NSCopying arrays of custom objects - ios

I have a Singleton object that manages all my lists. We'll call it ListStore.
ListStore has a mutable array, which stores Lists.
#interface ListStore : NSObject
#property (nonatomic, copy) NSMutableArray *lists; // an array of List objects
end
Lists has a mutable array, which stores Things.
#interface Wanderlist : NSObject <NSCoding, NSCopying>
#property (nonatomic, copy) NSMutableArray *things; // an array of Thing objects
#end
At any time, a background process might go through ListStore and loop through and process all Lists, while a user might be interacting with a List.
To guard against "object was mutated while being enumerated" type errors, I do this:
// all of this is in a background thread
NSArray *newLists = [[ListStore sharedStore] lists] copy];
for (List *list in newLists) {
// yay, no more crashes, because I'm enumerating over a copied object, so the user
// can do whatever they want while I'm here
for(Thing *thing in list.things) {
// oh crap, my copy and the original object both reference the same list.things,
// which is why i'm seeing the 'mutation while enumerating" errors still
...
}
}
I originally thought that because I made a copy into newLists that all of its members would be properly copied. I now understand that not to be the case: I'm still seeing the "object was mutated while enumerated" errors, but this time it's happening on list.things.
Can I use NSCopying with my setup so that when I say:
[[ListStore sharedStore] copy];
It calls copyWithZone: on Lists, so I can then copyWithZone: on things?
I tried to set it up like this but copyWithZone: wasn't getting called.
I know I could simply say NSArray *newList = [list.things copy] but I'd like to get a better understanding of NSCopying at the very least.

Right before submitting this question I clicked on a question in SO's list of related questions, and found my solution.
Figured it doesn't hurt to post my solution.
Instead of this:
NSArray *newLists = [[ListStore sharedStore] lists] copy];
I had to do:
NSArray *newLists = [[NSArray alloc] initWithArray:[[ListStore sharedStore] lists] copyItems:true];
From the NSArray docs:
- (id)initWithArray:(NSArray *)array copyItems:(BOOL)flag
flag:
If YES, each object in array receives a copyWithZone: message to create a copy of the object—objects must conform to the NSCopying protocol. In a managed memory environment, this is instead of the retain message the object would otherwise receive. The object copy is then added to the returned array.
Once I used initWithArray:copyItems:, it automatically sent copyWithZone to all my List objects, and I was able to then manually perform a copyWithZone on list.things.

