I have a class with a property sampleNames. It is of type NSSet. The instance variable I plan to use will be NSMutableSet.
the only reason I have for using NSMutableSet is convenience for myself. But I recall being 'told' that Mutable class should be used as sparingly as possible for some reason I cannot remember. I understand how to use both ways, so this si not so much a question about that.
What are the practical considerations when deciding whether or not to use a mutable class internally?
Please note, I am not at all interested in changing my PROPERTY to NSMutableSet. But the fact that I am using a different type internally made me think about why and I have no real justification other than convenience, Which I have found to be a bad justification by itself. Maybe it is the only reason, but i am not actively aware of what I am trading for the convenience.)
I think you are confused.
You use NSMutableSet when you need to change the contents of the set. You can only do that if it is mutable.
If you use NSSet, then you can't change the contents.
So ... your property declaration and your instance variable should be the SAME class. Either use NSSet if you don't need to change the contents or NSMutableSet if you do.
Having a different class for the property and the instance variable is a recipe for disaster ... it's not convenient and it is confusing = BAD ;-)
Related
The recommended practice is to use property, including private ones through class extension instead of ivar (except in init and dealloc) in the post ARC environment.
Aside from it being a recommended practice, what are the main drawbacks in someone using ivar instead of property? I am trying to convince some folks to make the switch but some have argued ivar works just as well and faster. So I would like to collect good solid arguments rather than giving soft statements such as "it's better, more consistent, etc."
There is no right answer to your question, just opinions. So you'll get varying answers, here's one to add to your collection :-)
Using private properties is is not recommended practice, it is largely a fad. :-)
A public property is part of the encapsulation of the class - how a property (or method) is implemented is not relevant to the user, only the behaviour.
A class does not need to hide how it is implemented from itself!
So the only use cases for private properties is where they provide some behaviour in a convenient way to the implementation of the class, not to hide that behaviour.
A private property with the copy attribute may be convenient if the class is, say, obtaining mutable strings from another class and needs to preserve their current values.
If the class wishes to lazily construct a value if it is needed but keep it after that time then a property can handle that conveniently. Of course a method or function can as well as a property is after all just a method call.
To make the choice think convenience/code design rather than encapsulation as you do for public properties. And most of the time you'll probably just use instance variables, just as you just use local variables.
HTH
There is not much difference in terms of performance. In reality, properties are instance variables with the accessors generated. So the reason why you want to do properties is because the code to generate the KVO notifications and the setter/getter methods are generated to you. So you have less time doing repetitive code on all your classes.
There are a few cases where using a private property is better or required over using an instance variable:
KVO - Since KVO requires getter/setter methods to do the work, you need a property (technically just the methods). Using KVO on a private property probably isn't too common.
Lazy loading or other "business logic" around the value. Using a property with custom setter/getter methods allows you to apply lazy loading and/or other logic/validation around the value.
Access to the value inside a block using a weak reference.
The last point is best covered with an example. As many people know, under certain conditions you can create a reference cycle in a block and this can be broken using a weak reference to self. The problem is that you can't access an ivar using the weak reference so you need a property.
__weak typeof(self) weakSelf = self;
[self.something someReferenceCycleBlock:^{
weakSelf->_someIvar = ... // this gives an error
weakSelf.someProperty = ... // this is fine
}];
Basically, use an ivar if none of these points will ever apply. Use private properties if any of these may apply over the lifetime of the class.
I was wondering what exactly are the differences between using the (get) accessor for reading the value of property and directly using the iVar?
Say I have a class which declares a property:
#interface Foo : NSObject
#property (strong) NSString *someString;
#end
And in the implementation I'm using it. Are there any differences between the following two lines:
someLabel.text = self.someString;
someLabel.text = _someString;
For set accessors it's clear. Afaik for strong properties the accessor takes care of retain and release (an interesting 'side question' would be if ARC changes that, i.e. does setting the iVar directly [assuming it's not an __weak iVar] also retain and release correctly using ARC), also KVO requires the use of accessors to work properly etc. But what about getters?
And if there's no difference, is there one way considered best practice?
Thx
As you know, calling self.someString is really [self someString]. If you chose to create a property then you should use the property. There may be other semantics added to the property. Perhaps the property is lazy loaded. Perhaps the property doesn't use an ivar. Perhaps there is some other needed side effect to calling the property's getter. Maybe there isn't now but maybe this changes in the future. Calling the property now makes your code a little more future proof.
If you have an ivar and a property, use the property unless you have explicit reason to use the ivar instead. There may be a case where you don't want any of the extra semantics or side effect of the property to be performed. So in such a case, using the ivar directly is better.
But ultimately, it's your code, your property, your ivar. You know why you added a property. You know any potential benefits of that property, if any.
I think this what you are looking for. Why use getters and setters?
There are actually many good reasons to consider using accessors rather than directly exposing fields of a class - beyond just the argument of encapsulation and making future changes easier.
