In Objective c, what is the differences between instance variables var1 and var2 showing below?
(This code is in one .m file, does it make any difference if the #interface is in a header file & #implementation is in implementation file? I mean any difference comparing with that in one file regarding to the two instance variables.)
#interface MyService {
NSString *var1;
}
#end
#implementation MyService {
NSString *var2;
}
#end
The difference between them is visibility. The variable defined in the #interface section is visible to any code which imports the interface. The variable declared in the #implementation section is only visible to code within the class implementation.
If the #interface is declared in the implementation file, it will act, for all practical purposes, the same as declaring it in the #implementation section.
Instance variables declared in the implementation are implicitly
hidden (effectively private) and the visibility cannot be changed -
#public, #protected and #private do not produce compiler errors (with
the current Clang at least) but are ignored.
you can found it here
I have deep searched for your question.Well asked Leem.fin brother.I tried sample one
#import "SecondViewController.h"
#interface SecondViewController ()
{
NSString *variableOne;
}
#end
#implementation SecondViewController
{
NSString *variableTwo;
}
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view.
[self setValueToString];
NSLog(#"The variable One is - %#",variableOne);
NSLog(#"The variable Two is - %#",variableTwo);
}
-(void)setValueToString
{
variableOne = #"iOS";
variableTwo = #"iPhone";
}
The printed results are
The variable One is - iOS
The variable Two is - iPhone
But when I tried to access these in Class method
+(void)changeStrings
{
variableOne = #"iPad"; //I get error here
variableTwo = #"iMac"; //I get error here
}
The error shows
Instance variable 'variableOne' accessed in class method
Instance variable 'variableTwo' accessed in class method
From above code I understood
Both are instance variables
That can be accessed only in instance methods
There is no difference between them
So Where to put
Difference between them
Difference between putting variable inside interface and implementation
Related
I am trying to access an Objective C singleton from Swift, however I only seem to get the initial value created in the init function of the singleton. The flightControllerState object exposed is updated in a delegate function and I can see that the value is properly updated on the Objective C side.
I have followed a few different posts here on SO and also this article on how to call the shared object from Swift. (I should also mention this is running inside a react native project if that may have any impact?)
EDIT updated swift code - I added the wrong line to the init method to grab shared instance - issue is still the same
Objective-C Singleton
#import DJISDK;
#interface RCTBridgeDJIFlightController : RCTEventEmitter<DJIFlightControllerDelegate> {
DJIFlightControllerState *flightControllerState;
}
#property(nonatomic, readonly) DJIFlightControllerState *flightControllerState;
+ (id)sharedFlightController;
#end
#implementation RCTBridgeDJIFlightController
DJIFlightControllerState *flightControllerState;
#synthesize flightControllerState;
+ (id)sharedFlightController {
static RCTBridgeDJIFlightController *sharedFlightControllerInstance = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedFlightControllerInstance = [[self alloc] init];
});
return sharedFlightControllerInstance;
}
- (id)init {
// I also tried this to make sure the shared instance was returned but no luck
//if (sharedFlightControllerInstance != nil) {
// return sharedFlightControllerInstance;
//}
if (self = [super init]) {
flightControllerState = nil;
}
return self;
}
-(void)flightController:(DJIFlightController *)fc didUpdateState:(DJIFlightControllerState *)state {
flightControllerState = state;
}
#end
Swift class calling singleton and accessing values
class VirtualStickController {
var flightControllerSharedInstance: RCTBridgeDJIFlightController
override init() {
self.flightControllerSharedInstance = RCTBridgeDJIFlightController.sharedFlightController()
}
func getFlightControllerState() {
if let state = flightControllerSharedInstance.flightControllerState {
print("FLIGHT CONTROLLER STATE: \(state)") // always null
} else {
print ("NULL")
}
}
DJIFlightControllerState *flightControllerState;
#synthesize flightControllerState;
There is no need to use #synthesize for properties in (modern) Objective-C except in special circumstance.
