Background Sprite Image keeps being centred on screen - objective-c

Question

I'm trying to implement a scrollable background into my current GameScene. This is supposed to be done via Gesture Recognition, which I'm already using for Taps and moving other scene objects.
Unlike pretty much every other result returned by my Google searches, I don't want an infinitely scrolling background. It just needs to move with your finger, and stay where it's been moved.
The Problem:
I can move the background SKSpriteNode in my scene, but as soon as I try to move it again it snaps to the center and your scrolling effectively becomes useless. It keeps resetting itself.
Here's what I've got so far for moving my Sprites:
-(void)selectTouchedNode:(CGPoint)location
{
SKSpriteNode *node = (SKSpriteNode *)[self nodeAtPoint:location];
if ([self.selectedNode isEqual:node]){
if (![self.selectedNode isEqual:self.background]){
self.selectedNode = NULL;
}
}
if ([node isKindOfClass:[SKLabelNode class]]){
self.selectedNode = node.parent;
} else {
self.selectedNode = node;
}
NSLog(#"Node Selected: %# | Position: %f, %f",node.name,node.position.x,node.position.y);
}
- (void)respondToPan:(UIPanGestureRecognizer *)recognizer {
if (recognizer.state == UIGestureRecognizerStateBegan) {
// Get Touch Location in the View
CGPoint touchLocation = [recognizer locationInView:recognizer.view];
// Convert that Touch Location
touchLocation = [self convertPointFromView:touchLocation];
// Select Node at said Location.
[self selectTouchedNode:touchLocation];
} else if (recognizer.state == UIGestureRecognizerStateChanged) {
// Get the translation being performed on the sprite.
CGPoint translation = [recognizer translationInView:recognizer.view];
// Copy to another CGPoint
translation = CGPointMake(translation.x, -translation.y);
// Translate the currently selected object
[self translateMotion:recognizer Translation:translation];
// Reset translation to zero.
[recognizer setTranslation:CGPointZero inView:recognizer.view];
} else if (recognizer.state == UIGestureRecognizerStateEnded) {
// Fetch Current Location in View
CGPoint touchLocation = [recognizer locationInView:recognizer.view];
// Convert to location in game.
CGPoint correctLocation = [self convertPointFromView:touchLocation];
// If the selected node is the background node
if ([self.selectedNode isEqual:self.background]) {
NSLog(#"Scrolling the background: Node is: %#",self.selectedNode.name);
// Set up a scroll duration
float scrollDuration = 0.2;
// Get the new position based on what is allowed by the function
CGPoint newPos = [self backgroundPanPos:correctLocation];
NSLog(#"New Position: %f, %f",newPos.x,newPos.y);
// Remove all Actions from the background
[_selectedNode removeAllActions];
// Move the background to the new position with defined duration.
SKAction *moveTo = [SKAction moveTo:newPos duration:scrollDuration];
// SetTimingMode for a smoother transition
[moveTo setTimingMode:SKActionTimingEaseOut];
// Run the action
[_selectedNode runAction:moveTo];
} else {
// Otherwise, just put the damn node where the touch occured.
self.selectedNode.position = correctLocation;
}
}
}
// NEW PLAN: Kill myself
- (void)translateMotion:(UIGestureRecognizer *)recognizer Translation:(CGPoint)translation {
// Fetch Location being touched
CGPoint touchLocation = [recognizer locationInView:recognizer.view];
// Convert to place in View
CGPoint location = [self convertPointFromView:touchLocation];
// Set node to that location
self.selectedNode.position = location;
}
- (CGPoint)backgroundPanPos:(CGPoint)newPos {
// Create a new point based on the touched location
CGPoint correctedPos = newPos;
return correctedPos;
}
What do I know so far?
I've tried printing the positions before the scrolling, when it ends, and when it gets initiated again.
Results are that the background does move positions, and once you try to move it again it starts at those new Coordinates, the screen has just repositioned itself over the centre of the sprite.
Supporting Illustration:

Answer 1

I'm not sure I 100% understand the situation you are describing, but I believe that it might be related to the anchor point of your background sprite node.
in your method:
- (void)translateMotion:(UIGestureRecognizer *)recognizer Translation:(CGPoint)translation
you have the line:
self.selectedNode.position = location;
Since your background sprite's anchor point is set to its center by default, any time you move a new touch, it will snap the background sprite's center to the location of your finger.
In other words, background.sprite.position sets the coordinates for the background sprite's anchor point (which by default is the center of the sprite), and in this case any time you set a new position, it is moving the center to that position.
The solution in this case would be to shift the anchor point of the background sprite to be directly under the touch each time, so you are changing the position of the background relative to the point of the background the touch started on.
It's a little hard to explain, so here's some sample code to show you:
1) Create a new project in Xcode using the Sprite Kit Game template
2) Replace the contents of GameScene.m with the following:
#import "GameScene.h"
#interface GameScene ()
#property (nonatomic, strong) SKSpriteNode *sprite;
#end
#implementation GameScene
-(void)didMoveToView:(SKView *)view {
/* Setup your scene here */
self.sprite = [SKSpriteNode spriteNodeWithImageNamed:#"Spaceship"];
self.sprite.position = CGPointMake(CGRectGetMidX(self.frame),
CGRectGetMidY(self.frame));
[self addChild:self.sprite];
}
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
/* Called when a touch begins */
UITouch *touch = [touches anyObject];
CGPoint nodelocation = [touch locationInNode:self.sprite];
//[self adjustAnchorPointForSprite:self.sprite toLocation:nodelocation];
CGPoint location = [touch locationInNode:self];
self.sprite.position = location;
}
-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
UITouch *touch = [touches anyObject];
CGPoint location = [touch locationInNode:self];
self.sprite.position = location;
}
-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
}
-(void)adjustAnchorPointForSprite:(SKSpriteNode *)sprite toLocation:(CGPoint)location {
// Remember the sprite's current position
CGPoint originalPosition = sprite.position;
// Convert the coordinates of the passed-in location to be relative to the bottom left of the sprite (instead of its current anchor point)
CGPoint adjustedNodeLocation = CGPointMake(sprite.anchorPoint.x * sprite.size.width + location.x, sprite.anchorPoint.y * sprite.size.height + location.y);
// Move the anchor point of the sprite to match the passed-in location
sprite.anchorPoint = CGPointMake(adjustedNodeLocation.x / self.sprite.size.width, adjustedNodeLocation.y / self.sprite.size.height);
// Undo any change of position caused by moving the anchor point
self.sprite.position = CGPointMake(sprite.position.x - (sprite.position.x - originalPosition.x), sprite.position.y - (sprite.position.y - originalPosition.y));
}
-(void)update:(CFTimeInterval)currentTime {
/* Called before each frame is rendered */
}
#end
3) Run the project in the sim or on a device and click / touch the top tip of the space ship and start dragging.
4) Notice that the spaceship snaps its center point to where your touch is. If you drag and release it to a new location, and the touch the screen again, it snaps its center back to your finger.
5) Now uncomment the line:
[self adjustAnchorPointForSprite:self.sprite toLocation:nodelocation];
and run the project again.
6) See how you can now drag the ship where you want it, and when you touch it later, it stays in place and follows your finger from the point you touched it. This is because the anchor point is now being adjusted to the point under the touch each time a new touch begins.
Hopefully this gives you a solution you can use in your game as well.