What does self mean in Objective-C? When and where should I use it?
Is it similar to this in Java?
self refers to the instance of the current class that you are working in, and yes, it is analagous to this in Java.
You use it if you want to perform an operation on the current instance of that class. For example, if you are writing an instance method on a class, and you want to call a method on that same instance to do something or retrieve some data, you would use self:
int value = [self returnSomeInteger];
This is also often used for accessor methods on an instance (i.e. setters and getters) especially with setter methods, if they implement extra functionality rather than just setting the value of an instance variable, so that you do not have to repeat that code over and over when you want to set the value of that variable, for example:
[self setSomeVariable:newValue];
One of the most common uses of self is during initialization of a class. Sample code might look like:
- (id)init
{
self = [super init];
if(self!=nil) {
//Do stuff, such as initializing instance variables
}
return self;
}
This invokes the superclass's (via super) initializer, which is how chained initialization occurs up the class hierarchy. The returned value is then set to self, however, because the superclass's initializer could return a different object than the superclass.
self is an implied argument to all Obj-C methods that contains a pointer to the current object in instance methods, and a pointer to the current class in class methods.
Another implied argument is _cmd, which is the selector that was sent to the method.
Please be aware that you only get self and _cmd in Obj-C methods. If you declare a C(++) method, for instance as a callback from some C library, you won't get self or cmd.
For more information, see the Using Hidden Arguments section of the Objective-C Runtime Programming guide.
Yes, it's exactly the same as "this" in Java - it points to the "current" object.
Two important notes:
The class itself, e.g. UIView (I'm NOT talking about a UIView object) is itself an object, and there is a self associated with it. So for example, you can reference self in a class method like this:
// This works
+(void) showYourself { [self performSelector: #selector(makeTheMostOfYourself)]; }
// Class method!
+(void) makeTheMostOfYourself { }
Note that the compiler does NOT raise any warnings or errors, even if the self you mean to reference is an object and not a class. It is VERY easy to cause crashes this way, for example:
// This will crash!
+(void) showYourself { [self performSelector: #selector(makeTheMostOfYourself)]; }
// Object method!
-(void) makeTheMostOfYourself { }
// This will crash too!
-(void) showYourself2 { [self performSelector: #selector(makeTheMostOfYourself2)]; }
// Class method!
+(void) makeTheMostOfYourself2 { }
Sadly, this makes class methods a bit harder to use, which is unfortunate because they are a valuable tool for encapsulation through information hiding. Just be careful.
Wow, that many half-correct answers and misleading hints. This let me answer the Q even there is a accepted answer for years:
First of all: It is really hard to compare a concept of messaging/calling in the context of an early binding, static typing language as Java with a late binding, dynamically typing languages as Objective-C. At one point this will break. I would say: No, this is not similiar, since the typing and dispatching concepts of both language are fundamental different so nothing can be similar to the other one. However, …
Then we should differ between the "two sides" of self.
A. Using self
When you use it in a message, it is simply an object reference as any other:
[self doSomething];
[anotherObject doSomething];
Technically both lines works identically (accept of having a different receiver, of course). This especially means, that the first line does not lead to an execution of a method inside the class of self, because self does not necessarily refer to "that class". As every message inside Objective-C (single exception: messages to super)this can lead to the execution of a method in a subclass:
#interface A : NSObject
- (void)doSomething;
- (void)doAnotherThing;
#end
#implementation
- (void)doSomething
{
[self doAntoherThing];
}
- (void)doAnotherThing
{
NSLog( #"A" );
}
#interface B : A
- (void)doSomething; // Not necessary, simply as a marker
#end
#implementation B
- (void)doAnotherThing
{
NSLog( #"B" );
}
In a code like this
B *b = [B new;]
[b doSomething];
The line
[self doAnotherThing];
in class A will lead to the execution of -doAnotherThing (B), because messages to self are late bound as every other message. The result on the console will b "B", not "A". Using self as a receiver you should not think of a single special rule. There is completely none.
(And the above example is a very good example for using self in class methods, because the same situation can occur on class methods. Using the class itself breaks polymorphism, what is one of the worst idea at all in OOP. DO use self in class methods, too.)
B. Getting self
What is self pointing to? It points to the object to whom the message is sent that caused the execution of the current method.
Having …
…[someObject doSomething]… // some object is a reference to an instance object
… as a message, a method is called, in the most simple case …
- (void)doSomething
{ … }
In such a case, self can point to an instance of the class, the method belongs to. And it can point to an instance of a subclass, the method belongs to, too. You don't know. (And this information is preserved using self to send a message as explained above.)
If the message is sent to a class object, self points to the class object, that was the receiver of the message. This is completely analogous. Therefore it is possible that self points to a subclass object:
#interface A : NSObject
+ (void)doSomething;
+ (void)doAnotherThing;
#end
#implementation
+ (void)doSomething
{
[self doAntoherThing];
}
+ (void)doAnotherThing
{
NSLog( #"A" );
}
#interface B : A
- (void)doSomething; // Not necessary, simply as a marker
#end
#implementation B
+ (void)doAnotherThing
{
NSLog( #"B" );
}
Having this classes
…[A doSomething]…
self inside -doSomething (A) points to the class object of B. Therefore [self doAnotherThing] of B(!) is executed. This is clearly different from
+ (void)doSomething
{
[A doAntoherThing];
}
The latter version causes relevant harm to the principles of OOP.
As a side note it is possible that self inside a class method of a root class points to an instance object of the root class or any subclass. You have to keep this in mind, when writing categories on NSObject.
self is an object pointer to the current instances dispatch table. It is an implicit first argument to every member function of an object, and is assigned when that function is called.
