Worrying about duplicates but can not seem to find and answer I can understand in any of the other posts, I just have to ask:
When I have in my .h:
#interface SecondViewController : UIViewController{
NSString *changeName;
}
#property (readwrite, retain) NSString *changeName;
then in my .m
#synthesize changeName;
-(IBAction)changeButton:(id)sender{
changeName = #"changed";
}
Is it the synthesized property or the instance variable that get changed when I press "changeButton" ?
You (and it seems some of the others that answered) are confusing properties with actual variables.
The way properties work is, they create METHODS (called setter and getter) that set or get/return ivars. And the do notation (self.string) actually INVOKES these methods. So a property can't be CHANGED, only the declared iVar is.
When you declare a property like so:
#property (nonatomic, retain) NSString *string;
And #synthesize it the following happens:
An iVar called string (of type NString*) is created
(if you do
#synthesize string = whateverYouWant
the iVar created
is called whateverYouWant - a convention is to name the iVars
the same as the property with preceding underscore (_string))
an accessor method is created like this
-(NSString*) string;
a setter is created like this
-(void) setString: (NSString*) newString;
Now what self.xxxx does is, it actually sends the message xxxx to self
(like [self xxxx]).
It works with ANY method, not just properties, though it should only
Be used with properties.
So when you do self.string = #"hello" it actually comes down to
[self setString: #"hello"];
(Note that the compiler actually knows you are trying to set and so the
setString message is sent instead of just string. If you accessed self.string
it would send [self string])
Thus you don't SET a property, you invoke the (synthesized) setter method that in
itself sets the iVar.
Accessing your iVar directly is ok, if you know what your doing.
Just calling
string = #"something else";
Will produce leaking code, since no memory management is done.
The synthesized accessors and setters actually do this for you, depending
on how you defined th property (retain,copy,assign).
Because the setter (for a retained property) doesn't just do
IVar = newValue
If you declared a retained property it actually looks something like this:
-(void) setString: (NSString*) newString {
if (string) [string release];
string = [newString retain];
}
So the property synthesize takes a bit of work off your hands.
EDIT
Since it still doesn't seem clear, the property that is declared is not to be thought
of like a variable. In the above example, when using
#synthesize string = _string;
there IS NO variable called "string". It's just the way you access the method structures
that set the iVar _string through the setter methods. Since string is no variable/object pointer, you cannot send messages to it ([string doSomething] won't work).
When you just synthesize the property using #synthesize string; the generated iVar gets
the same name as the property.
Calling [string doSomething] will then work, but it has nothing to do with the property. The "string" refers to the iVar. Hence th convention to name the iVars underscored, so
you don't accidentally access the iVar when you meant to use the getter/setter.
Both. Property uses instance variable as its storage. In your code you change the instance variable, but if you access the property (via self.changeName) you'd get the same value as instance variable.
Usually to distinguish between ivars and properties people use _ prefix for ivars. And then synthesizes properties like this:
#synthesize myProperty=_myProperty;
well, the var
it's always the var
in your case the property methods aren't used at all.
now, consider this case:
self.changeName = #"changed";
this way you are using the property, but that just means that you are using the methods "magically" created for you by the compiler, the setter and getter methods, where you, again, change the var (property doesn't exist, in reality, it's just a way to create the setter and getter methods for you)
Related
Hey I'm very new to Objective C programming and I'm stuck. How come when I create I function, it can't use the variables I created for the labels or textviews, etc. And whenever I call them in the viewDidLoad function, I have to do either self.(variableName) or _(variableName) and it won't let me do that outside of the viewDidLoad function. Is there a way to access them outside of it?
How come when I create I function, it can't use the variables I
created for the labels or textviews, etc.
