Delayed map loading

delay_loading_map.m

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    // loading the map seems to be the most taxing to the main thread, so
    // we hold off on loading it until absolutely necessary of if we think
    // we can do it without the user experiencing any scrolling hitches.
    // here, we're seeing if the user has scrolled down to the mapShell.
    // if so, we load the map.
    
    // top left of our mapShell
    float targetPixel = CGRectGetMinY(self.mapShell.frame);
    
    // which pixel on the bottom left is currently in view?
    float lastVisiblePixelOnScreen = scrollView.contentOffset.y + self.frame.size.height;
    
    // has the user scrolled below the mapShell?
    if (!self.mapHasLoaded && lastVisiblePixelOnScreen > targetPixel) {
        [self loadMap];
    }
}

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
    // loading the map seems to be the most taxing to the main thread, so
    // we hold off on loading it until absolutely necessary of if we think
    // we can do it without the user experiencing any scrolling hitches.
    // here, we're seeing if the user has scrolled and then lifted their finger, causing no
    // deceleration. if these conditions are met, it means we can safely load the map,
    // because the user has either stopped to look/read, or their adjusting their finger
    // to scroll more. either way, we've got enought time to load the map.
    if ( !self.mapHasLoaded && !decelerate) {
        [self loadMap];
    }
}

- (void)loadMap
{
    [self.map setDelegate:self];
    [self.map setCenterCoordinate:self.highlight.coordinate zoomLevel:13 animated:false];
    [self.map addAnnotation:self.highlight];
    [self.mapShell addSubview:self.map];
    self.mapHasLoaded = true;
}