This is probably a long shot, but I've got objects with a lot of properties. The values of these objects are populated from NSDictionary's created from a database request. Because of this, there may be NSNull values contained in those NSDictionaries that will automatically get assigned to the properties. I need the properties to automatically discard values/objects that aren't of the correct type. Currently I do it like this:
- (void) setViewID:(NSString *)viewID{
if (!viewID || [viewID isKindOfClass:[NSString class]]) _viewID = viewID;
}
But that ends up being a lot of extra code when I've got 30-50 properties. Is there a way to synthesize this behavior? It seems like it would be a common enough requirement, but I can't seem to find a way to do it aside from writing it all out.
Why not check for NSNull when you are going through the dictionary? E.g.
for (NSString *key in dictionary) {
id value = [dictionary objectForKey:key];
if (value == [NSNull null]) {
value = nil;
}
[self setValue:value forKey:key];
}
Related
In objective-C I find myself creating alot of Mutable objects and then returning them as non mutable objects. Is the way I am doing it here, simply returning the NSMutableSet as an NSSet a good practice? I was thinking maybe I should specify that i make a copy of it.
/** Returns all the names of the variables used in a given
* program. If non are used it returns nil */
+ (NSSet *)variablesUsedInProgram:(id)program
{
NSMutableSet* variablesUsed = [[NSMutableSet alloc]init];
if ([program isKindOfClass:[NSArray class]]) {
for (NSString *str in program)
{
if ([str isEqual:#"x"] || [str isEqual:#"y"] || [str isEqual:#"a"] || [str isEqual:#"b"])
[variablesUsed addObject:str];
}
}
if ([variablesUsed count] > 0) {
return variablesUsed;
} else {
return nil;
}
}
If I were you, I would do it this way.
+ (NSSet *)variablesUsedInProgram:(id)program
{
NSSet *variablesUsed;
if ([program isKindOfClass:[NSArray class]]) {
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"SELF = 'x' or SELF = 'y' or SELF = 'z'"];
variablesUsed = [NSSet setWithArray:[program filteredArrayUsingPredicate:predicate]];
}
int count;
return (count = [variablesUsed count]) > 0 ? variablesUsed : nil;
}
I find using predicate to filter array quite comprehensive and easy. Rather than dealing with creating a new mutable type and then testing certain condition, adding until the loop; in this scenario, it seems to be easier to use predicate. Hope this helps you.
It depends how much safety you require. If you return the object as an NSSet it will still be an NSMutableSet, so it could easily be cast back to one and modified.
Certainly, if you're creating a public API, I'd recommend returning a copy. For in internal project, perhaps the method signature already makes the intention clear enough.
Its, worth noting that, generally the performance impact of returning a copy is negligible - copying an immutable instance is effectively free whereas each copy sent to a mutable-passing-as-immutable will create another copy. So I would say its good practice to default to.
No. This is an absolutely correct OOP approach (it takes advantage of polymorphism). Every NSMutableSet is a proper NSSet. Don't copy superfluously.
Not a full answer here, consider NSProxy's one, but I want to clarify something.
In your case you create your object from scratch, and you don't set any ivar to point to that object. In my opinion in a good percentage of cases you don't need to make a copy of the mutable object returned. But if there is a good reason to deny the class client from mutating the class, then you should copy the variable.
Consider a property like this:
#property (nonatomic,assign) NSSet* set;
The class client could do this:
NSMutableSet* set= ... ; // inizialized to some value
classInstance.set= set;
// Mutate the set
Once mutated the set it could make the class be in an inconsistent state.
That's why when I have a property with the type of a class that has also a mutable version, I always put copy instead of assign in the property.
In using Scripting Bridge with Excel, when I get the value of a single cell, I don't know what class I'm going to get from the cell. A range of a single cell so far returns an NSString or NSNumber (a range of multiple cells always returns an NSArray and I need to coerce the values from there). In my work, I usually only want the string value of the cell, so to coerce the value of a single-cell range into what I need, I do this...
NSString *cellValue = [targetRange.value get];
if ([cellValue isKindOfClass:[NSString class]]) {
cellValue = [targetRange.value get];
} else if ([cellValue isKindOfClass:[NSNumber class]]) {
cellValue = [[targetRange.value get] stringValue];
}
My problem lies with the first line (we'll ignore the third line for the time being as I'm still doing a bit of refactoring). Would there be a better class to capture the value of the cell to test for class against? Those last three lines work fine, but I don't feel entirely comfortable doing that; it seems unintuitive but going with NSObject feels just as weird.
it seems unintuitive but going with NSObject feels just as weird.
In such cases you can use id:
The id type is completely nonrestrictive. By itself, it yields no information about an object, except that it is an object.
E.g.:
id cellValue = [targetRange.value get];
NSString *stringValue = nil;
if ([cellValue isKindOfClass:[NSString class]]) {
stringValue = cellValue;
} else if ([cellValue isKindOfClass:[NSNumber class]]) {
stringValue = [cellValue stringValue];
}
Note that in your case you could probably just use something like this:
NSString *stringValue = [NSString stringWithFormat:#"%#", [targetRange.value get]];
Are you asking if it's "okay" or "normal" to inspect an object's class at runtime and act differently depending on the class? If so, that's an emphatic yes - that's part of what makes Objective-C so powerful.
