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What does Objective-C property get resolved to in runtime? Will calling [obj valueForKey:#"property"] always yield the same result?
e.g.
obj.property
First, note that obj.property is precisely the same as [obj property]. Dot syntax is just syntactic sugar. While there are some small run-time implementation details related to properties that are different than other methods, for the purposes of this discussion, think only in terms of "I have an ivar named _key and a method called -key." The fact that you created that ivar and method by declaring a property is irrelevant for valueForKey:.
valueForKey: is a method, and it can be overridden by a class to return whatever it likes. The default behavior is that valueForKey: will first look for a method whose name matches the key, and will return the result of that. In the vast majority of cases, this means that if you have a property, then valueForKey:#"property" will return the value of it.
The full search path for the default implementation of valueForKey: is explained in "Accessor Search Implementation Details", but here is the "short" version:
get<Key>, <key>, is<Key> (yes, the first place it looks is getKey, which is a little embarrassing because you should not prefix getters with get unless they return values by reference, but there you go; it is the first thing checked.)
countOf<Key>, objectIn<Key>AtIndex:, and <key>AtIndexes. If a legal combination of these are found, then an NSArray-like proxy object is returned.
countOf<Key>, enumeratorOf<Key>, and memberOf<Key>:. If all three are found, then an NSSet-like proxy object is returned.
If accessInstanceVariablesDirectly is YES (the default), then ivars are checked, named _<key>, _is<Key>, <key>, or is<Key>. Yes, this is a way to access an object's private ivars.
If everything else failed, then it calls valueForUndefinedKey:, which is free to return a result (and this is in fact a very useful thing to do if you want a generic key/value store).
But nine times out of ten, you're going to get the value of the method named <key>.
Side note: valueForKey: returns an object. If the return is a number-like scalar (including BOOL), it will return an NSNumber. Otherwise it will return an NSValue. There is some special handling for NSPoint, NSRange, NSRect, and NSSize (on Mac; on iOS, only NSRange is handled specially I believe).
obj.property is the same as [obj property], not [obj valueForKey:#"property"];. The latter is part of a system called Key Value Coding that's separate from properties.
In my quest to update a Core Data model within my iOS project, I'm querying a server for JSON objects that correspond - to some extent - with the managed entities of my model. The end result I'm striving for is a reliable update solution from JSON output.
For the examples in this question, I'll name the core data managed object existingObj and the incoming JSON deserialized dictionary updateDict. The tricky part is dealing with these facts:
Not all properties of the existingObj are present in the updateDict
Not all properties of the updateDict are available in the extistingObj.
Not all types of existingObj's properties match the JSON deserialized properties. (some strings may need a custom Objective-C wrapper).
updateDict may contain values for keys that are uninitialized (nil) in existingObj.
This means that while iterating through the updated dictionaries, there has to be some testing of properties back and forth. First I have to test whether the properties of the updateDict exist in existingObj, then I set the value using KVC, like so:
// key is an NSString, e.g. #"displayName"
if ([existingObj respondsToSelector:NSSelectorFromString(key)) {
[existingObj setValue:[updateDict objectForKey:key] forKey:key];
}
Although this part works, I don't like the fact that I'm actually testing for displayName as a getter, while I'm about to call the setDisplayName: setter (indirectly via KVC). What I'd rather to is something like [existingObj hasWritablePropertyWithName:key], but something that does this I can't find.
This makes for subquestion A: How does one test for a property setter, if you only have the property's name?
The next part is where I'd like to automate the property identification based on their types. If both the updateDict and the existingObj have an NSString for key #"displayName", setting the new value is easy. However, if the updateDict contains an NSString for key #"color" that is #"niceShadeOfGreen", I'd like to transform this into the right UIColor instance. But how do I test the type of the receiving property in existingObj so I know when to convert values and when to simply assign? I was hoping for something along the lines of typeOfSelector:
if ([existingObj typeOfSelector:sel] == [[updateDict objectForKey:key] class]) {
// regular assignment
} else {
// perform custom assignment
}
Of course this is boguscode. I can't rely on testing the type of the existingObj-property's value, for it may be unitialized or nil.
Subquestion B: How does one test for the type of a property, if you only have the property's name?
I guess that's it. I figured this must be a dupe of something that's already on here, but I couldn't find it. Maybe you guys can?
