A category on MKPolyline adding a class method to create a new MKPolyline from a Google Maps Directions API encoded polyline string. See https://developers.google.com/maps/documentation/utilities/polylinealgorithm

MKPolyline+GMEncodedString.h

#import <MapKit/MapKit.h>

@interface MKPolyline (GMEncodedString)

+ (MKPolyline *)polylineWithGMEncodedString:(NSString *)encodedString;

@end

MKPolyline+GMEncodedString.m

#import "MKPolyline+GMEncodedString.h"

@implementation MKPolyline (GMEncodedString)

+ (MKPolyline *)polylineWithGMEncodedString:(NSString *)encodedString {
	
	const char *bytes = [encodedString UTF8String];
	NSUInteger length = [encodedString lengthOfBytesUsingEncoding:NSUTF8StringEncoding];
	NSUInteger idx = 0;
	
	NSUInteger count = length / 4;
	CLLocationCoordinate2D *coords = calloc(count, sizeof(CLLocationCoordinate2D));
	NSUInteger coordIdx = 0;
	
	float latitude = 0;
	float longitude = 0;
	while (idx < length) {
		char byte = 0;
		int res = 0;
		char shift = 0;
		
		do {
			byte = bytes[idx++] - 63;
			res |= (byte & 0x1F) << shift;
			shift += 5;
		} while (byte >= 0x20);
		
		float deltaLat = ((res & 1) ? ~(res >> 1) : (res >> 1));
		latitude += deltaLat;
		
		shift = 0;
		res = 0;
		
		do {
			byte = bytes[idx++] - 0x3F;
			res |= (byte & 0x1F) << shift;
			shift += 5;
		} while (byte >= 0x20);
		
		float deltaLon = ((res & 1) ? ~(res >> 1) : (res >> 1));
		longitude += deltaLon;
		
		float finalLat = latitude * 1E-5;
		float finalLon = longitude * 1E-5;
		
		CLLocationCoordinate2D coord = CLLocationCoordinate2DMake(finalLat, finalLon);
		coords[coordIdx++] = coord;
		
		if (coordIdx == count) {
			NSUInteger newCount = count + 10;
			coords = realloc(coords, newCount * sizeof(CLLocationCoordinate2D));
			count = newCount;
		}
	}
	
	MKPolyline *polyline = [MKPolyline polylineWithCoordinates:coords count:coordIdx];
	free(coords);
	
	return polyline;
}

@end