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