We know NSArray is immutable and NSMutableArray is mutable,because we can judge by it's name.But how can we know a custom class whether mutable?
I believe there's no special interface or API in Objective-C or OS library to do this.
You can start by checking the known types like so:
if ([obj isKindOfClass:[NSMutableArray class]]) {
return YES; // mutable
}
if ([obj isKindOfClass:[NSMutableSet class]]) {
return YES; // mutable
}
... etc.
And for your own custom objects you can create your own custom "marker" protocol:
#protocol MYMutable <NSObject>
#end
(you apply that protocol to your own objects)
And then check your objects like so:
if ([obj conformsToProtocol:#protocol(MYMutable)]) {
return YES;
}
This is not perfect solution, but it might cover 90% of the use cases.
Related
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.
i would like to add extra data to EKEvent, i tried NSDictionary (there is a lot of data to add) but it doesn't work..
sample code:
NSMutableDictionary *dictionary = [[NSMutableDictionary alloc]init];
[eventStore setValue:dictionary forKey:MAIN_DICTIONARY];
any ideas?
You're using setValue:forKey: in a wrong way. That a look here. There are different options to achieve what you want: category, subclassing or create a class that contains the EKEvent and the NSMutableDictionary. It depends on how you need to use the EKEvent.
You cannot do it this way, because even with key-value coding you can only set (declared or non declared) properties known by the instance. Basically the accessors (setter, getter) are executed. But there is no property MAIN_THREAD,no setter setMAIN_THREAD: in EKEvent.
If you want to extend instances of a foreign class that are created by the system (the instances, not the class), there are to ways to add data:
You create an own class, let's say MyEvent and give them a reference to the system instance (EKEvent) as a property plus the properties you need. When you get an instance of EKEvent you look-up your list of MyEventss using the identifier. With that you have the full access to your data.
You use associated objects. But you have to take care that they are not handled by the instance, i. e. while copying.
The first solution is better by far. Simple sample code:
#interface MyEvent : NSObject
#property (readonly) EKEvent* systemEvent;
#property id customProperty;
- (instancetype)eventForSystemEvent:(EKEvent*)systemEvent;
#end
#implemenation MyEvent
// Look-Up
NSMutableDictionary *eventLookUp;
+ (void)initialize
{
if( self == [MyEvent class])
{
eventLookUp = [NSMutableDictionary new];
}
}
- (instancetype)eventForSystemEvent:(EKEvent*)systemEvent
{
return eventLookUp[systemEvent.calendarItemIdentifier];
}
// Instance creation
- (instancetype)initWithSystemEvent:(EKEvent*)systemEvent
{
// Usual initializer
…
eventLookUp[systemEvent.calendarItemIdentifier] = systemEvent;
return self;
}
+ (instancetype)newEventWithSystemEvent:(EKEvent*)systemEvent
{
return [[self alloc] initWithSystemEvent:systemEvent];
}
#end
Typped in Safari
This question already has answers here:
How do I compare objects in Objective-C?
(4 answers)
Closed 8 years ago.
I have a list of Objects that I pull from a web service. When I update my UITableView, I retrieve the objects again from the web service, and compare them to each other for equality. I then remove the ones that are not present, and insert the new objects, then update my UITableView. How can I test to see if the new object equals the old object? I've created a test for clarity..
requestA should equal requestC, but fails.
Is this possible to do without looking at each property value as the objects have many values?
I was originally comparing the ID only, but this doesn't work as sometimes other property values change and the ID stays the same.
Request *requestA = [[Request alloc] init];
Request *requestB = [[Request alloc] init];
Request *requestC = [[Request alloc] init];
requestA.requestID = #"1";
requestA.productName = #"Clutch";
requestB.requestID = #"2";
requestB.productName = #"Wiper";
requestC.requestID = #"1";
requestC.productName = #"Clutch";
if (requestA == requestB)
NSLog(#"A == B");
if (requestA == requestC)
NSLog(#"A == C");
if ([requestA isEqual:requestB])
NSLog(#"A isEqual B");
if ([requestA isEqual:requestC])
NSLog(#"A isEqual C");
// Look at the pointers:
NSLog(#"%p", requestA);
NSLog(#"%p", requestB);
NSLog(#"%p", requestC);
isEqual: is a method declared in NSObject Protocol. From official docs of isEqual:
This method defines what it means for instances to be equal. For
example, a container object might define two containers as equal if
their corresponding objects all respond YES to an isEqual: request.
