The following viewDidLoad will (1) set two locations, (2) remove all the previous annotations, and (3) call user defined helper functions (to get route points and draw the route).

-(void)viewDidLoad
{
    [super viewDidLoad];

    // Origin Location.
    CLLocationCoordinate2D loc1;
    loc1.latitude = 29.0167;
    loc1.longitude = 77.3833;
    Annotation *origin = [[Annotation alloc] initWithTitle:@"loc1" subTitle:@"Home1" andCoordinate:loc1];
    [objMapView addAnnotation:origin];

    // Destination Location.
    CLLocationCoordinate2D loc2;
    loc2.latitude = 19.076000;
    loc2.longitude = 72.877670;
    Annotation *destination = [[Annotation alloc] initWithTitle:@"loc2" subTitle:@"Home2" andCoordinate:loc2];
    [objMapView addAnnotation:destination];

    if(arrRoutePoints) // Remove all annotations
        [objMapView removeAnnotations:[objMapView annotations]];

    arrRoutePoints = [self getRoutePointFrom:origin to:destination];
    [self drawRoute];
    [self centerMap];
}

The following is the MKMapViewDelegate method, which draws overlay (iOS 4.0 and later).

/* MKMapViewDelegate Meth0d -- for viewForOverlay*/
- (MKOverlayView*)mapView:(MKMapView*)theMapView viewForOverlay:(id <MKOverlay>)overlay
{
    MKPolylineView *view = [[MKPolylineView alloc] initWithPolyline:objPolyline];
    view.fillColor = [UIColor blackColor];
    view.strokeColor = [UIColor blackColor];
    view.lineWidth = 4;
    return view;
}

The following function will get both the locations and prepare URL to get all the route points. And of course, will call stringWithURL.

/* This will get the route coordinates from the Google API. */
- (NSArray*)getRoutePointFrom:(Annotation *)origin to:(Annotation *)destination
{
    NSString* saddr = [NSString stringWithFormat:@"%f,%f", origin.coordinate.latitude, origin.coordinate.longitude];
    NSString* daddr = [NSString stringWithFormat:@"%f,%f", destination.coordinate.latitude, destination.coordinate.longitude];

    NSString* apiUrlStr = [NSString stringWithFormat:@"http://maps.google.com/maps?output=dragdir&saddr=%@&daddr=%@", saddr, daddr];
    NSURL* apiUrl = [NSURL URLWithString:apiUrlStr];

    NSError *error;
    NSString *apiResponse = [NSString stringWithContentsOfURL:apiUrl encoding:NSUTF8StringEncoding error:&error];
    NSString* encodedPoints = [apiResponse stringByMatching:@"points:\\\"([^\\\"]*)\\\"" capture:1L];

    return [self decodePolyLine:[encodedPoints mutableCopy]];
}

The following code is the real magic (decoder for the response we got from the API). I would not modify that code unless I know what I am doing :)

- (NSMutableArray *)decodePolyLine:(NSMutableString *)encodedString
{
    [encodedString replaceOccurrencesOfString:@"\\\\" withString:@"\\"
                                  options:NSLiteralSearch
                                    range:NSMakeRange(0, [encodedString length])];
    NSInteger len = [encodedString length];
    NSInteger index = 0;
    NSMutableArray *array = [[NSMutableArray alloc] init];
    NSInteger lat=0;
    NSInteger lng=0;
    while (index < len) {
        NSInteger b;
        NSInteger shift = 0;
        NSInteger result = 0;
        do {
            b = [encodedString characterAtIndex:index++] - 63;
            result |= (b & 0x1f) << shift;
            shift += 5;
        } while (b >= 0x20);
        NSInteger dlat = ((result & 1) ? ~(result >> 1) : (result >> 1));
        lat += dlat;
        shift = 0;
        result = 0;
        do {
            b = [encodedString characterAtIndex:index++] - 63;
            result |= (b & 0x1f) << shift;
            shift += 5;
       } while (b >= 0x20);
        NSInteger dlng = ((result & 1) ? ~(result >> 1) : (result >> 1));
        lng += dlng;
        NSNumber *latitude = [[NSNumber alloc] initWithFloat:lat * 1e-5];
        NSNumber *longitude = [[NSNumber alloc] initWithFloat:lng * 1e-5];
        printf("\n[%f,", [latitude doubleValue]);
        printf("%f]", [longitude doubleValue]);
        CLLocation *loc = [[CLLocation alloc] initWithLatitude:[latitude floatValue] longitude:[longitude floatValue]];
        [array addObject:loc];
    }
    return array;
}

This function will draw a route and will add an overlay.

- (void)drawRoute
{
    int numPoints = [arrRoutePoints count];
    if (numPoints > 1)
    {
        CLLocationCoordinate2D* coords = malloc(numPoints * sizeof(CLLocationCoordinate2D));
        for (int i = 0; i < numPoints; i++)
        {
            CLLocation* current = [arrRoutePoints objectAtIndex:i];
            coords[i] = current.coordinate;
        }

        self.objPolyline = [MKPolyline polylineWithCoordinates:coords count:numPoints];
        free(coords);

        [objMapView addOverlay:objPolyline];
        [objMapView setNeedsDisplay];
    }
}

The following code will center align the map.

- (void)centerMap
{
    MKCoordinateRegion region;

    CLLocationDegrees maxLat = -90;
    CLLocationDegrees maxLon = -180;
    CLLocationDegrees minLat = 90;
    CLLocationDegrees minLon = 180;

    for(int idx = 0; idx < arrRoutePoints.count; idx++)
    {
        CLLocation* currentLocation = [arrRoutePoints objectAtIndex:idx];

        if(currentLocation.coordinate.latitude > maxLat)
            maxLat = currentLocation.coordinate.latitude;
        if(currentLocation.coordinate.latitude < minLat)
            minLat = currentLocation.coordinate.latitude;
        if(currentLocation.coordinate.longitude > maxLon)
            maxLon = currentLocation.coordinate.longitude;
        if(currentLocation.coordinate.longitude < minLon)
            minLon = currentLocation.coordinate.longitude;
    }

    region.center.latitude     = (maxLat + minLat) / 2;
    region.center.longitude    = (maxLon + minLon) / 2;
    region.span.latitudeDelta  = maxLat - minLat;
    region.span.longitudeDelta = maxLon - minLon;

    [objMapView setRegion:region animated:YES];
}

I hope this would help someone.