The property flightControllerState is an instance property and will be synthesised (with or without the #synthesize) using a hidden instance variable for its storage.
The variable flightControllerState is a global variable, it happens to have the same name as the property but has no connection whatsoever with it.
At a guess you are changing the global variable in Objective-C and expecting to see the result in Swift via the property, you won't.
Remove the global variable and then check the rest of your code.
Apart from that your code produces a valid shared instance which can be shared between Objective-C and Swift and changes made in one language will be visible in the other.
HTH
Regarding the titular question about how to access an Objective C singleton from Swift, I would recommend an alternative. Modern convention is to declare your sharedFlightController as a class property and declare init as NS_UNAVAILABLE:
#interface RCTBridgeDJIFlightController : NSObject
...
#property (nonatomic, readonly, class) RCTBridgeDJIFlightController *sharedFlightController;
- (instancetype)init NS_UNAVAILABLE;
#end
The implementation would implement a getter for this class property:
#implementation RCTBridgeDJIFlightController
+ (instancetype)sharedFlightController {
static RCTBridgeDJIFlightController *sharedFlightControllerInstance = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedFlightControllerInstance = [[self alloc] init];
});
return sharedFlightControllerInstance;
}
...
#end
Now, your Swift code can reference RCTBridgeDJIFlightController.shared, as is the convention with Swift singletons.
Regarding why you are receiving a nil for the status, there are one of two possible problems:
You Objective-C code has confusing combination of explicitly defined ivars, manual synthesis, and global variables. (See below.)
I would also suggest that you confirm whether flightController:didUpdateState: is ever getting called at all. (I don't see you ever setting the delegate of the flight controller.) Add a breakpoint or NSLog statement in that method and confirm.
On the first issue, above, I would suggest:
You should not use those commented lines in your init method. If you want to make sure that your singleton object is used, then declare init as NS_UNAVAILABLE.
Given that all your init method is doing is updating flightControllerState to nil, you can remove it entirely. In ARC, properties are initialized to nil for you.
You should not declare explicit ivar in your #interface. Let the compiler synthesize this automatically for you.
You should not #synthesize the ivar in your #implementation. The compiler will now automatically synthesize for you (and will use an appropriate name for the ivar, adding an underscore to the property name.
You should not declare that global in your #implementation.
If you want to use this sharedFlightController from Swift, you should define it to be a class property, not a class method. I know that that article suggested using a class method, but that really is not best practice.
Thus:
// RCTBridgeDJIFlightController.h
#import <Foundation/Foundation.h>
// dji imports here
NS_ASSUME_NONNULL_BEGIN
#interface RCTBridgeDJIFlightController : NSObject
#property (nonatomic, readonly, nullable) DJIFlightControllerState *flightControllerState;
#property (nonatomic, readonly, class) RCTBridgeDJIFlightController *sharedFlightController;
- (instancetype)init NS_UNAVAILABLE;
#end
NS_ASSUME_NONNULL_END
And
// RCTBridgeDJIFlightController.m
#import "RCTBridgeDJIFlightController.h"
#interface RCTBridgeDJIFlightController ()
#property (nonatomic, nullable) DJIFlightControllerState *flightControllerState;
#end
#implementation RCTBridgeDJIFlightController
+ (instancetype)sharedFlightController {
static RCTBridgeDJIFlightController *sharedFlightControllerInstance = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedFlightControllerInstance = [[self alloc] init];
});
return sharedFlightControllerInstance;
}
- (void)flightController:(DJIFlightController *)fc didUpdateState:(DJIFlightControllerState *)state {
NSLog(#"State updated");
self.flightControllerState = state;
}
#end
The end result is that you can now use it like so:
class VirtualStickController {
func getFlightControllerState() {
if let state = RCTBridgeDJIFlightController.shared.flightControllerState {
print("FLIGHT CONTROLLER STATE: \(state)")
} else {
print("NULL")
}
}
}
Note, because the sharedFlightController is now a class property, Swift/ObjC interoperability is smart enough so the Swift code can just reference shared, as shown above.
