I'm trying convert my code to Modern Objective-C style. How i read here http://www.techotopia.com/index.php/The_Basics_of_Modern_Objective-C": "In the case of Modern Objective-C, however, the synthesis takes place by default, making the use of #synthesize declarations unnecessary. When using default property synthesize, instance variable properties are accessible from within code using the property name prefixed with an underscore."
However, I have:
Relationship.h
#interface Relationship : NSObject <NSCoding>
//...
#property(nonatomic, weak) Person* first;
//...
#end`
OtherRelationship.h
#import "Relationship.h"
#interface OtherRelationship : Relationship
#end
OtherRelationship.m
#import "OtherRelationship.h"
#implementation OtherRelationship
#synthesize first = _first;
- (void)foo
{
NSLog(#"%#", _first);
}
and it's working. But when i delete
#synthesize first = _first;
i get "Use of undeclared identifier '_first'" error. Does inheritanced variables doesn't work with autosynthesize or should i looking for problem elsewhere?
The backing ivar in the superclass is #private to the subclass. That is, the subclass may call self.first, but not _first. If you want to #synthesize again, use a different name because you can't refer to _first. For example, replace with #synthesize first = _ffirst; or just drop the #synthesize.
Related
I'm following one of the iOS tutorials from Ray Wenderlich (Scarybugs part 1). But I notice for each property in the model, he always "#synthesize" it in the implementation.
Here is the example of the models:
#import <Foundation/Foundation.h>
#interface RWTScaryBugData : NSObject
#property (strong) NSString *title;
#property (assign) float rating;
- (id)initWithTitle:(NSString*)title rating:(float)rating;
#end
--
#import "RWTScaryBugData.h"
#implementation RWTScaryBugData
#synthesize title = _title;
#synthesize rating = _rating;
- (id)initWithTitle:(NSString*)title rating:(float)rating {
if ((self = [super init])) {
self.title = title;
self.rating = rating;
}
return self;
}
#end
--
#import <Foundation/Foundation.h>
#class RWTScaryBugData;
#interface RWTScaryBugDoc : NSObject
#property (strong) RWTScaryBugData *data;
#property (strong) UIImage *thumbImage;
#property (strong) UIImage *fullImage;
- (id)initWithTitle:(NSString*)title rating:(float)rating thumbImage:(UIImage *)thumbImage fullImage:(UIImage *)fullImage;
#end
--
#import "RWTScaryBugDoc.h"
#import "RWTScaryBugData.h"
#implementation RWTScaryBugDoc
#synthesize data = _data;
#synthesize thumbImage = _thumbImage;
#synthesize fullImage = _fullImage;
- (id)initWithTitle:(NSString*)title rating:(float)rating thumbImage:(UIImage *)thumbImage fullImage:(UIImage *)fullImage {
if ((self = [super init])) {
self.data = [[RWTScaryBugData alloc] initWithTitle:title rating:rating];
self.thumbImage = thumbImage;
self.fullImage = fullImage;
}
return self;
}
#end
I know "#synthesize" is basically to allocate an instance variable for a property, but it has been taken care of by default for every "#property" in ".h file" (although not visible).
My questions is: is it necessary to "#synthesize" every "#property" we have in our public API? (I tried deleting all the "#synthesize" in the implementation, and it still worked)
#synthesize is no longer needed. The compiler will synthesize the getter and setter as required with an instance variable named as _<propertyName> automatically. It creates the instance variable but more importantly it creates the getter and setter methods (for readwrite properties).
If you've manually provided the getter/setter for a property, then an instance variable won't be automatically synthesized, and you'll need to add the #synthesize statement. From the docs:
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;
As noted in the Objective-C Feature Availability Index, automatic synthesis of property instance variables was introduced with Xcode 4.4 (LLVM Compiler 4.0) and requires the modern runtime (all code on iOS, 64-bit code on OS X).
So, the tutorial is a bit dated, that's all.
hope this will help little more.
#property(nonatomic) NSString *name;
the #property is an Objective-C directive which declares the property
-> The "`nonatomic`" in the parenthesis specifies that the property is non-atomic in nature.
-> and then we define the type and name of our property.
-> prototyping of getter and setter method
now go to .m file
previously we have synthesis this property by using #synthesis , now it also NOT required , it automatically done by IDE.
-> this #synthesis now generate the getter and setter(if not readonly) methods.
and Then why we even write #synthesis in our code if it always done by IDE .
one of the basic use is :-
what our IDE do internally
#synthesis name=_name;
we use _name to access particular property but now you want synthesis by some other way like
firstname you can do it like
#synthesis name= firstname
or just by name
#synthesis name=name
So from it you can access this property as you want.