Related

Objective-C pattern for creating mutable copies

I have many "model" objects whose properties are defined as "readonly" and shared among various components.
In some cases I need to create local mutable copies of the objects (using them for local mutable state)
I rather not implement NSMutableCopy protocol as the object should be immutable after it is created. The modified object could be "passed" around after copy+mutate operations.
Is there a suggested mechanism , or should I just implement a constructor receiving the "changed" parameters?
For example an object which parses a JSON to native types :
#interface ImmutableObject : NSObject
// various "readonly" properties
...
-(instancetype)initWithJSON:(NSDictionary *)jsonDictionary;
#property (nonatomic, readonly) MyClass1 *prop1;
#property (nonatomic, readonly) MyClass2 *prop2;
...
#property (nonatomic, readonly) NSArray<MyClass100 *> *prop100;
#end
#implementation
-(instancetype)initWithJSON:(NSDictionary *)jsonDictionary {
self = [super init];
[self parseDictionaryToNative:jsonDictionary];
return self;
}
#end
Somewhere in code:
ImmutableObject *mutated = [immutableObject mutableCopy]; // best way to accomplish this?
// change some values...
mutated.prop1 = ... // change the value to something new
self.state = [mutated copy]; // save the new object
#spinalwrap is correct, but in this case there is no reason to create the extra copy before storing it. NSMutableArray is a subclass of NSArray, so can be used anywhere an NSArray can be used (and this is very common). Same for yours. In your particular case, you'd probably do it this way:
MutableObject *mutated = [immutableObject mutableCopy]; // create an instance of MutableObject
mutated.prop1 = ... // change the value to something new
self.state = mutated; // Since `state` is an immutable type,
// attempts to mutate this later will be compiler errors
This is safe because you know that this block of code is the only block that has a reference to the mutable version of the object (because you created it here).
That said, once you've created a mutable subclass, you now need to consider the possibility that any ImmutableObject you are passed might actually be a MutableObject, and so make defensive copies (just as is done with NSArray, NSString, etc.) For example:
- (void)cacheObject:(ImmutableObject *)object {
// Need to copy here because object might really be a MutableObject
[self.cache addObject:[object copy]];
}
This is made fairly efficient by implementing copy on ImmutableObject and return self, and implementing copy on MutableObject as an actual copy, usually like this:
ImmutableObject.m
- (ImmutableObject *)copy {
return self;
}
MutableObject.m
// as in spinalwrap's example
- (MutableObject *)mutableCopy {
MutableObject *instance = [MutableObject new];
instance.prop1 = [self.prop1 copy]; // depends what you want here and what kind of class the properties are... do you need a deep copy? that might be a bit more work.
// etc...
return instance;
}
// No need to duplicate code here. Just declare it immutable;
// no one else has a pointer to it
- (ImmutableObject *)copy {
return (ImmutableObject *)[self mutableCopy];
}
So the copy is almost free if the object was immutable already. I say "fairly efficient" because it still causes some unnecessary copies of mutable objects that are never mutated. Swift's copy-on-write system for value types was specifically created to deal with this problem in ObjC. But the above is the common pattern in ObjC.
note that NSMutableArray, NSMutableData etc. are different classes than their immutable counterparts. So in this case, you maybe should define a MutableObject class with the same interface as the ImmutableObject class (but with mutable properties) and use that if you want to have a mutable object.
MutableObject *mutated = [immutableObject mutableCopy]; // create an instance of MutableObject
mutated.prop1 = ... // change the value to something new
self.state = [mutated copy]; // creates an ImmutableObject
the implementation of ImmutableObject's mutableCopy could be something like:
- (MutableObject *) mutableCopy
{
MutableObject *instance = [MutableObject new];
instance.prop1 = [self.prop1 copy]; // depends what you want here and what kind of class the properties are... do you need a deep copy? that might be a bit more work.
// etc...
return instance;
}
and MutableObject's copy method could look like this:
- (ImmutableObject *) copy
{
ImmutableObject *instance = [ImmutableObject new];
instance.prop1 = [self.prop1 copy];
// etc...
return instance;
}
You're not forced to use the NSMutableCopy protocol formally, but you can.

How reference count work? [duplicate]

