In Objective-C, I wonder whether a custom setter will override the KVO support (willChangeValueForKey: and didChangeValueForKey;) and do I need to include calls to these explicitly in the custom setter?
What about Retain and Copy? Would I need to explcitly include release and retain or copy in the setter for properties with Retain or Copy attributes (for non-ARC code)?
KVO will work automatically if your accessors are in the standard format (-setFoo: and -foo). Retain and copy, however, you will need to manage yourself in your custom accessors.
This is because retain and copy are part of the synthesized accessors, whereas KVO is based solely on method names. Check here for a full description of KVO-compliant method naming.
Have a look at Apples Advanced memory management programming which tells you how to implement your own getters/setters properly:
The docs state:
Automatic support is provided by NSObject and is by default available for all properties of a class that are key-value coding
compliant. Typically, if you follow standard Cocoa coding and naming
conventions, you can use automatic change notifications—you don’t have
to write any additional code.
If you have a setter, just conform to the naming conventions.
Regarding copy/retain:
- (void)setFoo:(id)bar {
#synchronized (self) { // synchronize if you have to, can be omitted in most cases
if (bar_ != bar) { // bar_ is the ivar
[bar_ release]; // omit in ARC
bar_ = [bar copy]; // resp. retain in other variant
}
}
}
If your setter does not conform to the guildlines have a look at the "Manual Change Notification" section.
Related
So in some of the codes I see, they access an objects ivar directly instead of using accessors . What are the advantages of using them instead of accessors?
So how would this
thing = object->ivar
differ from this?
thing = object.ivar
Thanks.
First let me say, I totally loathe the Objective-C dot notation. It sacrifices understandability for brevity and that is a bad thing. In fact, the other two answers here both show evidence of the kind of confusion dot notation introduces.
Having got the rant out of the way, I'll now try to answer the question.
Under the hood, Objective-C objects are implemented as pointers to C structs. This is why
obj->ivar
sometimes works. Given that it's a C struct
(*obj).ivar
should also work exactly as you would expect for C. Having said that, you can make ivars private or protected, in which case using the above outside a scope where they are visible will cause a compiler error.
The dot operator when applied to an Objective-C object (which is a pointer don't forget) has a totally different meaning. It's syntactic sugar for sending an accessor message to the object meaning that:
foo = obj.property;
obj.property = foo;
is identical in effect to
foo = [obj property];
[obj setProperty: foo];
That is all there is to dot notation. If you go through your code changing all instances of the first form to instances of the second form, you have done everything the compiler does wrt dot notation.
In particular
you do not need a declared #property to use dot notation. You can declare the set and get accessors in the traditional way as Objective C methods, although it is definitely best practice to use #property declarations for things that are logically properties.
you do not need a backing instance variable. There's no reason why your getters and setters can't calculate values.
Given the above, the major difference between obj->ivar and obj.ivar is that the former modifies the ivar directly and latter invokes an accessor, this means that the latter can do any memory management stuff needed (retains, releases, copies etc) and can also invoke key value observing.
This is one thing with a huge difference between c/c++ and objective-c.
In C/C++ the . accesses the variable directly and the -> accesses the variable if it's a pointer to the variable, so basically it is the same.
In Objective-C the . is a shortcut to access the property using the setter and getter function and it is always using those functions. You can't access ivars with it if there is no property with that name.
Some say it's "dirty" to allow direct access to the variables. If more people work on the code it's "cleaner" to use accessors because it might be easier to debug where variables are changed since you can always break in the setter.
You can even do "bad" things with it, like:
NSArray *array = [NSArray alloc] init];
int count = array.count;
array.release;
this will technically work, because the array.release is a shortcut for [array release] but it is bad style to use . for other things then properties.
The advantage of properties is that they call methods that work with your ivars, in stead of calling the ivars directly, so you can do things like this:
-(void)setFrame:(CGRect)frame
{
if([self frameIsValid:frame])
{
if(self.flipsFrames)
{
frame.size = CGSizeMake(frame.size.height,frame.size.width);
}
[super setFrame:frame];
[delegate viewSubclass:self ChangedFrameTo:frame];
}
}
Four advantages shown here are:
The possibility to override
The possibility to check a given value
The possibility to alter a given value (use with caution)
A way to react to calls
Another advantage:
-(NSInteger) amountOfMinutes
{
return amountOfSeconds * 60;
}
You can use 1 ivar for multiple properties, saving memory and preventing/reducing redundancy, while keeping useful different formats.
