This question already has answers here:
Declaring a property with no instance variable
(2 answers)
Closed 9 years ago.
In modern versions of Xcode, variables are automatically created to back properties. For example, #property (nonatomic, assign) BOOL isOpen would automatically create BOOL _isOpen. Is there a way to prevent such variables from being created when the property is meant to be non-physical? In the following example of a non-physical isOpen property, _isOpen is not needed. It is actually detrimental, because I've had co-workers inadvertdently use _isOpen and wonder why nothing would happen.
- (void) setIsOpen:(BOOL)isOpen
{
if (isOpen) {
[self.specialView open]
} else {
[self.specialView close];
}
}
- (BOOL) isOpen
{
return self.specialView.alpha > 0.0;
}
If you implement both the setter & the getter the variable will not be created. From Apple's docs:
The compiler will automatically synthesize an instance variable in all situations where it’s also synthesizing at least one accessor method. If you implement both a getter and a setter for a readwrite property, or a getter for a readonly property, the compiler will assume that you are taking control over the property implementation and won’t synthesize an instance variable automatically.
Your co-workers should not be able to reference _isOpen at all. I've checked your code in Xcode 4.6.3 and it behaves as per spec - no variable is created.
You need to specify both the getter and the setter. Assuming you do that, no instance variable is created.
Note that you can specify a different name for the getter, too. For boolean properties, the is prefix is usually dropped, though in the case of "open" I might leave it. I think it's clearer with the prefix.
For other boolean properties, though, something like this might be preferred:
#property (nonatomic, assign, getter=isOpen) BOOL open;
Then:
implement setOpen and isOpen.
read using BOOL value = object.open or BOOL value = [object isOpen]
write using object.open = value or [object setOpen:value].
Using #dynamic is not necessary; the recent compiler will detect you've specified both the getter and setter. #synthesize should not be necessary either. (There are a few edge cases; if you run into a case where it is necessary, by all means use it there).
Note, however, that I'm talking about the modern runtime here. 32-bit OSX targets use the legacy runtime, which has different rules.
Related
This question already has answers here:
Under what conditions is #synthesize automatic in Objective-c?
(5 answers)
Closed 7 years ago.
I've only been in the Objective-C & Cocoa world for a year, so I wasn't around when properties weren't automatically synthesized
Whenever I create new classes in our project, I can declare #property BOOL testFlag without declaring an instance variable. Then, in the implementation, I could either call self.testFlag, [self testFlag], [self setTestFlag:NO]. But, I could also call _testFlag. I always assumed because properties are automatically synthesized with instance variables, that's why I can just append a "_" underscore before the property name to directly access the instance variable.
However, I have been refactoring really old files, that clearly were created before auto-synthesizing was a thing
So now in the header, I'd see something like this:
#interface testClass
{
BOOL _testFlag
}
#property BOOL testFlag;
In the implementation, there could be a custom setter/getter:
#implementation testClass
#synthesize testFlag = _testFlag;
-(void)setTestFlag:(BOOL)testFlag
{
_testFlag = testFlag;
}
-(BOOL)testFlag
{
return _testFlag;
}
But I thought because of auto-synthesizing, I could remove the _testFlag declaration in the header, and I could remove the #synthesize in the implementation. But when I do this, there are just a truck ton of errors; wherever somebody was directly trying to access the ivar _testFlag. (This actually includes the custom getter/setter above ^^, too)
Is there perhaps something I am missing with the project settings that prevent this old file from generating an implied instance variable for properties?
Remove the getter and setter to have automatic synthesis of the backing variable. When you supply both for the property, the assumption is that you're going to look after storage yourself.
This question already has answers here:
Good practice for disambiguating argument names versus instance variable names in Objective-C
(3 answers)
Closed 9 years ago.
I have been learning and using Objective-C for quite some time now (it also kind of was my first OOP language) and I finally would like to know how to correctly name synthesized properties.
Let's take the following scenario:
I have got a property called someVariable.
#property (nonatomic, retain) NSString *someVariable;
and synthesize it
#synthesize someVariable;
How would the custom setter look like conventionally ?
