gistfile1.m

// Using an animation grouping because we may be changing the duration
[NSAnimationContext beginGrouping];
	
// With the shift key down, do slow-mo animation
if ([[NSApp currentEvent] modifierFlags] & NSShiftKeyMask) {
    [[NSAnimationContext currentContext] setDuration:1.0];
} else {
    [[NSAnimationContext currentContext] setDuration:0.6];
}
    
NSRect originalFrame;
NSRect finalFrame;
    
// set final frame (ie: move to left by width)
originalFrame = _mainFeedViewController.view.frame;
finalFrame = originalFrame;
finalFrame.origin.x += originalFrame.size.width;
[_mainFeedViewController.view.animator setFrame:finalFrame];
    
originalFrame = _detailViewController.view.frame;
finalFrame = originalFrame;
finalFrame.origin.x += originalFrame.size.width;
[_detailViewController.view.animator setFrame:finalFrame];
    
[NSAnimationContext endGrouping];