There's not really an advantage to using ivars, except when you don't want to use a property so your class is more encapsulated. That does not make it impossible to reach, but it makes it clear it isn't supposed to be reached.
All ivars are private. There is no way to access them directly from outside the object. Therefore, both of your code samples are equivalent, in ObjC terms.
When you call object.ivar, what you are really doing is calling object's ivar selector. This may be either a getter method that you wrote yourself, or more likely, a synthesized getter method that you created with #synthesize.
thing, however, is an ivar. Your code would be calling the ivar selector on object and assigning the result directly to your instance's thing ivar.
If you had instead written it as self.thing = object.ivar, then you would be using your instance's setter method to assign to thing.
Some of the advantages of using accessors (specifically, synthesized properties) in ObjC are KVO/KVC compliance; better concurrency support; access control (readonly, readwrite); as well as all of the advantages that accessors give you in any other OO language.
I know declared property generates accessor method which is someway just syntax sugar.
I found quite a lot people use self.property = nil in their dealloc method.
1) In Apple's Memory Management document, p23
It says:
The only places you shouldn’t use accessor methods to set an instance variable are in init methods and dealloc.
why shouldn't?
2) In apple's Objective-C 2.0, p74
Declared properties fundamentally take the place of accessor method declarations; when you synthesize a property, the compiler only creates any absent accessor methods. There is no direct interaction with the dealloc method—properties are not automatically released for you. Declared properties do, however, provide a useful way to cross-check the implementation of your dealloc method: you can look for all the property declarations in your header file and make sure that object properties not marked assign are released, and those marked assign are not released.
Note: Typically in a dealloc method you should release object instance variables directly (rather than invoking a set accessor and passing nil as the parameter), as illustrated in this example:
- (void)dealloc { [property release]; [super dealloc]; }
If you are using the modern runtime and synthesizing the instance variable, however, you cannot access the instance variable directly, so you must invoke the accessor method:
- (void)dealloc { [self setProperty:nil]; [super dealloc]; }
What does the note mean?
I found [property release]; and [self setProperty:nil]; both work.
Setting a property can lead to notifications being sent to other objects that are observing that property. That could in turn lead to those objects attempting to do something further with your object. If you are in the middle of deallocating, this is probably not what you want to happen. So in general it is safer to release the relevant instance variable directly.
Note that this sort of problem will only arise in certain cases, so it is often perfectly possible to write code using self.property=nil in dealloc and for everything to work out fine. It's just not best practice.
In the Objective-C "modern runtime", it is possible to declare properties without ever specifying the ivar. The runtime will synthesise the storage to go along with the synthesised accessors. In this case you cannot release the ivar directly because as far as your code is concerned there isn't one. So you have no choice but to go the self.property=nil route.
New to Cocoa and Objective-c.
Do I need getters and setters if I'm depending on garbage collection?
For example is it safe to just modify instance variables directly without the dot syntax?
And in the dealloc method can I sent them to nil instead of release (or do I even have to release them)?
Property (Getters and setters) is a mean of encapsulation, and ivar is an implementation detail.
Accessing via properties allow you to change the internal design while keeping the interface unchanged.
Whether the program has GC enabled or not, you shouldn't access ivar directly externally (internally is fine).
(Also, the -dealloc method is ignored if GC (not ARC) is enabled. You could implement -finalize. But the GC should be smart enough to clean the no-longer-needed ivars, whether to set them to nil or not.)
From within the class itself you can directly modify the private variables of course. Think about accessors mainly to expose a public interface and avoid other classes to modify directly your object. This comes from the Object Oriented Paradigm and not with the memory management.
About the release it is better to understand clearly how it works using the retain count and what have a role on that count. Some method like init increase retains the object and some other do not. A lot static methods that return new ready to use object like imageWithNamed: return an autorelease object that you have to retain explicitly to own them.
