Sprite Kit - Apply Impulse to shoot projectile at character

Question

I am developing a game using Sprite-Kit (Objective-C). It's a game where you control a bird in flight and arrows and other bad projectiles are shot at you from the right/top/bottom sides of the screen. I am using physics instead of an SKAction in order to accomplish this as I want it to seem as life-like as possible. So I know to use applyImpulse on the projectile to shoot it towards the bird, but what I am wondering is how can gaurantee that the projectile will be shot directly at the bird, no matter the y-positioning of the bird and the y-positioning of the projectile prior to applyImpulse?

I'm having a very frustrating time accomplishing this so any help on this matter would be greatly appreciated. Thanks.

Answer 1

The basic steps are

1. Calculate vector components from the projectile launcher to the bird
2. Normalize the components (optional)
3. Create a vector by scaling the (normalized) components
4. Apply impulse to the projectile using the vector

Here's an example of how to do that

Obj-C

// Calculate vector components x and y
CGFloat dx = bird.position.x - launcher.position.x;
CGFloat dy = bird.position.y - launcher.position.y;

// Normalize the components
CGFloat magnitude = sqrt(dx*dx+dy*dy);
dx /= magnitude;
dy /= magnitude;

// Create a vector in the direction of the bird
CGVector vector = CGVectorMake(strength*dx, strength*dy);

// Apply impulse
[projectile.physicsBody applyImpulse:vector];

Swift

// Calculate vector components x and y
var dx = bird.position.x - launcher.position.x
var dy = bird.position.y - launcher.position.y

// Normalize the components
let magnitude = sqrt(dx*dx+dy*dy)
dx /= magnitude
dy /= magnitude

// Create a vector in the direction of the bird
let vector = CGVector(dx:strength*dx, dy:strength*dy)

// Apply impulse
projectile.physicsBody?.applyImpulse(vector)