I want to create instance variables dynamically at runtime, and I want to add these variables to a category. The number of the instance variables may change based on the configuration/properties file which I am using for defining them.
Any ideas??
3 Answers
Use Associative References - this is tricky, but that is the mechanism invented specifically for your use case.
Here is an example from the link above: first, you define a reference and add it to your object using objc_setAssociatedObject; then you can retrieve the value back by calling objc_getAssociatedObject.
static char overviewKey;
NSArray *array = [[NSArray alloc] initWithObjects:@ "One", @"Two", @"Three", nil];
NSString *overview = [[NSString alloc] initWithFormat:@"%@", @"First three numbers"];
objc_setAssociatedObject (
array,
&overviewKey,
overview,
OBJC_ASSOCIATION_RETAIN
);
[overview release];
NSString *associatedObject = (NSString *) objc_getAssociatedObject (array, &overviewKey);
NSLog(@"associatedObject: %@", associatedObject);
objc_setAssociatedObject (
array,
&overviewKey,
nil,
OBJC_ASSOCIATION_ASSIGN
);
[array release];
answered Dec 7, 2012 at 4:56
Sergey Kalinichenko
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Comments
I'd be inclined to just use a NSMutableDictionary (see NSMutableDictionary Class Reference). Thus, you would have an ivar:
NSMutableDictionary *dictionary;
You'd then initialize it:
dictionary = [NSMutableDictionary dictionary];
You can then save values to it dynamically in code, e.g.:
dictionary[@"name"] = @"Rob";
dictionary[@"age"] = @29;
// etc.
Or, if you are reading from a file and don't know what the names of the keys are going to be, you can do this programmatically, e.g.:
NSString *key = ... // your app will read the name of the field from the text file
id value = ... // your app will read the value of the field from the text file
dictionary[key] = value; // this saves that value for that key in the dictionary
And if you're using an older version of Xcode (before 4.5), the syntax is:
[dictionary setObject:value forKey:key];
4 Comments
Anil Kumar
But for this way, I have to modify my code everytime to add keys and it tends more towards a static reference.
Rob
@AnilKumarReddy No, it doesn't have to be static. By definition, being a mutable dictionary, you can programmatically add whatever keys and values you want at run time. I just showed some static examples, but you can use syntax like
dictionary[variableName] = variableValue;. It doesn't have to be static.Anil Kumar
Is there any other way to make the solution for a category, I want to add new colors to the UIColor class at runtime based on the details in json/file. So ideally want to add ivars to use them.
Rob
@AnilKumarReddy You can (see dasblinkenlight's discussion), but in my opinion that is a bad use of dbl's solution. I'd wager that dbl would agree that this is not what his technique is designed to solve. This color information you've downloaded is an attribute of the file you've downloaded, not a change or extension of the intrinsic behavior of the UIColor class. Personally, I'd simply store the RGB (or HSB or however it's specified in the file) as an key-value pair in the same
NSMutableDictionary that you're storing the rest of the stuff you've gotten from the file.Depends on exactly what you want to do, the question is vague but if you want to have several objects or several integers or so on, arrays are the way to go. Say you have a plist with a list of 100 numbers. You can do something sort of like this:
NSArray * array = [NSArray arrayWithContentsOfFile:filePath];
// filePath is the path to the plist file with all of the numbers stored in it as an array
That will give you an array of NSNumbers, you can then turn that into an array of just ints if you want like this;
int intArray [[array count]];
for (int i = 0; i < [array count]; i++) {
intArray[i] = [((NSNumber *)[array objectAtIndex:i]) intValue];
}
Whenever you want to get an integer from a certain position, lets say you want to look at the 5th integer, you would do this:
int myNewInt = intArray[4];
// intArray[0] is the first position so [4] would be the fifth
Just look into using a plist for pulling data, it will them be really easy to create arrays of custom objects or variables in your code by parsing the plist.
NSMutableDictionary, and then you can set values for whatever keys you want? The dictionary holds everything and the key names would serve a function similar to ivars.NSMutableDictionaryprobably makes sense.NSMutableDictionaryas an instance variable), or whether you're using a category to add functionality to an existing class that you don't own (e.g. you wanted to add someBase64encoding logic to theNSDataclass). You can't add theNSMutableDictionarytoNSData(you have to use associative references), but you obviously can add theNSMutableDictionaryto one of your own classes. You said "category", but that has a very specific meaning in Objective-C.