The best is to release and set to nil when you have transfer the ownership to another object (ex. an array)
ex.
NSString *myName = #"zgueb";
NSArray *myFamily = [[NSArray alloc] initWithObjects:myName, nil];
// now myName is retained by myFamily, i can release it
[myName release];
myName = nil;
Is it clear.
Vince.
I have several years of experience in Obj-c and Cocoa, but am just now getting back into it and the advances of Obj-C 2.0 etc.
I'm trying to get my head around the modern runtime and declaring properties, etc. One thing that confuses me a bit is the ability in the modern runtime to have the iVars created implicitly. And of course this implies that in your code you should always be using self.property to access the value.
However, in init* and dealloc(assuming you're not using GC) methods we should be using the iVar directly (in the current runtime).
So questions are:
Should we use property accessors in init* and dealloc with Modern Runtime?
If so, why is this different? Is it just because the compiler can't see the iVar?
If I need to override an accessor, can I still access that iVar that will be defined at runtime or do I have to define an actual iVar that the runtime will then use?
Again, if I can access the synthesized iVar, why can't I continue to do this for the init* and dealloc methods?
I read the docs several times, but they seemed a bit vague about all of this and I want to be sure that I understand it well in order to decide how I want to continue coding.
Hope that my questions are clear.
Quick summary of testing:
If you don't declare the ivar in legacy, compiler is completely unhappy
If you use #ifndef __OBJC2__ around ivar in legacy compiler is happy and you can use both ivar directly and as property
In modern runtime, you can leave the ivar undefined and access as property
In modern runtime, trying to access ivar directly without declaration gives error during compile
#private declaration of ivar, of course, allows direct access to ivar, in both legacy and modern
Doesn't really give a clean way to go forward right now does it?
In the current (OS X 10.5/GCC 4.0.1) compiler, you cannot directly access the runtime-synthesized ivars. Greg Parker, one of the OS X runtime engineers put it this way on the cocoa-dev list (March 12, 2009):
You can't in the current compiler. A
future compiler should fix that. Use
explicit #private ivars in the
meantime. An #private ivar should not
be considered part of the contract -
that's what #private means, enforced
by compiler warnings and linker
errors.
And why isn't there a way to
explicitly declare instance variables
in the .m file for the new runtime?
Three reasons: (1) there are some
non-trivial design details to work
out, (2) compiler-engineer-hours are
limited, and (3) #private ivars are
generally good enough.
So, for now you must use dot-notation to access properties, even in init and dealloc. This goes against the best practice of using ivars directly in these cases, but there's no way around it. I find that the ease of using runtime-synthesized ivars (and the performance benefits) outweigh this in most cases. Where you do need to access the ivar directly, you can use a #private ivar as Greg Parker suggests (there's nothing that prevents you from mixing explicitly declared and runtime-synthesized ivars).
Update With OS X 10.6, the 64-bit runtime does allow direct access to the synthesized ivars via self->ivar.
Since instance variables themselves can only be synthesized in the modern runtime (and must be declared in the #interface under 32-bit or pre-Leopard), it's safest / most portable to also declare the ivar
Should we use property accessors in init* and dealloc with Modern Runtime?
My rule of thumb is "possibly" for -init*, and "usually not" for -dealloc.
When initializing an object, you want to make sure to properly copy/retain values for ivars. Unless the property's setter has some side effect that makes it inappropriate for initialization, definitely reuse the abstraction the property provides.
When deallocating an object, you want to release any ivar objects, but not store new ones. An easy way to do this is to set the property to nil (myObject.myIvar = nil), which basically calls [myObject setMyIvar:nil]. Since messages to nil are ignored, there is no danger in this. However, it's overkill when [myIvar release]; is usually all you need. In general, don't use the property (or directly, the setter) in situations where deallocation should behave differently than setting the variable.
I can understand eJames' argument against using property accessors in init/dealloc at all, but the flipside is that if you change the property behavior (for example, change from retain to copy, or just assign without retaining) and don't use it in init, or vice versa, the behavior can get out of sync too. If initializing and modifying an ivar should act the same, use the property accessor for both.