1)
I would go ahead and say something like
-(void)setSomeVariable:(NSString *)someVar{
//input parameter MAY sound/look foreign due to the difference to the property
someVariable = someVar;
}
2) (illegal)
But I would like to name the formal parameter just like the property for the sake of readability and convenience. More like in Java like this:
-(void)setSomeVariable:(NSString *)someVariable{
//obviously illegal because this would call the setter over and over again
self.someVariable = someVariable;
}
3) (unconventional)
and according to what I have been reading in the past this
#synthesize someVariable = _someVariable;
is said to be unconventional and not supposed to be used.
So, am I correct in concluding that the way I have been doing it until now, is the only way to create a custom setter ?
3) is not unconventional, it's exactly what the compiler does if you don't provide the #synthesize statement.
This means that, without the #synthesize statement and the ivar declaration, you have an implicit ivar named _someVariable, and a custom setter would usually have a parameter named someVariable
-(void)setSomeVariable:(NSString *)someVariable {
_someVariable = someVariable;
}
Also note that providing custom setter and getter methods for a particular property indicates to the Xcode compiler to not provide the implicit ivar (here _someVariable). In the case of readonly properties, the same if true if you provide just the getter method.
WWDC 2012 session 405 provides a lot of details around Objective-C constructs for modern versions of the compiler.
EDIT
As H2CO3 has suggested in his answer, the code I wrote assumes you're using ARC. If you are using MRC, the setter method would rather be :
-(void)setSomeVariable:(NSString *)someVariable {
[someVariable retain];
[_someVariable release];
_someVariable = someVariable;
}
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Difference between self.ivar and ivar?
In Objective-C, what's the difference between [self setVariable: newStuff] and variable = newStuff?
When you have a class with a variable
#property (nonatomic) NSInteger num;
and you want to change the variable, typically you can do
[self setNum: newNum]
but you can also do
num = newNum
I know if you declare the variable readOnly, you can't use the first method to change it, but what's the concept behind it? Is it just because the second method with the setter can be called outside of its own class? Like if the class's instance was called 'sample'.
[sample setNum: newNum]
but then if you are changing the variable inside the class, either way is fine?
In Objective-C, what's the difference between [self setVariable:
newStuff] and variable = newStuff?
To be absolutely pedantic, one of them assigns the variable property the value in newStuff, whereas the other one assigns the value of newStuff to the iVar variable, but what I think you had in mind was a comparison between [self setVariable:
newStuff] and self.variable = newStuff. In that case, nothing is different, the compiler will expand case 2 out to case 1.
I know if you declare the variable readOnly, you can't use the first
method to change it, but what's the concept behind it? Is it just
because the second method with the setter can be called outside of its
own class? Like if the class's instance was called 'sample'.
readonly variables are important in cases where certain properties are private to the implementation of a class, but should be visible to other classes.
For example, if I were writing a Stack, I might want to expose the count of the number of items on the stack, but it would be a very bad idea for other classes to be able to write to the count variable. If I weren't smart and were using something like a count variable, I would want to be able to adjust the count of the semaphore internally (meaning you need it to be internally readwrite), so I declare a visibly readonly property so other classes can get it, but declare it internally readwrite so I can modify it:
//.h
#interface CFExampleStack : NSObject
#property (nonatomic, assign, readonly) int count; //readonly
#end
//.m
#interface CFExampleStack ()
#property (nonatomic, assign) int count; //readwrite
#end
Is it just because the second method with the setter can be called outside of its own class?
Well, that depends on how your instance variable is declared. By default, instance variables are #protected, i. e. they can be accessed from within the class and its subclasses only. However, if you explicitly declare an ivar as #public, then you can access it outside the class, using the C struct pointer member operator ->:
obj->publicIvar = 42;
However, this is not recommended, since it violates encapsulation.
Furthermore, if you use a custom setter method, then you have the opportunity to do custom actions when a property of an instance is updated. For example, if one changes the backgroundColor property of a UIView, it needs to redraw itself in addition to assigning the new UIColor object to its appropriate ivar, and for that, a custom setter implementation with side effects is needed.
Additionally, there are retained ("strong") and copied properties in case of instance variables that hold object. While writing a setter for a primitive type such as an integer is as simple as
- (void)setFoo:(int)newFoo
{
_foo = newFoo;
}
then, in contrast, a retained or copied property needs proper memory nanagement calls:
- (void)setBar:(Bar *)newBar
{
if (_bar != newBar) {
[_bar release];
_bar = [newBar retain]; // or copy
}
}
Without such an implementation, no reference counting would take place, so the assigned object could either be prematurely deallocated or leaked.
One more important difference...