In functions like init, you need to be careful that when you call the super class init you reassign self to be the return value as the super class init may redefine what self points to.
super is similar to self except it points to the superclass dispatch table.
Hi I've made my own custom class:
with this init method:
-(id)init:(NSString*) strFilename withImageData:(NSData*) objImageData withUploadTaskDelegate:(id<NSURLSessionTaskDelegate>) objUploadTaskDelegate
{
filename = strFilename;
uploadTaskDelegate = objUploadTaskDelegate;
imageData = objImageData;
return self;
}
I've made sure that the .h file also have the definition of this init file:
-(id)init:(NSString*) strFilename withImageData:(NSData*) objImageData withUploadTaskDelegate:(id<NSURLSessionTaskDelegate>) objUploadTaskDelegate;
however when I try to use it like this:
UploadQueueCellData *objTest =[UploadQueueCellData init:strImageName withImageData:objData withUploadTaskDelegate:objUploadDelegate];
it gives this error:
No known class method for selector
'init:withImageData:withUploadTaskDelegate:'
this is my 3th day trying to develop in objective c code so I'm sure I'm missing something but I do not seem to be able to spot what I'm doing wrong ?
Your init method is an instance method, not the class method, check below example:
For instance method, you've to create an instance before using it.
#interface MyClass : NSObject
+ (void)aClassMethod;
- (void)anInstanceMethod;
#end
To call a method of MyClass, for instance method:
[[MyClass alloc] anInstanceMethod]
For Class method:
[MyClass aClassMethod]
In your case do this:
[[UploadQueueCellData alloc] init:strImageName withImageData:objData withUploadTaskDelegate:objUploadDelegate]
Class Method
A class method is applied to the class as a whole. If you check the alloc method, that's a class method denoted by the + before the method declaration. It's a class method because it is applied to the class to make a specific instance of that class.
Instance Method
You use an instance method to modify a specific instance of a class that is unique to that instance, rather than to the class as a whole. init for example (denoted with a - before the method declaration), is an instance method because you are normally modifying the properties of that class after it has been created with alloc.
When you have a signature like this:
- (UIView *)fooView;
You can return any subclass of UIView * (e.g UIScrollView)
And when you have:
- (Class)anyClass;
You can return any class (not an instance, the class itself) but is there a way to only allow classes of a certain class or subclass? E.g in psuedo code:
- ([UIView class])bazClass;
So here it should only be able to return a class UIView of any of its subclasses.
As specified by other users, you can't.
If your goal is to instruct other programmers about what to return from a method in your code (overriden or delegate method), you can:
write in the comment (of course...)
create a typedef like this
.
typedef Class ClassOfKindUIView;
-(ClassOfKindUIView)class
{
return [super class];
}
This won't block anything, but it can be a "talking method", an escamotage, to make the programmers stop and think "what is this??", then cmd-click and read the docs :-)
What you're looking for is a sort of type bound on the return type.
Unfortunately this is not possible in Objective-C, so I'm afraid you're out of luck.
So in a regular Class, you can override the setter method for a class property:
-(void)setSortBy:(NSString *)sortBy {
// Do other stuff
_sortBy = sortBy;
}
using the _ prevents an infinite loop of the method calling its self.
I am trying to do something similar with a NSManagedObject class, but it does not give the option to use the underscore (_):
-(void)setHasNewData:(NSNumber *)hasNewData {
// update self.modifiyDate
_hasNewData = hasNewData;
}
Gives me an error and suggests I replace _hasNewData to hasNewData.
Is this how it should be done or will it give me an infinite loop?
I want it to update the NSManagedObject's property modifyDate anytime I set hasNewData.
Your first example for a "regular class" works if _sortBy is the instance variable
backing up the sortBy property (e.g. the default synthesized instance variable for
that property).
But Core Data properties are not backed up by instance variables.
When overriding Core Data accessors, you have to use the "primitive accessors",
and also trigger the Key-Value Observing notifications:
-(void)setHasNewData:(NSNumber *)hasNewData {
[self willChangeValueForKey:#"hasNewData"];
[self setPrimitiveValue:hasNewData forKey:#"hasNewData"];
[self didChangeValueForKey:#"hasNewData"];
// do other things, e.g.
self.modifyDate = ...;
}
More examples can be found in the "Core Data Programming Guide".
I have an IBAction instance method that is connected to a slider, and displays the slider value in a Text Field that has the name datacellR1. A copy of the code is below, followed by a question. Both methods are in the #implementation section of the View2Controller.m file.
- (IBAction)slider1Change:(id)sender
{
float pctVal1 = [slider1 floatValue]; // this works
[datacellR1 setFloatValue:pctVal1];
[View2Controller CalculateUpdatedTotal ]; // This method needs to work with the datacellR1 contents, but I can’t access it.
}
-(void)CalculateUpdatedTotal
{
// -------- do some work with datacellR1 ----
// This function fails with an error
float newValue = [datacellR1 floatValue];
//some other code goes here
}
The error in slider1Change is that CalculateUpdatedTotal method is not found. If I change CalculateUpdatedTotal from an instance method to a class method, the error is that the Instance variable datacellR1 accessed in a class method.
Any suggestions on how I can make this work?
CalculateUpdatedTotal, as written, is also an instance method. Therefore, to invoke it, you should pass the message to self, not the class (View2Controller):
[self CalculateUpdatedTotal];
By the way, it's conventional to begin method names in Objective-C with a lower-case letter.