If so, why is this different? Is it just because the compiler can't see the ivar?
The modern runtime deals with class size and layout more intelligently, which is why you can change the layout of ivars without having to recompile subclasses. It is also able to infer the name and type of the ivar you want from the name and type of the corresponding property. The Objective-C 2.0 Runtime Programming Guide has more info, but again, I don't know how deeply the details explained there.
If I need to override an accessor, can I still access that iVar that will be defined at runtime or do I have to define an actual iVar that the runtime will then use?
I haven't tested this, but I believe you're allowed to access the named ivar in code, since it actually does have to be created. I'm not sure whether the compiler will complain, but I would guess that since it will let you synthesize the ivar without complaining, it is also smart enough to know about the synthesized ivar and let you refer to it by name.
Again, if I can access the synthesized iVar, why can't I continue to do this for the init* and dealloc methods?
You should be able to access the property and/or ivar anytime after the instance has been allocated.
There is another SO question with similar information, but it isn't quite a duplicate.
The bottom line, from the Objective-C 2.0 documentation, and quoted from Mark Bessey's answer is as follows:
There are differences in the behavior that depend on the runtime (see also “Runtime Differences”):
For the legacy runtimes, instance variables must already be declared in the #interface block. If an instance variable of the same name and compatible type as the property exists, it is used—otherwise, you get a compiler error.
For the modern runtimes, instance variables are synthesized as needed. If an instance variable of the same name already exists, it is used.
My understanding is as follows:
You should not use property accessors in init* and dealloc methods, for the same reasons that you should not use them in the legacy runtime: It leaves you open to potential errors if you later override the property methods, and end up doing something that shouldn't be done in init* or dealloc.
You should be able to both synthesize the ivar and override the property methods as follows:
#interface SomeClass
{
}
#property (assign) int someProperty;
#end
#implementation SomeClass
#synthesize someProperty; // this will synthesize the ivar
- (int)someProperty { NSLog(#"getter"); return someProperty; }
- (void)setSomeProperty:(int)newValue
{
NSLog(#"setter");
someProperty = newValue;
}
#end
Which leads me to think that you would be able to access the synthesized ivar in your init* and dealloc methods as well. The only gotcha I could think of is that the #synthesize line may have to come before the definitions of your init* and dealloc methods in the source file.
In the end, since having the ivars declared in the interface still works, that is still your safest bet.
I am running into the same problem. The way I am working around not being able to access the synthesized instance variables is the following:
public header
#interface MyObject:NSObject {
}
#property (retain) id instanceVar;
#property (retain) id customizedVar;
#end
private header / implementation
#interface MyObject()
#property (retain) id storedCustomizedVar;
#end
#implementation MyObject
#synthesize instanceVar, storedCustomizedVar;
#dynamic customizedVar;
- customizedVar {
if(!self.storedCustomizedVar) {
id newCustomizedVar;
//... do something
self.storedCustomizedVar= newCustomizedVar;
}
return self.storedCustomizedVar;
}
- (void) setCustomizedVar:aVar {
self.storedCustomizedVar=aVar;
}
#end
It's not that elegant, but at least it keeps my public header file clean.
If you use KVO you need to define customizedVar as dependent key of storedCustomizedVar.
I'm relatively new to Obj-C (but not to programming) and have also been confused by this topic.
The aspect that worries me is that it seems to be relatively easy to inadvertently use the iVar instead of the property. For example writing:
myProp = someObject;
instead of
self.myProp = someObject;
Admittedly this is "user" error, but it's still seems quite easy to do accidentally in some code, and for a retained or atomic property it could presumably lead to problems.
Ideally I'd prefer to be able to get the runtime to apply some pattern to the property name when generating any iVar. E.g. always prefix them with "_".
In practice at the moment I'm doing this manually - explicitly declaring my ivars, and deliberately giving them different names from the properties. I use an old-style 'm' prefix, so if my property is "myProp", my iVar will be "mMyProp". Then I use #synthesize myProp = mMyProp to associate the two.
This is a bit clumsy I admit, and a bit of extra typing, but it seems worth it to me to be able to disambiguate a little bit more clearly in the code. Of course I can still get it wrong and type mMyProp = someObject, but I'm hoping that the 'm' prefix will alert me to my error.