Whenever you use self.prop KVC comes into play and you can observe the changes in the object, while _prop bypasses it.
I have this property:
#property (nonatomic, getter = getSolutionsCount, setter = setSolutionsCount:) NSInteger solutionsCount;
and implementation
- (NSInteger)getSolutionsCount {
return self.solutionsCount;
}
and I get EXC_BAD_ACCESS on this method - (NSInteger)getSolutionsCount.
What am I doing wrong here?
dot syntax is basically a shortcut for calling the getter. You have infinite recursion in your getter method.
What you need to do is return the instance variable directly:
- (NSInteger)getSolutionsCount {
//latest xcode makes variables with _property name automatically
return _solutionsCount;
//older versions of xcode or having written #synthesize solutionsCount
//will necessitate
//return solutionsCount;
}
Also just FYI objective-c convention is to have the getter method be defined as just the variable name. A getter which is the same as the property name is assumed if you don't write a getter in the property declaration
EDIT:
also i'm assuming this isnt the whole implementation for your getter because if it is let the compiler make it for you automatically, you don't need to write anything. (or by writing #synthesize propertyName = _propertyName in your implementation block with older versions of xCode)
The line self.solutionsCount is translated to [self getSolutionCount]. You are making a recursive call.
If you simply want to return the synthesized ivar then don't even implement this method. But if you do then simply call return _solutionCount;.
The problem is that self.solutionsCount is identical to [self getSolutionsCount], so your getter is directly recursive. You probably want to access the underlying ivar directly, to do so use self->_solutionsCount. Or, if you prefer not to explicitly use self, simply _solutionsCount.
There are several problems here:
According to the naming convention getters should not start with get. To read the value you need to use self.solutionsCount. Hence, you do not need to specify the name of the getter method in the property declaration.
You do not need to specify the name of the setter for it will be automatically generated.
The property should look like this:
#property (nonatomic, assign) NSInteger solutionsCount;
You do not need to write a custom getter implementation to make it work. Ask the compiler to synthesize the methods for you:
#synthesize solutionsCount;
If you want to have a direct access to the instance variable, ask compiler to sythesize it for you:
#synthesize solutionsCount = _solutionsCount;
Read objective-c and naming convention docs first. They will help enormously.
Good luck!
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Is there a difference between an “instance variable” and a “property” in Objective-c?
Difference between self.ivar and ivar?
What is the difference between declaring variables in brackets immediately after the #interface line, and defining properties below?
For example...
#interface GCTurnBasedMatchHelper : NSObject {
BOOL gameCenterAvailable;
BOOL userAuthenticated;
}
#property (assign, readonly) BOOL gameCenterAvailable;
Defining the variables in the brackets simply declares them instance variables.
Declaring (and synthesizing) a property generates getters and setters for the instance variable, according to the criteria within the parenthesis. This is particularly important in Objective-C because it is often by way of getters and setters that memory is managed (e.g., when a value is assigned to an ivar, it is by way of the setter that the object assigned is retained and ultimately released). Beyond a memory management strategy, the practice also promotes encapsulation and reduces the amount of trivial code that would otherwise be required.
It is very common to declare an ivar in brackets and then an associated property (as in your example), but that isn't strictly necessary. Defining the property and synthesizing is all that's required, because synthesizing the property implicitly also creates an ivar.
The approach currently suggested by Apple (in templates) is:
Define property in header file, e.g.:
#property (assign, readonly) gameCenter;
Then synthesize & declare ivar in implementation:
#synthesize gameCenter = __gameCenter;
The last line synthesizes the gameCenter property and asserts that whatever value is assigned to the property will be stored in the __gameCenter ivar. Again, this isn't necessary, but by defining the ivar next to the synthesizer, you are reducing the locations where you have to type the name of the ivar while still explicitly naming it.
{
BOOL gameCenterAvailable;
BOOL userAuthenticated;
}
the above two are called member Variables
They can't be accessed outside the class.(Important point) (unless you provide custom getters and setters)
if you make a #property then the variable can be read inside the class as well as outside the class..so the setters and getters are generated for you..automatically
then declaring the same as a member variable isn't required..
It is just done to increase Readability .. you can read it easily than reading
#property (non..)
When you define a property a getter and setter is created for you. When you access them usingobject.member setters and getters are called automatically.
When you declare variable in interface setters and getters are not written for you. you can also specify some visibility modifiers to them like #private,#public etc.