For one thing, you need to differentiate between a function and an instance method. In Objective-C, classes can have instance variables (variables that are part of an instance of that class) and instance methods (similar to functions that are associated with an instance of that class). Classes can also have properties, which are used rather like instance variables in that they're values associated with an object, but they're accessed through accessor methods. Functions, on the other hand, aren't part of any class. So, a class has an interface where instance variables and methods are declared, like this:
#interface Person : NSObject
{
NSString *firstName;
NSString *lastName;
}
#property (readonly) NSString *fullName;
#property (strong) NSArray *friends;
#property (assign) int age;
- (id)initWithFirstName:(NSString*)first lastName:(NSString*)last;
- (void)addFriend:(Person*)friend;
#end
And also an implementation, like this:
#implementation Person
- (id)initWithFirstName:(NSString*)first lastName:(NSString*)last
{ /* code goes here */ }
- (void)addFriend:(Person*)friend
{ /* code goes here */ }
- (NSString *)fullName
{ return [NSString stringWithFormat:#"%# %#", firstName, lastName; }
#end
Those things in the implementation are instance methods, as denoted by the - at the beginning and the fact that they're defined in an #implementation block. (If they had + instead of -, they'd be class methods instead of instance methods -- I'll let you read about that in the docs.) Properties are accessed by calling an appropriate accessor methods using either normal method calls or dot notation, so if you have:
Person *george = [[Person alloc] initWithFirstName:#"George" lastName:#"Bailey"]
all of these are valid:
NSString *name1 = george.fullName;
NSString *name2 = [george fullName];
george.age = 45;
[george setAge:45];
int years1 = george.age;
int years2 = [george age];
Also, self is a pointer to "the current object". You can use it in instance methods so that objects can call their own methods and access their own properties. For example, the Person class could contain a method like this:
(NSString *)nameAndAge
{
NSString *nameAndAge = [NSString stringWithFormat:#"%#: %d", self.fullName, self.age];
}
Functions, on the other hand, aren't part of any class, use C function syntax rather than Objective-C method syntax, and aren't defined in an #implementation block:
BOOL isMiddleAged(Person* person)
{
return (person.age > 30) && (person.age < 60);
}
You can't use self in function because a function isn't associated with an object, so there's nothing for self to point to. You can, however, use properties of other objects you know about, such as person.age in the example above.
And whenever I call them in the viewDidLoad function, I have to do
either self.(variableName) or _(variableName) and it won't let me do
that outside of the viewDidLoad function.
You must be accessing properties of your view controller. As explained above, self.(variableName) is the way to access properties. _(variableName) refers to a variable (often generated by the compiler) that stores the value of the property. (You shouldn't normally access those variables directly outside initialization methods and -dealloc -- use the property accessors instead.) You can use those properties in any instance method of the class, not just -viewDidLoad. You can also access properties of other objects by replacing self with the name of a pointer to the object, just as I did with person in isMiddleAged().
Seems like your are using autosythesized property. Using Auto Synthesized property you need not to #syhtesize objects.
#sythesize object = _object; will be implicitly implement in this case.
So you can access object using self.object or _object.
You can #synthesize to avoid using objects via self.varName or _varName .You can directly use it using varName.
I am looking for a shorthand way of setting my properties directly to an NSMutableDictionary that is a instance variable. ie:
KVCModle.h:
#interface KVModel : NSObject {
NSMutableDictionary * data;
}
#property(nonatomic,assign)NSString * string1;
#property(nonatomic,assign)NSString * string2;
#end
KVCModel.m
#import "KVModel.h"
#implementation KVModel
-(id)init
{
self = [super init];
if(self)
{
data = [[NSMutableDictionary alloc] init];
}
return self;
}
-(NSString *)string1
{
return [data objectForKey:#"string1"];
}
-(NSString *)string2
{
return [data objectForKey:#"string2"];
}
-(void)setString1:(NSString *)_string1
{
[data setObject:_string1 forKey:#"string1"];
}
-(void)setString2:(NSString *)_string2
{
[data setObject:_string2 forKey:#"string2"];
}
-(void)dealloc
{
[data release];
[super dealloc];
}
#end
I have tried to override setValue:ForKey: and valueForKey:, but those aren't called, they allow you to directly set properties without using the property syntax.