It would feel much nicer if I could just declare the property and let the compiler/runtime do the rest, but when I have lots of code my gut instinct tells me that I'll make mistakes that way if I still have to follow manual rules for init/dealloc.
Of course there are also plenty of other things I can also do wrong...
When is self needed for class properties? For example:
self.MyProperty = #"hi there";
vs
MyProperty = #"hi there";
MyProperty is an NSString set as (nonatomic, copy). Is there any difference in memory management for the above two?
What about when there is no property and the variable MyProperty is declared in the header file? Is a property needed if it is never referenced outside of the class? Does it make a difference to memory management?
Yes, there is a difference for both memory and performance.
MyProperty = #"hi there";
This is considered a direct assignment. There is practically no memory or performance impact. Of course, that's not to say it's best practice - that's a different question :)
#property(nonatomic, copy) NSString *MyProperty;
// ...
self.MyProperty = #"hi there";
This statement has a significant impact on memory and performance. This is essentially equivalent to:
-(void)setMyProperty(NSString *)newValue {
if (MyProperty != newValue) {
[MyProperty release];
MyProperty = [newValue copy];
}
}
The old value is released and the new value is copied into MyProperty. This is acceptable and especially typical when dealing with strings when the string your assigning is mutable (ie, it could change later).
If, as in your example, you're simply assigning a static string (#"hi there"), there is nothing wrong with directly assigning the string value; it's more efficient however the difference in performance is trivial.
You can declare a property with #property as retain, copy, or assign (default is assign). You can then generate "accessor" (getter/setter) methods by using #synthesize. Here is what the setter methods look like that are generated when you do so:
// #property(nonatomic, assign)
-(void)setMyProperty(NSString *)newValue {
MyProperty = newValue;
}
// #property(nonatomic, retain)
-(void)setMyProperty(NSString *)newValue {
if (property != newValue) {
[property release];
property = [newValue retain];
}
// #property(nonatomic, copy)
-(void)setMyProperty(NSString *)newValue {
if (property != newValue) {
[property release];
property = [newValue copy];
}
}
More information on ObjectiveC Declared Properties.
"You can use the #synthesize and #dynamic directives in #implementation blocks to trigger specific compiler actions. Note that neither is required for any given #property declaration.
Important: If you do not specify either #synthesize or #dynamic for a particular property, you must provide a getter and setter (or just a getter in the case of a readonly property) method implementation for that property."
In other words, if you declare a property but don't synthesize the property, you won't be able to use [self MyProperty] or self.MyProperty unless you define 'MyProperty' and 'setMyProperty' methods. If you don't declare a property then you simply have an instance variable.
Note: #dynamic doesn't generate the accessors. It's really used if you're dynamically (ie, magically) resolving accessor methods via loading code or dynamic method resolution.
The difference is that
self.MyProperty = #"hi there"
is dot-notation call that will call the generated accessor, which will handle the retain counts correctly (equivalent to [self setMyProperty:#"hi there"]), whereas
MyProperty = #"hi there"
is a direct assignment to your member variable, which doesn't release the old value, retain the new one, or do anything else your accessor does (e.g., if you have a custom setter that does extra work).
So yes, there is a big difference in memory management and in behavior in general between the two. The latter form is almost always wrong, unless you know specifically why you are doing it and that you are handling the retain counts correctly yourself.
If you use automatic Key-Value Observing (or any Cocoa technology that builds on it - like bindings, ...), it is also important use the setter. The observer would not receive any notification if you assign to the ivar.
If you bind "MyProperty" to a NSTextfield and you change your "MyProperty" ivar via code, the bound textfield would still display the old value as it did not receive any change notification.
To access a variable, there is often no need to use the dot notation. Thus, in code generated by the XCode templates, you will see things like:
[flipsideViewController viewWillAppear:YES];
There is no need to write self.flipsideViewController here, because the accessor method typically does nothing except handing you the variable.
