I have a situation here where I would want to nil a strong property with a weak reference property. My class looks like this;
#interface MyClass()
#property(nonatomic, strong) Man *man;
#property(nonatomic, strong) Women *women;
#property(nonatomic, weak) Person *passedPerson
#end
I pass this object everytime to the new view controller, sometimes I pass men while some time I pass women but I always keep track of the passed object in weak reference, ie. passedPerson.
Now, when the viewcontroller return, based on the currently passed object, I would like to nil it through the weak reference pointer. Is it even possible ? This is not my exact situation but I have many objects like this which needs to be nilled out when I finish with them in the presenting view controller. How can this be done ?
Weak reference is used when you need to break a reference cycle. In the example you show, there is no reference cycle. But I assume you want to keep the passedPerson.
You could use weak references to man and women and keep a strong reference on the passedPerson. Then you can nil out the passedPerson as you are finished with them.
Related
What are the differences between strong and weak in #property declarations of pointers to objects?
Also, what does nonatomic mean?
It may be helpful to think about strong and weak references in terms of balloons.
A balloon will not fly away as long as at least one person is holding on to a string attached to it. The number of people holding strings is the retain count. When no one is holding on to a string, the ballon will fly away (dealloc). Many people can have strings to that same balloon. You can get/set properties and call methods on the referenced object with both strong and weak references.
A strong reference is like holding on to a string to that balloon. As long as you are holding on to a string attached to the balloon, it will not fly away.
A weak reference is like looking at the balloon. You can see it, access it's properties, call it's methods, but you have no string to that balloon. If everyone holding onto the string lets go, the balloon flies away, and you cannot access it anymore.
A strong reference (which you will use in most cases) means that you want to "own" the object you are referencing with this property/variable. The compiler will take care that any object that you assign to this property will not be destroyed as long as you point to it with a strong reference. Only once you set the property to nil will the object get destroyed (unless one or more other objects also hold a strong reference to it).
In contrast, with a weak reference you signify that you don't want to have control over the object's lifetime. The object you are referencing weakly only lives on because at least one other object holds a strong reference to it. Once that is no longer the case, the object gets destroyed and your weak property will automatically get set to nil. The most frequent use cases of weak references in iOS are:
delegate properties, which are often referenced weakly to avoid retain cycles, and
subviews/controls of a view controller's main view because those views are already strongly held by the main view.
atomic vs. nonatomic refers to the thread safety of the getter and setter methods that the compiler synthesizes for the property. atomic (the default) tells the compiler to make the accessor methods thread-safe (by adding a lock before an ivar is accessed) and nonatomic does the opposite. The advantage of nonatomic is slightly higher performance. On iOS, Apple uses nonatomic for almost all their properties so the general advice is for you to do the same.
strong: assigns the incoming value to it, it will retain the incoming value and release the existing value of the instance variable
weak: will assign the incoming value to it without retaining it.
So the basic difference is the retaining of the new variable.
Generaly you want to retain it but there are situations where you don't want to have it otherwise you will get a retain cycle and can not free the memory the objects. Eg. obj1 retains obj2 and obj2 retains obj1. To solve this kind of situation you use weak references.
A dummy answer :-
I think explanation is given in above answer, so i am just gonna tell you where to use STRONG and where to use WEAK :
Use of Weak :-
1. Delegates
2. Outlets
3. Subviews
4. Controls, etc.
Use of Strong :-
Remaining everywhere which is not included in WEAK.
strong and weak, these keywords revolves around Object Ownership in Objective-C
What is object ownership ?
Pointer variables imply ownership of the objects that they point to.
When a method (or function) has a local variable that points to an object, that variable is said to own the object being pointed to.
When an object has an instance variable that points to another object, the object with the pointer is said to own the object being pointed to.
Anytime a pointer variable points to an object, that object has an owner and will stay alive. This is known as a strong reference.
A variable can optionally not take ownership of an object that it points to. A variable that does not take ownership of an object is known as a weak reference.