Here is code I am referring to.
// Person.h
#interface Person : NSObject {
NSString *firstName;
NSString *lastName;
}
#end
// Person.m
#implementation Person
- (id)init {
if (![super init]) return nil;
firstName = #"John";
lastName = #"Doe";
}
#end
// MyClass.m
#implementation MyClass
.....
- (NSArray *)getPeople {
NSMutableArray *array = [[NSMutableArray alloc] init];
int i;
for (i = 0; i < 10; i++) {
Person *p = [[Person alloc] init];
[array addObject:p];
}
return array;
}
.....
#end
Now, I know there is no memory-management going on in this sample code. What would be required?
In the getPeople loop, I am alloc'ing a Person (retainCount 1), then adding it to array. The retain count is now 2, right? If it is two, should I be [p release]'ing after adding it to the array, bringing the retainCount back down to 1?
Am I right in that it is the caller's responsibility to release the array returned by the method? (Which would also free the memory of the Person's, and their instance variables, assuming their counts are at 1).
I have read Apple's memory management document, but I guess what I am most unclear about, is what increases an objects retain count? I think I grasp the idea of who's responsibility it is to release, though. This is the fundamental rule, according to Apple:
You take ownership of an object if you create it using a method whose name begins with “alloc” or “new” or contains “copy” (for example, alloc, newObject, or mutableCopy), or if you send it a retain message. You are responsible for relinquishing ownership of objects you own using release or autorelease. Any other time you receive an object, you must not release it.
bobDevil's sentence "only worry about the retain counts you add to the item explicitly" made it click for me. After reading the Ownership policy at Apple, essentially, the object/method that created the new object, is the one responsible for releasing /it's/ interest in it. Is this correct?
Now, let's say I a method, that receives an object, and assigns it to a instance variable. I need to retain the received object correct, as I still have an interest in it?
If any of this is incorrect, let me know.
You are correct that the retain count is 2 after adding it to an array. However, you should only worry about the retain counts you add to the item explicitly.
Retaining an object is a contract that says "I'm not done with you, don't go away." A basic rule of thumb (there are exceptions, but they are usually documented) is that you own the object when you alloc an object, or create a copy. This means you're given the object with a retain count of 1(not autoreleased). In those two cases, you should release it when you are done. Additionally, if you ever explicitly retain an object, you must release it.
So, to be specific to your example, when you create the Person, you have one retain count on it. You add it to an array (which does whatever with it, you don't care) and then you're done with the Person, so you release it:
Person *p = [[Person alloc] init]; //retain 1, for you
[array addObject:p]; //array deals with p however it wants
[p release]; //you're done, so release it
Also, as I said above, you only own the object during alloc or copy generally, so to be consistent with that on the other side of things, you should return the array autoreleased, so that the caller of the getPeople method does not own it.
return [array autorelease];
Edit:
Correct, if you create it, you must release it. If you invest interest in it (through retain) you must release it.
Retain counts are increased when you call alloc specifically, so you'll need to release that explicitly.
factory methods usually give you an autoreleased object (such as [NSMutableArray array] -- you would have to specifically retain this to keep it around for any length of time.).
As far as NSArray and NSMutableArray addObject:, someone else will have to comment. I believe that you treat a classes as black boxes in terms of how they handle their own memory management as a design pattern, so you would never explicitly release something that you have passed into NSArray. When it gets destroyed, its supposed to handle decrementing the retain count itself.
You can also get a somewhat implicit retain if you declare your ivars as properties like #property (retain) suchAndSuchIvar, and use #synthesize in your implementation. Synthesize basically creates setters and getters for you, and if you call out (retain) specifically, the setter is going to retain the object passed in to it. Its not always immediately obvious, because the setters can be structured like this:
Person fart = [[Person alloc] init];
fart.firstName = #"Josh"; // this is actually a setter, not accessing the ivar
// equivalent to [fart setFirstName: #"Josh"], such that
// retainCount++
Edit:
And as far as the memory management, as soon as you add the object to the array, you're done with it... so:
for (i = 0; i < 10; i++) {
Person *p = [[Person alloc] init];
[array addObject:p];
[p release];
}
Josh
You should generally /not/ be worried about the retain count. That's internally implemented. You should only care about whether you want to "own" an object by retaining it. In the code above, the array should own the object, not you (outside of the loop you don't even have reference to it except through the array). Because you own [[Person alloc] init], you then have to release it.
Thus
Person *p = [[Person alloc] init];
[array addObject:p];
[p release];
Also, the caller of "getPeople" should not own the array. This is the convention. You should autorelease it first.
NSMutableArray *array = [[[NSMutableArray alloc] init] autorelease];
You'll want to read Apple's documentation on memory management: http://developer.apple.com/documentation/Cocoa/Conceptual/MemoryMgmt/MemoryMgmt.html

Objective-C: Get an objects class and cast the object to it dynamically

Let's say I have a collection of objects of different types/classes, i.e. an NSArray.
I know that all of this objects inherit from NSManagedObject and all of them have a property named "uuid".
Now, I want to loop over this array, retrieve each objects uuid and add it to another array, like this:
NSMutableArray *objectUUIDs = [[NSMutableArray alloc] initWithCapacity:0];
for (NSObject *object in objects) {
// somehow cast the object to its class, so that I can send get its uuid
}
Actually, I could check for the class by [object class] in an if-else-clause, and cast it respectively, but as I have 30 something classes, I would like to do something more generic, like (in pseudo code):
// give me the object's true class instead of NSManagedObject
// add the object uuid to my objectUUIDs array
As you say all objects are NSManagedObjects and all have a property uuid, you can use Key-Value-Coding
NSMutableArray *objectUUIDs = [#[] mutableCopy];
for (NSManagedObject *object in objects) {
[objectUUIDs addObject: [object valueForKey:#"uuid"]];
}
or
NSArray *objectUUIDs = [objects valueForKey:#"uuid"];
NSArray's -valueForKey doc
if you enumerate over an collection of objects of different classes you shouldn't type the enumerated object NSObject, but use either the closest common super class, or id — the generic objective-C object type. The reason is that you can send any message to an object typed with id, and you can do further testing. With other more concrete classes you must ensure a message is under stud by an given method.
You state
// give me the object's true class instead of NSManagedObject
The object doesn't change during casting. if you put a instance of MyFabulousManagedObject (subclass of NSManagedObject) in an array, and later you cast it to NSManagedObject, it is actually still an instance of MyFabulousManagedObject
While vikingosegundo's solution is probably best, there's also this possibility:
Define a protocol which has this property (or anything you know for sure is shared among all the objects):
#protocol FooBarProtocol
#property NSUUID *uuid;
#end
Now iterate over the original array as such:
NSMutableArray *objectUUIDs = [NSMutableArray array];
for (id<FooBarProtocol> object in objects) {
[objectUUIDs addObject:object.uuid];
}
Here, we're simply casting the objects all to objects that conform to FooBarProtocol, although it may be true that all of your objects already conform to a protocol that defines this property, or perhaps all have a common superclass with this property defined.
The main point here is that you just need to cast them to anything that defines the property.
Note that as written, this will crash if the object actually doesn't have a uuid property. Might be a good idea to add:
if ([object respondsToSelector:#selector(uuid)]) {
[objectUUIDs addObject:object.uuid];
}
And this also avoids having a massive chain of if statements to check all the different classes. We don't care what sort of class it is. We only care that it can give us a UUID.