I'm sure my confusion here is just a result of being stuck in a "Java mindset" and not understanding how Obj-C differs in this case.
In Java, I can declare a variable in a class, like this, and each instance of that class will have it's own:
MyClass {
String myVar;
MyClass() {
// constructor
}
}
In Obj-C I tried to do the same thing by declaring a variable only in the .m file like this:
#import "MyClass.h"
#implementation MyClass
NSString *testVar;
#end
My expectation here was that this variable has a scope limited to this class. So I created a second class (identical):
#import "MySecondClass.h"
#implementation MySecondClass
NSString *testVar;
#end
What I'm seeing (and has me baffled) is that changing the variable in one class, affects the value seen in the other class. In fact, if I set a breakpoint, and then "Jump to Definition" of the variable, it takes me to th
I've created an extremely small Xcode project that demonstrates the problem here.
Change this:
#implementation MyClass
NSString *testVar;
#end
to:
#implementation MyClass {
NSString *testVar;
}
// methods go here
#end
and you'll get what you expected.
As you had it, you are actually creating a global variable. The two global variables were combined into one by the linker which is why both changed when you set one. The variable in curly braces will be a proper (and private) instance variable.
Edit: After being downvoted for no apparent reason, I thought I'd point out the "old" way of doing things, and the new way.
The old way:
SomeClass.h
#interface SomeClass : UIViewController <UITextFieldDelegate> {
UITextField *_textField;
BOOL _someBool;
}
#property (nonatomic, assign) BOOL someBool;
// a few method declarations
#end
SomeClass.m
#implementation SomeClass
#synthesize someBool = _someBool;
// the method implementations
#end
Now the new and improved way with the modern Objective-C compiler:
SomeClass.h
#interface SomeClass : UIViewController
#property (nonatomic, assign) BOOL someBool;
// a few method declarations
#end
SomeClass.m
#interface SomeClass () <UITextFieldDelegate>
#end
#implementation SomeClass {
UITextField *_textField;
}
// the method implementations
#end
The new way has several advantages. The primary advantage is that none of the implementation specific details about the class appear in the .h file. A client has no need to know what delegates the implementation needs. The client has no need to know what ivars I use. Now, if the implementation needs a new ivar or it needs to use a new protocol, the .h file doesn't change. This mean less code gets recompiled. It cleaner and much more efficient. It also makes for easier editing. When I'm editing the .m file and realize I need a new ivar, make the change in the same .m file I'm already editing. No need to swap back and forth.
Also note the implementation no longer needs an ivar or #synthesize for the property.
What you probably want (unless you're using a very old OS and compiler) is to just use property syntax. I.e.:
#interface MyClass : NSObject
// method declarations here ...
#property (copy) NSString* myVar;
// ... or here.
#end
This will do what you intended to do. This will implicitly synthesize an instance variable and a getter/setter pair for this variable. If you manually wanted to create the instance variable (you generally don't need that unless you need your code to work on very old MacOS versions), this is what the above code does under the hood to create the ivar:
#interface MyClass : NSObject
{
NSString* _myVar;
}
// method declarations here.
#end
Note the curly braces, which tell the compiler that this is not just a global variable somewhere in between the methods, but actually an instance variable that belongs to this object.
If you are creating the property only for internal use and don't want clients of your class to mess with it, you can hide this away a little bit in everything but the oldest ObjC compilers by using a class extension which "continues" the class declaration from the header, but can be placed separate from it (so usually in your implementation file). A class extension looks like a category without a name:
#interface MyClass ()
#property (copy) NSString* myVar;
#end
And you can either put your property declaration in there, or even ivar declarations (again wrapped in curly brackets). You can even declare the same property as readonly in the class interface, and then re-declare it identical, but as readwrite in the extension, so that clients only read it, but your code can change it.
Note that, if you didn't use ARC (that is, you've switched off the default of Automatic Reference Counting), you would have to set all your properties to nil in your dealloc method (unless they're set to weak or assign of course).
NB - All the above are #interface sections. Your actual code will go in separate #implementation sections. This is so you can have header files (.h) you can hand off to your class's clients that just contain the portions you intend them to use, and hide away implementation details in the implementation file (.m) where you can change them without having to worry someone might have accidentally used them and you'll break other code.
