I am looking for a basic pattern where I can move some shared code. I have an NSManagedObject PurchaseOrder which is stored in Core Data. This can be edited and changed in several different views. Most of the time it is always the same type of change, PurchaseOrder is updated with data from another NSManagedObject Client.
I am wanting to move this type of code to a single place so I can call something like:
-(void) updatePurchaseOrder: (PurchasOrder *) aPurchaseOrder withClient:(Client *) aClient withManagedObjectContext: (NSManagedObjectContext *) aManagedObjectContext {
// update code goes here
}
Is this a bad idea or a recipe for disaster? I do have a shared instance class I am using already (small bit of bit below):
static SharedFunctions* singletonInstance = nil;
-(id)init
{
if ((self = [super init]))
{
}
return self;
}
+(SharedFunctions*)sharedInstance
{
#synchronized(self) {
if (singletonInstance == nil)
{
singletonInstance = [[SharedFunctions alloc] init];
}
}
return singletonInstance;
}
Which I call function like this:
[[[SharedFunctions] sharedInstance] myMethod];
This has been working well for me. Thoughts?
Where is your updatePurchaseOrder method defined? In this situation, I would create a custom PurchaseOrder subclass of the NSManagedObject and implement an updateWithClient method.
#interface PurchaseOrder : NSManagedObject {
}
-(void)updateWithClient:(Client *)client;
#end
So you can simply call [aPurchaseOrder updateWithClient:aClient]; where you need to. Seems more OO, unless I'm missing something?
If you've not made a custom subclass for PurchaseOrder, you could do the same thing in a category of NSManagedObject.
Related
I'm working on an iPhone app and facing some troubles with my shared singleton class.
I'm using a shared singleton to store two variables
int gameRuns and int totalScore
'gamRuns' just increments every time the user loads the app, and 'totalScore' is obvious :D
the issue is as follows, I load the singleton and init using my own method when the app loads using this code:
+ (SingletonLevelState*)sharedLevelStateInstance {
static SingletonLevelState *sharedLevelStateInstance;
#synchronized(self) {
if(!sharedLevelStateInstance) {
//Init a singleton
sharedLevelStateInstance = [[SingletonLevelState alloc] init];
sharedLevelStateInstance->gameRuns = 1;
sharedLevelStateInstance->totalScore = 0;
}
}
return sharedLevelStateInstance;
}
This is working great as I can reference this class from any other class and always get a pointer to the same object, so this works fine from other objects:
sharedLevelState = [SingletonLevelState sharedLevelStateInstance];
sharedLevelStateInstance.gameRuns++;
Now I added the NSCoder protocol, and added the two methods initWithCoder and encodeWithCoder as follows :
- (void) encodeWithCoder: (NSCoder *)coder
{
//encode level data
[coder encodeInt:self->gameRuns forKey:#"gameRuns"];
[coder encodeInt:self->totalScore forKey:#"totalScore"];
}
- (id) initWithCoder: (NSCoder *) coder
{
if(self = [super init]){
self->gameRuns = [coder decodeIntForKey:#"gameRuns"];
self->totalScore = [coder decodeIntForKey:#"totalScore"];
}
return self;
}
Now when the app loads, I check to see if we already have a saved sate, if it exists, I just unarchive the class with that file, if not, I init that class using my custom method above, then set its defaults, encode it to file so we have a saved state, here's the code:
//Load Level state
sharedLevelStateInstance = [SingletonLevelState sharedLevelStateInstance];
//Check if file is saved
NSFileManager *fm = [[NSFileManager alloc] init];
NSString *gameStatePath = [NSString stringWithString:[self getSavePath]];
if([fm fileExistsAtPath:gameStatePath]){
[self loadState];
sharedLevelStateInstance.gameRuns = sharedLevelStateInstance.gameRuns+1;
NSLog(#"Loaded %d times", [sharedLevelStateInstance gameRuns]);
}
[fm release];
Now the last line in the if statement works perfectly, it increments every time I load the app as expected and I feel really happy lol.
