Keeping the main character in view

We will need to constantly check whether the zombies are in view, so we need to implement the update method that will perform the appropriate calculations. So add the following block to GameScene.m:

- (void)update:(NSTimeInterval)delta {
    // Keep the zombie on view
    [self setZombieOnView:_zombie.position];
}

Implement the setZombieOnView method with the following lines:

- (void)setZombieOnView:(CGPoint)position {
    // Establish the maximum positions on screen
    NSInteger positionX = MAX(position.x, _screenSize.width / 2);
    NSInteger positionY = MAX(position.y, _screenSize.height / 2);
    // Calculate the limits in both axes
    float backgroundWidth = _background0.contentSize.width + _background1.contentSize.width + _background2.contentSize.width + _background3.contentSize.width;
    positionX = MIN(positionX, backgroundWidth - (_screenSize.width / 2));
    positionY = MIN(positionY, _screenSize.height / 2);

    // Initialize current position...