PS - Note that NSStrings and other objects that you want the immutable flavor of, but that also exist in a mutable flavor (i.e. NSMutableString) should always be copy properties, because that will turn an NSMutableString into an NSString so that nobody on the outside can change the mutable string underneath you. For all other object types, you generally use strong (or retain if not ARC). For your class's owner (e.g. its delegate) you usually use weak (or assign if not ARC).
In Java
MyClass {
String myVar;
MyClass() {
// constructor
}
}
In Objective-c
MyClass.h
#interface MyClass : NSObject{
NSString* str; // Declaration
}
#end
MyClass.m
#implementation MyClass
-(void)initializieTheString
{
//Defination
}
#end
In objective-c, you define the variable as private by doing like so
MyClass.h
#interface MyClass : NSObject{
NSString* _myTestVar; // Declaration
}
#end
and refer to it in the implementation class by doing like so
MyClass.m
#import "MyClass.h";
#implementation MyClass
-(void)initializieTheString
{
_myTestVar= #"foo"; //Initialization
}
#end
I'm very new to Objective C. (Two days now). When read about #synthesize, it seemed to overlap with my understanding #property (which I thought I understood) ... So, some details need to be ironed out in my mind ... it's bugging me.
Please correct me if I'm wrong about differences of #property and #synthesize:
If you declare a #property in your #interface, then you're telling the world that users can expect to use standard getters and setters for that property. Futhermore, XCode will make generic getters and setters for you. ... BUT, To what degree does that happen with the #property declaration? ( I.E. does that mean "completely" ... like unseen declarations for it in your #interface, and also unseen code in your #interface?
-Or-
Does #property take care of the unseen code declarations in your #interface only - whereas #synthesize takes care of the unseen code implementation in your #implementation section? )
First, note that the latest version of Xcode does not require #synthesize at all anymore. You can (and should) just omit it. That said, here's what the pieces do.
#property is a declaration of accessors. It is just a declaration. There is very little difference between the following:
#property (nonatomic, readwrite, strong) NSString *something;
vs.
- (NSString *)something;
- (void)setSomething:(NSString)aSomething;
The main difference is that declaring these methods using #property lets the compiler automatically generate (synthesize) the implementations for you. There is no requirement that you let the compiler do it for you. You are absolutely free to implement something and setSomething: by hand, and it is common to do. But, if you don't implement them by hand, the compiler will automatically create an ivar for you called _something and create a reasonable implementation for the getter and setter.
In older versions of Xcode, you had to explicitly request the auto-generation using the #synthesize keyword. But that is no longer required. Today, the only reason to use #synthesize is if you want the ivar to have a non-standard name (never do that).
A key point here is that the methods something and setSomething: are just methods. There is nothing magical about them. They're not special "property methods." They're just methods that by convention access a piece of state. That piece of state is often stored in an ivar, but does not need to be.
To be even more clear: object.something does not mean "return the ivar named _something from object." It means "return the result of [object something], whatever that does." It is common for that to return the value of an ivar.
You should declare all of your state (internal and external) using #property declarations, and you should avoid directly declaring ivars. You should also always access your properties via their accessors (self.something), except in the init and dealloc methods. In init and dealloc, you should directly use the ivar (_something).
#property declares a property on your class with whatever atomicity and setter semantics you provide.
With Xcode 4.4, autosynthesis is available wherein you are provided with a backing ivar from your property without declaring it in #synthesize. This ivar has the form of _propertyName where your property name is propertyName.
