I take for example the UIButton interface.
Here the first rows of #private definition :
#private
CFMutableDictionaryRef _contentLookup;
UIEdgeInsets _contentEdgeInsets;
UIEdgeInsets _titleEdgeInsets;
And here 2 of these ivar, that are defined as properties:
#property(nonatomic) UIEdgeInsets contentEdgeInsets;
#property(nonatomic) UIEdgeInsetstitleEdgeInsets;
However these 2 properties are not defined on the ivars i found in private method (which have suffix _).
I'm not sure to understand how could be implemented setter and getter for these 2 properties to refer to the private ivars.
And a second question... i used to create properties for ivar, thus, if i have an ivar FOO i can create a #property for FOO. Is it a normal behavior create property for a non existing ivar ? (in this case contentEdgeInsets is not an attribute for this class... on the contrary _contentEdgeInset is defined in #interface and this's a valid ivar). Ok what i missed with this argument ?
When you #synthesize these properties you do so like
#synthesize contentEdgeInsets = _contentEdgeInsets;
^property name ^iVar name
Check out the Property Implementation Directives section in the documentation.
By default, a property will use the ivar whose name is the same as that of the property, but it's also possible to specify an ivar of a different name. Do this in your #synthesize statement in the class implementation.
In the modern runtime, used pretty much everywhere at this point, you don't actually have to declare the ivar at all -- if you synthesize accessors for a property and there's no matching ivar, the runtime will provide one.
Finally, properties with #dynamic rather than #synthesized accessors don't necessarily need an ivar at all -- you're providing the accessors in this case, so you're free to derive the value of the property however you like.
Related
I have class A, which exposes a way of getting and setting an object of type Foo. In property parlance, I generally declare this in the interface:
#property (nonatomic, strong) Foo * foo;
This (in modern ObjC) generates both the accessors and an ivar, _foo for storage.
If I want to do custom work in the accessors, I can implement one or both of them myself. But what if I not only want to do custom work, I actually don't want the ivar? In other words, I'm doing something else with the Foo object, like handing it back and forth to another internal object that I'm composed with. I don't actually need to keep storage for foo in the instance of A at all.
It seems like I have two choices:
Declare the property, implement both accessors, and simply ignore the fact that the compiler creates storage for _foo, and never use it.
Declare my accessors explicitly: - (Foo *)foo and - (void)setFoo:(Foo *)foo in the interface, like I used to in pre-modern ObjC.
The first seems inelegant at runtime, and the second seems inelegant in the declaration (where I'd probably now have a mix of properties and property-like accessors).
Is there a way to declare a property and have it serve as purely a declaration?
Use the #dynamic keyword in the implementation file. The usual discussion of #dynamic describes it as not creating the accessors at compile time. Not usually mentioned is that is also has the effect of doing nothing to create storage for the property, which is exactly what is desirable in this case.
#implementation A
#dynamic foo;
- (Foo *)foo
{
// get a Foo from somewhere and return it.
}
- (void)setFoo:(Foo *)foo
{
// do something with foo
}
#end
(Note: answered my own question since I discovered this while writing up the question and it seemed interesting and nonobvious.)
If you override both the setter and getter and don't use the variable within the setter and getter, a variable will not be created.
For example, if you have a class for which you want a firstName and lastName property, but perhaps also a setter and getter for a fullName property, if your fullName setter simply parses a string into firstName and lastName and sets these properties to that (and never stores the full string into a fullName variable), and your fullName getter simply returns the concatenated firstName + lastName and never uses a fullName variable, one will never be created.
This is according to Apple's official documentation. Scroll down to "You Can Implement Custom Accessor Methods"
#dynamic is probably the way to go.
However, there are two other ways you can do this as well:
An Unimplemented Category
You can use an un-implemented category to declare a property, but not get the backing storage:
#interface Foo : NSObject
#end
#interface Foo (UnimplementedProperties)
#property (strong) id bar;
#end
#implementation Foo
#end
By not declaring the implementation of the category (#implementation Foo (UnimplementedProperties)), the properties won't be synthesized.
A Protocol
You can declare the properties on a protocol, and then make your class conform to that protocol. This has the same effect as the unimplemented category: the methods are declared, but the properties are not synthesized.
#protocol FooProperties <NSObject>
#property (strong) id bar;
#end
#interface Foo : NSObject <FooProperties>
#end
When I declare a property in objective-C, I also add the synthesize clause to get accessors
#interface StoreManager ()
#property (nonatomic, copy) NSString *writeStoresTimer;
#implementation StoreManager
#synthesize writeStoresTimer
I usually use the following syntax to set value to a property
[self setWriteStoresManager:#"Data"];
Is the above statement same as self.writeStoresTimer = #"Data" ? Will this also call the set-accessor
Is the above statement same as self.writeStoresTimer = #"Data"?