See the NSData, NSDictionary, NSArray, and NSString class
specifications for examples of the use of this method.
If two objects are equal, they must have the same hash value. This
last point is particularly important if you define isEqual: in a
subclass and intend to put instances of that subclass into a
collection. Make sure you also define hash in your subclass.
Thus, as Salavat Khanov pointed out in his answer:
You need to implement -isEqual: and -hash methods for your Request class.
You want to do something like this:
// TestRequest.h
#interface TestRequest : NSObject
#property (nonatomic) NSString *requestID;
#property (nonatomic) NSString *productName;
#end
// TestRequest.m
#import "TestRequest.h"
#implementation TestRequest
- (BOOL)isEqual:(TestRequest *)object {
if (self == object) {
return YES;
}
if (![self.requestID isEqual:object.requestID]) {
return NO;
}
if (![self.productName isEqual:object.productName]) {
return NO;
}
return YES;
}
- (NSUInteger)hash {
// this is a very simple hash function
return [self.requestID hash] ^ [self.productName hash];
}
#end
or you can use a custom method:
- (BOOL)isEqualToRequest:(TestRequest *)otherRequest {
return [self.requestID isEqualToString:otherRequest.requestID] &&
[self.productName isEqualToString:otherRequest.productName];
}
Check this answer: How do I compare objects in Objective-C?
You need to implement -isEqual: and -hash methods for your Request class.
You need to overwrite isEqual: of your Request object to specify the properties to compare.
Write sth. like this:
- (BOOL)isEqual:(id)other {
if (other == self) return YES;
if (!other || ![other isKindOfClass:[self class]]) return NO;
if (![(id)[self name] isEqual:[other name]]) return NO;
// add other checks if needed
return YES;
}
First off. == is a check for "are these two objects actually the SAME OBJECT". I.e. They are just two pointers to the same but of memory.
You need to be using the isEqual method. However, in order to do this properly you need to override the method in the class.
Something like...
- (BOOL)isEqual:(Request *)otherObject
{
return [self.productName isEqual:otherObject.productName]
&& [self.requestID isEqual:otherObject.requestID];
}
I'm not really sure exactly how to describe what I want to do - the best I can do is provide some code as an example:
- (void) doStuffInLoopForDataArray:(NSArray *)arr forObjectsOfClass:(NSString *)class
{
for ([class class] *obj in arr)
{
// Do stuff
}
}
So I might call this like
NSArray *arr = [NSArray arrayWithObjects:#"foo",#"bar", nil];
[self doStuffInLoopForDataArray:arr forObjectsOfClass:#"NSString"];
and I would expect the code to be executed as if I had wrote
- (void) doStuffInLoopForDataArrayOfStrings:(NSArray *)arr
{
for (NSString *obj in arr)
{
// Do KVC stuff
}
}
Is there a way to get this kind of behavior?
I don't see much point in passing the class to the method. Run your loop as:
for (id obj in arr) {
and check the methods you want to call exist. Passing the class is only really useful if you want to check that the objects in the array are actually of that class, but you couldn't then do much with that information.
Another approach would be to create a single superclass that all the classes I'd like to use this method for inherit from. I can then loop using that superclass.
So if I want to be able to loop for MyObject1 and MyObject2, I could create a BigSuperClass, where MyObject1 and MyObject2 are both subclasses of BigSuperClass.
- (void) doStuffInLoopForDataArray:(NSArray *)arr
{
for (BigSuperClass *obj in arr)
{
// Do stuff
}
}
This loop should work for arrays of MyObject1 objects, arrays of MyObject2 objects, or arrays of BigSuperClass objects.
The more I've been thinking about this, the more I'm leaning towards this being the best approach. Since I can setup my BigSuperClass with all the #propertys and methods I'd be interested in as part of my // Do Stuff, which means I won't have to check respondsToSelector as with the other answers. This way just doesn't feel quite as fragile.
