For example:
myVC.bunnies = self.myBunnies;
or
[self getBunniesWithCompletion:^(NSArray *bunnies) {
self.myBunnies = bunnies;
}];
Is the object copied, or it's just its reference that is copied, and if it gets destroyed everything will break?
The property is declared like so:
#property (strong, nonatomic) NSArray *myBunnies;
When you do this assignment, a reference is copied, and a reference count is incremented. All of this is done implicitly through ARC, because you declared your property strong.
If you would like a copy to be made, change the declaration to
#property (copy, nonatomic) NSArray *myBunnies
This would be more expensive, but the array inside your object would be insulated from the array passed into the setter.
It totally depends on the way you declared the property:
#property(strong) : the object reference count is increased, meaning that the object self keeps a reference to the affected object so that this object is not released (until the reference has been released by setting it to nil)
#propery(weak) : the object reference is simply assigned but the reference count is not increased, meaning that self does not retain a reference to it
#property(copy) : the object is copied (using <NSCopying>'s copy method) and thus a new instance is stored, independant from the first one
I strongly recommand you to read the Advanced Memory Managment Programming Guide in the Apple Doc. It is not totally up-to-date as some parts of the doc still describe the way it works before ARC was the standard, but it's still always interesting to read to understand those mecanisms.
Related
I have a model class which has below properties.
#interface CountryModel : NSObject
#property (nonatomic) NSInteger id;
#property (nonatomic, strong) NSString *country;
#property (nonatomic, strong) NSString *dialCode;
#property (nonatomic) BOOL isInEurope;
#end
I'm getting the below data from web service.
{
"id": 123,
"country_name": "India",
"dialCode": "+91"
}
Then I'm binding values to the array, Now my question is how memory management will work for the model class? Because there are few has a strong reference. Can anyone explain me.
Simple answer: automatically (provided you haven't disable ARC)
Slightly longer answer (provided you haven't disabled ARC):
When you assign values to the properties country and dialCode of an instance of CountryModel then that instance will have strong references to the referenced NSString values.
If you assign a new value to either of these properties then the previous strong reference will be dropped and replaced by a strong reference to the new value.
When the instance of CountryModel is no longer strongly reference by anything then the strong references it contains will be dropped.
When a strong reference is dropped if there are no other strong references to the reference object then that object becomes reclaimable and the memory it occupies returned to the available memory pool.
Note: if an NSString reference refers to a literal string then the literal string itself is immortal and will never be reclaimed. If you are trying to gian an understanding of when memory is released by monitoring memory usage, tracking dealloc calls, etc. do not use literal strings as your test objects – it is best to use a user-defined type.
But I've disabled ARC... Then memory will get reclaimed when you manually instruct it to be. If you're getting leaks turning on ARC is your best choice, if you cannot then you've some debugging to do!
HTH
Addendum after comment
Trying to follow reference counts and expecting them to match with actual executing code doesn't work well as there are often references you do not know about in compiled code. For this reason you will see many writers discouraging tracking reference counts.
Instead think in terms of strong references making an ownership stake in an object, and as long as there is an owner the object survives.
This might seem to be essentially the same thing, but there is a subtle difference in how the two are viewed: the reference count belongs to the object; while the strong reference attribute, which asserts an ownership stake, is part of the belongs to the referencing variable.
In your comment example a has an ownership stake in the CountryModel object it references - the only ownership stake you have so far created to that object. The CountryModel object has ownership stakes in the objects that country and dialcode reference, but may not be the only such stakeholders – in that fragment we've no idea what other reference variables assert an ownership stake in the objects those two variables reference.
Later when you create temparray the object reference stored in a is copied into the array and temparray claims and ownership stake in the referenced CountryModel object – while a continues to assert its own ownership stake as well. So now you have two variables you have created asserting an ownership stake in the CountryModel object.
A similar explanation applies to b & c.
When there is no ownership stake claimed by any variable/property over an object then that object can be destroyed. As part of that destruction any variables/properties that are part of that object are destroyed and any ownership stakes they asserted over references stored in them are withdrawn.
If that process results in more objects having no more ownership stakes asserted over them then those objects too can be destroyed; the process continuing to destroy unwanted objects until the only remaining objects are those where some variable/property asserts an ownership stake over them.
