SpriteKit .sks files and subclassing - objective-c

Question

Apple demoed this code in their WWDC 2014 Session 608 video on best practices for SpriteKit.
AppDelegate.m
+ (instancetype)unarchiveFromFile:(NSString *)file {
/* Retrieve scene file path from the application bundle */
NSString *nodePath = [[NSBundle mainBundle] pathForResource:file ofType:#"sks"];
/* Unarchive the file to an SKScene object */
NSData *data = [NSData dataWithContentsOfFile:nodePath
options:NSDataReadingMappedIfSafe
error:nil];
NSKeyedUnarchiver *arch = [[NSKeyedUnarchiver alloc] initForReadingWithData:data];
[arch setClass:self forClassName:#"SKScene"];
SKScene *scene = [arch decodeObjectForKey:NSKeyedArchiveRootObjectKey];
[arch finishDecoding];
return scene;
}
I understand the gist of what it's doing, but what I'm confused about is how to utilize this code in for any other .sks file. I tried calling the unarchiveFromFile method from my GameScene.m class, but to no avail. I read the post here on this topic, but it did not clarify things.
EDIT
As per what was suggested by Okapi, I tried the following in a new OS X SceneKit project in the GameScene.m class:
#import "GameScene.h"
#implementation GameScene
-(void)didMoveToView:(SKView *)view {
SKNode *nodeInScene2 = [self childNodeWithName:#"object1"];
for (SKNode *blah in [SKScene unarchiveFromFile:#"Scene2"].children) {
[nodeInScene2 addChild:blah];
}
}
-(void)update:(CFTimeInterval)currentTime {
/* Called before each frame is rendered */
}
#end
I have 2 .sks files. The first is called GameScene.sks and that has a sprite in there called "object1". I would like to add children stored in "Scene2.sks". The loop in the didMoveToView method gives me an error. What am I doing wrong? This is what Apple did in their WWDC 608 video, but perhaps I'm missing something since I can't find their project online.

Answer 1

+unarchiveFromFile: is defined as a category on SKScene in GameViewController.m. You would need to copy that code if you want to use it somewhere else.
#implementation SKScene (Unarchive)
+ (instancetype)unarchiveFromFile:(NSString *)file {
/* Retrieve scene file path from the application bundle */
NSString *nodePath = [[NSBundle mainBundle] pathForResource:file ofType:#"sks"];
/* Unarchive the file to an SKScene object */
NSData *data = [NSData dataWithContentsOfFile:nodePath
options:NSDataReadingMappedIfSafe
error:nil];
NSKeyedUnarchiver *arch = [[NSKeyedUnarchiver alloc] initForReadingWithData:data];
[arch setClass:self forClassName:#"SKScene"];
SKScene *scene = [arch decodeObjectForKey:NSKeyedArchiveRootObjectKey];
[arch finishDecoding];
return scene;
}
#end

Answer 2

This one hung me up for a while too. I ended up declaring unarchiveFromFile in every subclass that needed it, then calling it when I wanted to transition scenes. Something like this...
if (contactQuery == (playerCategory | proceedCategory)) {
SKScene *levelTwo = [LevelTwo unarchiveFromFile:#"LevelTwo"];
[self.view presentScene:levelTwo transition:[SKTransition doorsCloseHorizontalWithDuration:0.5]];
}
Full sample code can be downloaded here.