Sorry for the simple question.
When I see a definition of a property inside the h file, but outside of the class #interface scope, what does it mean ?
#property (nonatomic, readonly) RMMapContents *mapContents;
Here is the code:
#class RootViewController;
#class RMMapContents;
#interface MapTestbedAppDelegate : NSObject <UIApplicationDelegate> {
UIWindow *window;
//MAIN VIEW
//==============
RootViewController *rootViewController;
// NETWORK DATA
// =============
NSMutableArray *photoTitles; // Titles of images
NSMutableArray *photoSmallImageData; // Image data (thumbnail)
NSMutableArray *photoURLsLargeImage; // URL to larger image
NSMutableData *receivedData;
NSURLConnection *theConnection;
NSURLRequest *request;
}
#property (nonatomic, retain) IBOutlet UIWindow *window;
#property (nonatomic, retain) IBOutlet RootViewController *rootViewController;
#property (nonatomic, readonly) RMMapContents *mapContents;
#end
Inside a function I see this line:
- (void)foo:(xyz *)abc{
..
RMMapContents *mapContents = [self mapContents];
..
}
So, taking it from C++, the mapContents seem like it is not a global scope var (after all, that's why they call them properties, right?), but isn't defining the same name again inside the function weird a bit?
I hope someone can clarify a little here.
Thanks!
The scope of the #interface block extends upto the #end keyword and is not restricted to the braces {}.
So the #property declaration lies very much inside the scope of the #interface and like cli_hlt rightly answered, it acts like a substitute to setter and getter methods for the mapContents property.
so a property named mapContents, would have setters and getters which look like this :
- (void)setMapContents; //setter
- (RMMapContents *)mapContents; //getter
and would can be accessed from within the class using these methods:
[self setMapContents:newContents];
AND
RMMapContents *contents = [self mapContents];
Well, a property is not just a variable. A property is a variable plus its setter and getter methods. A property is usually said to be backed by a variable, which usually(but not always) has the same name as the property itself.
So there are basically three scenarios:
The developer has redefined the backing variable, look for something like:#synthesize mapContents=mapContents_, at the beginning of the implementation -> no problem here.
The compiler defined the variable to be something you don't now but not equal to mapContents - > no problem.
The property backing variable is indeed called "mapContents", so then the local definition hides the global definition (look for a compiler warning here). But by calling [self mapContents] you will not access the global variable but call the getter, which in turn will access the class variable (because then the local mapContents is out of scope)
Hope this helps.
global var mapContents is readonly,in foo function , create a new pointer,then you can change the value of inner var.
Look for a method in your class with a name mapContents that will return a initialization to your RMMapContents class.
Basically this line RMMapContents *mapContents = [self mapContents]; says that initializing an instance of RMMapContents called mapContens using the method mapContents.
Related
You can declare a variable like this.
Case1:
#interface MyClass : NSObject
{
NSString *str;
}
#end
Also, if you want to set its property, you can do
Case2:
#interface MyClass : NSObject
{
NSString *str;
}
#property (nonatomic, strong) NSString *str;
#end
And in the .m,
#synthesize str;
My understanding with the difference between Case 1 and Case 2 is that synthesized and propertied variables in Case 2 can be accessed from another class when this another class instantiates this class.
What are other differences between Case 1 and 2? Say when these variables are just used only in its .m file. The fact that you are setting 'str' properties probably makes a difference, but how? If you don't set property, how are they going to be released with ARC?
The differences are fairly confusing in this case because of the way it is set up.
Also it is using what is now old practises.
The new suggested way of doing this (suggested by Apple) is to do this...
MyClass.h
#interface MyClass : NSObject
#property (nonatomic, strong) NSString *str;
#end
MyClass.m
#import "MyClass.h"
#implementation MyClass
#end
You no longer need the #synthesize as Xcode (since 4.5) will auto generate these for you.
Doing this sets up the property called str and an iVar called _str.
You now no longer need to worry about defining multiple ivars and properties etc... Just use the property and that's it done.
An example setter method for the property str would look like this...
- (void)setStr:(NSString*)str
{
_str = str;
}
I've kind of been confused about properties. Some people say to always use setters and getters for ivars, even within the ivar's class. So if "name" is an ivar, when referring to it, one should always use "self.name". Always. Even if you're in the same class that "name" is declared in.
First, is that correct advice?
Second, what if I wish to reap the automatic memory management that comes with declaring "name" as a property and synthesizing it, but I don't want to give other classes access to change "name"? I guess it would be sort of a private property?
Thanks!
Yes, you should always try to use the property accessors when possible. Using ARC alleviates these concerns somewhat, but it's still good style. As for your second question, you can declare the property as readonly in the public header file and redefine it in a class extension:
In MyClass.h:
#interface MyClass : NSObject
#property (strong, readonly, nonatomic) id foo;
#end
In MyClass.m:
#interface MyClass()
#property (strong, readwrite, nonatomic) id foo;
#end
#implementation MyClass
#synthesize foo = _foo;
// The rest of your code goes here.
