Question

I am developing a game that uses the gyroscope to keep enemies in relatively the same place, with the following code:

if([motionManager isGyroAvailable])
{
    [motionManager setGyroUpdateInterval:0.05];

    [motionManager startGyroUpdatesToQueue:[NSOperationQueue mainQueue]
                               withHandler:^(CMGyroData *gyroData, NSError *error)
     {
         valueX3 = gyroData.rotationRate.y* 50;
         valueY3 = gyroData.rotationRate.x* 50;

         int newX3 = (int)(enemyufoG.center.x +valueY3);
         int newY3 = (int)(enemyufoG.center.y -valueX3);

         CGPoint newCenter2 = CGPointMake(newX3, newY3);

         enemyufoG.center = newCenter2;

         valueX2 = gyroData.rotationRate.y* 50;
         valueY2 = gyroData.rotationRate.x* 50;

         int newX2 = (int)(enemyufoR.center.x +valueY2);
         int newY2 = (int)(enemyufoR.center.y -valueX2);

         CGPoint newCenter = CGPointMake(newX2, newY2);

         enemyufoR.center = newCenter;

         valueX = gyroData.rotationRate.y* 50;
         valueY = gyroData.rotationRate.x* 50;

         int newX = (int)(enemyAlien.center.x +valueY);
         int newY = (int)(enemyAlien.center.y -valueX);

         CGPoint newCenter3 = CGPointMake(newX, newY);

         enemyAlien.center = newCenter3;
     }];
}

Once you shoot an enemy that is in the crosshairs of the gun, it hides the UIImageView, then uses NSTimer to call a different method that shows it again. I would like to have the enemies reappear in random positions on the screen.

CGPoint pos = enemyAlien.center;

if ((pos.x > 254) && (pos.x < 304) && (pos.y > 140) && (pos.y < 160 && _ammoCount != 0))
{
    enemyAlien.hidden = YES;
    [dangerBar setProgress:dangerBar.progress-0.10];
    _killCount = _killCount+3;
    [killCountField setText: [NSString stringWithFormat:@"%d", _killCount]];

    timer = [NSTimer scheduledTimerWithTimeInterval: 4.0
                                             target: self
                                           selector: @selector(showAlien)
                                           userInfo: nil
                                            repeats: NO];
}

- (void) showAlien {

enemyAlien.hidden = NO;

}

When I try and use enemyAlien.center = enemyAlien.center + arc4random()%100; above enemyAlien.hidden = NO, I get the following error:

'Invalid operands to binary expression ('CGPoint (aka 'struct CGPoint') and 'unsigned int').

Answer 1

You’re trying to add an integer to a cgpoint.
try this.

enemyAlien.center = CGPointMake(enemyAlien.center.x + (arc4random()%100),enemyAlien.center.y + (arc4random()%100));

although this will only move the alien in a diagonal direction. You should probably change it to the following for a better experience.

enemyAlien.center = CGPointMake((arc4random()%SCREEN_WIDTH),(arc4random()%SCREEN_HEIGHT));

where SCREEN_WIDTH and SCREEN_HEIGHT are the dimensions of your playing field.