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Destructible terrain: presentScene()

It's time for the most challenging part of our project, but as per usual I've tried to keep things as simple as possible because the fun is in getting results not in learning algorithms. We're going to add collision detection to our code so that players can carve chunks out of the buildings or, better, blow up their opponents.

You will, as always, need to assign self to be the delegate of your scene's physics world so that you can get notified of collisions. So, put this in didMove(to:):

physicsWorld.contactDelegate = self

Make sure you modify your class definition to say that you conform to the SKPhysicsContactDelegate protocol.

When it comes to implementing the didBegin() method, there are various possible contacts we need to consider: banana hit building, building hit banana (remember the philosophy?), banana hit player1, player1 hit banana, banana hit player2 and player2 hit banana.

This is a lot to check, so we're going to eliminate half of them by eliminating whether "banana hit building" or "building hit banana". Take another look at our category bitmasks:

enum CollisionTypes: UInt32 {
    case banana = 1
    case building = 2
    case player = 4

They are ordered numerically and alphabetically, so what we're going to do is create two new variables of type SKPhysicsBody and assign one object from the collision to each: the first physics body will contain the lowest number, and the second the highest.

So, if we get banana (collision type 1) and building (collision type 2) we'll put banana in body 1 and building in body 2, but if we get building (2) and banana (1) then we'll still put banana in body 1 and building in body 2.

Once we have eliminated half the checks, we're going to optionally unwrap both the bodies. They are optional because they might be nil, and this is highly likely in our project. The reason it's likely is because we might get "banana hit building" and "building hit banana" one after the other, but when either of these happens we'll destroy the banana so the second one will definitely be nil.

If the banana hit a player, we're going to call a new method named destroy(player:). If the banana hit a building, we'll call a different new method named bananaHit(building:), but we'll also pass in the contact point. This value tells us where on the screen the impact actually happened, and it's important because we're going to destroy the building at that point.

That's all you need to know, so here's the code for didBegin():

func didBegin(_ contact: SKPhysicsContact) {
    let firstBody: SKPhysicsBody
    let secondBody: SKPhysicsBody

    if contact.bodyA.categoryBitMask < contact.bodyB.categoryBitMask {
        firstBody = contact.bodyA
        secondBody = contact.bodyB
    } else {
        firstBody = contact.bodyB
        secondBody = contact.bodyA

    guard let firstNode = firstBody.node else { return }
    guard let secondNode = secondBody.node else { return }

    if == "banana" && == "building" {
        bananaHit(building: secondNode, atPoint: contact.contactPoint)

    if == "banana" && == "player1" {
        destroy(player: player1)

    if == "banana" && == "player2" {
        destroy(player: player2)

Note: We haven’t written those functions yet, so Xcode will complain.

If a banana hits a player, it means they have lost the game: we need to create an explosion (yay, particles!), remove the destroyed player and the banana from the scene, then… what? Well, so far we've just left it there – we haven't looked at how to make games restart.

There are a number of things you could do: take players to a results screen, take them to a menu screen, and so on. In our case, we're going to reload the level so they can carry on playing. We could just delete all the buildings and generate it all from scratch, but that would be passing up a great opportunity to learn something new!

SpriteKit has a super-stylish and built-in way of letting you transition between scenes. This means you can have one scene for your menu, one for your options, one for your game, and so on, then transition between them as if they were view controllers in a navigation controller.

To transition from one scene to another, you first create the scene, then create a transition using the list available from SKTransition, then finally use the presentScene() method of our scene's view, passing in the new scene and the transition you created. For example, this will cross-fade in a new scene over 2 seconds:

let newGame = GameScene(size: self.size)
let transition = SKTransition.crossFade(withDuration: 2)
self.view?.presentScene(newGame, transition: transition)

In the destroy(player:) method we're going to execute the scene transition after two seconds so that players have a chance to see who won and, let's face it, laugh at the losing player. But when we create the new game scene we also need to do something very important: we need to update the view controller's currentGame property and set the new scene's viewController property so they can talk to each other once the change has happened.

We also need to call the changePlayer() method when a player is destroyed. We haven't written this method yet, but it transfers control of the game to the other player, then calls the activatePlayer() method on the game view controller so that the game controls are re-shown. Calling this method here ensures that the player who lost gets the first turn in the new game.

First, here's the code for destroy(player:):

func destroy(player: SKSpriteNode) {
    if let explosion = SKEmitterNode(fileNamed: "hitPlayer") {
        explosion.position = player.position


    DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
        let newGame = GameScene(size: self.size)
        newGame.viewController = self.viewController
        self.viewController.currentGame = newGame

        newGame.currentPlayer = self.currentPlayer

        let transition = SKTransition.doorway(withDuration: 1.5)
        self.view?.presentScene(newGame, transition: transition)

Important: after calling changePlayer(), we must set the new game's currentPlayer property to our own currentPlayer property, so that whoever died gets the first shot.