I have made preprocessor macros to make this work in the past, but I am not interested in typing at all, and would like to avoid as much of it as I can in the future. Is there a way to make this work that I am not familiar with?
I have thought about using NSManagedObject, but I am not sure if I can get what I want out of that.
EDIT:
source
If you're trying to access the properties with code like foo = obj.foo and obj.foo = foo, that's why it doesn't work.
Property-access syntax is synonymous with message syntax; the former is exactly the same as foo = [obj foo], and the latter is exactly the same as [obj setFoo:foo]. There is no KVC code to intercept. Properties are at the language level; KVC is at the framework level.
You'll need to intercept the accessor messages instead. Consider implementing the resolveInstanceMethod: class method, in which you “resolve” the selector by adding a method implementation to the class using the Objective-C runtime API. You can add the same implementation(s) for many different selectors.
For your purpose, have a function or method that examines the selector (using NSStringForSelector and regular NSString-examining techniques) and returns two facts: (1) the property name, and (2) whether it's a getter (foo, isFoo) or setter (setFoo:). Then, have two more methods, one a dynamic getter and the other a dynamic setter. When the selector names a getter, add it with your dynamic-getter method; when the selector names a setter, add it with your dynamic-setter method.
So how do the dynamic-getter and -setter methods work? They'll need to know what property to dynamically get and set, but they also need to take no arguments (getter) or one argument (setter, which takes the value), in order to match the original property-access message. You might be wondering how these generic implementations can know what property to get or set. The answer is: It's in the selector! The selector used to send the message is passed to the implementation as the hidden argument _cmd, so examine that selector the same way as before to extract the name of the property you should dynamically get or set. Then, the dynamic getter should send [data objectForKey:keyExtractedFromSelector] and the dynamic setter should send [data setObject:newValue forKey:keyExtractedFromSelector].
Two caveats:
You may still get complaints from the compiler when you use the property-access syntax to access a “property” that you have not declared in the class's #interface. This is normal and intentional; you're really only supposed to use property-access syntax to access known formal properties. What you're doing, while I found it fun to solve, is technically an abuse of the property-access syntax.
This will only work for object values. KVC does the boxing and unboxing for primitive values, such as integers; since KVC is not involved, no free boxing and unboxing. If you have declared formal properties (see 1), you'll need to introspect them using the Objective-C runtime API, and do the boxing and unboxing yourself with your findings.
This piqued my curiosity, so I went ahead and used Peter Hosey's suggestion of overriding +resolveInstanceMethod: to generate the getters and setters. I posted the resulting object (DDDynamicStorageObject) to a github repository:
https://github.com/davedelong/Demos
What you basically want is your own implementation of the NSManagedObject machinery. I have done something similar. Look here: https://gist.github.com/954035 HTH
(Updated the code to remove the dependency on the non-existant NSString+Utilities.h)
(Added missing ReleaseAndZero() macro)
For the love of all that is sacred - do not use an NSDictionary as a place to stuff every conceivable property of a model object. Ivars are easier to debug, and much much clearer to other developers (including your future self).
If you want to use a dictionary, use a dictionary and some statically defined keys - but if you want a model object, use some ivars
I come to the same problem today just like you. So I find your question posted here.
The answers above used the +resolveInstanceMethod: is a little bit hard for me. :)
My understanding is that as long as we setup the property, we would have getter and setter method, so I use the setter method to implement it.
BDLink.h
#property (nonatomic, strong) NSString *type;
#property (nonatomic, strong) NSString *displayName;
#property (nonatomic, strong) NSString *linkURI;
BDLink.m
- (id)initWithLinkInfoDictionary:(NSDictionary *)linkInfoDict {
for (NSString *key in linkInfoDict) {
const char *rawName = [key UTF8String];
NSString *setMethodString = [NSString stringWithFormat:#"set%c%s:", toupper(rawName[0]), (rawName+1)];
SEL setMethod = NSSelectorFromString(setMethodString);
if ([self respondsToSelector:setMethod]) {
[self performSelector:setMethod withObject:linkInfoDict[key]];
}
}
return self;
}
Hope it would be helpful. My first answer, :)
For the following two lines in an obj-c class:
self.instanceVar = X
instanceVar = X
Is the only difference that the 'self' version calls the synthesized 'setter', whereas the latter simply sets the instanceVar and doesn't go through the setter?