HTH more than it confuses!
is there a way I can force a property in a class declared as strong #property (strong, nonatomic) to be deallocated?
I received a report from a crash where one of the properties are being deallocated, but then the app is trying to access them from a CompletionBlock in a AFHTTPRequest. But since the property was deallocated, the app crashed.
I was unable to reproduce the scenario so I need something that will allow me to force the property to be deallocated only for testing the scenario.
I tried to use dealloc method but ARC forbid it.
Thanks
You can't "force" deallocation of an object under ARC. You can, however, (sort of) recommend deletion by assigning any object to nil and not using it in the rest of the scope of a method. If you're having problems with accessing deallocated objects, then you need precisely the opposite: To retain the owning object so it doesn't deallocate any properties.
The only way to forcefully release the memory assigned in a strong property is setting it to nil.
Means:
#property (nonatomic, strong) NSArray *testArray;
testArray = //array return from some method;
testArray = nil; //It'll release the memory associated
set nil to that object for forcefully release the memory
ex :
#property (nonatomic , strong) NSString *str;
str = nil;
I just resolved the same problem myself right now.
I had set the obvious reference to nil at several points in my app, still the object would not go away.
The object used timers, which all have a strong reference to the object. Once I invalidated those, dealloc finally ran!
As everyone else has posted all strong references to an object must be nil to run dealloc. Also, be absolutely thorough in tracking down those references!
When I'm setting a property with an object that is currently in a retained NSArray, will it only store the pointer (light-weight), or will it copy the contents to the property?
From what I know, it seems like it would only assign the pointer, but I'm not sure.
Also, under what circumstances would using *pointer = *otherPointer or the set methods (setDelegate, for instance) copy the value, instead of just passing the pointer, if any.
Shouldn't it always just pass the pointer address?
It always passes the pointer, as you said. Unless you are specifically adding a de-referencing sign, this will always be the case.
However, when you add a property to a class, and set the setter to copy:
#property (nonatomic, copy) id obj;
When using the dot syntax or the setter, This will be translated to:
_obj = [otherObj copy];
Here, it will depend whether the object in question supports copying itself, or will it fall back to it's super class NSObject or another intermediate class's copy.
Moreover, Collection classes NSDictionary and NSArray do a shallow copy, as in they copy the references to their objects only, so you have two collections pointing to the same set of objects.
As someone that's new to Objective-C can someone give me an overview of the retain, assign, copy and any others I'm missing, that follow the #property directive? What are they doing and why would I want to use one over another?
Before you know about the attributes of #property, you should know what is the use of #property.
#property offers a way to define the information that a class is intended to encapsulate.
If you declare an object/variable using #property, then that object/variable will be accessible to other classes importing its class.
If you declare an object using #property in the header file, then you have to synthesize it using #synthesize in the implementation file. This makes the object KVC compliant. By default, compiler will synthesize accessor methods for this object.
accessor methods are : setter and getter.
Example:
.h
#interface XYZClass : NSObject
#property (nonatomic, retain) NSString *name;
#end
.m
#implementation XYZClass
#synthesize name;
#end
Now the compiler will synthesize accessor methods for name.
XYZClass *obj=[[XYZClass alloc]init];
NSString *name1=[obj name]; // get 'name'
[obj setName:#"liza"]; // first letter of 'name' becomes capital in setter method
List of attributes of #property
atomic, nonatomic, retain, copy, readonly, readwrite, assign, strong, getter=method, setter=method, unsafe_unretained
atomic is the default behavior. If an object is declared as atomic then it becomes thread-safe. Thread-safe means, at a time only one thread of a particular instance of that class can have the control over that object.
If the thread is performing getter method then other thread cannot perform setter method on that object. It is slow.
#property NSString *name; //by default atomic`
#property (atomic)NSString *name; // explicitly declared atomic`
nonatomic is not thread-safe. You can use the nonatomic property attribute to specify that synthesized accessors simply set or return a value directly, with no guarantees about what happens if that same value is accessed simultaneously from different threads.
For this reason, it’s faster to access a nonatomic property than an atomic one.
#property (nonatomic)NSString *name;
retain is required when the attribute is a pointer to an object.
