Objective c - create a variable name at runtime and evaluate its value - ios

Question

I have a few keys defined as static vars:
static NSString icon_0 = #"image_0.png";
static NSString icon_1 = #"some_image_with_a_different_name.png";
static NSString icon_3 = #"picure_of_a_bear.png";
now inside a datasource method where I get the indexpath i would like to create the variable name from a string:
-(UICollectionviewCell*)cellForIndexPath:(NSIndexPath *)path
{
NSString *varName = [NSString stringWithFormat:#"icon_%d",path.row];
// here I need the static NSString which corresponds to the var name created
// i.e
NSString imageName;
if (indexPath.row == 0)
{
imageName = #"image_0.png";
}
// would be much nicer to do something like
NSString *imageName = [varName evaluate]; // get the content out of it...
}
How can I do this on static variable?
I tried
NSString *iconName = [self valueForKey:str];
but it isn't an iVar so not working...

Answer 1

i'd not use static vars but a static dictionary like this:
runnable example:
#import <Foundation/Foundation.h>
NSDictionary *DDImageName(NSString *varName);
NSDictionary *DDImageName(NSString *varName) {
static NSDictionary *dict = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
//TODO
//add all names and the image names here
dict = #{#"icon_0": #"image_0.png",
#"icon_1": #"some_image_with_a_different_name.png",
#"icon_2": #"picure_of_a_bear.png"};
});
return dict[varName];
}
//demo only
int main(int argc, char *argv[]) {
#autoreleasepool {
NSString *varName = #"icon_0";
NSString *imgName = DDImageName(varName);
NSLog(#"imageName for %# = %#", varName, imgName);
}
}

Answer 2

If you make your variables instance variables or properties of an object, then you could use key value coding (KVC) to read and write values to them:
-(UICollectionviewCell*)cellForIndexPath:(NSIndexPath *)path
{
NSString *varName = [NSString stringWithFormat:#"icon_%d",path.row];
// here I need the static NSString which corresponds to the var name created
// i.e
NSString imageName;
if (indexPath.row == 0)
{
[self setValue = #"image_0.png" forKey: varName];
}
}
or
string = [self valueForKey: varName];
As #Daij-Djan points out, though, it's probably better to refactor your code to save your information to a dictionary rather than trying to manipulate your instance variables using string variable names. KVC is fairly slow, and will crash your program if a key doesn't exist at runtime, so it's fragile.