Thanks
Yes. The implication of this is that the synthesized getter will wrap additional code depending on how the property is specified - so use of assign / retain / copy along with nonatomic / atomic change the behaviour.
Imagine the following:
#property( retain ) NSString * myprop;
If you set it by self.myprop, the NSString instance will be retained.
If you set directly the instance variable, this will not be the case.
So always use the self., unless you're absolutely sure...
This is an excellent question and understanding the difference between setting a variable through its accessor rather than directly assigning it is very important.
Here's what happens: when you declare a #property (nonatomic, retain) NSString *variable in the header, you add a property to your object. Simple enough. Calling #synthesize does the following thing: it generates two methods in your class, setVariable: and getVariable. Of course, if you name your property "name", the methods will be setName: and getName.
Now, it is important for you to understand what happens in the setVariable: method. The method is declared something like this:
- (void)setVariable:(NSString *)theVariable {
if (variable != nil) {
[variable release];
}
// variable is the class member,
// theVariable is the object that was sent by the method parameter
variable = [theVariable retain];
}
When you call self.variable = #"test"; you will actually call [self setVariable:#"test"] which is exactly the method that was generated by the #synthesize call!
When you call variable = #"test"; you do just that - you assign a string to a variable, without retaining it or anything.
If you were to call self.variable = nil the current value of the variable would be released and variable will be assigned to nil, but if you were to call variable = nil you would just ditch the reference to the previously assigned value (object). Therefore, if you would be calling
self.variable = #"test";
// wrong, do not do this in this case
variable = nil;
you would be be generating a memory leak because the #"test" object that was assigned to variable and retained through its accessor is never going to be released. Why's that? Because the setter (setVariable:) never gets called to know to release the previously held value.
For the sake of example, here's what getVariable looks like:
- (void)getVariable {
// variable is the class member
return variable;
}
Let me know if you have further questions.
Yes. self.instanceVar accesses the value through the property.
Although it is not necessarily the synthesized property. You can write your own get and set methods that can be called.
I have seen the following piece of code:
//example.h
MKMapView * mapView1;
#property (nonatomic, retain) MKMapView * mapView;
//example.m
#synthesize mapView = mapView1
What is the relation between mapView and mapView1?
Does it create a set and get method for mapView1?
In your example, mapView1 is an instance variable (ivar), a piece of memory storage that belongs to an instance of the class defined in example.h and example.m. mapView is the name of a property. Properties are attributes of an object that can be read or set using the dot notation: myObject.mapView. A property doesn't have to be based on an ivar, but most properties are. The #propertydeclaration simply tells the world that there is a property called mapView.
#synthesize mapView = mapView1;
This line tells the compiler to create a setter and getter for mapView, and that they should use the ivar called mapView1. Without the = mapView1 part, the compiler would assume that the property and ivar have the same name. (In this case, that would produce a compiler error, since there is no ivar called mapView.)
The result of this #synthesize statement is similar to if you had added this code yourself:
-(MKMapView *)mapView
{
return mapView1;
}
-(void)setMapView:(MKMapView *)newMapView
{
if (newMapView != mapView1)
{
[mapView1 release];
mapView1 = [newMapView retain];
}
}
If you do add that code to the class yourself, you can replace the #synthesize statement with
#dynamic mapView;
The main thing is to have a very clear conceptual distinction between ivars and properties. They are really two very different concepts.
#synthesize creates a getter and a setter for the variable.
This lets you specify some attributes for your variables and when you #synthesize that property to the variable you generate the getter and setter for the variable.
The property name can be the same as the variable name. Sometimes people want it to be different so as to use it in init or dealloc or when the parameter is passed with the same variable's name.