How to expose an NSMutableArray as an NSArray as a return type from a method

I have a model class which contains an NSMutable array of objects. The controller classes need to have access to this array, however that access should be read only.
How should this be implemented? Should the model expose the array as a (readonly) NSMutable array and use consts, or expose it as an NSArray? If the latter how can the NSArray be created efficiently from the NSMutableArray i.e. how should the NSArray contain a reference to the NSMutableArray/its contents rather than have duplicate copies? (the NSMutableArray is guaranteed to persist in memory while the controllers access it).
You can just return your NSMutableArray directly:
- (NSArray *)method
{
return myMutableArray;
}
NSMutableArray is a subclass of NSArray, so the controllers will be able to perform all of the NSArray operations on it already. If you're really concerned that somebody might be trying to pull tricks on you, you could use:
return [NSArray arrayWithArray:myMutableArray];
to return an immutable copy.
There is no read only NSMutableArray. If you are thinking that defining a property as read only makes the returned object immutable, this is not the case.
Expose a read-only property of type NSArray, and in the accessor return a new array as follows:
return [NSArray arrayWithArray:mutableArray];
Note that the objects within the array, if mutable, will still be changeable.
If you want to continue passing your mutable array, but want to ensure that the other method does not overwrite your existing array, you could always pass a copy
return [myMutableArray copy];
This way, you are returning a type NSArray (since NSMutableArray is a subclass) and ensure that myMutableArray itself is not getting tampered with.
NSMutableArray inherits from NSArray, so there is nothing to be done to expose NSMutableArray as an NSArray except to (probably implicitly) cast it. Note that it's still the same object, so if someone wants to be clever, they can downcast it or even just try to call NSMutableArray methods on it.
While this might be overkill, you could create your own custom class that has a #private ivar of an array, and then you could create readonly properties/methods to access data only.
i.e (implementation snippet):
#interface CustomArray : NSObject
{
#private
NSArray *array;
}
#end
- (id)initWithNSArray:(NSMutableArray *)array;
- (id)getObjectAtIndex:(int)index;
#implementation
// implement methods here
#end

Proper way of creating new objects which are copies of NSDictionary and NSArray objects defined in app delegate