#end
This will allow you to call [self setFoo:foo] all day inside of MyClass’s implementation, but not other classes.
For ivars which are accessed externally, I generally use properties to access the ivar from within the class, for ivars which are only used internally (usually BOOL, NSUInteger, NSInteger, etc), I use the ivar directly. I do however access an consistently within the class (i.e. if I'm using a property to access it, I always use a property).
For the second part of your question. You can create a readonly property in the class's interface definition and within the same file as the implementation create a category with the read-write property. For example:
MyClass.h
#interface MyClass : NSObject
{
NSString * name;
}
#property (nonatomic, readonly) NSString * name;
#end
MyClass.m
#interface MyClass()
#property (nonatomic, retain) NSString * name;
#end
#implementation MyClass
#synthesize name;
-(void)dealloc
{
[name release];
[super dealloc];
return;
}
#end
Keep in mind, that although another class accessing the method -setName: may cause compile warnings or errors, another class may still call -(id)performSelector:withObject: with without an error.
For instance:
MyClass * test = [[MyClass alloc] init];
test.name = #"David";
is functionally the same as:
MyClass * test = [[MyClass alloc] init];
[test performSelector:#selector(setName:) withObject:#"David"];
I was going through the AVCam project from WWDC and I'm curious about the following code. I thought you were supposed to declare an object, then #property (nonatomic,retain), then synthesize.
The Demo code does it a little differently, I'll post some code (just a sample), anyone know what this does and when you should use it? Can anyone explain its significance and when to use it?
#interface AVCamCaptureManager : NSObject {
#private
// Capture Session
AVCaptureSession *_session;
AVCaptureVideoOrientation _orientation;
// Identifiers for connect/disconnect notifications
id _deviceConnectedObserver;
id _deviceDisconnectedObserver;
}
#property (nonatomic,readonly,retain) AVCaptureSession *session;
#property (nonatomic,assign) AVCaptureVideoOrientation orientation;
#property (nonatomic,readonly,retain) AVCaptureAudioChannel *audioChannel;
#property (nonatomic,assign) NSString *sessionPreset;
in the implementation file:
#interface AVCamCaptureManager ()
#property (nonatomic,retain) AVCaptureSession *session;
#property (nonatomic,retain) AVCaptureDeviceInput *videoInput;
#property (nonatomic,retain) AVCaptureDeviceInput *audioInput;
#end
#implementation AVCamCaptureManager
#synthesize session = _session;
#synthesize orientation = _orientation;
#dynamic audioChannel;
#dynamic sessionPreset;
#dynamic focusMode;
- (id) init
{
#property (nonatomic,readonly,retain) AVCaptureSession *session;
This is a property that is readonly externally. Internally, it will have a setter that retains the new value.
#property (nonatomic,assign) AVCaptureVideoOrientation orientation;
This is a property that does a simple assignment to store the new value (since you can't -copy or -retain primitives).
#property (nonatomic,readonly,retain) AVCaptureAudioChannel *audioChannel;
This is a property that is readonly externally. Internally, it will have a setter that retains the new value.
#property (nonatomic,assign) NSString *sessionPreset;
This is a property that does a simple assignment to store the new string value. This is normally not a good idea, unless you're only allowing pre-defined constants for the presets. When dealing with NSString properties, you generally want them to be copy unless you have a good reason against it.
In the implementation file:
#property (nonatomic,retain) AVCaptureSession *session;
#property (nonatomic,retain) AVCaptureDeviceInput *videoInput;
#property (nonatomic,retain) AVCaptureDeviceInput *audioInput;
These are used in conjunction with the properties declared in the header, except now it's readwrite. By declaring the version in the header as readonly, anyone using the class will not have access to the setSession: method. We re-declare this property internally so that we can have access to the setter (and the setter retains the new value). Also, if the property is not present in the header, the user won't know it exists, but we'll still be able to use it internally.
#synthesize session = _session;
#synthesize orientation = _orientation;
This means you want the compiler to generate the appropriate setters and getters for the session and orientation properties, and that you want those properties to store their values in the _session and _orientation instance variables, respectively.
#dynamic audioChannel;
#dynamic sessionPreset;
#dynamic focusMode;
This means that the implementations for the setters and getters will be provided at runtime. You usually don't use #dynamic properties yourself, other than the ones provided by the Core Data framework.
In this code, the session property is defined as readonly outside of the class and readwrite inside the class. By overriding the session property in an unnamed category before synthesizing it, a set accessor will also be synthesized, but the header file says it is readonly so other classes don't know about the set accessor. The category also defines two new properties which are not visible to other classes at all.
The retain and assign keywords tell the compiler how the accessor methods should work. Retain means that the set accessor should retain the value of the property, and the assign keyword tells it to set the property without retaining it. There is also copy, which copies the value and is often used to make sure a mutable value isn't set. The default is assign, but the compiler will issue a warning if nothing is specified for an object property.
