How to query an NSDictionary using an expression written in NSString? - ios

Question

I want to be able to run the following hypothetical function called evaluateExpression:on: and get "John" as answer.
NSDictionary *dict = #{"result": #[#{#"name": #"John"}, #{#"name": #"Mary"}]};
NSString *expression = #"response['result'][0]['name']";
NSString *answer = [self evaluateExpression: expression on: dict];
Is this possible?

Answer 1

There's an NSObject category that extends valueForKeyPath to give valueForKeyPathWithIndexes. It lets you write this:
NSDictionary *dict = #{#"result": #[#{#"name": #"John"}, #{#"name": #"Mary"}]};
NSString *path = #"result[0].name";
NSString *answer = [dict valueForKeyPathWithIndexes:path];
XCTAssertEqualStrings(answer, #"John");
The category is by psy, here: Getting array elements with valueForKeyPath
#interface NSObject (ValueForKeyPathWithIndexes)
-(id)valueForKeyPathWithIndexes:(NSString*)fullPath;
#end
#import "NSObject+ValueForKeyPathWithIndexes.h"
#implementation NSObject (ValueForKeyPathWithIndexes)
-(id)valueForKeyPathWithIndexes:(NSString*)fullPath
{
NSRange testrange = [fullPath rangeOfString:#"["];
if (testrange.location == NSNotFound)
return [self valueForKeyPath:fullPath];
NSArray* parts = [fullPath componentsSeparatedByString:#"."];
id currentObj = self;
for (NSString* part in parts)
{
NSRange range1 = [part rangeOfString:#"["];
if (range1.location == NSNotFound)
{
currentObj = [currentObj valueForKey:part];
}
else
{
NSString* arrayKey = [part substringToIndex:range1.location];
int index = [[[part substringToIndex:part.length-1] substringFromIndex:range1.location+1] intValue];
currentObj = [[currentObj valueForKey:arrayKey] objectAtIndex:index];
}
}
return currentObj;
}
#end
Plain old valueForKeyPath will get you close, but not exactly what you asked for. It may be useful in this form though:
NSDictionary *dict = #{#"result": #[#{#"name": #"John"}, #{#"name": #"Mary"}]};
NSString *path = #"result.name";
NSString *answer = [dict valueForKeyPath:path];
XCTAssertEqualObjects(answer, (#[#"John", #"Mary"]));

Answer 2

After spending quite a while trying to tackle this problem in the most efficient way, here's another take I came up with: Use javascript.
The above approach posted by Ewan definitely takes care of the problem, but I had some additional custom features I wanted to add, and objective-c approach became too complex very quickly. I ended up writing eval code in javascript and integrating it into objective-c using JavascriptCore. With that, it becomes as simple as one line of eval() call.