I am wondering what the correct way is to make a copy of an object defined in the app delegate or a singleton object. In short, I am making an app which requires a user to login. This login view is just a modal view controller on top of the 'real' app, which consists of a tabbarcontroller, plus some tableview controllers. After a successful login, there is send a data request to a remote server, and the modal view controller is dismissed, revealing the tabbar controller and table views holding the XML data. To parse the incoming data, I have created a singleton object named DataParser, which has interface
...
#interface DataParser : NSObject {
// Data objects that hold the data obtained from XML files
NSMutableDictionary *personnel;
NSMutableDictionary *schedule;
NSMutableDictionary *today;
}
#property (nonatomic, retain) NSMutableDictionary *personnel;
#property (nonatomic, retain) NSMutableDictionary *schedule;
#property (nonatomic, retain) NSMutableDictionary *today;
...
Now in these dictionaries I store (mutable) dictionaries and arrays holding NSString objects with the parsed XML data. Since I do not want to modify these original objects holding the parsed data (that is to say, I only want to modify them at the login stage, but not in any of the tableview controllers), I am creating a new dictionary object which holds a copy of the content of one of the dictionaries above in each tableview controller. So for instance, in the loadView of a view controller called ScheduleViewController I have
...
#interface ScheduleViewController : UITableViewController {
NSDictionary *copyOfSchedule;
}
#property (nonatomic, retain) NSDictionary *copyOfSchedule;
...
#end
#implementation ScheduleViewController
#synthesize copyOfSchedule;
- (void)loadView {
[super loadView];
DataParser *sharedSingleton = [DataParser sharedInstance];
self.copyOfSchedule = [NSDictionary dictionaryWithDictionary:sharedSingleton.schedule];
}
...
Now this seems to work fine. The only difficulty arises however, when the user 'logs out', which entails popping the login modal view controller back on the stack. When the user presses the login button again, then a new XML data request is send to the server and the dictionaries in the singleton object get refreshed with the (new) data (I check if they contain any data, if so I call removeAllObjects before filling them up again with newly parsed data). At this point the dictionaries in all view controllers should be updated too, however I am not quite sure how to go about this the right way. I have noticed that loadView is not always called again in this case and so to this end I have added the same code as above in loadView to every viewWillAppear method. After navigating back and forth between the different views or navigating back and forth between child views of a tableview a couple of times, I receive an EXC_BAD_ACCESS error however. I suspect this has to do with not properly retaining the copies of the original dictionaries, but I don't seem to be able to find a solution around this. Instead of using dictionaryWithDictionary, which I suspect is not the right way to go anyway, I also tried a different approach, where instead of using objects of type NSDictionary in ScheduleViewController I use NSMutableDictionary. So:
...
#interface ScheduleViewController : UITableViewController {
NSMutableDictionary *copyOfSchedule;
}
#property (nonatomic, retain) NSMutableDictionary *copyOfSchedule;
...
#end
#implementation ScheduleViewController
#synthesize copyOfSchedule;
- (void)loadView {
[super loadView];
DataParser *sharedSingleton = [DataParser sharedInstance];
self.copyOfSchedule = [[NSMutableDictionary alloc] initWithDictionary:sharedSingleton.schedule];
}
- (void)viewWillAppear {
DataParser *sharedSingleton = [DataParser sharedInstance];
[self.copyOfSchedule removeAllObjects];
[self.copyOfSchedule addEntriesFromDictionary:sharedSingleton.schedule];
[self.tableView reloadData];
}
...
But this doesn't get rid of the EXC_BAD_ACCESS errors. To make a very long story short: what would be the best way to go about making independent copies of objects defined in a singleton object or app delegate and which can be dynamically updated at request? Since I am already rather into the project and lots is going on, I realize that my question may be a bit vague. Nonetheless I hope there is somebody who could enlighten me somehow.
Deep copies are often made recursively. One way to do it would be to add -deepCopy methods to NSDictionary and NSArray. The dictionary version might go like this:
- (NSDictionary*)deepCopy
{
NSMutableDictionary *temp = [self mutableCopy];
for (id key in temp) {
id item = [temp objectForKey:key];
if ([item respondsToSelector:#sel(deepCopy)] {
// handle deep-copyable items, i.e. dictionaries and arrays
[temp setObject:[item deepCopy] forKey:key]
}
else if ([item respondsToSelector:#(copy)]) {
// most data objects implement NSCopyable, so will be handled here
[temp setObject:[item copy] forKey:key];
}
else {
// handle un-copyable items here, maybe throw an exception
}
}
NSDictionary *newDict = [[temp copy] autorelease];
[temp release]
return newDict;
}
I haven't tested that, so be a little careful. You'll want to do something similar for NSArray.
Note that views are not copyable.
It is quite a typical pattern that you build an array or dictionary with some code, so clearly it must be mutable while you add bits to it, and when you're done you don't want it ever to change. To do this:
Have a property like
#property (...) NSArray* myArray;
When you calculate the contents of myArray, use a mutable array to build it, like
NSMutableArray* myMutableArray = [NSMutableArray array];
When you're done building the array, just use
self.myArray = [NSArray arrayWithArry:myMutableArray];

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