Objective-C #property and #synthesize
#property
generates get/set method
today(from Xcode v4.4 with the LLVM v4.0) #property additionally uses #synthesize inside
#synthesize propertyName = _propertyName
#synthesize:
generates a new iVar or link with existing iVar
generates an implementation of the get/set method with an appropriate iVar
[Case when #synthesize can be used]
#property
#interface SomeClass : NSObject
#property NSString *foo;
#end
//generated code
#interface SomeClass : NSObject
- (NSString *)foo;
- (void)setFoo:(NSString)newFoo;
#end
#synthesize pattern
#synthesize <property_name> = <variable_name>;
//Using
//1. Specify a variable. New variable(variableName) will be generated/linked with existing
#synthesize propertyName = variableName
//if variableName is not exist it generates:
//NSString *variableName;
//read access
NSString *temp = variableName;
//2. Default. New variable(propertyName - the same name as a property) will be generated/linked with existing
#synthesize propertyName
//is the same as
//#synthesize propertyName = propertyName
//if propertyName is not exist it generates:
//NSString *propertyName;
//read access
NSString *temp = propertyName;
//if you specify not-existing <property_name> you get
//Property implementation must have its declaration in interface '<class_name>' or one of its extensions
previously you had to use next syntax:
#interface SomeClass : NSObject
{
//1. declare variable
NSString *_foo;
}
//2. create property
#property NSString *foo;
#end
#implementation SomeClass
//3. link property and iVar
#synthesize foo = _foo;
#end
But today you can use next syntax
#interface SomeClass : NSObject
//1. create property
#property NSString *foo;
#end
Next, the same code, will be generated for both cases
#interface SomeClass : NSObject
{
//variable
NSString *_foo;
}
//getter/setter
- (void)setFoo:(NSString *)newFoo;
- (NSString *)foo;
#end
#implementation SomeClass
- (void)setFoo:(NSString *)newFoo
{
_foo = newFoo;
}
- (NSString *)foo
{
return _foo;
}
#end
If you have a property in your public interface like the following
#interface MyClass : NSObject
#property(strong) NSString *myProp;
#end
And then synthesize it, in effect synthesizing the variable:
#implementation MyClass
#synthesize myProp = _myProp; // or just leave it at the default name..
#end
What is the visibility of the instance variable _myProp? That is, is this considered #public, #protected or #private? I'm guessing since MySubClass could inherit from MyClass then it would also get the properties (naturally), but would it also inherit the instance variable visibility?
What difference does it make if I put the property in a class extension? That would hide the property from subclasses, and I'm guessing the instance variable, too. Is this documented anywhere?
A synthesized ivar is completely invisible to all code that cannot see the #synthesize line (which basically means anything outside of the .m file). It's not #protected, it's not #private, it's simply unknown. With a #private ivar, other code trying to access it will be told that it's private, but with a synthesized ivar, other code trying to access it will be told that the field simply doesn't exist.
As a thought experiment, try imagining a situation where the ivar acted like it was #protected. You make a subclass, and you muck about with the ivar there. Now you go back to the superclass and change #synthesize myProp to #synthesize myProp=foo. What happens in the subclass? When the compiler processes the subclass, it cannot see the #synthesize line, so it would have no idea that you just changed the name of the ivar. In fact, it cannot even tell if the property is backed by an ivar at all, or if it's implemented with custom-written accessor methods. I hope it's obvious why this means that the subclass cannot possibly access the ivar, and neither can any other class.
That said, I'm not quite sure what the compiler does if you write code in the same .m file that tries to access the ivar. I expect it will treat the ivar as #private (since the compiler can, in fact, see that the ivar exists).
Also, none of this has any bearing on the runtime methods. Other classes can still use the obj-c runtime methods to dynamically look up your class's ivar list and muck about with it.
If it is declared in your interface it is virtually public when using the #property declarative. If you want to use #property declaratives and keep them property truly private, you should create a private category in your implementation.
MyClass.h
#interface MyClass : NSObject {
#private
NSObject* foo;
}
#end
MyClass.m
#import "ClassWithPrivateProperty.h"
#interface MyClass ()
#property (nonatomic,retain) NSObject* foo;
#end
#implementation MyClass
#synthesize foo;
// class implementation...
#end
A synthesized variable acts as if declared #private:
#interface Garble : NSObject
#property (copy) NSString * s;
#end
#implementation Garble
#synthesize s;
#end
#interface Bargle : Garble
#end
#implementation Bargle
- (void) useS {
NSLog(#"%#", s); // error: instance variable 's' is private
}
#end
I swear I've seen this in the docs, but I can't find it right now. Will update if I track it down.
You can create a dynamic property and indicate it to the compiler that its instantiation would be at run time.
And then in your subclass write your own getter or synthesize the property.
#interface BaseClass : NSObject
#property (nonatomic, strong) NSString *ThisWillBeSynthesizedInRespectiveSubclasses;
#end
#implementation BaseClass
#dynamic ThisWillBeSynthesizedInRespectiveSubclasses;
#end
In Sub classes
#interface Subclass : BaseClass
#end
#implementation Subclass
#synthesize ThisWillBeSynthesizedInRespectiveSubclasses = _ThisWillBeSynthesizedInRespectiveSubclasses;
#end
or you write your own setter / getter methods.
Hope this helps !
Other classes have access to everything that they #include. In other words, to everything that is inside your header.
If something appears only in your implementation file, other classes (including subclasses) don't know it exists. A synthesized property is like that. Other classes know only about the property (a property means a getter and a setter method) but they don't know anything about the inner implementation of its methods.
