I have an object which contains an array.
Without coredate or a database, how do I model the relationship back to the parent from any of the children. Do I have to explicitly store the parent id in the child? Or is there a way to get the "parent" class of this array?
Objective-C (unlike Qt and a few other object models) has no "parent-child" or "ownership" relationship. Yes, sometimes we say that the object that retains an object "owns" it, but that's more of a shared ownership, since multiple objects can retain a given object, but none of the "owners" is more "distinguished" than the others and hence (in the plain Objective-C model) the "parent".
The first thing to consider is whether your "parent/child" concept really makes sense in this multiple-owner environment. If so, then you need to maintain the child-to-parent pointers yourself, somehow.
Without subclassing NSArray and adding an extra property, which I think is a bad idea, I guess you have two choices:
Make the object at index 0 a pointer to your containing object
Don't store your array directly in your "parent" object, store it in a NSDictionary which also has a #parent key, holding a reference to the parent.
To be honest I'm not sure what you are trying to achieve here - when would you need to find out this information that wouldn't be better done by simply working with the parent object itself?
UPDATE
I've just seen this answer to a separate question: Create Custom UIButton class
Associative references sound like they could be what you need? You can associate a reference to your containing object to the NSArray. I don't know anything else about them, though, I just saw that answer and thought of this question.
NSArray * array = [[NSArray alloc]init];
id parrent = [array super];
Related
Let me consider following piece of code:
Person *peter = [people chooseRandomPerson];
where "people" is class instance, which was already initialised, and "chooseRandomPerson" is method which returns object of "Person" type. I wonder if it's the same as following:
Person *peter = [[Person alloc] init];
peter = [people chooseRandomPerson];
If not, what is the difference. If not, can I use such a substitution anytime? I am sorry, if the question is elementary, but I wasn't able to find an answer.
The second creates an object, puts it into the variable peter, and then immediately discards it* to store the return value of chooseRandomPerson.
The variable peter is a place to put a thing; the object is the thing you get back from the alloc/init or the chooseRandomPerson. You don't need to create a thing in order to have a place to put it; the declaration Person *peter; creates the place -- the variable -- by itself.
*If you weren't using ARC, this would be a leak, but if you're asking this you'd better be using ARC.
The answer by Josh Caswell is correct. But your question is not entirely clear.
In your question, you never define what is "people". You say "class instance" by which you presumably meant "object". But an object/instance of what class? Using the plural word "people" suggests a collection of objects. You should clarify this in your question.
Object Creating Sibling Objects
If you are asking "Can an object return a new instance of its own class?", the answer is "Yes". In your example, "peter" a Person object can produce a new Person object, "Sally".
However, an object creating sibling objects (new objects of the same class) is somewhat unusual in my experience. More common would be a using the Factory design pattern to produce instances.
There is one common use of an object creating sibling objects: Immutable objects. Instead of changing one field of data in a DateTime object, an entirely new object is created with most of its data based on the original.
I would like to check if my NSMutableArray contains my custom object. But if I understand correct contains functions searches for the same object in array (placed at the same memory point)
if(![objectArray containsObject:objToCheck])
{
[objectArray addObject:objToCheck];
}
I know that objectArray has identical object with identical variable values compared to objToCheck, yet such if always returns false. Is there a way to check this without writing custom loop and comparing objects by their parameters?
Override the [NSObject isEqual:] method (actually it's part of the NSObject protocol) of your custom object and check whatever instance variables make sense to you for an object to be considered equal.
Here's an Apple Cocoa Competency article on the subject.
You might try creating a temporary NSSet from your array and testing against that for membership.
I am trying to create a mutable array in objetive c to hold references to objects. The objects in the array are regularly updated through user interaction and i want the array to automatically reflect changes made to the objects as they occur. Does anyone know if there is a way to do this? Perhaps store pointers to the objects instead of the objects themselves in the array? Any help would be much appreciated
Thanks in advance
Edit: I should mention that the objects are not exactly being updated in the strict sense of the word. They are being reinitialized. For ex if i had a controller:
MyController = [MyController alloc] initWith.....]]
the above call is made again with different init parameters.
The array always stores the pointers.... It holds a strong reference to it or sends it a retain message (if using non ARC).
So
[myMutableArray addObject: anObject];
adds the pointer to it.
If you now change anObject's properties and access it later through the array, it will
give you the pointer to just that object with the changes to its properties.
Edit:
No, if you alloc/init, you are creating a new object instance and allocate new memory for it on the heap (ie, it's another pointer to a new memory address).