Yes, it is exactly the same.
Indeed, if you want to override the default setter method that is automatically synthesized, you define:
- (void)setWriteStoresTimer:(NSString *)string;
and then your custom implementation will be called through the dot-syntax.
From Apple reference:
You can think of a property declaration as being equivalent to declaring two accessor methods. Thus
#property float value;
is equivalent to:
- (float)value;
- (void)setValue:(float)newValue;
Yes. They both do the same thing.
From Apple's Documentation:
Objective-C provides a dot (.) operator that offers an alternative to
square bracket notation ([]) to invoke accessor methods. Dot syntax
uses the same pattern that accessing C structure elements uses:
You can see the full documentation in The Objective C Programming Language
Yes self. and set uses accessors
only time you dont is when your within the same class and just use the pointers name
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.
Well ! I got confused about the way of declaring variables & implementing its properties.
The .h File contents
#interface XYZAppDelegate : NSObject <UIApplicationDelegate> {
}
#property (nonatomic, retain) IBOutlet UIWindow *window;
#property (nonatomic, retain) IBOutlet XYZViewController *viewController;
#end
The .m File Contents
#import "XYZAppDelegate.h"
#import "XYZViewController.h"
#implementation XYZAppDelegate
#synthesize window=_window;
#synthesize viewController=_viewController;
My questions/Queries are as follows.
Don't we require to declare variables if we put property ? ( Using property, we can indirectly declare variable - is it like that ? )
What are the additional features other than this ? ( In coding specific )
Why does everybody insist to use _ before each property accessor ? ( Other than security threats ? Has it become coding standard ? Whats the reason behind it? )
You do not have to declare the
variable. It is done automatically,
I believe by #synthesize. One
advantage to declaring it is that the
debugger will automatically list it.
Weigh this against the ugliness of
redundant definition.
Other features: read only properties,
assigned (unretained) values.
The underscore is a convention for
naming member variables that are
differently named than properties and
method variables. Apple's samples
sometimes use this convention and
sometimes do not. I view it as
usually unnecessarily verbose as a
programmer can easily tell the
difference between myVariable and
self.myVariable.
1) Don't we require to declare variables if we put property ? ( Using property, we can indirectly declare variable - is it like that ? )
No. You aren't required to declare variables for the corresponding properties. You are required to use the #synthesize propertyName command which tells the compiler to create those variables for you.
2) Why does everybody insist to use _ before each property accessor ? ( Other than security threats ? Has it become coding standard ? Whats the reason behind it? )
Most people (all?) have been stung by memory management nightmares. Some of these are caused by sloppy/lazy/zero-sleep coding. Using #synthesize propertyName = _propertyName allows the programmer to immediately know that the underscored variable is private to the class, and is unretained. It prevents issues where you specifically allocate or copy an object to store in the property, or accidentally assign an autoreleased object to the ivar.
Consider:
1) An autoreleased object being assigned to an unretained ivar.
#synthesize propertyName;
propertyName = [NSString stringWithFormat:#"I've just made %#", "a boo-boo."];
"propertyName" now references an object that will soon not exist, which will create EXEC_BAD_ACCESS errors down the road (as soon as it's referenced again).
2) A retained object being set to the retained property.
#synthesize propertyName;
self.propertyName = [[NSString alloc] initWithFormat:#"I just created %#", #"a leak"]
Now we've created an NSString object, and set it to the propertyName property, which itself is retaining the variable. Now the object is double retained and won't be properly released.
Both of these issues are easy to combat (even when tired, albeit less so) when you properly name your ivars with an underscore. It's not a fool-proof method, but it makes it considerably easier to manage the retain counts of objects in your head.
(1) No, not with the new 64-bit only features. The variables are declared for you, automatically.
(2) I don't know what you're asking here.
(3) It's just a convention, so that the variable name is different from the accessor name. Makes it clearer which you're dealing with.
I'm currently using the iOS 5 SDK trying to develop my app.
I'm trying to make an NSString a property, and then to synthesize it in the .m file (I have done this before with no issues). Now, I came across this: "Semantic Issue: Property's synthesized getter follows Cocoa naming convention for returning 'owned' objects."
This is my code:
.h
#interface ViewController : UIViewController {
NSString *newTitle;
}
#property (strong, nonatomic) NSString *newTitle;
.m
#synthesize newTitle;
Does anyone have a clue how I could fix this?
Thanks!!
My guess is that the compiler version you’re using follows the memory management rules for declared properties, too — more specifically, for declared properties’ accessors:
You take ownership of an object if you create it using a method whose name begins with “alloc”, “new”, “copy”, or “mutableCopy”.