The setter method will increase retain count of the object, so that it will occupy memory in autorelease pool.
#property (retain)NSString *name;
copy If you use copy, you can't use retain. Using copy instance of the class will contain its own copy.
Even if a mutable string is set and subsequently changed, the instance captures whatever value it has at the time it is set. No setter and getter methods will be synthesized.
#property (copy) NSString *name;
now,
NSMutableString *nameString = [NSMutableString stringWithString:#"Liza"];
xyzObj.name = nameString;
[nameString appendString:#"Pizza"];
name will remain unaffected.
readonly If you don't want to allow the property to be changed via setter method, you can declare the property readonly.
Compiler will generate a getter, but not a setter.
#property (readonly) NSString *name;
readwrite is the default behavior. You don't need to specify readwrite attribute explicitly.
It is opposite of readonly.
#property (readwrite) NSString *name;
assign will generate a setter which assigns the value to the instance variable directly, rather than copying or retaining it. This is best for primitive types like NSInteger and CGFloat, or objects you don't directly own, such as delegates.
Keep in mind retain and assign are basically interchangeable when garbage collection is enabled.
#property (assign) NSInteger year;
strong is a replacement for retain.
It comes with ARC.
#property (nonatomic, strong) AVPlayer *player;
getter=method If you want to use a different name for a getter method, it’s possible to specify a custom name by adding attributes to the property.
In the case of Boolean properties (properties that have a YES or NO value), it’s customary for the getter method to start with the word “is”
#property (getter=isFinished) BOOL finished;
setter=method If you want to use a different name for a setter method, it’s possible to specify a custom name by adding attributes to the property.
The method should end with a colon.
#property(setter = boolBool:) BOOL finished;
unsafe_unretained There are a few classes in Cocoa and Cocoa Touch that don’t yet support weak references, which means you can’t declare a weak property or weak local variable to keep track of them. These classes include NSTextView, NSFont and NSColorSpace,etc. If you need to use a weak reference to one of these classes, you must use an unsafe reference.
An unsafe reference is similar to a weak reference in that it doesn’t keep its related object alive, but it won’t be set to nil if the destination object is deallocated.
#property (unsafe_unretained) NSObject *unsafeProperty;
If you need to specify multiple attributes, simply include them as a comma-separated list, like this:
#property (readonly, getter=isFinished) BOOL finished;
The article linked to by MrMage is no longer working. So, here is what I've learned in my (very) short time coding in Objective-C:
nonatomic vs. atomic
- "atomic" is the default. Always use "nonatomic". I don't know why, but the book I read said there is "rarely a reason" to use "atomic". (BTW: The book I read is the BNR "iOS Programming" book.)
readwrite vs. readonly
- "readwrite" is the default. When you #synthesize, both a getter and a setter will be created for you. If you use "readonly", no setter will be created. Use it for a value you don't want to ever change after the instantiation of the object.
retain vs. copy vs. assign
"assign" is the default. In the setter that is created by #synthesize, the value will simply be assigned to the attribute. My understanding is that "assign" should be used for non-pointer attributes.
"retain" is needed when the attribute is a pointer to an object. The setter generated by #synthesize will retain (aka add a retain count) the object. You will need to release the object when you are finished with it.
"copy" is needed when the object is mutable. Use this if you need the value of the object as it is at this moment, and you don't want that value to reflect any changes made by other owners of the object. You will need to release the object when you are finished with it because you are retaining the copy.
After reading many articles I decided to put all the attributes information together:
atomic //default
nonatomic
strong=retain //default
weak= unsafe_unretained
retain
assign //default
unsafe_unretained
copy
readonly
readwrite //default
Below is a link to the detailed article where you can find these attributes.
Many thanks to all the people who give best answers here!!
Variable property attributes or Modifiers in iOS
Here is the Sample Description from Article
atomic
-Atomic means only one thread access the variable(static type).
-Atomic is thread safe.
-but it is slow in performance
-atomic is default behavior
-Atomic accessors in a non garbage collected environment (i.e. when using retain/release/autorelease) will use a lock to
ensure that another thread doesn't interfere with the correct setting/getting of the value.
-it is not actually a keyword.