Can you explain what you're confused about here? My only guess is you are confused about audioChannel and sessionPreset, as they have no ivars and are declared #dynamic (there's also focusMode, but I don't even see a #property declaration for that in the code you pasted).
In any case, I expect that if you read the rest of the code you'll find that there are getters for -audioChannel and -sessionPreset that have been written, as well as a setter for -setSessionPreset:. Assuming that's the case, then the #dynamic declarations are completely unnecessary. #dynamic is only necessary to tell the compiler that the methods will exist at run-time; if they exist at compile-time then you don't need any directive whatsoever.
From what I have experienced it seems as if objects cannot be shared data members in objective c. I know you can init a pointer and alloc the object in each method but I cannot seem to figure out how one can say define a NSMutableString as a data member and allow all of the methods to use and modify its data as in c++. Is this true or am I missing something?
To define an instance variable (member), edit your .h file:
#interface MyClass : NSObject {
// ivars go here
NSObject *member;
}
// methods go here
#end
Then, in your .m file, from any instance method (one which begins with -), you can access this variable.
- (void)doThingWithIvar {
[member doThing];
}
If you want to access the variable from outside the object itself, you'll need accessors. You can do this easily with Obj-C properties:
#interface MyClass : NSObject {
// ivars go here
NSObject *member;
}
// methods go here
#property (nonatomic, retain) NSObject *member;
#end
And in the .m:
#implementation MyClass
#synthesize member;
// ...
#end
The #synthesize line creates getter/setter methods for the ivar. Then you can use property syntax:
MyClass *thing = ...;
NSLog(#"%#", thing.member); // getting
thing.member = obj; // setting
(Note that I specified (retain) for the #property; if your member isn't an Objective-C object you won't want that. And if your property's class has a mutable counterpart, you'll want (copy) instead.)
It sounds like you want to synthesize (create getter/setter methods) a property for a member variable. I just found this cheat sheet, go down to the section called, "Properties", should give a quick overview.
Other than that Apple's documentation should give you more info.
What do #synthesize and #property do in Xcode? Please provide an explanation in really simple terms?
You asked for simple terms:
#property declares a property in your
class header
#property (nonatomic, retain) NSString *myString;
#synthesize creates your setter and
getter for your property (accessor
methods)
Without synthesize you have to write
your own setter and getter
implemention, like getMyString or
setMyString (capitalize the first
character of your property)
Sam: Just an advice: http://www.cocoadevcentral.com/d/learn_objectivec/ is a pretty solid resource to learn about basics like properties.
Good Luck!
Properties and synthesized accessors are new features in Objective-C 2.0.
When you declare a #property you declare somewhat an instance var. Then you #synthesize accessor methods (i.e. getter and setter) for that property.
There are also #dynamic accessors if you're interested.
You should really do your homework on this. Apple has nifty pdf for that.
Think of all objective-c magic as just a "smarter macro", like a "smarter #define statement"
#property if you notice is always in the h file,
#synthesize is always in the m file.
So in the background
#property (whatever) NSString *myString;
becomes a declaration of 2 methods and a private variable;
void set_myString:(NSString *) str;
(NSString*) get_myString;
declarations in the header file
to make them do something their implementation is added into m file when you type in
#synthesize myString;
which becomes something like
void set_myString:(NSString *)str
{
myString = str;
}
(NSString *) get_myString
{
return (myString);
}
But it's smarter than this
depending on if you say "retain" "strong" or "weak"
it will either just return the pointer to the myString or it will copy the myString into a new object
So all of this is done automatically by a compiler just by reading your declarations.
Which is quite useful and saves a lot of time
By default all our variables are Private so we can't acess out of the class.
if we want to use our instance variable in out of the class.
When you declare a #property you declare somewhat an instance var. Then you #synthesize accessor methods (i.e. getter and setter) for that property.
There are also #dynamic accessors if you're interested.
it simply sets the property's setter variable name in it's own class.
For example lets say I have this: #property (nonatomic, copy) NSArray* viewControllers;
Well if i want to access the setter _viewController i wouldn't set a synthesize variable.
but if i want to access the viewController variable by the name viewController instead of _viewController, I would do #synthesize viewController;.
If I wanted to use it as a completely different name I could do this #synthesize viewControllers = viewControlololer; but that's only setter. As you will notice [self viewControllers] only works and not [self viewControlololer];
So I don't understant why everyone writes sets the "setter and getter" of a property. It doesn't change the getter variable at all... unless that means [self viewController] is aware of viewControlololer (obviously).
Actually properties are synthesized, either implicitly or explicitly. Properties are implicitly synthesized. So there's no need to use synthesized unless you wanna change the name of the variable to something different than _property_name.
There are other use cases, for example if you don't want an instance variable to back your property.
Properties are explicitly synthesized by using the #synthesized directive.
(Answer extracted from the Big Nerd Ranch guide)