I got this global variable
define AN_MOBILE_HOSTNAME #"(hostname).mob"
And I want to change the hostname with a segmented control variable, that defines different regions (Europe-USA-Australia)
Thanks
Sorry for my ignorance, im a begginer
You can't change Pre-Processor macros after compile time. You are going to need to set a global NSString for what you want accomplished.
.h
extern NSString *AN_MOBILE_HOSTNAME;
#interface SomeClass
{
}
#end
.m
NSString *AN_MOBILE_HOSTNAME;
#implementation SomeClass
-(id)init
{
//Do the if(self) stuff
{
AN_MOBILE_HOSTNAME = #"(hostname).mob";
}
}
-(void)FunctionToChangeStuff:(NSString*)somethingElse
{
AN_MOBILE_HOSTNAME = somethingElse;
}
#end
Or you could make the object a class variable if you have a singleton class that your program knows about.
This question already has answers here:
Autosynthesized property 'delegate' will use synthesized instance variable '_delegate', not existing instance variable 'delegate'
(2 answers)
Closed 9 years ago.
I'm new to protocol delegates and I am trying to understand the difference between the 2 notations below.
In my protocol delegate file, I have defined
#interface SampleProtocol : NSObject
{
id <SampleProtocolDelegate> delegate;
}
which gives me a warning, so I changed this piece of code to
#interface SampleProtocol : NSObject
{
id <SampleProtocolDelegate> _delegate;
}
and it works as I had intended it to. So I would like to know why the syntax is id _delegate whereas most tutorials I have seen is just id delegate in the protocol definition?
You have a property named delegate also, but have not included an explicit #synthesize. Implicit synthesize creates a backing instance variable with the name _propertyName. If you create an instance property with the name propertyName, the compiler is kind enough to warn you that there will be two instance variables.
If you have a property, you don't have to create the instance variable yourself. The compiler will create one for you. Leaves your code cleaner.
You always want to name ivars with a leading underscore. If you create properties, the compiler will automatically create the underscore ivar for you. To illustrate:
This code:
// header
#interface MyGreatClass : NSObject
#property NSInteger someNumber;
#end
// implementation
#implementation MyGreatClass
#end
Is the same as this:
// header
#interface MyGreatClass : NSObject {
NSInteger _someNumber;
}
- (void)setSomeNumber:(NSInteger)someNumber;
- (NSInteger)someNumber;
#end
// implementation
#implementation MyGreatClass
- (void)setSomeNumber:(NSInteger)someNumber
{
_someNumber = someNumber;
}
- (NSInteger)someNumber
{
return _someNumber
}
#end
No need for the #synthesize keyword.
One more thing: Unless you are creating a class that represents something like a network protocol, be aware that a protocol is similar to an interface in Java: https://developer.apple.com/library/ios/documentation/cocoa/conceptual/ProgrammingWithObjectiveC/WorkingwithProtocols/WorkingwithProtocols.html
Here is my TextValidator class:
//TextValidator.h
#import <Foundation/Foundation.h>
#interface TextValidator : NSObject
- (BOOL) isValidPassword:(NSString *)checkPassword;
- (BOOL) isValidEmail:(NSString *)checkString;
- (BOOL) isEmpty:(NSString *)checkString;
#end
// TextValidator.m
#import "TextValidator.h"
#implementation TextValidator
- (BOOL) isEmpty:(NSString *)checkString
{
return YES;
}
- (BOOL) isValidPassword:(NSString *)checkPassword
{
return YES;
}
- (BOOL) isValidEmail:(NSString *)checkString
{
return YES;
}
#end
This is the way I try to initialise the TextValidator class in ViewController.m:
//ViewController.h
#import <Foundation/Foundation.h>
#interface SignUpViewController : UIViewController <UITextFieldDelegate>
#end
//ViewController.m
#import "SignUpViewController.h"
#import "TextValidator.h"
#interface SignUpViewController ()
#property TextValidator *myValidator;
#end
#implementation SignUpViewController
- (void)viewDidLoad
{
[[self.myValidator alloc] init]; //iOS error: No visible #interface for 'TextValidator' declares the selector 'alloc'*
[super viewDidLoad];
}
#end
When I try to compile the code I get the following error:
No visible #interface for 'TextValidator' declares the selector 'alloc'.
