I'm just starting to learn Objective-C, one thing I'm trying to learn is good Property use. I'm currently trying to create some properties with custom setters. This is how far I've gotten:
#interface MyClass : NSObject
#property (nonatomic, assign) int myNumber;
#end
#implementation MyClass
#dynamic myNumber;
- (int)myNumber {
return ???;
}
- (void)setMyNumber:newNumber {
myNumber = newNumber;
// custom stuff here
}
I really just want to implement a custom setter, I'm fine with the getter being default. However, how do I access the variable directly? If I put "return self.myNumber", won't that just call the getter method and infinite loop?
Property access functions are only called when using the x.p notation. You can access the instance variable backing the property with just p (in Objective C, all members have the class instance variables in scope). You can, if you really want, also access via the pointer deference notation ->. So, any of these two:
return p;
return self->p;
However, you needn't use #dynamic here. #synthesize is smart, and will only create defaults if you've not provided them. So feel free to just
#synthesize p;
Which will create the getter, but not the setter in this case.
- (int)myNumber {
return myNumber;
}
Related
This question already has answers here:
Error accessing generated ivars when I override setters and getters in Modern Objective-C
(3 answers)
Closed 5 years ago.
I'm learning the Swift programing language and during this I sometimes get in touch with the old Objective-C programming language and its code.
I'm an absolutely beginner and therefore I have some question for better understanding the Setter and Getter.
So, I know that I can create an instance variable through curly braces in the .h file but normally I use properties. These properties are backed by an instance variable and offer automatically a Getter and Setter Method.
Example:
Vehicle .h file:
#interface Vehicle : NSObject
#property int myProperty;
#end
Because I created this property I don't have to declare a Getter and Setter method in the vehicle.m file because they are automatically created by the compiler. So I can create a vehicle-object, set and get the value.
Example
main.m
Vehicle *myvehicle = [[vehicle alloc] init];
[myvehicle myProperty] // myvehicle.myProperty
[myvehicle setMyProperty : 10] // myvehicle.myProperty = 10;
Now I read that it is possible to override the automatically created Getter and Setter method of my created property "myProperty". When declaring my own version of the Getter and Setter I have to declare two methods in the vehicle.h and vehicle.m file. In the vehicle.m file I don't call the object by using the self keyword but by using it's automatically created instance variable (_myProperty). Is it right?
I tried it but alway get an error and I don't know why and what is the point.
Example
Vehicle .h file:
#interface Vehicle : NSObject
#property int myProperty;
-(int) myProperty; //my new Getter method
-(void) setMyProperty: (int)updatedMyProperty; //My new Setter method
#end
vehicle .m file:
#implementation Vehicle
-(int) myProperty {
if (! _myProperty) {
_myProperty = NO;
}
return _myProperty;
}
-(void) setMyProperty: (int)updatedMyProperty {
if (_myProperty == updatedMyProperty) return;
_myProperty = updatedMyProperty;
}
#end
I always get the error "Use of undeclared identifier" and I don't know why. If I understand right I don't have to declare the ivar or its name using #synthesize because the compiler automatically creates the ivar called _myProperty for me. I just have to use #synthesize when I want to change the ivar's name.
I'm not sure why I get stuck and what the point is. Could you explain it?
Thanks in advance!
If you implement all of the accessor methods, the compiler will no longer automatically synthesize the ivar for you. In this case, you have to explicitly do so yourself. E.g.
#synthesize myProperty = _myProperty;
This is only necessary when you have manually implemented all of the accessor methods. The reason is that the compiler is smart enough to know that if you're taking over the accessor methods, you may well not need the ivar, namely that you might be doing something radically different, e.g. computing values from some other property, setting/getting values from some different store, etc. You may want the compiler to synthesize the ivar (in which case you add the above #synthesize statement), but it's equally likely that you've implemented the accessor methods because no backing ivar is needed (in which case you'd omit the above #synthesize statement).