Note, that the access specifiers (public/private/protected) in obj-c are only a hint to the compiler that even if something appears in the header file, it can't be accessed. The runtime does not check it in any way.
What happens if you put it into a class extension? Note that a property is a set of two methods. You just hide the methods from every class which includes your class main header but not the class extension header.
We use this for example to declare a property as readonly and in class continuation we declare it as readwrite. Then, we can use the setter only from inside of the class.
If you have a property in your public interface like the following
#interface MyClass : NSObject
#property(strong) NSString *myProp;
#end
And then synthesize it, in effect synthesizing the variable:
#implementation MyClass
#synthesize myProp = _myProp; // or just leave it at the default name..
#end
What is the visibility of the instance variable _myProp? That is, is this considered #public, #protected or #private? I'm guessing since MySubClass could inherit from MyClass then it would also get the properties (naturally), but would it also inherit the instance variable visibility?
What difference does it make if I put the property in a class extension? That would hide the property from subclasses, and I'm guessing the instance variable, too. Is this documented anywhere?
A synthesized ivar is completely invisible to all code that cannot see the #synthesize line (which basically means anything outside of the .m file). It's not #protected, it's not #private, it's simply unknown. With a #private ivar, other code trying to access it will be told that it's private, but with a synthesized ivar, other code trying to access it will be told that the field simply doesn't exist.
As a thought experiment, try imagining a situation where the ivar acted like it was #protected. You make a subclass, and you muck about with the ivar there. Now you go back to the superclass and change #synthesize myProp to #synthesize myProp=foo. What happens in the subclass? When the compiler processes the subclass, it cannot see the #synthesize line, so it would have no idea that you just changed the name of the ivar. In fact, it cannot even tell if the property is backed by an ivar at all, or if it's implemented with custom-written accessor methods. I hope it's obvious why this means that the subclass cannot possibly access the ivar, and neither can any other class.
That said, I'm not quite sure what the compiler does if you write code in the same .m file that tries to access the ivar. I expect it will treat the ivar as #private (since the compiler can, in fact, see that the ivar exists).
Also, none of this has any bearing on the runtime methods. Other classes can still use the obj-c runtime methods to dynamically look up your class's ivar list and muck about with it.
If it is declared in your interface it is virtually public when using the #property declarative. If you want to use #property declaratives and keep them property truly private, you should create a private category in your implementation.
MyClass.h
#interface MyClass : NSObject {
#private
NSObject* foo;
}
#end
MyClass.m
#import "ClassWithPrivateProperty.h"
#interface MyClass ()
#property (nonatomic,retain) NSObject* foo;
#end
#implementation MyClass
#synthesize foo;
// class implementation...
#end
A synthesized variable acts as if declared #private:
#interface Garble : NSObject
#property (copy) NSString * s;
#end
#implementation Garble
#synthesize s;
#end
#interface Bargle : Garble
#end
#implementation Bargle
- (void) useS {
NSLog(#"%#", s); // error: instance variable 's' is private
}
#end
I swear I've seen this in the docs, but I can't find it right now. Will update if I track it down.
You can create a dynamic property and indicate it to the compiler that its instantiation would be at run time.
And then in your subclass write your own getter or synthesize the property.
#interface BaseClass : NSObject
#property (nonatomic, strong) NSString *ThisWillBeSynthesizedInRespectiveSubclasses;
#end
#implementation BaseClass
#dynamic ThisWillBeSynthesizedInRespectiveSubclasses;
#end
In Sub classes
#interface Subclass : BaseClass
#end
#implementation Subclass
#synthesize ThisWillBeSynthesizedInRespectiveSubclasses = _ThisWillBeSynthesizedInRespectiveSubclasses;
#end
or you write your own setter / getter methods.
Hope this helps !
Other classes have access to everything that they #include. In other words, to everything that is inside your header.
If something appears only in your implementation file, other classes (including subclasses) don't know it exists. A synthesized property is like that. Other classes know only about the property (a property means a getter and a setter method) but they don't know anything about the inner implementation of its methods.
Note, that the access specifiers (public/private/protected) in obj-c are only a hint to the compiler that even if something appears in the header file, it can't be accessed. The runtime does not check it in any way.
What happens if you put it into a class extension? Note that a property is a set of two methods. You just hide the methods from every class which includes your class main header but not the class extension header.
We use this for example to declare a property as readonly and in class continuation we declare it as readwrite. Then, we can use the setter only from inside of the class.