Have a look for a detailed explanation here Demystifying #property and attributes
Here, Apple Documentation has explained the difference between weak and strong property using various examples :
https://developer.apple.com/library/ios/documentation/cocoa/conceptual/ProgrammingWithObjectiveC/EncapsulatingData/EncapsulatingData.html#//apple_ref/doc/uid/TP40011210-CH5-SW3
Here, In this blog author has collected all the properties in same place. It will help to compare properties characteristics :
http://rdcworld-iphone.blogspot.in/2012/12/variable-property-attributes-or.html
strong is the default. An object remains “alive” as long as there is a strong pointer to it.
weak specifies a reference that does not keep the referenced object alive. A weak reference is set to nil when there are no strong references to the object.
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.
Strong: Basically Used With Properties we used to get or send data from/into another classes.
Weak: Usually all outlets, connections are of Weak type from Interface.
Atomic: Such type of properties are used in conditions when we don't want to share our outlet or object into different simultaneous Threads. In other words, Atomic instance make our properties to deal with one thread at a time.
Hopefully it helpful for you.
I can't understand why it is correct to define a delegate with weak pointer :
#property (nonatomic,weak) id delegate;
I can't realize why isn't necessary to retain a reference to the delegate... i don't want the object that i use as the delegate to be deallocated... thus, i would prefer using a strong reference not a weak!
In many cases the delegate is the same object where the instance of my class will be created, in this case creating a weak reference would be a great solution to avoid retain cycle... but what if I choose a totally different object as the delegate ?
I searched for other questions on stack overflow but I can't find something that can help me to fully understand this situation.
The reason that objects weakly retain their delegates is to avoid retain cycles. Imagine the following scenario: object a creates b and retains it, then sets itself as b's delegate. a is released by its owner, leaving a retain cycle containing a and b. This is actually a very common scenario. Consider a view controller that owns a view and acts as that view's delegate. In this case, the view should not retain the controller—as a mater of proper MVC architecture and to prevent retain cycles.
Though retain cycles are a valid concern, the reasoning for a weak reference is more related to apple's perspective on how to use the delegation pattern with uikit and other elements out of the box which is explained here:
http://developer.apple.com/library/IOs/documentation/General/Conceptual/DevPedia-CocoaCore/Delegation.html
Specifically:
"The main value of delegation is that it allows you to easily customize the behavior of several objects in one central object."
If the delegate deals with managing the delegated tasks of several objects then those objects need not retain the delegate and should not bear the responsibility of dealllocating the delegate as it might be used by other objects. The weak reference enforces the concept that the management of the delegate is not the delegators responsibility.
An example in objective c is a one delegate being used for multiple table views, like when using a table view and a searchdisplaycontroller with a uisearchbar. Apples examples use the controller as the delegate, but reasoning still holds when using one custom delegate for both the main table view and the results table view for your search. That custom delegate would likely be retained by your controller in order to be provided to both table views.
This is fundamentally different from the basic delegation pattern that is referred to in other languages where the delegate is often created by the delegator and each instance may manage a its own delegate instance.
This is to avoid retain cycles. Apple offers an informative guide on advanced memory management explaining the situation and how best to deal with it. In ARC, they are now known as strong reference cycles, which are explained in the Transitioning to ARC Release Notes.
Previously you would define a property for a delegate like this,
#property (nonatomic, assign) id delegate;
But in ARC, you can define it thus,
#property (nonatomic, unsafe_unretained) id delegate;
Or, for example, if you have a protocol named <MyObjectDelegate>, you can also define the delegate in this way,
#property (nonatomic, weak) id <MyObjectDelegate> delegate;
In other words, in ARC if you have a protocol, you can declare a delegate weak. Otherwise, unsafe_unretained.
As a common practice, if we have two objects holding references with each other, we make the "child" object in a "parent-children" relationship a weak reference.
For delegation patters in iOS, the delegated object is the parent, because there is no need for the delegate caller to exist without the delegated object. For example, you have a sentence object with delegate object for method sentenceShouldEnd. Your paragraph object is the delegated object for your sentence object. Obviously the paragraph object is actually the parent, and in your sentence object you should keep your delegate as a weak reference.