Anyway, staying with your simple example, you get something like:
#interface Vehicle : NSObject
#property (nonatomic) int myProperty; // if you don't write atomic accessor methods, you really should be explicit that this is nonatomic
// as an aside, even if you implement accessor methods, you don't have to declare them here
//
// -(int) myProperty; //my new Getter method
// -(void) setMyProperty: (int)updatedMyProperty; //My new Setter method
#end
And
#implementation Vehicle
// since you implemented all of the accessor properties, you have to manually synthesize the ivar
#synthesize myProperty = _myProperty;
- (int) myProperty {
// do whatever you want here; note, the following doesn't make sense
//
// if (! _myProperty) {
// _myProperty = NO;
// }
return _myProperty;
}
- (void)setMyProperty:(int)updatedMyProperty {
if (_myProperty == updatedMyProperty) return;
_myProperty = updatedMyProperty;
}
#end
Clearly, there's no point in writing these particular accessor methods in the above example, because you're not offering any new functionality, so you wouldn't. You'd just avail yourself of the auto-synthesized accessor methods.
But in those cases that you really need to write your own accessor methods, then you have to explicitly tell the compiler whether you need it to synthesize the ivar for you, too, or not.
New to Objective-C, and i am basically from c++ background. I am learning now objective-c and would like to get confirmation of what i understood is write or wrong?. Kindly advise.
I have the following class:
#interface Test: NSObject
{
int instance1;
}
#property int instance1;
- (void) sayHello;
#end
Class 'Test' has a instance variable instance1. If the member function ie: sayHello wants to access the variable, it has to happen through getter/setter functions. So, There are two ways to get it :
User can define.
We can get the help from the compiler?. How?.
declare the same variable as a property, and synthesize it, the the compiler
gets the code of getter/setter for us for that particular variable.
So, Untimately, getter/setter is the only way to access the variable in the method implementation, ie. both self.instance1 = 100; and instance1 = 100 need getter/setter.
Having missed both 1. and 2., there is no way to access the instance1 variable.
Also, instance1 is a pubic variable can can be accessed outside of the class with object instance.
Test *t = [[ Test alloc] init];
t.instance1 = 200;
Questions:
Is there any way to make instance1 is "private", so that I can not access the instance
variable outside the class?
Is there anything wrong in my understanding?
If the member function ie: sayHello wants to access the variable, it has to happen through getter/setter functions.
It doesn't have to. You can access ivars directly, without using accessor methods:
- (void)sayHello {
instance1 = 123;
}
You can define private ivars by declaring them in the implementation file, not the header:
#implementation Test {
int privateVar;
}
// ... additional implementation, methods etc.
#end
Note, that since Xcode 4.4 you don't have to declare your ivars anymore. You simply declare a property. The ivar and the accessor methods will be synthessized automatically.
For more details, I recommend reading my answer to this question: Declaration of variables
ion SomeDelegate.h
#interface SomeDelegate : NSWindowController {
#private
int fLanguage;
int fDBID;
bool fEndEditingIsReturn;
#public
int fIsMyLastMSG;
}
#property int language;
In SomeDelegate.mm
#implementation SomeDelegate
#synthesize language=fLanguage;
In my example you get private and public variables, private variable fLanguage has a property for synthesize accessor methods.
I was wondering what the point of #property and #synthesise were. At the moment I use the following to declare something:
//Class.m
#import "Class.h"
CCNode *node;
#implementation
//init, etc..
But I have seen others use:
#property (nonatomic, etc..) CCNode* node;
#synthesise (nonatomic, etc..) node;
//I am not too sure on how this type of declaration works, please correct me on how it's done.
They both seem to work in the same way, what are the advantages of the #property and #synthesise way? Do they do different things, if so, what?
#property and #synthesize are two objective C keyword that allow you to easily create your properties and therefore avoid to write by hand getters and setters methods of the property.
The #property define the property itself, should be placed in the header file and can get some attributes (as for example : strong, nonatomic, retain assign, copy), the #synthesize should be placed into the implementation file and tell the compiler to generate the body of getter and setter method.
