I wrote a setter and getter method following Apple's conventions and noticed that despite having no variable I can still access the setter and getter using the dot syntax. Is this normal behavior? What enables this feature?
Example:
// Header definition. Keep in mind there is no class variable or #property for height.
- (void)setHeight:(float)height;
- (float)height;
// else using the dot syntax.
object.height = 10.0f;
A property-access expression is equivalent to a message expression:
[object setTexture:tex];
A property declaration is equivalent to one (readonly) or two (readwrite/default) instance-method declarations. Keywords like retain tell the compiler how to implement the method if you tell it to do so (#synthesize).
However, you can skip the property declaration and declare the methods directly, as shown in your question. You can't synthesize their implementations, since you need a property declaration for that (otherwise, it wouldn't know what memory-management policy to use: assign, retain, or copy), but you can always implement the methods yourself.
Then, even though you declared and implemented the methods yourself, since property-access syntax and message syntax are equivalent to each other, you can use the methods whichever way you want: With a message expression, or with a property-access expression.
Some would consider it bad form, though, to use property access expressions on anything but a formal #property (e.g., myString.length or myArray.count or myView.frame). It definitely is bad form to use a property-access expression to send a message that doesn't access any kind of property; foo.retain.autorelease, for example, is bad and wrong: It reeks of trying to pretend you're programming some other language than Objective-C.
Incidentally, a property and a variable are unrelated. A #property will ordinarily be backed by an instance variable, but this is not required: You could store the property's value inside another object, or convert it to and from some other format, or both. Likewise, accessing a property (which is an accessor message) and accessing an instance variable (which is just accessing a variable, nothing more) are very different.
Related
I'm wondering what the difference between a class's public global variable and a class's property is (Objective-C primarily iOS programming). Only thing I notice is that you have to use pointer notation -> to access a class's global variable rather than a dot.
I've read that changing code from using globals to using properties can be a program breaking change. Is that true and if so, why?
Thanks!
Edit:
Block.h
Public Global Variable (I think?) [Edit: I now understand this is an Instance Variable, thanks]
#interface Block : GameObject {
#public
int type;
SKEmitterNode *particles;}
Property
#property (nonatomic) CGFloat x;
No, this is not a "global variable".
It is called an instance variable.
A property often (but not necessarily) has an associated instance variable, but the modern compilers hide that from you.
The big difference between using an instance variable is, that a property is always accessed through its accessors (in your case setX:(CGFLoat)x?and -(CGFloat)x`.
If you wanted, you could overwrite these accessors and do special handling, say, whenever the variable is accessed.
It is always possible to bypass the accessors by using the instance variable directly. In a case of an auto-synthesized iVar, this would be _x.
Note that the -> is not necessary in either case
Even a class property is backed by a class variable even though it is not global.
But with a property one has additional gatekeepers guarding access to the variable:
You can make the property readonly.
Finetune memory semantics (copy, assign, etc).
By using KVO it is easy to let changes propagate automatically.
I'm just wondering about semantics. When is something truly a "property" of an object? I noticed in a lot of Apple's APIs, they explicitly define getters and setters instead of using properties (e.g. URL and setURL on NSURLRequest/NSMutableURLRequest; surely the URL seems like a "property" of an URL request, right?) I'm wondering if there's some subtle thing that I'm missing or if Apple just doesn't like properties all that much. =P
UPDATE: As of iOS 8, Apple has converted most (if not all) of their non-property getters and setters to properties. (Probably done so that Swift compatibility would be easier.)
or if Apple just doesn't like properties all that much.
The real cause is that most of the Foundation framework (let's not forget you're talking about NS* classes) is old as dirt - they have been around there since the NeXT times... At that time, the Objective-C language didn't sport the #property keyword - to emulate properties, it was necessary for the programmers to declare and implement getter and setter methods manually, this applied to Apple's code as well.
Now the Foundation framework is so fundamental that it hasn't changed a lot. It hasn't been radically rewritten and as far as I'm concerned, programmers who wrote it didn't bother rewriting all the code using the new syntax. You can see that recently added classes do in fact feature declared properties instead of getters and setters, but that's not true for older classes.
Anyway, properties declared manually and those declared using #property and #synthesize are completely equivalent. That said, there's a very minor difference when accessing them, but that doesn't belong to the declaration thingy: if you write
someObject.someProperty
in your code, the someObject must have a complete and concrete type, so if a property named someProperty is nonexistent, you'll get a compiler error. In contrast,
[someObject someProperty]
and
[someObject setSomeProperty:]
allow you the method call even if it's undeclared.
Edit:
I ask what the semantic difference between them is
So by "semantic difference", you meant "when it should be used" rather than "does it run differently". I see. Well... Conceptually, properties represent state. A property is a particular characteristic of an object that may change over time. It's just an unrelated fact that properties are accessed using accessor methods in Objecive-C.
If you write a method that acts on the obejct (other than setting a property, of course), there's a fair chance you should be declaring and calling it as a method. If you access (read or write) an attribute of an object, that better fits the task of a property.
Having a getter and setter lets you use messages to access the item
[myObject someProperty] or [myObject setSomeProperty: someNewValue]
Making something a #property gives you the additionally ability to use dot notation to call the getter and setter. This is because #property chooses method names that make the class key-value-coding compliant for the particular value.
myObject.someProperty or myObject.someProperty = someNewValue
While it is possible to do this manually, it is considered best-practice to use #property when you want to use the dot notation. Over time, the behind-the-scenes bahaviours of #property and #synthesize have changed quite a bit particularly in regard to auto-creating storage for the associated pointer. Using #property makes it easier to keep up with Apple's changes in convention with little or no change in your code.
Additionaly, using #property makes your code much easier to read.
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 have a very basic question regarding properties in Objective-C.
I can only access object's properties via dot notation (Obj.MyProp) if I #synthesize myProp. Is that correct?
Would it be true to say that if I use my own setter method, I will no longer be able to refer to property in dot notation?
Basically I am looking for C# type of functionality where I can write my own custom getter/setter and yet provide an additional code which I need to execute when the property is set.
#property creates automatic message declarations, just like writing
(int)thing;
(void)setThing:(int)value;
#synthesize automatically creates the implementations, i.e.
(int)thing {
return thing;
}
(void)setThing:(int)value {
thing = value;
}
If you give a definition yourself, it overrides the #synthesized version. So as long as you name a method correctly, it will work, with or without #synthesize in there.
Dot notation works with either synthesized or custom method implementations.
This is not correct. You can still use dot-notation even if you write custom getters or setters provided of course that your getters and setters maintain the correct method naming for the property.
From the docs:
#synthesize
You use the #synthesize keyword to tell the compiler that it should
synthesize the setter and/or getter
methods for the property if you do not
supply them within the #implementation
block.
It only synthesizes if you haven't already written them. If you've written them, they don't get synthesized.
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...