@interface APAParallaxSprite : SKSpriteNode

@property (nonatomic) BOOL usesParallaxEffect;

@property (nonatomic) CGFloat virtualZRotation;

/* If initialized with this method, sprite is set up for parallax effect; otherwise, no parallax. */

- (id)initWithSprites:(NSArray *)sprites usingOffset:(CGFloat)offset;

- (void)updateOffset;

@end

@interface APAParallaxSprite ()
// 指定顶部Node与底部Node最大偏移。

@property (nonatomic) CGFloat parallaxOffset;

@end

@implementation APAParallaxSprite

#pragma mark - Initialization

- (id)initWithSprites:(NSArray *)sprites usingOffset:(CGFloat)offset {

    self = [super init];

    if (self) {

        _usesParallaxEffect = YES;

        // Make sure our z layering is correct for the stack.

        CGFloat zOffset = 1.0f / (CGFloat)[sprites count];

        // All nodes in the stack are direct children, with ordered zPosition.

        CGFloat ourZPosition = self.zPosition;

        NSUInteger childNumber = ;

        for (SKNode *node in sprites) {

            node.zPosition = ourZPosition + (zOffset + (zOffset * childNumber));

            [self addChild:node];

            childNumber++;

        }

        _parallaxOffset = offset;

    }

    return self;

}

#pragma mark - Copying

- (id)copyWithZone:(NSZone *)zone {

    APAParallaxSprite *sprite = [super copyWithZone:zone];

    if (sprite) {

        sprite->_parallaxOffset = self.parallaxOffset;

        sprite->_usesParallaxEffect = self.usesParallaxEffect;

    }

    return sprite;

}

#pragma mark - Rotation and Offsets

- (void)setZRotation:(CGFloat)rotation {

    // Override to apply the zRotation just to the stack nodes, but only if the parallax effect is enabled.

    if (!self.usesParallaxEffect) {

        [super setZRotation:rotation];

        return;

    }

    if (rotation > 0.0f) {

        self.zRotation = 0.0f; // never rotate the group node

        // Instead, apply the desired rotation to each node in the stack.

        for (SKNode *child in self.children) {

            child.zRotation = rotation;

        }

        self.virtualZRotation = rotation;

    }

}

- (void)updateOffset {

    SKScene *scene = self.scene;

    SKNode *parent = self.parent;

    if (!self.usesParallaxEffect || parent == nil) {

        return;

    }

    CGPoint scenePos = [scene convertPoint:self.position fromNode:parent];

    // Calculate the offset directions relative to the center of the screen.

    // Bias to (-0.5, 0.5) range.

    CGFloat offsetX =  (-1.0f + (2.0 * (scenePos.x / scene.size.width)));

    CGFloat offsetY =  (-1.0f + (2.0 * (scenePos.y / scene.size.height)));

    CGFloat delta = self.parallaxOffset / (CGFloat)self.children.count;

    int childNumber = ;

    for (SKNode *node in self.children) {

        node.position = CGPointMake(offsetX*delta*childNumber, offsetY*delta*childNumber);

        childNumber++;

    }

}

@end