However, the problem arises when I try to get a reference of the singleton in another class by doing the following:
sharedLevelStateInstance = [SingletonLevelState sharedLevelStateInstance];
NSLog(#"Played: %d times", sharedLevelStateInstance.gameRuns);
It always counts back to 1, I know what happens but I'm not sue what's the best way to solve it, when I initWithCoder the singleton, It's not returning a static object, it creates a new one, when I init my sharedLevelStateInstance, it calls my first custom method, initializing it to the defaults hardcoded.
So StackOverflow, can you please help me ?!
I just need to know what's the best way to get a reference to the same object without allocating a new one every time I initWithCoder !
Thanks :)
So, you code should probably look like this:
if(self = [[SingletonLevelState sharedLevelStateInstance] retain])
Which sets the variables of the singleton, and returns the singleton. Be sure to retain the singleton, so that when the NSCoder releases this instance, it doesn't fully deallocate your singleton.
I get the error
* Terminating app due to uncaught exception 'NSGenericException', reason: '* Collection <__NSCFSet: 0x6b66390> was mutated while being enumerated.'
when adding an new delegate to my class. Or at least, that's where I think the problem is.
This is my code: MyAppAPI.m
[...]
static NSMutableSet *_delegates = nil;
#implementation MyAppAPI
+ (void)initialize
{
if (self == [MyAppAPI class]) {
_delegates = [[NSMutableSet alloc] init];
}
}
+ (void)addDelegate:(id)delegate
{
[_delegates addObject:delegate];
}
+ (void)removeDelegate:(id)delegate
{
[_delegates removeObject:delegate];
}
[...]
#end
MyAppAPI is a singleton which I can use throughout my application. Wherever I can (or should be able to) do: [MyAppAPI addDelegate:self].
This works great, but only in the first view. This view has a UIScrollView with PageViewController which loads new views within itself. These new views register to MyAppAPI to listen to messages until they are unloaded (which in that case they do a removeDelegate).
However, it seems to me that it dies directly after I did a addDelegate on the second view in the UIScrollView.
How could I improve the code so that this doesn't happen?
Update
I'd like to clarify me a bit further.
What happens is that view controller "StartPage" has an UIScrollView with a page controller. It loads several other views (1 ahead of the current visible screen).
Each view is an instans PageViewController, which registers itself using the addDelegate function shown above to the global singleton called MyAppAPI.
However, as I understand this viewcontroller 1 is still reading from the delegate when viewcontroller 2 registers itself, hence the error shows above.
I hope I made the scenario clear. I have tried a few things but nothing helps.
I need to register to the delegate using addDelegate even while reading from the delegates. How do I do that?
Update 2
This is one of the reponder methods:
+ (void)didRecieveFeaturedItems:(NSArray*)items
{
for (id delegate in _delegates)
{
if ([delegate respondsToSelector:#selector(didRecieveFeaturedItems:)])
[delegate didRecieveFeaturedItems:items];
}
}
Scott Hunter is right. This error is thrown when you try to edit a list while iterating.
So here is an example of what you may be doing.
+ (void)iteratingToRemove:(NSArray*)items {
for (id delegate in _delegates) {
if(delegate.removeMePlease) {
[MyAppAPI removeDelegate:delegate]; //error you are editing an NSSet while enumerating
}
}
}
And here is how you should handle this correctly:
+ (void)iteratingToRemove:(NSArray*)items
{
NSMutableArray *delegatesToRemove = [[NSMutableArray alloc] init];
for (id delegate in _delegates) {
if(delegate.removeMePlease) {
[delegatesToRemove addObject:delegate];
}
}
for(id delegate in delegatesToRemove) {
[MyAppAPI removeDelegate:delegate]; //This works better
}
[delegatesToRemove release];
}
The error suggests that, while some code somewhere is in the middle of going through your list, you are modifying the list (which explains the crash after addDelegate is called). If the code doing the enumerating is the one modifying the list, then you just have to put off the modifications until the enumeration is done (say, by collecting them up in a different list). Without knowing anything about the code doing the enumerating, can't say much more than that.