Example :
#property (retain) NSString *name;
#synthesize name;
nonatomic
-Nonatomic means multiple thread access the variable(dynamic type).
-Nonatomic is thread unsafe.
-but it is fast in performance
-Nonatomic is NOT default behavior,we need to add nonatomic keyword in property attribute.
-it may result in unexpected behavior, when two different process (threads) access the same variable at the same time.
Example:
#property (nonatomic, retain) NSString *name;
#synthesize name;
Explain:
Suppose there is an atomic string property called "name", and if you call [self setName:#"A"] from thread A,
call [self setName:#"B"] from thread B, and call [self name] from thread C, then all operation on different thread will be performed serially which means if one thread is executing setter or getter, then other threads will wait. This makes property "name" read/write safe but if another thread D calls [name release] simultaneously then this operation might produce a crash because there is no setter/getter call involved here. Which means an object is read/write safe (ATOMIC) but not thread safe as another threads can simultaneously send any type of messages to the object. Developer should ensure thread safety for such objects.
If the property "name" was nonatomic, then all threads in above example - A,B, C and D will execute simultaneously producing any unpredictable result. In case of atomic, Either one of A, B or C will execute first but D can still execute in parallel.
strong (iOS4 = retain )
-it says "keep this in the heap until I don't point to it anymore"
-in other words " I'am the owner, you cannot dealloc this before aim fine with that same as retain"
-You use strong only if you need to retain the object.
-By default all instance variables and local variables are strong pointers.
-We generally use strong for UIViewControllers (UI item's parents)
-strong is used with ARC and it basically helps you , by not having to worry about the retain count of an object. ARC automatically releases it for you when you are done with it.Using the keyword strong means that you own the object.
Example:
#property (strong, nonatomic) ViewController *viewController;
#synthesize viewController;
weak (iOS4 = unsafe_unretained )
-it says "keep this as long as someone else points to it strongly"
-the same thing as assign, no retain or release
-A "weak" reference is a reference that you do not retain.
-We generally use weak for IBOutlets (UIViewController's Childs).This works because the child object only
needs to exist as long as the parent object does.
-a weak reference is a reference that does not protect the referenced object from collection by a garbage collector.
-Weak is essentially assign, a unretained property. Except the when the object is deallocated the weak pointer is automatically set to nil
Example :
#property (weak, nonatomic) IBOutlet UIButton *myButton;
#synthesize myButton;
Strong & Weak Explanation, Thanks to BJ Homer:
Imagine our object is a dog, and that the dog wants to run away (be deallocated).
Strong pointers are like a leash on the dog. As long as you have the leash attached to the dog, the dog will not run away. If five people attach their leash to one dog, (five strong pointers to one object), then the dog will not run away until all five leashes are detached.
Weak pointers, on the other hand, are like little kids pointing at the dog and saying "Look! A dog!" As long as the dog is still on the leash, the little kids can still see the dog, and they'll still point to it. As soon as all the leashes are detached, though, the dog runs away no matter how many little kids are pointing to it.
As soon as the last strong pointer (leash) no longer points to an object, the object will be deallocated, and all weak pointers will be zeroed out.
When we use weak?
The only time you would want to use weak, is if you wanted to avoid retain cycles
(e.g. the parent retains the child and the child retains the parent so neither is ever released).
retain = strong
-it is retained, old value is released and it is assigned
-retain specifies the new value should be sent -retain on assignment and the old value sent -release
-retain is the same as strong.
-apple says if you write retain it will auto converted/work like strong only.
-methods like "alloc" include an implicit "retain"
Example:
#property (nonatomic, retain) NSString *name;
#synthesize name;
assign
-assign is the default and simply performs a variable assignment
-assign is a property attribute that tells the compiler how to synthesize the property's setter implementation
-I would use assign for C primitive properties and weak for weak references to Objective-C objects.
Example:
#property (nonatomic, assign) NSString *address;
#synthesize address;
unsafe_unretained
-unsafe_unretained is an ownership qualifier that tells ARC how to insert retain/release calls
-unsafe_unretained is the ARC version of assign.
Example:
#property (nonatomic, unsafe_unretained) NSString *nickName;
#synthesize nickName;
copy
-copy is required when the object is mutable.
-copy specifies the new value should be sent -copy on assignment and the old value sent -release.