A property named newTitle, when synthesised, yields a method called -newTitle, hence the warning/error. -newTitle is supposed to be a getter method for the newTitle property, however naming conventions state that a method whose name begins with new returns an object that’s owned by the caller, which is not the case of getter methods.
You can solve this by:
Renaming that property:
#property (strong, nonatomic) NSString *theNewTitle;
Keeping the property name and specifying a getter name that doesn’t begin with one of the special method name prefixes:
#property (strong, nonatomic, getter=theNewTitle) NSString *newTitle;
Keeping both the property name and the getter name, and telling the compiler that, even though the getter name starts with new, it belongs to the none method family as opposed to the new method family:
#ifndef __has_attribute
#define __has_attribute(x) 0 // Compatibility with non-clang compilers
#endif
#if __has_attribute(objc_method_family)
#define BV_OBJC_METHOD_FAMILY_NONE __attribute__((objc_method_family(none)))
#else
#define BV_OBJC_METHOD_FAMILY_NONE
#endif
#interface ViewController : UIViewController
#property (strong, nonatomic) NSString *newTitle;
- (NSString *)newTitle BV_OBJC_METHOD_FAMILY_NONE;
#end
Note that even though this solution allows you to keep newTitle as both the property name and the getter name, having a method called -newTitle that doesn’t return an object owned by the caller can be confusing for other people reading your code.
For the record, Apple have published Transitioning to ARC Release Notes, in which they state:
You cannot give a property a name that begins with new or copy.
They’ve already been notified that their statement is not quite accurate: the culprit is the getter method name, not the property name.
Edit 17 Jan 2015: I’ve just noticed a recent commit to Clang that suggests option 3 above (using objc_method_family(none)), including a fix-it, for the general case where a property name matches one of the special method family prefixes. Xcode will likely incorporate this change eventually.
Unacceptable Object Names
newButton
copyLabel
allocTitle
Acceptable Object Names
neueButton
mCopyLabel
_allocTitle
#arc #auto-synthesized #xcode-4.6.1
** EDIT **
Apparently you can't use mutableCopy either.
The name of the member starting with new is what triggers the warning. Change the name to editedTitle and the warning will go away. I was unable to find documentation confirming this but through testing was able to determine that member variables that begin with 'new' aggravate the compiler.
ARC does not allow to use "New...." in property name. but you can use "newTitle" by changing getter name.
#property (nonatomic, strong, getter=theNewTitle) NSString *newTitle;
It doesn't look like what Bavarious was suggesting was what you wanted to do. All you want to do is declare an instance variable NewTitle and then synthesize the property. We used to have to declare the instance variable and property. No more.
Now, I believe the right way of doing this is the following:
.h
#interface ViewController : UIViewController
#property (nonatomic, strong) NSString *newTitle;
.m
#synthesize newTitle = _newTitle; // Use instance variable _newTitle for storage
The instance variable for the property newTitle is synthesized. You don't want your instance variable to be the same as your property - too easy to make mistakes.
See Example: Declaring Properties and Synthesizing Accessors
In CoreData if you use "new..." in attribute (compile normally) it will crash randomly with a "bad access" exception.
There is no crash log and the line shown with the "All Exceptions Breakpoint" will not help you at all.
Writing a setter manually with the name same as the property's removed this warning.
NS_RETURNS_NOT_RETAINED is used to solve the naming problem.
#property (nonatomic, copy) NSString *newTitle NS_RETURNS_NOT_RETAINED;
We can find its definition as follows:
#define NS_RETURNS_NOT_RETAINED __attribute__((ns_returns_not_retained))
The 'ns_returns_not_retained' attribute is the complement of 'ns_returns_retained'. Where a function or method may appear to obey the Cocoa conventions and return a retained Cocoa object, this attribute can be used to indicate that the object reference returned should not be considered as an "owning" reference being returned to the caller. The Foundation framework defines a macro NS_RETURNS_NOT_RETAINED that is functionally equivalent to the one shown below.
Besides the issue that you should/can't use "new" in front of you property names, let say one more thing: Try to avoid "new" in front of names in general. "New" is dependent on time. Currently it is new for you, but some time later you maybe want to implement something new again. So using "new" in names is always bad. Try to think this way: In the programming world, "new" is always creating something: a new instance of something.
In your case when you want to assign a different title then the current name your property titleReplacement.
One more thing: Try to name functions and methods with the verb first, like setSomething or getSomething.
But in properties try to name the object first, like heightMinimum, heightMaximum, etc. -> when you use your inspector when you are coding, you always looking for objects. Try it out. ;-)
try this:-
#property (nonatomic,retain) NSString *newTitle;