I came up with an idea while I was typing up this question, figured I might as well finish it. I just need to change how I'm doing my loop, and I don't really need to send in the class.
- (void) doStuffInLoopForDataArray:(NSArray *)arr
{
for (int i=0; i < [arr count]; i++)
{
// Do stuff
}
}
I should note that part of my // Do stuff is checking to make sure if ([[arr objectAtIndex:i] respondsToSelector:...]) before I actually try to do anything with it - and from what I understand that should prevent any nasty crashes.
My app crashed after I called addImageToQueue. I added initWithObjects: forKeys: count: but it doesn't helped me.
Terminating app due to uncaught exception 'NSInvalidArgumentException',
reason: '*** -[NSDictionary initWithObjects:forKeys:count:]:
method only defined for abstract class.
Define -[DictionaryWithTag initWithObjects:forKeys:count:]!'
my code
- (void)addImageToQueue:(NSDictionary *)dict
{
DictionaryWithTag *dictTag = [DictionaryWithTag dictionaryWithDictionary:dict];
}
#interface DictionaryWithTag : NSDictionary
#property (nonatomic, assign) int tag;
- (id)initWithObjects:(id *)objects forKeys:(id *)keys count:(NSUInteger)count;
#end
#implementation DictionaryWithTag
#synthesize tag;
- (id)initWithObjects:(id *)objects forKeys:(id *)keys count:(NSUInteger)count
{
return [super initWithObjects:objects forKeys:keys count:count];
}
#end
Are you subclassing NSDictionary? That's not a common thing to do in Cocoa-land, which might explain why you're not seeing the results you expect.
NSDictionary is a class cluster. That means that you never actually work with an instance of NSDictionary, but rather with one of its private subclasses. See Apple's description of a class cluster here. From that doc:
You create and interact with instances of the cluster just as you would any other class. Behind the scenes, though, when you create an instance of the public class, the class returns an object of the appropriate subclass based on the creation method that you invoke. (You don’t, and can’t, choose the actual class of the instance.)
What your error message is telling you is that if you want to subclass NSDictionary, you have to implement your own backend storage for it (for example by writing a hash table in C). It's not just asking you to declare that method, it's asking you to write it from scratch, handling the storage yourself. That's because subclassing a class cluster directly like that is the same as saying you want to provide a new implementation for how dictionaries work. As I'm sure you can tell, that's a significant task.
Assuming you definitely want to subclass NSDictionary, your best bet is to write your subclass to contain a normal NSMutableDictionary as a property, and use that to handle your storage. This tutorial shows you one way to do that. That's not actually that hard, you just need to pass the required methods through to your dictionary property.
You could also try using associative references, which "simulate the addition of object instance variables to an existing class". That way you could associate an NSNumber with your existing dictionary to represent the tag, and no subclassing is needed.
Of course, you could also just have tag as a key in the dictionary, and store the value inside it like any other dictionary key.
From https://stackoverflow.com/a/1191351/467588, this is what I did to make a subclass of NSDictionary works. I just declare an NSDictionary as an instance variable of my class and add some more required methods. It's called "Composite Object" - thanks #mahboudz.
#interface MyCustomNSDictionary : NSDictionary {
NSDictionary *_dict;
}
#end
#implementation MyCustomNSDictionary
- (id)initWithObjects:(const id [])objects forKeys:(const id [])keys count:(NSUInteger)cnt {
_dict = [NSDictionary dictionaryWithObjects:objects forKeys:keys count:cnt];
return self;
}
- (NSUInteger)count {
return [_dict count];
}
- (id)objectForKey:(id)aKey {
return [_dict objectForKey:aKey];
}
- (NSEnumerator *)keyEnumerator {
return [_dict keyEnumerator];
}
#end
I just did a little trick.
I'm not sure that its the best solution (or even it is good to do it).
#interface MyDictionary : NSDictionary
#end
#implementation MyDictionary
+ (id) allocMyDictionary
{
return [[self alloc] init];
}
- (id) init
{
self = (MyDictionary *)[[NSDictionary alloc] init];
return self;
}
#end
This worked fine for me.