Cheers, EP.
P.S. If you'd have a better way to synchronize custom Objective-C objects to deserialized JSON objects, please do share! In the end, the result is what counts.
If you want to query whether an object has a setter for a given KVC key called key which corresponds to a declared property, you need to check whether it responds to a selector method called setKey: (starts with set, capitalise the first character in key, add a trailing colon). For instance,
NSString *key = #"displayName";
NSString *setterStr = [NSString stringWithFormat:#"set%#%#:",
[[key substringToIndex:1] capitalizedString],
[key substringFromIndex:1]];
if ([obj respondsToSelector:NSSelectorFromString(setterStr)]) {
NSLog(#"found the setter!");
[obj setValue:someValue forKey:key];
}
Two remarks:
Even though properties can have setters with names that do not follow the pattern described above, they wouldn’t be KVC compliant, so it is safe to check for set<Key>: since you’re using KVC to set the corresponding value.
KVC doesn’t use the setter method only. If it doesn’t find a setter method, it checks whether the class allows direct access to instance variables and, if so, use the instance variable to set the value. Also, if no setter method or instance variable is found, it sends -setValue:forUndefinedKey: to the receiver, whose class might have overridden the standard implementation that throws an exception. This is described in the Key-Value Coding Programming Guide.That said, if you’re always using properties, checking for the setter method should be safe.
As for your second question, it is not possible to query the runtime to know the actual Objective-C class of a property. From the runtime perspective, there’s an implementation specific type encoding for properties and general types (such as method parameters/return types). This type encoding uses a single encoding (namely #) for any Objective-C object, so the type encoding of an NSString property is the same as the type encoding of a UIColor property since they’re both Objective-C classes.
If you do need this functionality, one alternative is to process your classes and add a class method that returns a dictionary with keys and corresponding types for every property (or the ones you’re interested in) declared in that class and superclasses, or maybe some sort of description language. You’d have to do this on your own and rely on information not available during runtime.
I'm trying to pass an NSString by reference but it doesn't work.
This is the function:
+(void)fileName:(NSString *) file
{
file = #"folder_b";
}
and this is the call:
NSString *file;
[function fileName:file];
nslog(#"%#",file); // and there is nothing in the string....
What I must do to pass my string by reference?
If you want to return a value, then return a value. Pass by reference in Cocoa/iOS is largely limited to NSError**.
Given:
+(void)fileName:(NSString *) file
Then do:
+(NSString *) fileName;
And be done with it.
If you need to return more than one value at a time, that begs for a structure or, more often, a class.
In Objective-C, pass by reference smells like you are doing it wrong.
Pass by reference in Objective-C is reserved largely for returning NSError* information about a recoverable failure, where the return value of the method itself indicates whether or not the requested task succeeded or failed (you can pass NULL as the NSError** argument to allow the method to optimize away creating said error metadata).
Pass by references is also used to retrieve interior state of objects where the return value is effectively a multi-value. I.e. methods from AppKit like the following. In these cases, the pass-by-reference arguments are typically either optional or are acting as secondary return values.
They are used quite sparingly across the API. There is certainly use for pass by reference, but -- as said above -- doing so should be quite rare and rarer still in application code. In many cases -- and in some of the cases below, potentially -- a better pattern would be to create a class that can encapsulate the state and then return an instance of said class instead of pass by reference.
NSWorkspace.h:- (BOOL)getInfoForFile:(NSString *)fullPath application:(NSString **)appName type:(NSString **)type;
NSTextView.h:- (void)smartInsertForString:(NSString *)pasteString replacingRange:(NSRange)charRangeToReplace beforeString:(NSString **)beforeString afterString:(NSString **)afterString;
NSAttributedString.h:- (BOOL)readFromURL:(NSURL *)url options:(NSDictionary *)options documentAttributes:(NSDictionary **)dict;
NSNib.h:- (BOOL)instantiateWithOwner:(id)owner topLevelObjects:(NSArray **)topLevelObjects NS_AVAILABLE_MAC(10_8);
NSSpellChecker.h:- (NSRange)checkGrammarOfString:(NSString *)stringToCheck startingAt:(NSInteger)startingOffset language:(NSString *)language wrap:(BOOL)wrapFlag inSpellDocumentWithTag:(NSInteger)tag details:(NSArray **)details NS_AVAILABLE_MAC(10_5);
I believe you're looking for:
+ (void)fileName:(NSString **)file
{
*file = #"folder_b";
}
What's really done here is we're working with a pointer to a pointer to an object. Check C (yup, just plain C) guides for "pointer dereference" for further info.