These two keyword are extremely useful when coupled with the right use of their attributes, because they take care of the generation of the property code and most of all they take care of the memory management of the property.
#property - create the declaration of your getter and setter.
#synthesize - provide the definition of getter and setter based upon the parameters which are passed inside property.
Check this out, there are a lot more details about the same present there - https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocProperties.html
on using #property the compiler will take care of declaring getter and setter methods based on readonly and readwrite
readonly -> getterMethod
readwrite -> both setter and getter method
on using #synthesize the compiler will take care of defining getter and setter methods
If you have an instance variable (ivar) in your class, you can't access it from other classes usually. So you have to make public accessor methods (getters and setters). They look something like this:
Setter:
- (void)setMyVariable:(SomeClass *)newValue {
if (newValue != myVariable) {
[myVariable release];
myVariable = [newValue retain];
}
}
Getter:
- (SomeClass *)myVariable {
return myVariable;
}
This was the way you had to do it before Objective-C 2.0. Now you can use #property and #synthesize to speed this up. It's basically just a shortcut.
In the header you use #property to define what kind of setters you want. Should the setter retain the passed value (like in my example) or copy or just assign?
And in the implementation you just write #synthesize to make the compiler include the automatically created getters and setters at that position. Usually at the top of your implementation.
My feeling is that all iVars should have an associated underscore synthesised property (using an _iVar prevents accidental direct access), and all access to the iVars, apart from init and dealloc methods, should via the property.
IMHO the big win is memory management - it's safer and much easier as there is no need to remember which iVars have been retained.
And think of how much work is required to code an accessor - 4 lines for getter and 2 for a setter.
At some point in the future #synthesize is likely to be optional, so all you'll need is the #property.
I'm trying to add a property without creating an instance variable. Is it possible to do this? Or can you do something similar in a way that's not a property?
Example:
#interface RandomClass()
#property (nonatomic) int value;
#end
#implementation RandomClass
#synthesize value = _value;
// Here I override the default methods #synthesize
-(int)value
{
return 8; // Actually I'm returning something more complex, so a "define" won't work
}
-(void)setValue:(int)value
{
self.someOtherValue = value;
}
In the code above, I'm not using the instance variable _value! Is there a way to do this without creating the variable?
Remove the line
#synthesize value = _value;
Since you're implementing the getter/setter yourself, the #synthesize isn't helpful.
#synthesize serves two jobs. The first job is to connect the property to a backing ivar, synthesizing the ivar if it doesn't already exist. The second job is to synthesize the getter/setter. If you don't need the backing ivar, and if you're providing implementations for the getter/setter yourself, then you don't need the #synthesize at all.
I'm new to C, new to objective C. For an iPhone subclass, Im declaring variables I want to be visible to all methods in a class into the #interface class definition eg
#interface myclass : UIImageView {
int aVar;
}
and then I declare it again as
#property int aVar;
And then later I
#synthesize aVar;
Can you help me understand the purpose of three steps? Am I doing something unnecessary?
Thanks.
Here, you're declaring an instance variable named aVar:
#interface myclass : UIImageView {
int aVar;
}
You can now use this variable within your class:
aVar = 42;
NSLog(#"The Answer is %i.", aVar);
However, instance variables are private in Objective-C. What if you need other classes to be able to access and/or change aVar? Since methods are public in Objective-C, the answer is to write an accessor (getter) method that returns aVar and a mutator (setter) method that sets aVar:
// In header (.h) file
- (int)aVar;
- (void)setAVar:(int)newAVar;
// In implementation (.m) file
- (int)aVar {
return aVar;
}
- (void)setAVar:(int)newAVar {
if (aVar != newAVar) {
aVar = newAVar;
}
}
Now other classes can get and set aVar via:
[myclass aVar];
[myclass setAVar:24];
Writing these accessor and mutator methods can get quite tedious, so in Objective-C 2.0, Apple simplified it for us. We can now write:
// In header (.h) file
#property (nonatomic, assign) int aVar;
// In implementation (.m) file
#synthesize aVar;
...and the accessor/mutator methods will be automatically generated for us.