From the documentation:
You use the #synthesize keyword to tell the compiler that it should synthesize the setter and/or getter methods for the property if you do not supply them within the #implementation block.
As I just run into this problem when editing legacy code I want to make additional notes to the existing answers one needs to be aware of.
Even with a newer compiler version it sometimes does make a difference if you omit #synthesize propertyName or not.
In the case you declare an instance variable without underscore while still synthesizing it, such as:
Header:
#interface SomeClass : NSObject {
int someInt;
}
#property int someInt;
#end
Implementation:
#implementation SomeClass
#synthesize someInt;
#end
self.someInt will access the same variable as someInt. Not using a leading underscore for ivars does not follow the naming conventions but I just came into a situation where I had to read and modify such code.
But if you now think "Hey, #synthesize is not important any more as we use a newer compiler" you are wrong! Your class then will result in having two ivars, namely someInt plus an autogenerated _someInt variable. Thus self.someInt and someInt will not address the same variables any more. If you don't expect such behavior as I did this might get you some headache to find out.
As per apple documentation #Synthesize is used only to rename instance variables. For example
#property NSString *str;
#synthesize str = str2;
Now in the class you can not use _str as the above line has renames the instance variable to str2
#property allows objects to be used by objects in other classes, or in other words makes the object public.
When you create a property in #interface, that property will be automatically back by an instance variable named as _propertyName.
So when you create a property named as firstName, behind the scene compiler will create an instance variable named as _firstName by default. Compiler will also create the getter and setter method for you(i.e. firstName, setFirstName).
Now when you synthesize the property by #synthesize firstName, you are simply telling the compiler rename my instance variable(_firstName) by firstName. If you want to rename your backed up instance variable by different name you can simply assign different name while synthesizing the property name(i.e. #synthesize firstName = myFirstName), by doing this your property is backed up by an instance variable named as myFirstname.
So, in short, most of the time #synthesize used to rename your instance variable backed up by your property.
See the apple docs
Basically the synthesize creates a setMapView and mapView methods which set and get mapView1
It creates getter and setter for your object. You can access with something like this :
MKMapView* m = object.mapView;
or
object.mapView = someMapViewObject
mapView1 is the name of the ivar in the class, mapView is the name for the getter / setter method(s).
I'm trying to understand how strategies some folks use to distinguish instance vars vs. properties. A common pattern is the following:
#interface MyClass : NSObject {
NSString *_myVar;
}
#property (nonatomic, retain) NSString *myVar;
#end
#implementation MyClass
#synthesize myVar = _myVar;
Now, I thought the entire premise behind this strategy is so that one can easily distinguish the difference between an ivar and property. So, if I want to use the memory management inherited by a synthesized property, I'd use something such as:
myVar = #"Foo";
The other way would be referencing it via self.[ivar/property here].
The problem with using the #synthesize myVar = _myVar strategy, is I figured that writing code such as:
myVar = some_other_object; // doesn't work.
The compiler complains that myVar is undeclared. Why is that the case?
Thanks.
Properties are just setters and getters for ivars and should (almost) always be used instead of direct access.
#interface APerson : NSObject {
// NSString *_name; // necessary for legacy runtime
}
#property(readwrite) NSString *name;
#end
#implementation APerson
#synthesize name; // use name = _name for legacy runtime
#end
#synthesize creates in this case those two methods (not 100% accurate):
- (NSString *)name {
return [[_name copy] autorelease];
}
- (void)setName:(NSString *)value {
[value retain];
[_name release];
_name = value;
}
It's easy now to distinguish between ivars and getters/setters. The accessors have got the self. prefix. You shouldn't access the variables directly anyway.
Your sample code doesn't work as it should be:
_myVar = some_other_object; // _myVar is the ivar, not myVar.
self.myVar = some_other_object; // works too, uses the accessors
A synthesized property named prop is actually represented by two methods prop (returning the current value of the property) and setProp: (setting a new value for prop).