Here are the some of the reasons I am aware of:
Encapsulation of behavior associated with getting or setting the
property - this allows additional functionality (like validation) to
be added more easily later.
Hiding the internal representation of the
property while exposing a property using an alternative
representation.
Insulating your public interface from change -
allowing the public interface to remain constant while the
implementation changes without effecting existing consumers.
Controlling the lifetime and memory management (disposal) semantics
of the property - particularly important in non-managed memory
environments (like C++ or Objective-C).
Providing a debugging
interception point for when a property changes at runtime - debugging
when and where a property changed to a particular value can be quite
difficult without this in some languages.
Improved interoperability
with libraries that are designed to operate against property
getter/setters - Mocking, Serialization, and WPF come to mind.
Allowing inheritors to change the semantics of how the property
behaves and is exposed by overriding the getter/setter methods.
Allowing the getter/setter to be passed around as lambda expressions
rather than values.
Getters and setters can allow different access
levels - for example the get may be public, but the set could be
protected.
I am not a very experienced person to answer this question, even though I am trying to give my views and my experience by seeing source code which is around 10yrs older.
In earlier codes they were creating ivars and property/synthesise. Nowadays only property/synthesise is used.One benefit I see is of less code and no confusion.
Confusion!!! Yes, if ivars and its property are of different name, it does create a confusion to other person or even to you if you are reading your own code after a while. So use one name for ivar and property.
By using property KVO/KVB/KVC are handled automatically, thats for sure.
#property/#synthesise sets your ivar to 0/nil etc.
Also helpful if your subclass contains same ivar.
For mutable objects Dont make properties.
I have a situation where I'm keeping references to ivars which need to be persistent. In one object, I have an array of pointers to ivars in another object, which are used over the entire lifetime of the program. In other words, I'm not just passing a reference to retrieve a value -- I'm keeping the pointers around.
Is this a valid? Is it possible that the ivars might move? Are there cases where objects instantiated objects are moved around at runtime unbeknownst to the program? Or, do objects stay exactly where they are created. If the later is the case, is there any reason not to use references the way I am?
I'm using ARC.
Note: This probably wasn't a good way to design this to begin with, but... it's all done and working 99%! (except for a nasty crash which reboots the entire phone... )
Objects and their instance variables don't move once created. However, you also need to keep a strong reference to the object that holds the ivar. Otherwise, the object might be deallocated, leaving you with a dangling pointer.
Note that it is generally a very bad idea to have pointers to another object's insntance variables.
While there's no technical problem with accessing the ivars from outside (as rob stated) there's still the architectural design to consider: The approach you've taken breaks encapsulation. Additionally it is very uncommon for Objective-C.
So regarding maintainability of your code I would recommend to refactor the code. In Objective-C there's no friend declaration as in C++, so it's unusual to access ivars from outside the declaring class.
Let's say an object of class A wants to access the ivars of an object of class B persistently (in your example).
What you normally do is create a property (with the strong annotation, like #property (strong) ClassB *myBVar) in class A to reference an object of class B.
If you want to set or read B's properties you use the dot notation or call the getter/setter methods:
myBVar.name = #"Jim";
NSLog(#"Name:%#",myBVar.name);
[myBVar setName:#"Jim"];
NSLog(#"Name:%#",[myBVar name]);
You never call a ivar directly as it's implementation might change.
In Objective-C, we can add #property and #synthesize to create a property -- like an instance variable with getter and setter which are public to the users of this class.
In this case, isn't it just the same as declaring an instance variable and making it public? Then there won't be the overhead of calling the getter and setter as methods. There might be a chance that we might put in validation for the setter, such as limiting a number to be between 0 and 100, but other than that, won't a public instance variable just achieve the same thing, and faster?
Even if you're only using the accessors generated by #synthesize, they get you several benefits:
Memory management: generated setters retain the new value for a (retain) property. If you try to access an object ivar directly from outside the class, you don't know whether the class might retain it. (This is less of an issue under ARC, but still important.)
Threadsafe access: generated accessors are atomic by default, so you don't have to worry about race conditions accessing the property from multiple threads.
Key-Value Coding & Observation: KVC provides convenient access to your properties in various scenarios. You can use KVC when setting up predicates (say, for filtering a collection of your objects), or use key paths for getting at properties in collections (say, a dictionary containing objects of your class). KVO lets other parts of your program automatically respond to changes in a property's value -- this is used a lot with Cocoa Bindings on the Mac, where you can have a control bound to the value of a property, and also used in Core Data on both platforms.
In addition to all this, properties provide encapsulation. Other objects (clients) using an instance of your class don't have to know whether you're using the generated accessors -- you can create your own accessors that do other useful stuff without client code needing changes. At some point, you may decide your class needs to react to an externally made change to one of its ivars: if you're using accessors already, you only need to change them, rather than make your clients start using them. Or Apple can improve the generated accessors with better performance or new features in a future OS version, and neither the rest of your class' code nor its clients need changes.