To your point you assign the delegate to self, your understanding is wrong. We should never assign delegate to itself. Why you buy your ticket yourself if you feel it is necessary to hire an agent to buy the ticket for you? You are saying two completely different concepts. When you define a delegate object as property, it's used a weak reference in the object it is defined in(lets say A, i.e. the delegate object is a property of A). The delegate is assigned when you init A(let's say in B), then most likely you would assign A.delegate to self, which is acturally B. You see the parent-child relationship here?? You alloc memory for A in B. You hold A in B. A does not exist without B. You are not assigning the delegate to A!!!!
Can anyone tell me what is the difference among three property of delegation. I searched in google but did not get any satisfactory answer.
Please also tell me which is the best option and why?
#property (nonatomic, strong) id <GameAddViewControllerDelegate> delegate;
#property (nonatomic, weak) id <GameAddViewControllerDelegate> delegate;
#property (nonatomic, assign) id <GameAddViewControllerDelegate> delegate;
The difference is same as with strong, weak and assign specifiers.
Points to be noted : Any object never retains the delegate. Hence strong and retain should not be used.
weak and assign are allowed or even you can go with unsafe_unretained.
Why not to use retain?
Why use weak or assign?
Weak
weak applies to the delegate object (which has reference counts and
all the stuff), but weak references don't increase refcount. But once
the delegate object is deallocated (from anywhere in the code), any
weak reference to that object is set to nil. This is extremely useful,
because if you use only strong and weak references, you can't end up
with an invalid pointer (pointer to an already deallocated object).
Assign
assign is usually used for ints, floats and other non-object types.
You can of course assign an object reference to such a variable, but
if the object is deallocated, you will still have a pointer to it's
memory (which is garbage now, and will hurt you when you use it).
Strong
Strong will keep the object in the heap until it don't point to it
anymore. In other words " I'am the owner, you cannot dealloc this
before i'm fine with that same as retain" You use strong only if you
need to retain the object.
In case of delegation, weak preferred
You generally want to assign delegates rather than retain them, in order to avoid circular retain counts where object A retains object B and object B retains object A. (You might see this referred to as keeping a "weak reference" to the delegate.) For example, consider the following common pattern:
-(void)someMethod {
self.utilityObject = [[[Bar alloc] init] autorelease];
self.utilityObject.delegate = self;
[self.utilityObject doSomeWork];
}
if the utilityObject and delegate properties are both declared using retain, then self now retains self.utilityObject and self.utilityObject retains self.
Also see this detailed answer on stackoverflow
Should one use
#property (nonatomic, weak, readonly)
or
#property (nonatomic, readonly)?
Weak has the advantage of nil-ing out the instance if it gets deallocated, but is readonly implying weak? Should one explicit declare a property as weak if it want the weak behaviour?
If you want to keep a pointer to an object that you don't own but want it to be valid only as long as it exists, then you want to use a weak pointer because when it gets deallocated by the owner, your pointer will automatically get set to nil and won't be pointing to memory that it shouldn't be.
These both have differnect meaning, readonly doesn't make any differnce if it is weak or strong.
#property (nonatomic, weak, readonly)
#property (nonatomic, readonly)
You can also find some reference here.
Weak or strong is by no means related to readonly or readwrite. None implies the other.
A strong relation takes ownership. A weak does not but it receives the service of being nullified upon deletion of the related object.
Readonly suppresses a setter (afaik). The property cannot be changed from outside its class.
Readwrite (wich is the default if none is stated) allows changes to the property.
That's basically it. That are two settings which are not related to each other. They work in all thinkable comibnations.
I have a project with ARC but I disable it and now I have problem with properties as:
#property (weak, nonatomic) IBOutlet UIView *frame;
now I have an error for "weak", what kind I can replace? retain?
thanks
The closest thing to weak under manual reference counting is assign. Keep in mind, that unlike weak under ARC, the value of assign properties is not automatically zeroed when the instance pointed to by the property is deallocated. So, you need to be careful to discard assign references to objects before they're deallocated.
Why did you disable ARC?
You should replace it with assign. And strong with retain. retain means the reference counter will be increased for every object stored there. And if you replace weak with retain you can get reference cycle and leak some memory.