(...But as has been pointed out repeatedly, in this particular example, there's no reason to pass by reference at all: just return a value.)
Passing a pointer to your object is the Objective C (and C) way of passing by reference.
I agree with 'bbum' that a perceived need to pass by reference is a signal to think about what you are doing; however, it is by no means the case that there are not legitimate reasons to pass by reference.
You should not create classes willy-nilly every time you have a function or method that needs to return more than one value. Consider why you are returning more than one value and if it makes sense to create a class for that then do so. Otherwise, just pass in pointers.
-Just my 2 cents
Try this
+(void)filename:(NSString **)file {
*file=#"folder_b";
}
and send the file as &file like:
NSString *file;
[function fileName:&file];
nslog(#"%#",file);
hope this will work.
I suspect this is because NSString is immutable. Have you tried NSMutableString?
I have a database model class that is a NSObject. I have a set of these objects in a NSMutableArray. I use indexOfObject: to find a match. Problem is the model object's memory address changes. So I am overriding the hash method to return the model's row ID. This however does not fix it. I also have to override the isEqual: method to compare the value of the hash method.
What does the isEqual: method use to determine equality by default?
I'm assuming it uses the memory address. After reading the isEqual: documentation I thought it used the value from the hash method. Obviously, that is not the case as my attempt to override that value did not solve my initial problem.
As you've correctly guessed, NSObject's default isEqual: behaviour is comparing the memory address of the object. Strangely, this is not presently documented in the NSObject Class Reference, but it is documented in the Introspection documentation, which states:
The default NSObject implementation of isEqual: simply checks for pointer equality.
Of course, as you are doubtless aware, subclasses of NSObject can override isEqual: to behave differently. For example, NSString's isEqual: method, when passed another NSString, will first check the address and then check for an exact literal match between the strings.
The answer about default implementation of isEqual: is comprehensive one. So I just add my note about default implementation of hash. Here it is:
-(unsigned)hash {return (unsigned)self;}
I.e it's just the same pointer value which is used in isEqual:. Here's how you can check this out:
NSObject *obj = [[NSObject alloc] init];
NSLog(#"obj: %#",obj);
NSLog(#"hash: %x",obj.hash);
The result will be something like this:
obj: <NSObject: 0x16d44010>
hash: 16d44010
Best Regards.
BTW in iOS 8 hash became a property not a method, but it's there.
I would assume that NSObject isEquals uses the == operator, and hash uses the memory address.
isEquals method should never uses hash as an absolute test for equality. It is guaranteed to have two objects having similar hashCode, if you search for enough objects (just create more than 2^32 different objects, and at least two of them will have the same hash).
In other words, hash requires the following spec: If two objects are equals, then their hash needs to be equal; however, if two objects' hash values are equals, they are not necessarily equal.
As a tip, you always should override isEquals and hashCode together.
The documentation for -hash says it must not change while a mutable object is stored in a collection, and similarly the documentation for -isEqual: says the -hash value must be the same for equal objects.
Given this, does anybody have any suggestions for the best way to implement -hash such that it meets both these conditions and yet is actually calculated intelligently (i.e. doesn't just return 0)? Does anybody know how the mutable versions of framework-provided classes do this?
The simplest thing to do is of course just forget the first condition (about it not changing) and just make sure I never accidentally mutate an object while it's in a collection, but I'm wondering if there's any solution that's more flexible.
EDIT: I'm wondering here whether it's possible to maintain the 2 contracts (where equal objects have equal hashes, and hashes don't change while the object is in a collection) when I'm mutating the internal state of the object. My inclination is to say "no", unless I do something stupid like always return 0 for the hash, but that's why I'm asking this question.
Interesting question, but I think what you want is logically impossible. Say you start with 2 objects, A and B. They're both different, and they start with different hash codes. You add both to some hash table. Now, you want to mutate A, but you can't change the hash code because it's already in the table. However, it's possible to change A in such a way that it .equals() B.
In this case, you have 2 choices, neither of which works:
Change the hashcode of A to equal B.hashcode, which violates the constraint of not changing hash codes while in a hash table.
