Freedom of NSExpression

expression.m

#import <Foundation/Foundation.h>
#import <objc/runtime.h>

/**
 Joins the given components into a single string by concatenating each component
 in order.
 */
NSString *MGLJoinComponents(Class self, SEL _cmd, NSArray<NSString *> *components) {
    return [components componentsJoinedByString:@""];
}

/**
 Adds to NSExpression’s built-in repertoire of functions.
 */
void endowExpressions() {
    // NSExpression’s built-in functions are backed by class methods on a
    // private class, so use a function expression to get at the class.
    // http://funwithobjc.tumblr.com/post/2922267976/using-custom-functions-with-nsexpression
    NSExpression *functionExpression = [NSExpression expressionWithFormat:@"sum({})"];
    NSString *className = NSStringFromClass([functionExpression.operand.constantValue class]);
    
    // Effectively categorize the class with some extra class methods.
    Class NSPredicateUtilities = objc_getMetaClass(className.UTF8String);
#pragma clang push
#pragma clang diagnostic ignored "-Wundeclared-selector"
    class_addMethod(NSPredicateUtilities, @selector(mgl_join:), (IMP)MGLJoinComponents, "@@:@");
#pragma clang pop
}

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        endowExpressions();
        
        NSExpression *expression = [NSExpression expressionWithFormat:@"mgl_join({age, 'McDonald'})"];
        NSString *value = [expression expressionValueWithObject:@{@"age": @"Old"} context:nil];
        NSCAssert([value isEqualToString:@"OldMcDonald"], @"Evaluated expression should concatenate the strings.");
    }
    return 0;
}

That’s pretty interesting!!