The self.prop syntax is syntactic sugar for calling one of these accessors. In your example, you can do any one of the following to set the property myVar:
self.myVar = #"foo"; // handles retain/release as specified by your property declaration
[self setMyVar: #"foo"]; // handle retain/release
_myVar = #"Foo"; // does not release old object and does not retain the new object
To access properties, use self.propname. To access instance variables use just the instance variable's name.
The problem with using the #synthesize myVar = _myVar strategy, is I figured that writing code such as:
myVar = some_other_object; // doesn't work.
The compiler complains that myVar is undeclared. Why is that the case?
Because the variable myVar is undeclared.
That statement uses the syntax to access a variable, be it an instance variable or some other kind. As rincewind told you, to access a property, you must use either the property-access syntax (self.myVar = someOtherObject) or an explicit message to the accessor method ([self setMyVar:someOtherObject]).
Otherwise, you're attempting to access a variable, and since you don't have a variable named myVar, you're attempting to access a variable that doesn't exist.
In general, I name my properties the same as my instance variables; this is the default assumption that the #property syntax makes. If you find you're fighting the defaults, you're doing it wrong (or your framework sux, which is not the case for Cocoa/Cocoa-touch in my opinion).
The compiler error you're getting is because property use always has to have an object reference, even inside your own class implementation:
self.stuff = #"foo"; // property setter
[stuff release]; // instance variable
stuff = #"bar"; // instance variable
return self.stuff; // property getter
I know that many Cocoa programmers disagree, but I think it's bad practice to use properties inside your class implementation. I'd rather see something like this:
-(void) someActionWithStuff: (NSString*) theStuff {
// do something
[stuff release];
stuff = [theStuff copy];
// do something else
}
than this:
-(void) someActionWithStuff: (NSString*) theStuff {
// do something
self.stuff = theStuff;
// do something else
}
I prefer to do memory management as explicitly as possible. But even if you disagree, using the self.stuff form will clue in any experienced Objective-C programmer that you're calling a property rather than accessing an instance variable. It's a subtle point that's easy for beginners to gloss over, but after you've worked with Objective-C 2.0 for a while, it's pretty clear.
Don,
According to the "rules", you should call Release for every Copy, Alloc, and Retain. So why are you calling Release on stuff? Is this assuming it was created using Alloc, Copy, or Retain?
This brings up another question: Is it harmful to call Release on a reference to an object if it's already been released?
Since Apple reserves the _ prefix for itself, and since I prefer to make it more obvious when I am using the setter and when I am using the ivar, I have adopted the practive of using a prefix of i_ on my ivars, so for example:
#interface MyClass : NSObject {
NSString *i_myVar;
}
#property (nonatomic, retain) NSString *myVar;
#synthesize myVar = i_myVar;
i_myVar = [input retain];
self.myVar = anotherInput;
[i_myVar release]
Since it is quite important to know when you are using the setter and when you are using the ivar, I find the explicitly different name is safer.
In your question, it should be:
self.myVar = #"Foo"; // with setter, equivalent to [self setMyVar:#"Foo"]
and
_myVar = some_other_object; // direct ivar access - no memory management!
Remember that you should not use setters/getters in init/dealloc, so you need to do your direct ivar access (and careful memory management) iin those methods.
what's wrong with simply using
#interface MyClass : NSObject
#property NSString *prop;
#end
nonatomic and retain are not required, retain is the default, and atomic/nonatomic isn\t important unless XCode tells you with a warning.
it is NOT necessary to declare the iVar, one will be created for you named _prop, if you really want to use one (i don't see why to be honest)
#synthesize is NOT required.
when (and you should) using ARC you don't have to bother with retain and release either.
keep it simple !
furthermore, if you have a method like this one
- (void)aMethod:(NSString*)string
{
self.prop = string;
// shows very clearly that we are setting the property of our object
_aName = string;
// what is _aName ? the _ is a convention, not a real visual help
}
i would always use properties, more flexible, easier to read.