I'm trying to build MVC Application with Objective-C, I'm trying to allocate and init my model once in my superclass witch is UIViewController, my idea is to do it once in superclass and to have access from every subclass of my superclass.
superclass.h
#property (nonatomic, strong) Lecturer *lecturer;
superclass.m
- (void) viewDidLoad {
[super viewDidLoad];
}
#pragma mark - GET
- (Lecturer *)lecturer {
if (!_lecturer) {
_lecturer = [Lecturer alloc]init];
}
return _lecturer;
}
My idea is to call self.lecturer from all subclasses and set/get the lecturer class property's but every time when i call self.lecturer its creating a new instance, i know i can use SINGLETONE but is there any way i can do it differently without singletone design pattern?
Thanks for attention.
How about using static variable?
- (Lecturer *)lecturer {
static dispatch_once_t onceToken;
static Lecturer *o;
dispatch_once(&onceToken, ^{
o = [[Lecturer alloc] init];
});
return o;
}
In addition dispatch_once helps to do it thread-safely.
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 have a table view controller that a user can select from and it passes a dictionary to a UITabBarController.
How do I get the different views to access the same data stored in the UITabController?
In my UITabBarController
#interface MasterTabController : UITabBarController
#property (nonatomic,strong) NSMutableDictionary * detailDictionary;
#end
Is it common practice to keep passing the same data dictionary around? I want to be able to manipulate the data so I can later post it online.
Should I create a singleton? Can I call just the detailDictionary from bView?
It sounds like you want to want to have three different table views, in there different tabs, and set the delegates to their views, and the data source to a custom class in your model.
You can create singleton class like this
+ (id)sharedManager {
static MyManager *sharedMyManager = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedMyManager = [[self alloc] init];
});
return sharedMyManager;
}
- (id)init {
if (self = [super init]) {
someProperty = [[NSString alloc] initWithString:#"Default Property Value"];
}
return self;
}
For reference http://www.galloway.me.uk/tutorials/singleton-classes/
Say I have the following singleton:
#interface ABCSingleton: NSObject
#property (nonatomic, copy) NSString *name;
#property (nonatomic, strong) ABCViewController *mainViewController;
#end
#implementation ABCSingleton
+ (ABCSingleton *)sharedInstance {
static ABCSingleton *instance = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
instance = [ABCSingleton new];
});
return instance;
}
- (void)doSomething {
}
#end
If doSomething contained this code:
- (void)doSomething {
self.mainViewController.tapBlock = ^() {
self.name = #"abc";
};
}
... it would create a retain cycle since ABCSingleton owns mainViewController, which owns tapBlock, which owns ABCSingleton.
What if instead of using self, I used sharedInstance?
- (void)doSomething {
self.mainViewController.tapBlock = ^() {
[ABCSingleton sharedInstance].name = #"abc";
};
}
Would it still create a retain cycle? (An explanation as to why, or why not, would be appreciated!)
To the specific question, this is a retain loop in the first case, and equivalent to a retain loop in the second case (equivalent in that mainViewController will never be released).
This indicates a deeper design problem. A singleton should never reference a view controller. Singletons are by nature model objects. Model objects should never reference controller objects directly. See Model-View-Controller for an introduction. This is a key design pattern in Cocoa.
Instead, the view controller should know about the model (the singleton in this case). doSomething should modify the model, and the view controller, when it comes on the screen, should read the model to set tapBlock. The view controller can also observe the model (via KVO, notifications, or delegation) while it is onscreen to know when to change its values.
I know there are several threads on this, but none answer my questions.
I've implemented my singleton class like this (being aware of the controversy about singletons):
+ (MyClass*) sharedInstance {
static MyClass *_sharedInstance = nil;
static dispatch_once_t oncePredicate;
dispatch_once(&oncePredicate, ^{
_sharedInstance = [[MyClass alloc] init];
});
return _sharedInstance;
}
- (instancetype)init{
self = [super init];
if (self) {
//setup code
}
return self;
}
I tried instantiating a different object and compared it to the one returned by sharedInstance with '==' and they were indeed different.
Questions:
Shouldn't creating more than one object of the singleton class be impossible? Isn't that the point? Singleton implementation in Java prevents it.
And if so, how? Should I make a setup method and call it instead of having the init implemented and doing it?
Is this correct implementation?
Your observation is correct, many of the "singleton" patterns you see in Objective-C are not singletons at all but rather a "shared instance" model where other instances can be created.
