这个方法来自SpriteKit

Question

我一直在关注SpriteKit的教程，对于我的生活，我不知道这个方法来自哪里。在StartGameLayer中，我完整地使用了这段代码：

#import "StartGameLayer.h"

@interface StartGameLayer()
@property (nonatomic, retain) SKSpriteNode* playButton;
@end


@implementation StartGameLayer

- (id)initWithSize:(CGSize)size
{
    if(self = [super initWithSize:size])
    {
        SKSpriteNode* startGameText = [SKSpriteNode spriteNodeWithImageNamed:@"FlappyBirdText"];
        startGameText.position = CGPointMake(size.width * 0.5f, size.height * 0.8f);
        [self addChild:startGameText];

        SKSpriteNode* playButton = [SKSpriteNode spriteNodeWithImageNamed:@"PlayButton"];
        playButton.position = CGPointMake(size.width * 0.5f, size.height * 0.30f);
        [self addChild:playButton];

        [self setPlayButton:playButton];

    }

    return self;
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [touches anyObject];
    CGPoint location = [touch locationInNode:self];

    if ([_playButton containsPoint:location])
    {
        if([self.delegate respondsToSelector:@selector(startGameLayer:tapRecognizedOnButton:)])
        {
            [self.delegate startGameLayer:self tapRecognizedOnButton:StartGameLayerPlayButton];
        }
    }
}
@end

setPlayButton:(SKSpriteNode* )方法在哪里设置？在帮助文件中，它表示它已在此文件中声明，但这是我看到它的唯一实例，它只是让我最简单的一点就是如何创建此方法。