-copy is like retain returns an object which you must explicitly release (e.g., in dealloc) in non-garbage collected environments.
-if you use copy then you still need to release that in dealloc.
-Use this if you need the value of the object as it is at this moment, and you don't want that value to reflect any changes made by other
owners of the object. You will need to release the object when you are finished with it because you are retaining the copy.
Example:
#property (nonatomic, copy) NSArray *myArray;
#synthesize myArray;
Atomic property can be accessed by only one thread at a time. It is thread safe. Default is atomic .Please note that there is no keyword atomic
Nonatomic means multiple thread can access the item .It is thread unsafe
So one should be very careful while using atomic .As it affect the performance of your code
prefer this links about properties in objective-c in iOS...
https://techguy1996.blogspot.com/2020/02/properties-in-objective-c-ios.html
A 101 question
Let's say i'm making database of cars
and each car object is defined as:
#import <UIKit/UIKit.h>
#interface Car:NSObject{
NSString *name;
}
#property(nonatomic, retain) NSString *name;
Why is it #property(nonatomic, retain) NSString *name; and not #property(nonatomic, assign) NSString *name;?
I understand that assign will not increment the reference counter as retain will do. But why use retain, since name is a member of the todo object the scope of it is to itself.
No other external function will modify it either.
There's no such thing as the "scope of an object" in Objective-C. Scope rules have nothing to do with an object's lifetime — the retain count is everything.
You usually need to claim ownership of your instance variables. See the Objective-C memory management rules. With a retain property, your property setter claims ownership of the new value and relinquishes ownership of the old one. With an assign property, the surrounding code has to do this, which is just as mess in terms of responsibilities and separation of concerns. The reason you would use an assign property is in a case where you can't retain the value (such as non-object types like BOOL or NSRect) or when retaining it would cause unwanted side effects.
Incidentally, in the case of an NSString, the correct kind of property is usually copy. That way it can't change out from under you if somebody passes in an NSMutableString (which is valid — it is a kind of NSString).
and don't forget to access it via
self.name = something;
because
name = something;
will not care about the generated setter/getter methods but instead assign the value directly.
Without retain there is no guarantee the NSString* you are setting name with will live any longer than the assignment statement itself. By using the retain property for the synthesized setter you're allowing it to tell the memory management system that there is at least one more object interested in keeping the NSString* around.
For those who are looking for it, Apple's documentation on property attributes is here.
The self. in:
self.name = something;
is important! Without it, you are accessing the variable directly and bypassing the setter.
The older style (correct me if I am wrong) would have been:
[self setName:something];
Anyway, this notation was the (vaguely familiar sounding) advice that I really needed when I went looking for proper #properties on NSStrings. Thanks Axel.
After reading so many Articles, SO posts and made demo apps to check Variable property attributes, I decided to put all the attributes information together
atomic //default
nonatomic
strong=retain //default
weak= unsafe_unretained
retain
assign //default
unsafe_unretained
copy
readonly
readwrite //default
so below is the detailed article link where you can find above mentioned all attributes, that will defiantly help you.
Many thanks to all the people who give best answers here!!
Variable property attributes or Modifiers in iOS
retain = strong
it is retained, old value is released and it is assigned
retain specifies the new value should be sent -retain on assignment and the old value sent -release
retain is the same as strong.
apple says if you write retain it will auto converted/work like strong only.
methods like "alloc" include an implicit "retain"
Example:
#property (nonatomic, retain) NSString *name;
#synthesize name;
assign
assign is the default and simply performs a variable assignment
assign is a property attribute that tells the compiler how to synthesize the property's setter implementation
I would use assign for C primitive properties and weak for weak references to Objective-C objects.
Example:
#property (nonatomic, assign) NSString *address;
#synthesize address;
Google's Objective-C Style Guide covers this pretty well:
Setters taking an NSString, should always copy the string it accepts.
Never just retain the string. This avoids the caller changing it under you without your knowledge. Don't assume that because you're accepting an NSString that it's not actually an NSMutableString.
Would it be unfortunate if your class got this string object and it then disappeared out from under it? You know, like the second time your class mentions that object, it's been dealloc'ed by another object?
That's why you want to use the retain setter semantics.