To sum up:
int aVar; declares an instance variable aVar
#property (nonatomic, assign) int aVar; declares the accessor and mutator methods for aVar
#synthesize aVar; implements the accessor and mutator methods for aVar
This declares an instance variable in your object:
#interface myclass : UIImageView {
int aVar;
}
Instance variables are private implementation details of your class.
If you want other objects to be able to read or set the value of the instance variable (ivar), you can declare it as a property:
#property int aVar;
This means that the compiler expects to see setter and getter accessor methods for the property.
When you use the #synthesize keyword, you are asking the compiler to automatically generate setter and getter accessor methods for you.
So, in this case the compiler will generate code similar to this when it encounters the #synthesize keyword:
- (int) aVar
{
return aVar;
}
- (void)setAVar:(int)someInt
{
aVar = someInt;
}
By default on the iPhone (and on the 32-bit runtime on the Mac), #synthesize requires an instance variable to be present in order to store the property's value. This ivar is usually named the same as the property, but doesn't have to be, for instance you could do this:
#interface myclass : UIImageView {
int aVar;
}
#property int someValue;
#synthesize someValue = aVar;
Neither #synthesize nor #property are actually required, you can create your own getter and setter methods, and as long as you create them using Key-Value Coding-compliant syntax, the property will still be usable.
The requirement for an ivar to be present as well as the #property declaration is due to the fragile base class limitation of the 32-bit Objective-C runtime on both the Mac and iPhone. With the 64-bit runtime on the Mac you don't need an ivar, #synthesize generates one for you.
Note that there are numerous keywords you can use with your #property declaration to control what sort of synthesized accessor code is created, such as readonly for a getter-only accessor, copy, atomic, nonatomic and so on. More information is in the Objective-C 2.0 Programming Language documentation.
Classes can have instance variables (ivars). These are in the first section, and are only visible to code in that class, not any outside code. I like to prefix them with an underscore to show their internal-ness. In low level terms, the ivars are added as an additional member to the struct that the class you are creating uses internally.
The second declaration, #property, is a declared property. It is not required (except when you are using #synthesize), but it helps other programmers (and the compiler!) know that you are dealing with a property, and not just two methods -setAVar and -aVar, which is the alternative way of doing this.
Thirdly, the #synthesize actually creates the methods to set and access the property from outside the class. You can replace this with your own setter and getter methods, but only do that if you need to, as the built-in ones have some features that you would otherwise have to code yourself. In fact, using the #synthesize aVar = _someVariable; syntax, you can have your property actually reference a differently named instance variable!
Short version:
The #property is just a hint to the compiler and other programmers that you are making a property and not just getter/setter methods. The instance variables are internal to the class, and otherwise cannot be normally accessed from outside it. The #synthesize just creates simple getters and setters for you, to go with your #property, but you can also just code those getters and setters yourself, like any other method.
Class A
#interface myclass : UIImageView {
int aVar;
}
If you declare like this then you can only use this variable within your class A.
But suppose in Class B
A *object=[A new];
object.aVar---->not available
For this you should **declare aVar as a property in Class A**
so class A should look like
Class A
#interface myclass : UIImageView {
int aVar;
}
#property int iVar;
and
.m file
#synthesize iVar;
So now you can use this iVar in another class Suppose B
Class B
#import "Class A.h"
enter code here
A *object=[A new];
object.aVar---->available
means
object.aVar=10;
#interface declares the instances variables of a class in obj-c. You need it to create an instance variable. However the variable is not visible outside the class by default (as the field is by default protected).
#property tells the compiler to specify a particular property accessor (get/set) method. However, you will need to use #synthesize to actually have the compiler generate the simple accessors automatically, otherwise you are expected to create them on your own.
I recently started learning iphone apps. As per my knowledge #property is used in .h file as a setter method and #synthesize in .m file as getter method. In Java we use setter and getter methods, same as Java, in Objective C we use #property and #synthesize.
Please forgive me If u think I mislead you.