In the old MRC days Apple used to have sample code showing how to implement a true singleton.
The code you have is the recommended pattern for ARC and thread-safe singletons, you just need to place it in the init method:
- (instancetype) init
{
static MyClass *initedObject;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
initedObject = [super init];
});
return initedObject;
}
This code will ensure that there is only ever one instance of MyClass regardless of how many [MyClass new] or [[MyClass alloc] init] calls are made.
That is all you need to do, but you can go further. First if you wish to have a class method to return the singleton it is simply:
+ (instancetype) singletonInstance
{
return [self new];
}
This method ends up calling init which returns the singleton, creating it if needed.
If MyClass implements NSCopying then you also need to implement copyWithZone: - which is the method which copy calls. As you've a singleton this is really simple:
- (instancetype) copyWithZone:(NSZone *)zone
{
return self;
}
Finally in Objective-C the operations of allocating a new object instance and initialising it are distinct. The above scheme ensures only one instance of MyClass is initialised and used, however for every call to new or alloc another instance is allocated and then promptly discarded by init and cleaned up by ARC. This is somewhat wasteful!
This is easily addressed by implementing allocWithZone: (like copy above this is the method alloc actually ends up calling) following the same pattern as for init:
+ (instancetype) allocWithZone:(NSZone *)zone
{
static MyClass *allocatedObject;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
allocatedObject = [super allocWithZone:zone];
});
return allocatedObject;
}
The first time an instance is created then allocWithZone: will allocate it and then init will initialise it, all subsequent calls will return the already existing object. No discarded unneeded allocations.
That's it, a true singleton, and no harder than the faux-singletons that are so common.
HTH
You can't make the init method private, like you would do in Java with the constructor. So nothing stops you from calling [[MyClass alloc] init] which indeed creates a different object. As long as you don't do that, but stick to the sharedInstance method, your implementation is fine.
What you could do: have the init method raise an exception (e.g. with [self doesNotRecognizeSelector:#_cmd]) and perform the initialization in a different method (e.g. privateInit) which is not exposed in the header file.
With objective-c, you can prevent your singleton class to create more than one object. You can prevent alloc and init call with your singleton class.
#import <Foundation/Foundation.h>
#interface SingletonClass : NSObject
+ (id) sharedInstance;
- (void) someMethodCall;
- (instancetype) init __attribute__((unavailable("Use +[SingletonClass sharedInstance] instead")));
+ (instancetype) new __attribute__ ((unavailable("Use +[SingletonClass sharedInstance] instead")));
#end
#import "SingletonClass.h"
#implementation SingletonClass
+ (id) sharedInstance{
static SingletonClass * sharedObject = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedObject = [[self alloc] initPrivate];
});
return sharedObject;
}
- (instancetype)init {
#throw [NSException exceptionWithName:NSInternalInconsistencyException reason:[NSString stringWithFormat:#"You can't override the init call in class %#", NSStringFromClass([self class])] userInfo:nil];
}
- (instancetype)initPrivate {
if (self = [super init]) {
}
return self;
}
- (void) someMethodCall{
NSLog(#"Method Call");
}
#end
1# If you will try to call init or new methods on SingletonClass, then these methods would not be available to call.
2# If you comment out mentioned below methods in header file and try to call the init on SingletonClass method then app will be crashed with reason "You can't override the init call in class SingletonClass".
- (instancetype) init __attribute__((unavailable("Use +[SingletonClass sharedInstance] instead")));
+ (instancetype) new __attribute__ ((unavailable("Use +[SingletonClass sharedInstance] instead")));
Just use this code to create single object to Singleton Pattern and prevent alloc init call for singleton pattern from other classes. I had tested this code with xCode 7.0+ and its working fine.
To prevent creating multiple objects of single class, you need to do following things. you just fine to created singleton object. but while calling init, copy, mutable copy, you need to handle such way.
- (instancetype)init{
if (!_sharedInstance) {
_sharedInstance = [MyClass sharedInstance];
}
return _sharedInstance;
}
- (id)copy{
if (!_sharedInstance) {
_sharedInstance = [MyClass sharedInstance];
}
return _sharedInstance;
}
the same things for mutable copy as well. so this implementation make sure that once one instance is available throughout..
May this help you.