The changePlayer() method is trivial, so here it is:

func changePlayer() {
    if currentPlayer == 1 {
        currentPlayer = 2
    } else {
        currentPlayer = 1

    viewController.activatePlayer(number: currentPlayer)

Now it's time for the real work. How do we allow our exploding bananas to create holes in buildings? Surprisingly, it's not that hard. I'm going to split it into two parts: a bananaHit(building:) game scene method that handles creating the explosion, deleting the banana and changing players, and a hit(at:) building node method that handles damaging the building. The first one is easy, so put this into the game scene:

func bananaHit(building: SKNode, atPoint contactPoint: CGPoint) {
    guard let building = building as? BuildingNode else { return }
    let buildingLocation = convert(contactPoint, to: building)
    building.hit(at: buildingLocation)

    if let explosion = SKEmitterNode(fileNamed: "hitBuilding") {
        explosion.position = contactPoint
    } = ""
    banana = nil


Note: We haven’t written hit(at:) yet, so don’t worry about Xcode’s error message.

The only new thing in there is the call to convert(), which asks the game scene to convert the collision contact point into the coordinates relative to the building node. That is, if the building node was at X:200 and the collision was at X:250, this would return X:50, because it was 50 points into the building node.

If you're curious why I use = "", it's to fix a small but annoying bug: if a banana just so happens to hit two buildings at the same time, then it will explode twice and thus call changePlayer() twice – effectively giving the player another throw. By clearing the banana's name here, the second collision won't happen because our didBegin() method won't see the banana as being a banana any more – its name is gone.

And now for the part where we handle destroying chunks of the building. With your current knowledge of Core Graphics, this is something you can do with only one new thing: blend modes. When you draw anything to a Core Graphics context, you can set how it should be drawn. For example, should it be be drawn normally, or should it add to what's there to create a combination?

Core Graphics has quite a few blend modes that might look similar, but we're going to use one called .clear, which means "delete whatever is there already." When combined with the fact that we already have a property called currentImage you might be able to see how our destructible terrain technique will work!

Put simply, when we create the building we save its UIImage to a property of the BuildingNode class. When we want to destroy part of the building, we draw that image into a new context, draw an ellipse using .clear to blast a hole, then save that back to our currentImage property and update our sprite's texture.

Here's a full break down of what the method needs to do:

  1. Figure out where the building was hit. Remember: SpriteKit's positions things from the center and Core Graphics from the bottom left!
  2. Create a new Core Graphics context the size of our current sprite.
  3. Draw our current building image into the context. This will be the full building to begin with, but it will change when hit.
  4. Create an ellipse at the collision point. The exact co-ordinates will be 32 points up and to the left of the collision, then 64x64 in size - an ellipse centered on the impact point.
  5. Set the blend mode .clear then draw the ellipse, literally cutting an ellipse out of our image.
  6. Convert the contents of the Core Graphics context back to a UIImage, which is saved in the currentImage property for next time we’re hit, and used to update our building texture.
  7. Call configurePhysics() again so that SpriteKit will recalculate the per-pixel physics for our damaged building.

Here's that in code – put this method into the BuildingNode class:

func hit(at point: CGPoint) {
    let convertedPoint = CGPoint(x: point.x + size.width / 2.0, y: abs(point.y - (size.height / 2.0)))

    let renderer = UIGraphicsImageRenderer(size: size)
    let img = renderer.image { ctx in
        currentImage.draw(at: .zero)

        ctx.cgContext.addEllipse(in: CGRect(x: convertedPoint.x - 32, y: convertedPoint.y - 32, width: 64, height: 64))
        ctx.cgContext.drawPath(using: .fill)

    texture = SKTexture(image: img)
    currentImage = img


We haven’t used the abs() function before, but its job is quite simple: it makes negative number positive. So, if you pass it 1000 it sends back 1000, but if you pass in -1000 it still sends back 1000.

That's it for destructible terrain! There's one curious quirk of SpriteKit's physics implementation: if you slice a building in two with lots of bananas, only one half will respond to physics because it won't put two (now separate) physics bodies into one. Fortunately, the chances of that happening are pretty slim unless you're an appalling shot!

There is just one more thing to do with the game before we're finished: what if the banana misses the other player and misses all the other buildings? If you put in a 45° angle and full velocity, changes are it will shoot right off the screen, at which point the game won't end. We're going to fix this by using the update() method: if the banana is ever way off the screen, remove it and change players:

override func update(_ currentTime: TimeInterval) {
    guard banana != nil else { return }

    if abs(banana.position.y) > 1000 {
        banana = nil

That's it, your game is finished. Go and play!

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