So a good rule of thumb is to use dot notation when you are setting a variable (absolutely necessary unless you want to do your own retaining and releasing), but not when you're accessing it:
self.aString = #"Text text text";
NSLog (aString); // No need for self.aString here
NSString* tmpString = aString; // Here neither
When you're using non-object types, like int or float or many others, you can get away with not using the dot notation/setter method. In these cases, there is nothing to retain, so the setter method will do little apart from just assigning the value.
However, synthesized getters and setters do more than just retaining and releasing. As others have mentioned, they are also the engine that keeps the KVO system running. Thus, you should use the proper setters even on ints, floats and the rest.
What about the accessors then? In more advanced contexts, a class might respond to a request for a variable's value even when the variable doesn't exist. To quote the exalted Objective-C manual, classes might provide "method implementations directly or at runtime using other mechanisms [than simple accessor methods] such as dynamic loading of code or dynamic method resolution."
(One way of implementing this sort of on-the-fly response to messages is by overriding NSObject methods like methodSignatureForSelector: and forwardInvocation: .)
For this reason, using properly declared interfaces (whether synthesized or not) is always a good idea when you're working on something big. But it's completely ok to access ivars directly, as long as you set them using the proper API.
(Note: I'm not a Cocoa guru, so corrections are more than welcome.)
For the second part of the question, property definition is not needed, it is a help to us . The #synthesize directive on property generates accessor methods for properties so we don't have to do it manually, and because:
This code instructs the compiler to
generate, or synthesize, the accessor
methods. The compiler will generate
the accessor methods using
well-tested, fast algorithms that are
ready for multi-core and
multi-threaded environments, including
locking variables in setter methods.
Not only does using properties reduce
the amount of code that you have to
write, it replaces that code with the
best possible accessors for today's
modern multi-core systems. Later, if
you need to provide an alternative
implementation for a property
accessor, you can simply add the
appropriate code to your class.
http://developer.apple.com/leopard/overview/objectivec2.html
The nonatomic will avoid use of locking when accessing variables, if you don't specify anything then default is atomic. Locking is useful on multithreaded systems. The copy specifies what code should be generated for accessors, copy will copy the object, retain will retain new object and release old one, assign is good for simple variables like int to just plain assign values.
So when you define your property as you did above (nonatomic,copy) and then use self.MyProperty = #"Hey" you're actually calling generated accessor that will make a copy of the new variable as opposed to just assigning it. You can override accessor and add checking to it.
Because of the above I would say that defining property has benefits even when the variable is not used outside of the class.
I believe to access properties you should use self.MyProperty instead of just MyProperty but I can't point you to explanation why.
Might be something to do with the fact that compiler will generate from
self.MyProperty = #"Hey";
this:
[self setMyProperty: #"Hey"];
But I'm only speculating here.
Whether you call self.MyProperty or MyProperty it should not affect memory management (I would still prefer the first - self.MyProperty).
See Objective-C 2.0 Overview for some high level description from Apple.
As a supplement to the other answers, try to think of it this way:
self.MyProperty = #"hi there";
or
[self setMyProperty:#"hi there"];
(which are equivalent) both call a method, whereas
MyProperty = #"hi there";
Simply sets a variable.
This is an old question, though it used to be "When do I write [self setMyProperty:#"hi there"]?" (Note that self.MyProperty = #"hi there" is exactly equivalent to this.)
The answer I've always heard (and which makes good sense) is always use the accessor; never write MyProperty = #"hi there". There are several reasons:
Memory management is handled for you; you don't have to worry about proper retaining/releasing/copying.
It's easier to modify your code in the future; if at some point you realize that changing MyProperty needs to have a particular side effect, you can add to the setter method without finding every time you set MyProperty.
If you ever have problems with MyProperty, it's easy to add logging code to the setter (or even getter) to find out every time it's changed (or even accessed).
Summary: it's safest and most flexible to always use [self setMyProperty:#"hi there"] or self.MyProperty = #"hi there", and never use MyProperty = #"hi there".
Still not clear on when to use the accessors and when to do direct assignment on ivars ? I have seen lot of Apple examples which directly access the ivars. So using properties for all ivars seems pedantic overkill.
It seems only significant ivars which need to be around longer and are accessed outside tend to use the properties.
Would appreciate some Cocoa gurus to step in and clarify.