If you're asking if there's a better way of determining an object's class, then no, not really. I mean you can directly compare classes ( == ) but that's not always wise, given class clusters, subclasses, etc.
I have a NSDictionary with a NSString and NSArray
I have to save only the NSArray in a variable without knowing the key.
Is this possible?
If I'm understanding you correctly, you have a dictionary that contains both an NSString and an NSArray, and you want to extract just the NSArray, without knowing what the key is.
One way to do that is to look through the dictionary with fast enumeration:
NSString *key;
for(key in someDictionary){
id someObject = [someDictionary objectForKey: key];
}
and then look at the objects to see which one is an NSArray:
if ([someObject isKindOfClass:[NSArray class]]) {
// do something with the array
}
(obligatory warning: explicitly checking an object's class is often a sign of a flawed design. In most cases, you should be checking for behavior (-respondsToSelector), not class identity)
I have an NSDictionary in which I use my own classes (NSObject subclasses) as keys and would like to make sure that I do not include the same key twice. However, because NSDictionary copies its keys, if I try to check whether an object is in the dictionary, it never thinks it is.
For example,
MyClass* obj = [[MyClass alloc] init];
NSMutableDictionary* dict = [NSMutableDictionary dictionary];
[dict setObject:someObj forKey:obj];
if ([[dict allKeys] contains:obj]) // always returns false
// obj is already in dict
else
// add obj to dict etc.
Similarly, if I want to change the object associated with this key, it seems to create a new entry.
// dict is empty
// say [obj description] gives 'MyClass : 0x1' - (impossible address?)
[dict setObject:someObj forKey:obj];
// dict: { 'MyClass : 0x2' = someObjDesc }
[dict setObject:someOtherObj forKey:obj];
// dict: { 'MyClass : 0x2' = someObjDesc , 'MyClass : 0x3' = someOtherObjDesc }
Also, this same thing leads to not being able to access the items in the dictionary from the original object
[dict setObject:someObj forKey:obj];
[dict objectForKey:obj]; // returns null
So, as far as the uniqueness is concerned, would I be best off keeping track of the keys in a separate array or is there a better way of doing this.
I considered implementing an isEqual method based on a unique variable (such as a name) but didn't think that was the Right Thing to do.
Background (in case it turns out that maybe I'm just using the wrong thing entirely):
I want to keep track of information about a group of people going to different places. So each person at each place has some info. What I've done is used nested dictionaries so the key to the main dictionary is a Person object and the object another dictionary. This latter dictionary has key Place and info as the object. I think this is Java syntax but something like > (the array holds the info). I want to be able to add a Person only if the don't already exist, add a Place (for each person), change the array.
Any help on any of this would be greatly appreciated!
You should always use NSStrings as keys for dictionaries, especially if you are new at objective-C. There are a few things that I can see you are doing wrong with your current implementation - you would need to read up on key requirements for NSDictionaries.
You can do what you want with strings as keys - person's name, etc.
The objects in a dictionary have all the info about a certain person:
NSDictionary* personsInfo = [mainDict objectForKey:#"Jane Smith"];
NSString* addressLine1 = [personsInfo objectForKey#"addressLine1"];
--Tom
The simple answer would be to make it so that the MyClass doesn't actually copy anything.
That would be something like changing:
- (id) copyWithZone:(NSZone *)zone {
MyClass * foo = (MyClass *)[super copyWithZone:zone];
[foo configureCopy];
return foo;
}
To:
- (id) copyWithZone:(NSZone *)zone {
return [self retain];
}
I have NSMutableArray with different objects in it of different classes. Now I want to get the class name, related stuff and also check if the respective object is NSString or not. How should I go about it?
I was trying something like the following. It wasn't working of course.
for(NSString *string in array){
NSLog(#"Name of the class : %#", [NSString stringWithCString:class_getName(Class id)];
If you're on Mac OS X, you can use [object className], it returns an NSString
for(id obj in array) NSLog(#"Name of the class: %#", [obj className]);
To check if it's a NSString, you should use something like this:
for(id obj in array) {
if ([obj isKindofClass:[NSString class]]) {
// do something
}
}
for(id object in array){
NSLog(#"Name of the class: %#", [object className]);
NSLog(#"Object is a string: %d", [object isKindOfClass:[NSString class]]);
}
Take a look at the NSObject class and protocol for other interesting methods.
I have NSMutableArray with different objects in it of different classes. Now I want to get the class name & related stuff & also check if the respective object is NSString or not.
Hold up. Why do have an array of different typed objects in the first place? Could you redo your design to avoid getting into that situation?
As others have said, -isKindOfClass: works. One downside is it generally leads to brittle code. Here your loop needs to know about all the classes that could be in the array. Sometimes this is the best you can do though.
Designs that use -respondsToSelector: tend to be a little more robust. Here your loop would need to know about the behaviors it depends on of classes in the array.