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.
Related
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:
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.
This question already has answers here:
Do declared properties require a corresponding instance variable?
(6 answers)
Closed 9 years ago.
Why in many fragment code declare instance variable like and for what? what different about property and non property
#import <Foundation/Foundation.h>
#interface class1:NSObject
{
NSMutableString *currentData;
}
#property (nonatomic, retain) NSMutableString * currentData;
What you saw is an "old code"... but sometimes you still need to support old versions (e.g. 10.5).
A property is simply a couple of getter and setter (well.. it depends on the attributes you choose: e.g. readonly will generate only a getter).
But a property operates (and so it needs) an instance variable. Usually what you see in the implementation file is something like
#implementation class1
#synthesize currentData = currentData;
#end
This means create getter and setter which uses currentData as variable.
For newer version you don't need to create the instance variable and you can just type the property and synthesize statement. In the most recent language version you don't even need the synthesize statement. Automatically an instance variable named _propertyName (underscore + name of the property) is created.
BTW: sometimes you still need to make your own getter and/or setter. Classic naming convention applies (e.g. - (void)setCurrentData: (NSMutableString*)newData; for setter and - (NSMutableString*)currentData; for getter), but same rules as before for properties: if you support only the most recent OSes you can just write the #property statement and right your getter and setter by using the "underscored" variable...
This question already has answers here:
What does #synthesize window=_window do?
(3 answers)
Closed 9 years ago.
I'm just starting to use Objective-C and I need to clarify something
When I #synthesize a #property, it is common convention to do the following:
#interface Class : ParentClass
#property propertyName
#end
#implementation
#synthesize propertyName = _propertyName;
#end
I've seen plenty of questions and answers suggesting that "_propertyName" is widely accepted as the "correct" way to synthesize properties. However, does it serve ANY purpose? Or is it merely to increase readability and identify instance variables?
It makes it so that if you accidentally leave off "self." you get a nice compiler error instead of silently not having your methods called.
From http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/ProgrammingWithObjectiveC/EncapsulatingData/EncapsulatingData.html
You Can Customize Synthesized Instance Variable Names
As mentioned earlier, the default behavior for a writeable property is
to use an instance variable called _propertyName.
If you wish to use a different name for the instance variable, you
need to direct the compiler to synthesize the variable using the
following syntax in your implementation:
#implementation YourClass #synthesize propertyName =
instanceVariableName; ... #end
Also:
Note: 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. If you still
need an instance variable, you’ll need to request that one be
synthesized: #synthesize property = _property;
By doing this the generated accessor actually got to know which variable(iVar) to use.
Yea, It increases the readability & also separates the private & public variables to understand & use. Private variable of Class generally written in "propertyName" format.You can say it is a coding convention where Private Variable Names use '' as prefix and Public Variables or Property Names are lowerCamelCase.
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.