Swift3 SpriteKit：以角度射击子弹

Question

Currently my app has an enemy that shoots a bullet which travels from the enemy at whatever x and y position it may be towards the bottom of the screen - the size of the bullet. This is an SKAction that happens only once in the enemy trajectory from top to bottom and this trajectories are all at random X positions.

What I want to do is to shoot 4 bullets, at 4 different angles. 45, 135, 225 and 270 degrees.

Currently my fireBullet function is as follows:

func fireBullet() {
    let bullet = EnemyBullet(imageName: bulletImage, bulletSound: bulletSound)
    bullet.name = "Bullet"
    bullet.setScale(1)
    bullet.position = CGPoint(x: enemy.position.x, y: enemy.position.y - bullet.size.height)
    bullet.physicsBody = SKPhysicsBody.init(rectangleOf: bullet.size)
    bullet.physicsBody!.affectedByGravity = false
    bullet.physicsBody!.categoryBitMask = PhysicsCategories.enemy
    bullet.physicsBody!.collisionBitMask = PhysicsCategories.none
    bullet.physicsBody!.contactTestBitMask = PhysicsCategories.player
    gameScene.addChild(bullet)

    let moveBullet = SKAction.moveTo(y: -gameScene.size.height * 0.1, duration: bulletTimeInScreen)
    let deleteBullet = SKAction.removeFromParent()

    let bulletSequence = SKAction.sequence([moveBullet, deleteBullet])
    bullet.run(bulletSequence)
}

I'm assuming that I can change bullet for bullet1 and repeat for bullet2, 3 and 4. With that I can probably use zRotation to rotate each bullet to that specific angle.

What I can not figure out is in which direction to move the bullet. Specifically, how to figure out the X and Y of the SKAction.moveTo from the enemy's position at the time.

I have searched and read many articles but none that can point me in the right direction.

Answer 1

I found a way to do it thanks to this post Shoot bullet in the Direction the Ship is Facing Swift and Sprite Kit by KnightOfDragon which by the way, pointed me in the right direction with trigonometry.

Basically I changed the variable name from bullet to bullet1 and added bullet2, 3 and 4 changing the zRotation within the same function. The moveBullet1 SKAction fires the bullet in the direction the zRotation points at.

The degrees use the extension shown below. I can't remember where in this site I found that one.

func fireBullet() {
    let bullet1 = EnemyBullet(imageName: bulletImage, bulletSound: bulletSound)
    bullet1.name = "Bullet"
    bullet1.setScale(scale)
    bullet1.position = CGPoint(x: enemy.position.x, y: enemy.position.y)
    bullet1.zRotation = CGFloat(45.degreesToRadians)
    bullet1.physicsBody = SKPhysicsBody.init(rectangleOf: bullet1.size)
    bullet1.physicsBody!.affectedByGravity = false
    bullet1.physicsBody!.categoryBitMask = PhysicsCategories.enemy
    bullet1.physicsBody!.collisionBitMask = PhysicsCategories.none
    bullet1.physicsBody!.contactTestBitMask = PhysicsCategories.player
    gameScene.addChild(bullet1)

    let moveBullet1 = SKAction.move(to: CGPoint(
        x: distance * cos(bullet1.zRotation) + bullet1.position.x,
        y: distance * sin(bullet1.zRotation) + bullet1.position.y),
        duration: bulletTimeInScreen)
    let deleteBullet1 = SKAction.removeFromParent()

    let bullet1Sequence = SKAction.sequence([moveBullet1, deleteBullet1])

    bullet1.run(bullet1Sequence)
}


extension Int {
    var degreesToRadians: Double { return Double(self) * .pi / 180 }
    var radiansToDegrees: Double { return Double(self) * 180 / .pi }
}