TextValidator class inherits from NSObject and as far as I know init and alloc functions are already defined at the base class. So why does the program gives such an error?
Note that, I already checked this topic and it doesn't work for me.
My psychic debugger, without reading your code, tells me you're calling alloc on an object instance, rather than a class. The alloc method is a static method defined by classes (typically inherited from NSObject) that returns a new instance of the underlying object. You can't ask an instance to allocate itself!
Now looking at the code, I see that you want:
self.myValidator = [[TextValidator alloc] init];
to construct a new instance, and assign it to the myValidator property.
Replace
[[self.myValidator alloc] init];
with
self.myValidator = [[TextValidator alloc] init];
The error signals that you have not implemented the alloc instance method for self.myValidator, which is true. But that's a class method that applies for all NSObject objects.
Your syntax of creating object is incorrect. Correct code:
self.myValidator = [[TextValidator alloc] init];
If you experience this randomly (like when you are changing branches), not because you forgot to declare selector.
Go to file inspector > Target Membership
uncheck the targets
then check it again
This will refresh your project.pbxproj
Then if you build, you'll see your real problem
For Others :
Check the varible name is not like the class name.
Well it happend to me.
XXXViewController * XXXViewController = [[XXXViewController alloc] init];
Don't tell anyone like I did right now.
For those who get the error of "no visible #interface for declares the selector ..."
such an error usually happens when you have mistyped the name of the method, or that method doesn't belong to that class at all and doesn't exist in your class
I had this problem today and solved it on my own. Basically you could also not be satisfying all the requirements of the function / procedure.
Go into the class itself and make sure your declaring all the requirements.
I took the class out of the header library and compared it word for word to verify it matches the function using it.
I have 2 classes geoViewController and geoMainViewController
I have a method in the geoMainViewController called getFoo
It looks like this:
- (NSString *)getFoo
{
NSString* foo = #"This is foo";
return foo;
}
I am trying to call getFoo from the geoViewController class.
I have #import "geoMainViewController.h" in my geoViewController m file.
I am trying instantiate the geoMainViewController class and call the getFoo method from the viewDidLoad in my geoViewController class like this:
- (void)viewDidLoad
{
[super viewDidLoad];
geoMainViewController* mainVC = [[geoMainViewController alloc] init];
NSString* myFoo = [mainVC getFoo];
}
It seems to be instantiating the geoMainViewController class fine but I am getting an error on NSString* myFoo = [mainVC getFoo];
The error is - no visible #interface for 'geoMainViewController' declares the selector 'getFoo'
I am sure I am missing a step because I am very new to Objective C. I am just not sure what I am doing wrong.
Any help on this would be great.
Thanks!
In your geoMainViewController.h you should declare the selector to be visible:
-(NSString *)getFoo;
Did you put - (NSString *)getFoo in your geoMainViewController.h ?
You have to make those methods visible to the outside of your object through the .h file, so other objects know which selectors they respond to. Did the autoComplete fill in the message per chance?
#import <Foundation/Foundation.h>
#interface
{
}
#property (nonatomic,strong) ;
#property (nonatomic,strong) ;
#property (nonatomic, strong) ;
- (NSString *)getFoo
#end
EDIT: (You could also just make Foo a property by the way)
Did you declare it in your header file?
Header file contains all the function declarations in the .h file and you only include the .h file in your class. So it depends on .h file. .h file will have all the functions as the .m file.
Hope it helps you.
You are misunderstanding how to use a view controller. While you can technically create an instance of a view controller in order to call one of its methods, you shouldn't do so. The normal approach is that the view controller is part of the view hierarchy and you can call methods on it when you have access to that instance. You are missing something fundamental here.
Your actual error is a missinh method declaration, I would suspect, but you have bigger problems to solve first.