#VeryPoliteNerd just mark the init and new methods as unavailable on the .h:
- (instancetype)init __attribute__((unavailable("Use +[MyClass sharedInstance] instead")));
+ (instancetype)new __attribute__((unavailable("Use +[MyClass sharedInstance] instead")));
This will cause the compiler to complain if a caller tries to manually instantiate this objects
This works for me:
static AudioRecordingGraph * __strong sharedInstance;
+(instancetype)sharedInstance {
#synchronized(self) {
if(!sharedInstance) {
sharedInstance = [AudioRecordingGraph new];
}
return sharedInstance;
}
}
I wanted to add observation for a modern implementation of Objective-C singletons for optimal Swift interoperability. But let me start with the code. Let’s assume for a second that the class was to manage a series of requests, so I might call it a RequestManager:
// RequestManager.h
NS_ASSUME_NONNULL_BEGIN
#interface RequestManager : NSObject
#property (nonatomic, strong, readonly, class) RequestManager *sharedManager;
- (instancetype)init NS_UNAVAILABLE;
- (instancetype)copy NS_UNAVAILABLE;
- (instancetype)mutableCopy NS_UNAVAILABLE;
- (void)someMethod;
#end
NS_ASSUME_NONNULL_END
And
// RequestManager.m
#implementation RequestManager
+ (instancetype)sharedManager {
static RequestManager *_sharedInstance = nil;
static dispatch_once_t oncePredicate;
dispatch_once(&oncePredicate, ^{
_sharedInstance = [[self alloc] init];
});
return _sharedInstance;
}
- (instancetype)init {
self = [super init];
if (self) {
//setup code
}
return self;
}
- (void)someMethod { ... }
#end
Note:
Note, I made the singleton a class property rather than a class method.
From the Objective-C side, it is a distinction without difference, but from Swift, you can do things like MyClass.shared without the noise of the (). This is the pattern that Apple has adopted with all of their singletons.
One might give the singleton a more meaningful name. For example, if the class was a RequestManager, the singleton might be called sharedManager rather than sharedInstance.
If you do this, Swift will automatically detect the salient portion of the instance name and will expose it to Swift as shared, not sharedInstance or whatever.
If you really want to use the name sharedInstance in your Objective-C code, then will just need to supply an explicit NS_SWIFT_NAME of shared to adopt the nice, concise, and consistent singleton naming practice in Swift:
#property (nonatomic, strong, readonly, class) RequestManager *sharedInstance NS_SWIFT_NAME(shared);
Note the use of NS_UNAVAILABLE. This prevents callers from accidentally instantiating their own copies. E.g. from Objective-C:
Or from Swift:
Probably needless to say, I’ve audited this for nullability, namely using the NS_ASSUME_NONNULL_BEGIN/END to make the default non-null. Or, obviously, you could just mark the singleton property as nonnull.
Calling alloc / init to get a second instance of a Singleton class is considered a blatant programming error. To avoid this kind of programming error, you don't write complicated code to prevent it, you do code reviews and tell off everyone trying to do it, as you would with any programming error.
This works for me :
static DataModel *singleInstance;
+ (DataModel*)getInstance{
if (singleInstance == nil) {
singleInstance = [[super alloc] init];
}
return singleInstance;
}
You can call it with
_model = [DataModel getInstance];
I am trying to create a singleton NSMutableArray so I can use it on TableView as Titles. How to declare it in a TableView?
e.g.
cell.textLabel.text = ??? //Don't know how to declare it from the singleton.
Here is my singleton.m:
#import "Singleton.h"
#implementation Singleton
#synthesize Trees;
+(Singleton *) theTrees {
static dispatch_once_t pred;
static Singleton *theTrees = nil;
dispatch_once(&pred, ^ {
theTrees = [[Singleton alloc] init];
theTrees.Trees = [[NSMutableArray alloc] init];
});
return theTrees;
}
-(id) init {
self = [super init];
if(self) {
// Variables
[Trees addObject:#"Berzas"];
[Trees addObject:#"Liepa"];
[Trees addObject:#"Drebule"];
}
return self;
}
#end
Please point me in the right direction, thank you.
It seems there is no need to create a singleton class here. Just have a class or instance method somewhere
- (NSDictionary *)trees
and the implementation about the same as your method that returns a singleton, but just creating a singleton NSDictionary once.
Properties like "Trees" should be written in lowercase: "trees", not "Trees".
Instance variables should start with an underscore, so
[_trees addObject:#"Berzas"];
That's done so that you always can distinguish between using accessor methods and accessing instance variables; they are not the same thing. Usually you would only access instance variables in your init method, and when implementing getters and setters for properties.