A simple solution, don't use a mutable set. They are dangerous for a variety of reasons, including this one.
You can use -copy and -mutableCopy to convert between mutable and non-mutable versions of NSSet (and many other classes). Beware all copy methods return a new object with a retain count of 1 (just like alloc), so you need to release them.
Aside from having less potential for bugs, non-mutable objects are faster to work with and use less memory.
[...]
static NSSet *_delegates = nil;
#implementation MyAppAPI
+ (void)initialize
{
if (self == [MyAppAPI class]) {
_delegates = [[NSSet alloc] init];
}
}
+ (void)addDelegate:(id)delegate
{
NSMutableSet *delegatesMutable = [_delegates mutableCopy];
[delegatesMutable addObject:delegate];
[_delegates autorelease];
_delegates = [delegatesMutable copy];
[delegatesMutable release];
}
+ (void)removeDelegate:(id)delegate
{
NSMutableSet *delegatesMutable = [_delegates mutableCopy];
[delegatesMutable removeObject:delegate];
[_delegates autorelease];
_delegates = [delegatesMutable copy];
[delegatesMutable release];
}
[...]
#end
Scott Hunter is right - it's a problem with modifying the NSSet while you're enumerating over the set's items. You should have a stack trace from where the application crashes. It probably has a line where you're adding to/remove from the _delegates set. This is where you need to make the modification. It's easy to do. Instead of adding to/deleting from the set, do the following:
NSMutableSet *tempSet = [_delegates copy];
for (id delegate in _delegates)
{
//add or remove from tempSet instead
}
[_delegates release], _delegates = tempSet;
Additionally, NSMutableSet is not thread safe, so you should call your methods always from the main thread. If you haven't explicitly added any extra threads, you have nothing to worry about.
A thing to always remember about the Objective-C "fast enumeration".
There is 2 big difference between "fast enumeration" and a for loop.
"fast enumeration" is quicker than a for loop.
BUT
You can't modify the collection your enumerating over.
You can ask your NSSet for - (NSArray *)allObjects and enumerate over that array while modifying your NSSet.
You get this error when a thread tries to modify (add,delete) the array while other thread is iterating over it.
One way to solve this using NSLock or synchronizing the methods. That ways add, remove and iterate methods cannot be called in parallel.
But this will have effect on performance and/or responsiveness because any add/delete will have to wait for the thread that was iterating over the array.
A better solution inspired from Java's CopyOnWriteArrayList would be to create a copy of the array and iterate over the copy. So the only change in your code will be:-
//better solution
+ (void)didRecieveFeaturedItems:(NSArray*)items
{
NSArray *copyOfDelegates = [_delegates copy]
for (id delegate in copyOfDelegates)
{
if ([delegate respondsToSelector:#selector(didRecieveFeaturedItems:)])
[delegate didRecieveFeaturedItems:items];
}
}
Solution using locks with performance impact
//not a good solution
+ (void)addDelegate:(id)delegate
{
#synchronized(self){
[_delegates addObject:delegate];
}
}
+ (void)removeDelegate:(id)delegate
{
#synchronized(self){
[_delegates removeObject:delegate];
}
}
+ (void)didRecieveFeaturedItems:(NSArray*)items
{
#synchronized(self){
for (id delegate in _delegates)
{
if ([delegate respondsToSelector:#selector(didRecieveFeaturedItems:)])
[delegate didRecieveFeaturedItems:items];
}
}
}
I have two objects of class WidgetClass in my stored model. They are saved each time the app exits and reloaded each time it starts. I want to update my model to make one of them a WidgetSubclass object. WidgetSubclass will be a subclass of WidgetClass.
WidgetClass has quite a lot of ivars. WidgetSubclass will add few or none.