What exactly are you trying to accomplish? There sure is a way, if you provide a little more detail.
If you alloc/init the object with the same class, why not just create a method to change the object's properties:
Instead of
myObject = [[MyClass alloc] initWithParameter1: one parameter2: two];
You could create a method that changes these properties:
[myObject updateParameter1: anotherOne parameterTwo: anotherTwo];
And, of course, the advantage of a mutable array is, that you can change its contents, so like #Eli Gregory pointed out, you can replace an object with another one (or rather the pointers to it).
Because you want to point to a newly allocated and initialized object, you can't 'update' the pointer, what you can do is 'replace' the pointer with a new one at a certain index.
A method you could use to do this is:
- (void)replaceObjectAtIndex:(NSUInteger)index withObject:(id)anObject
so it would look something like:
NewViewController *new = [[NewViewController alloc] init..];
[myArray replaceObjectAtIndex:x withObject:new];
I've got a NSMutableArray (containing NSMutableDictionary instances) bound to an NSArrayController (the NSArrayController is in turn bound to NSTableView columns).
What is the most Cocoa-, and KVO- friendly way of, programmatically :
adding a new empty object (NSMutableDictionary) to the array?
removing currently selected object? (after removing, the previous item - if exists - should be selected)
I've always been doing this in a way I don't particularly like - and I'm sure it's not the best way around (too many lines of code for something so simple : in Cocoa that indicates a wrong take on the subject :-)).
My code (quite an overkill, actually) :
Adding to the Array
NSMutableArray* oldParams = [paramsArray mutableCopy];
[oldParams addObject:[NSMutableDictionary dictionaryWithObjectsAndKeys:#"Parameter",#"Parameter",#"",#"Value", nil]];
[self setParamsArray:oldParams];
[paramsController setSelectionIndex:[paramsArray count]-1];
Removing currently selected object from the Array
if ([paramsArray count]>0)
{
int s = [paramsController selectionIndex];
NSMutableArray* oldParams = [paramsArray mutableCopy];
[oldParams removeObjectAtIndex:s];
[self setParamsArray:oldParams];
if (s<=[paramsArray count]-1)
[paramsController setSelectionIndex:s];
else
[paramsController setSelectionIndex:[paramsArray count]-1];
}
So, what are your opinions on that?
Given that the array controller is bound to a property named paramsArray on some object, the best approach is to define the key-value coding indexed accessors on that object's class. Then, use those accessors to mutate the to-many relationship represented by the property in a KVO-compliant manner.
For example, implement -insertObject:inParamsArrayAtIndex: and then use that to add an object. If you like the convenience of NSMutableArray's -addObject: method, you can write an -addObjectToParamArray method that forwards to -insertObject:inParamsArrayAtIndex:.
By the way, "paramsArray" is a poor name for a property. The property name shouldn't encode the type used to implement it. If you look at the templates for the indexed accessor names, you'll see that Apple is expecting to-many relationship properties to just be a plural noun like "params" (no "Array"). For example, -paramsAtIndexes: is better than -paramsArrayAtIndexes:.
You have to think of your array as the controller's backing store, and that it's managing it for you.
Adding an object:
[[self accountsArrayController] addObject:accountDictionary];
Removing the currently selected object:
[[self accountsArrayController] remove:nil];
You'll have to write another line or two to make that previous item selected, but that's an exercise I leave to you.
I want to use the object's reference value as a key into a dictionary, as opposed to a copy of value of the object. So, I essentially want to store an object associated with a particular instance of another object in a dictionary and retrieve that value later.
Is this possible? Is it completely against the idea of NSDictionary? I can tell that I am probably approaching this the wrong way because the dictionary wants me to implement NSCopying on the object itself, which doesn't really make sense in terms of what I'm doing. I can see that what I should really be doing is wrapping the pointer value, but that seems a little mad.
Advice would be appreciated.
I think you can use [NSValue valueWithPointer:object].
NSMutableDictionary has been designed to only deal with Objective-C object instances. For example, when you call setObject:forKey: method calls copyWithZone: on the key and retain on the value.
If you want to have a dictionary structure and to be able to deal with arbitrary key and value, then you can go with CFMutableDictionary. You can describe precisely what is done with key and values; it is flexible enough to deal with arbitrary pointer or event char * strings.
This did the trick for me
aDictionary[#((intptr_t)object)] = ...;
You can use the address in memory of myObejct as a key in myDictionary
NSString *myObject_addressInMemory = [NSString stringWithFormat:#"%p", myObject];
myDictionary[myObject_addressInMemory] = someValue;