Don't change the hashcode, in which case A.equals(B) but they don't have the same hashcodes.
It seems to me that there's no possible way to do this without using a constant as a hashcode.
My reading of the documentation is that a mutable object's value for hash can (and probably should) change when it is mutated, but should not change when the object hasn't been mutated. The portion of the documentation to which to refer, therefore, is saying, "Don't mutate objects that are stored in a collection, because that will cause their hash value to change."
To quote directly from the NSObject documentation for hash:
If a mutable object is added to a
collection that uses hash values to
determine the object’s position in the
collection, the value returned by the
hash method of the object must not
change while the object is in the
collection. Therefore, either the hash
method must not rely on any of the
object’s internal state information or
you must make sure the object’s
internal state information does not
change while the object is in the
collection.
(Emphasis mine.)
The question here isn't how to meet both of these requirements, but rather which one you should meet. In Apple's documentation, it is clearly stated that:
a mutable dictionary can be put in a hash table but you must not change it while it is in there.
This being said, it seems more important that you meet the equality requirement of hashes. The hash of an object should always be a way to check if an object is equal to another. If this is ever not the case, it is not a true hash function.
Just to finish up my answer, I'll give an example of a good hash implementation. Let's say you are writing the implementation of -hash on a collection that you have created. This collection stores an array of NSObjects as pointers. Since all NSObjects implement the hash function, you can use their hashes in calculating the collection's hash:
- (NSUInteger)hash {
NSUInteger theHash = 0;
for (NSObject * aPtr in self) { // fast enumeration
theHash ^= [aPtr hash];
}
return theHash;
}
This way, two collection objects containing the same pointers (in the same order) will have the same hash.
Since you are already overriding -isEqual: to do a value-based comparison, are you sure you really need to bother with -hash?
I can't guess what exactly you need this for of course, but if you want to do value-based comparison without deviating from the expected implementation of -isEqual: to only return YES when hashes are identical, a better approach might be to mimick NSString's -isEqualToString:, so to create your own -isEqualToFoo: method instead of using or overriding -isEqual:.
The answer to this question and the key to avoiding many cocoa-bugs is this:
Read the documentation carefully. Place every word and punctuation on a golden scale and weight it as it was the world's last grain of wheat.
Let's read the documentation again:
If a mutable object is added to a collection that uses hash values to determine the object’s position in the collection, [...]
(emphasis mine).
What the writer of the docs, in his/hers eternal wisdom, mean by this is that when you are implementing a collection, like a dictionary, you shouldn't use the hash for positioning since that can change. In other words it has little to do with implementing -hash on mutable Cocoa objects (which all of us thought it had, assuming the documentation has not changed in the last ~10 years since the question was asked).
That is why dictionaries always copy their keys - so they can guarantee
that the hash value won't change.
You will then ask the question: But, good sir, how does NSMapTable and similar handle this?
The answer to this is according to the documentation:
"Its keys or values may be copied on input or may use pointer identity for equality and hashing."
(emphasis mine again).
Since we were so easily fooled by the documentation last time, let's run a little experiment to see for ourselves how stuff actually work:
NSMutableString *string = [NSMutableString stringWithString:#"so lets mutate this"];
NSString *originalString = string.copy;
NSMapTable *mutableStrings = [NSMapTable strongToStrongObjectsMapTable];
[mutableStrings setObject:originalString forKey:string];
[string appendString:#" into a larger string"];
if ([mutableStrings objectForKey:string] == nil)
NSLog(#"not found!");
if ([mutableStrings objectForKey:originalString] == nil)
NSLog(#"Not even the original string is found?");
for (NSString *inCollection in mutableStrings)
{
NSLog(#"key '%#' : is '%#' (null)", inCollection, [mutableStrings objectForKey:inCollection]);
}
for (NSString *value in NSAllMapTableValues(mutableStrings))
{
NSLog(#"value exists: %#", value);
}
Surprise!
So, instead of using pointer equality, they focus on the words "may" here which in this case mean "may not", and simply copy the hash value when adding stuff to the collection.
(All this is actually good, since it would be quite difficult to implement NSHashMap, or -hash, otherwise).
In Java, most mutable classes simply don’t override Object.hashCode() so that the default implementation returns a value that is based on the address of the object and doesn’t change. It might just be the same with Objective C.