What is the most efficient way to accomplish the update? I am not using core data.
Couple of things.
If the subclass does not add any ivars to the superclass, you can actually get away with the following:
WidgetSubclass* widget = (WidgetSubclass*)[[WidgetClass alloc]initWithCoder: someCoder];
Class object_setClass(widget, [WidgetSubclass class]);
There is some risk that changes in the runtime could break the above code. So here is a safer way:
Foo.m:
-(void) copyIvarsTo: (Foo*) foo {
[super copyIvarsTo: foo];
foo.ivar1 = [self.objectIvar1 copy];
foo.ivar2 = [self.objectIvar2 copy];
foo.floatIvar = self.floatIvar;
// etc. Method works fine if foo is actually a member of a subclass.
}
-(Foo*) copy {
Foo* clone = [[self class]alloc];
[self copyIvarsTo: clone];
return clone;
}
Now I can have the following NSObject category method:
-(NSObject*) wj_copyWithSubclass: (Class) subclass {
if (![self respondsToSelector: #selector(copyIvarsTo:)])
return nil;
NSAssert([subclass isSubclassOfClass: [self class]], #"call copyWithSubclass only on subclasses");
NSObject* clone = [subclass alloc];
[self copyIvarsTo: clone];
return clone; // at this point, clone has copied all the ivars that are members of the receiver's class. Other ivars have their default values. Calling code needs to handle that.
}
I initialized a class in my singleton called DataModel. Now, from my UIViewController, when I click a button, I have a method that is trying to access that class so that I may add an object to one of its dictionaries. My get/set method passes back the pointer to the class from my singleton, but when I am back in my UIViewController, the class passed back doesn't respond to methods. It's like it's just not there. I think it has something to do with the difference in passing pointers around classes or something. I even tried using the copy method to throw a copy back, but no luck.
UIViewController:
ApplicationSingleton *applicationSingleton = [[ApplicationSingleton alloc] init];
DataModel *dataModel = [applicationSingleton getDataModel];
[dataModel retrieveDataCategory:dataCategory];
Singleton:
ApplicationSingleton *m_instance;
DataModel *m_dataModel;
- (id) init {
NSLog(#"ApplicationSingleton.m initialized.");
self = [super init];
if(self != nil) {
if(m_instance != nil) {
return m_instance;
}
NSLog(#"Initializing the application singleton.");
m_instance = self;
m_dataModel = [[DataModel alloc] init];
}
NSLog(#"ApplicationSingleton init method returning.");
return m_instance;
}
-(DataModel *)getDataModel {
DataModel *dataModel_COPY = [m_dataModel copy];
return dataModel_COPY;
}
For the getDataModel method, I also tried this:
-(DataModel *)getDataModel {
return m_dataModel;
}
In my DataModel retrieveDataCategory method, I couldn't get anything to work. I even just tried putting a NSLog in there but it never would come onto the console.
Any ideas?
Most likely you are sending messages that get ignored, e.g. they're being sent to objects which don't exist/aren't the one you're looking for, and for some reason aren't crashing. This occurs in the case of messaging nil, or possibly other illegitimate values. Although you seem to expect that the m_ variables will be initialized to 0, this is not good form, and furthermore you are not following a very typical objc pattern for your singletons -- m_dataModel should be an ivar of m_instance, and m_instance should probably be declared static, as you probably don't want it accessed from other files directly. In addition, the most likely source of your bug is somehow the -init method, which should never be called on a singleton -- instead do something like this:
+ (ApplicationSingleton *)sharedInstance {
static ApplicationSingleton *instance = nil;
if(!instance) {
instance = [[self alloc] init]; //or whatever custom initializer you would like, furthermore some people just put the initialization code here and leave -init empty
}
return instance;
}
the code you have now leaks because you allocate an object (self) and don't release it before returning a potentially different instance (the shared one if one already exists), such that the newly allocated one is typically lost.
