assigning myName to dummyName which is a Weak reference, self.dummyName = myName; Unlike Strong reference Weak will hold the myName only till there is Strong reference to myName. See below code to understand Weak reference,
-(void)displayLocalVariable
{
NSString myName = @"ABC";
NSLog(@"My name is = %@", myName);
self.dummyName = myName;
}
In above code there is Weak reference to myName(i.e. self.dummyName is having Weak reference to myName) but there is no Strong reference to myName, hence self.dummyName will not be able to hold the myName value.
Now again consider the below code,
-(void)displayLocalVariable
{
NSString myName = @"ABC";
NSLog(@"My name is = %@", myName);
self.username = myName;
self.dummyName = myName;
}
In above code self.username has a Strong reference to myName, hence self.dummyName will now have a value of myName even after method ends since myName has a Strong reference associated with it.
Now whenever we make a Strong reference to a variable it's retain count get increased by one and the variable will not get deallocated retain count reaches to 0.
Hope this helps.