Overhead Is Not a Real Issue
To answer your last question, yes there will be overhead—but the overhead of pushing one more frame and popping it off the stack is negligible, especially considering the power of modern processors. If you are that concerned with performance you should profile your application and decide where actual problems are—I guarantee you you'll find better places to optimize than removing a few accessors.
It's Good Design
Encapsulating your private members and protecting them with accessors and mutators is simply a fundamental principle of good software design: it makes your software easier to maintain, debug, and extend. You might ask the same question about any other language: for example why not just make all fields public in your Java classes? (except for a language like Ruby, I suppose, which make it impossible to expose instance variables). The bottom line is that certain software design practices are in place because as your software grows larger and larger, you will be saving yourself from a veritable hell.
Lazy Loading
Validation in setters is one possibility, but there's more you can do than that. You can override your getters to implement lazy loading. For example, say you have a class that has to load some fields from a file or database. Traditionally this is done at initialization. However, it might be possible that not all fields will actually be used by whoever is instantiating the object, so instead you wait to initialize those members until it's requested via the getter. This cleans up initialization and can be a more efficient use of processing time.
Helps Avoid Retain Cycles in ARC
Finally, properties make it easier to avoid retain loops with blocks under ARC. The problem with ivars is that when you access them, you are implicitly referencing self. So, when you say:
_foo = 7;
what you're really saying is
self->_foo = 7;
So say you have the following:
[self doSomethingWithABlock:^{
_foo = 7;
}];
You've now got yourself a retain cycle. What you need is a weak pointer.
__block __weak id weakSelf = self;
[self doSomethingWithABlock:^{
weakSelf->_foo = 7;
}];
Now, obviously this is still a problem with setters and getters, however you are less likely to forget to use weakSelf since you have to explicity call self.property, whereas ivars are referenced by self implicitly. The static analayzer will help you pick this problem up if you're using properties.
#property is a published fact. It tells other classes that they can get, and maybe set, a property of the class. Properties are not variables, they are literally what the word says. For example, count is a property of an NSArray. Is it necessarily an instance variable? No. And there's no reason why you should care whether it is.
#synthesize creates a default getter, setter and instance variable unless you've defined any of those things yourself. It's an implementation specific. It's how your class chooses to satisfy its contractual obligation to provide the property. It's just one way of providing a property, and you can change your implementation at any time without telling anyone else about it.
So why not expose instance variables instead of providing getters and setters? Because that binds your hands on the implementation of the class. It makes other acts rely on the specific way it has been coded rather than merely the interface you've chosen to publish for it. That quickly creates fragile and inter-dependent code that will break. It's anathema to object-oriented programming.
Because one would normally be interested in encapsulation and hiding data and implementations. It is easier to maintain; You have to change one implementation, rather than all. Implementation details are hidden from the client. Also, the client shouldn't have to think about whether the class is a derived class.
You are correct... for a few very limited cases. Properties are horrible in terms of CPU cycle performance when they are used in the inner loops of pixel, image and real-time audio DSP (etc.) code. For less frequent uses, they bring a lot of benefits in terms of readable maintainable reusable code.
#property and #synthesize is set are getting getter and setter methods
other usage is you can use the that variable in other classes also
if you want to use the variable as instance variable and your custom getter and setter methods you can do but some times when you set the value for variable and while retrieving value of variable sometimes will become zombie which may cause crash of your app.
so the property will tell operating system not to release object till you deallocate your object of class,
hope it helps
I have static global NSNumber value and i need to observe it. If it was member of some object, i would have no problems whatsoever. But what do i do with global scope? I guess i could use
[globalVar addObserver:self forKeyPath:**integerValue** options:... ]
but that seems ugly because i might as well use "intValue" KeyPath and i need to observe NSNumber, not it's int part, even if it's the only part of it i''m using now. Making this particular variable part of some class doesn't seem like a "right" think to do. Thanks!
Easy answer: you can't. Observing is a software mechanism which fundamentally involves method calls, doing a store (i.e. a machine instruction) into a global variable provides no hook to hang the mechanism on.
The best option is to re-think your design. Think of storing the value in a singleton class and accessing/observing it there.
Hard answer: write your own mutable version of NSNumber (an instance of which is immutable) and have this class implement the key-value observing protocol (this class might just be a wrapper with an NSNumber instance variable). Now store and instance of this class in your global variable and add any observers you like to it.
The usual way to do this kind of thing is by making it a value reachable from some globally-available object, such as NSApp, or its delegate.
CRD is right, "In KVO only the property is observed, not the value" Typically dictionaries or custom objects are KVO but not the leaves (values) themselves (numbers, strings). Facing a similar problem, I finally extended the NSNumber class making it KVO compliant.
Now if you are looking for different approaches to achieve notifications in your app, I would strongly suggest to read this article.