Shooting objects in a circular pattern SpriteKit - ios

Question

I don't really know how to explain what i'm after, so I drew some (very) artistic diagrams to help convey the idea. I'll also try and explain it the best I can.
I'm essentially trying to 'shoot' bullets/lasers/whatever from a circle in the center of the screen, and for it to repeat this at a rather rapid rate. Here are two pictures which kind of show what i'm trying to achieve: (Don't have enough reputation to post them here.
(1) http://i.imgur.com/WpZlTQ7.png
This is kind of where I want the bullets to shoot from, and how many I'd like.
(2) http://i.imgur.com/psdIjZG.png
This is pretty much the end result, I'd like them to repeatedly fire and make the screen kind of look like this.
Can anyone refer me to what I should be looking at in order to achieve this?

Answer 1

When dealing with circles, it's usually easier to use polar coordinates. In this case, each direction can be represented with a magnitude and an angle, where the magnitude is the amount of the force/impulse to apply to the bullet and angle is the direction to shoot the bullet.
The basic steps are
Determine the number of directions
Determine the angle increment by dividing 2*PI (360 degrees) by the number of directions
Start at angle = 0
Shoot bullet in the direction specified by angle
Convert the angle and magnitude to cartesian coordinates to form a vector
Apply an impulse or a force to the bullet using the vector
Increment angle by angle increment
Here's an example of how to do that in Obj-C:
#implementation GameScene {
CGFloat angle;
SKTexture *texture;
CGFloat magnitude;
CGFloat angleIncr;
}
-(void)didMoveToView:(SKView *)view {
/* Setup your scene here */
long numAngles = 15;
magnitude = 1;
angleIncr = 2 * M_PI / numAngles;
angle = 0;
texture = [SKTexture textureWithImageNamed:#"Spaceship"];
SKAction *shootBullet = [SKAction runBlock:^{
SKSpriteNode *bullet = [SKSpriteNode spriteNodeWithTexture:texture];
bullet.size = CGSizeMake(8, 8);
bullet.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:bullet.size.width/2];
bullet.physicsBody.affectedByGravity = NO;
bullet.position = self.view.center;
bullet.zRotation = angle-M_PI_2;
[self addChild:bullet];
CGFloat dx = magnitude * cos(angle);
CGFloat dy = magnitude * sin(angle);
CGVector vector = CGVectorMake(dx,dy);
[bullet.physicsBody applyImpulse:vector];
angle = fmod(angle+angleIncr,2*M_PI);
}];
SKAction *wait = [SKAction waitForDuration:0.25];
SKAction *shootBullets = [SKAction sequence:#[shootBullet, wait]];
[self runAction:[SKAction repeatActionForever:shootBullets]];
}

Answer 2

From what I understand, you want this sprite to shoot a bunch of projectiles every X seconds.
SKAction *ShootProjectiles = [SKAction runBlock:^{
//Create Projectile1
projectile1.physicsBody.applyImpulse(CGVectorMake(1, 0)); //Shoot directly right
//Create Projectile2
projectile2.physicsBody.applyImpulse(CGVectorMake(1, 1)); //Shoot diagnally Up and to the right
//Follow this pattern to create all projectiles desired to be shot in one burst
}];
SKAction *delayBetweenShots = [SKAction scaleBy:0 duration:5];
SKAction* ShootSequence= [SKAction repeatActionForever:[SKAction sequence:#[ShootProjectiles, delayBetweenShots]]];
[self runAction: ShootSequnce];
What this does is create as many projectiles as you desire and fire them in the direction of the vectors you define. It then waits 5 seconds (the scaleBy:0 action does nothing except delay) and then repeats over and over again until you remove the action.