I am a little confused by this snippet of code (presented in the CocoaFundamentals guide) that overrides some of the methods when creating a singleton instance.
static id sharedReactor = nil;
+(id)sharedInstance {
if(sharedReactor == nil) sharedReactor = [[super allocWithZone:NULL] init];
return sharedReactor;
}
.
+(id)allocWithZone:(NSZone *)zone {
return[[self sharedInstance] retain];
}
-(id)retain {
return self;
}
In the code where the singleton instance is created the +sharedInstance method calls [super allocWithZone:NILL] from the superclass (which in my case is NSObject) The allocWithZone above is only called if you attempt to use it to create a new singleton.
The bit I am confused about is the use of retain, especially seeing as retain is also overridden to return self. Can anyone explain this, could it not be written:
+(id)allocWithZone:(NSZone *)zone {
return [self sharedInstance];
}
-(id)retain {
return self;
}
EDIT_001:
Based on comments and reading various posts on the web I have decided to go with the following (see below) I have chosen to go for a shared singleton approach where if needed I would have the option of creating a second or third instance. Also at this stage as I am only using the singleton for the model portion of MVC for a simple iPhone app I have decided to leave thread safety out. I am aware its important and as I get more familiar with iPhone programming I will likely use +initialize instead (keeping in mind the subclass issue where it can be called twice) Also I have added a dealloc, firstly to log a message should the singleton be released, but also to clean things up properly should the singleton be no longer required.
#interface SharedManager : NSObject
+(id)sharedInstance;
#end
#implementation SharedManager
static id myInstance = nil;
+(id)sharedInstance {
if(myInstance == nil) {
myInstance = [[self alloc] init];
}
return myInstance;
}
-(void)dealloc {
NSLog(#"_deal: %#", [self class]);
[super dealloc];
myInstance = nil;
}
#end
In testing I found that I had a set the static variable to nil in the dealloc or it maintained its pointer to the original object. I was initially a little confused by this as I was expecting the scope of the static to be the instance, I guess its the class instead, which makes sense.
cheers gary
First, don't use this code. There is almost never a reason to do all this for a simple singleton. Apple is demonstrating a "Forced Singleton," in that it is impossible to create two of them. It is very rare to really need this. You can almost always use the "shared singleton" approach used by most of the Cocoa objects that have a singleton constructor.
Here's my preferred way of implementing shared singleton:
+ (MYManager *)sharedManager
{
static MYManager *sharedManager = nil;
if (sharedManager == nil)
{
sharedManager = [[self alloc] init];
}
return sharedManager;
}
That's it. No other code is required. Callers who use +sharedManager will get the shared instance. Callers who call +alloc can create unique instances if they really want to. This is how such famous "singletons" as NSNotificationCenter work. If you really want your own private notification center, there is no reason the class should forbid it. This approach has the following advantages:
Less code.
More flexible in cases where a non-shared instance is useful.
Most importantly: the code does what it says it does. A caller who thinks he's making a unique instance with +alloc doesn't encounter surprising "spooky action at a distance" behavior that requires him to know an internal implementation detail of the object.
If you really need a forced singleton because the object in question maps to a unique resource that cannot be shared (and it's really rare to encounter such a situation), then you still shouldn't use +alloc trickery to enforce it. This just masks a programming error of trying to create a new instance. Instead, you should catch the programming error this way:
+ (MYManager *)sharedManager
{
static MYManager *sharedManager = nil;
if (sharedManager == nil)
{
sharedManager = [[self alloc] initSharedManager];
}
return sharedManager;
}
- (id)init
{
NSAssert(NO, #"Attempting to instantiate new instance. Use +sharedManager.");
return nil;
}
// Private method. Obviously don't put this in your .h
- (id)initSharedManager
{
self = [super init];
....
return self;
}
There is a good example of different singleton methods with comments here on SO:
What does your Objective-C singleton look like?
If it helps, the example has a different approach to allocWithZone: which returns nil.