Soft body CCSprite originally created by Sébastien Dabet (Gist: https://gist.github.com/sdabet/08a6f41415c5e3b8be0f and article: http://2sa-studio.blogspot.se/2014/05/soft-bodies-with-cocos2d-v3.html) updated for the new rendering engine (Cocos2d-SpriteBuilder >3.1). This is the slightly optimized Swift version set up for use with SpriteBuilder.

Soft body CCSprite Swift

import Foundation
import CoreGraphics

class SwiftBubble:CCSprite {
    
    static let π = CGFloat(M_PI)
  
    //Number of segments (the more the smoother circle) and segment radius
    static let numSegments = 60
    let physicsBodyRadius:CGFloat = 2.0
    
    //Control stiffness and damping
    let innerStiffness:CGFloat = 1500
    let innerDamping:CGFloat = 50
    let outerStiffness:CGFloat = 1000
    let outerDamping:CGFloat = 50
    
    //This is set in didLoadFromCCB
    var bubbleRadius:CGFloat!
    
    //Vertices and texCoords
    var vertices = [ccVertex2F]()
    var texCoords = [ccTex2F]()
    
    //Colors can be set up once in this implementation
    let texColor = ccColor4F(r:1.0,g:1.0,b:1.0,a:1.0)
    
    var verticesHasBeenSetUp = false
    
    func didLoadFromCCB()
    {
        bubbleRadius = contentSize.width/2

        let center = CGPointMake(bubbleRadius,bubbleRadius)
        
        physicsBody = CCPhysicsBody(circleOfRadius: physicsBodyRadius, andCenter: center)
        physicsBody.allowsRotation = false
        
        let childDist = bubbleRadius - physicsBodyRadius
        
        for i in 0...SwiftBubble.numSegments-1
        {
            let childAngle = CGFloat(i) * 2.0 * SwiftBubble.π / CGFloat(SwiftBubble.numSegments)
            let child = CCNode()
            child.physicsBody = CCPhysicsBody(circleOfRadius:physicsBodyRadius, andCenter:CGPointZero)
            child.physicsBody.allowsRotation = false
            child.position = CGPointMake(bubbleRadius + childDist * cos(childAngle), bubbleRadius + childDist * sin(childAngle))
            addChild(child)
            
            CCPhysicsJoint(springJointWithBodyA: physicsBody,
                bodyB: child.physicsBody,
                anchorA: center,
                anchorB: CGPointZero,
                restLength: childDist,
                stiffness: innerStiffness,
                damping: innerDamping)
            
        }
        for (index, child) in enumerate(children as! [CCNode])
        {
            let previous = index == 0 ? children[SwiftBubble.numSegments-1] as! CCNode : children[index-1] as! CCNode
            
            CCPhysicsJoint(
            springJointWithBodyA: child.physicsBody,
            bodyB: previous.physicsBody,
            anchorA: CGPointZero,
            anchorB: CGPointZero,
            restLength: childDist * 2.0 * SwiftBubble.π/CGFloat(SwiftBubble.numSegments),
            stiffness: outerStiffness,
            damping: outerDamping
            )
        }
        //In this implementation, the texture coordinates only has to be set up once
        setUpTexCoords()
        //Set up vertices for the first draw call
        setUpVertices()
    }
    func setUpVertices()
    {
        if verticesHasBeenSetUp {return}
            
        //First, the center vertex
        vertices.append(ccVertex2F(x: GLfloat(bubbleRadius), y: GLfloat(bubbleRadius)))
        //Then the vertices creating the circle
        for child in children as! [CCNode] {
            vertices.append(ccVertex2F(x: GLfloat(child.position.x + physicsBodyRadius * (child.position.x-bubbleRadius)/bubbleRadius), y: GLfloat(child.position.y + physicsBodyRadius * (child.position.y-bubbleRadius)/bubbleRadius)))
        }
        //Lastly, the first vertex again (it'll be connected with the last)
        vertices.append(vertices[1])
        
        verticesHasBeenSetUp = true

    }
    func updateVertices()
    {
        if(!verticesHasBeenSetUp) {
            setUpVertices()
        }
        for(index, child) in enumerate(children as! [CCNode])
        {
            vertices[index+1] = ccVertex2F(x: GLfloat(child.position.x + physicsBodyRadius * (child.position.x-bubbleRadius)/bubbleRadius), y: GLfloat(child.position.y + physicsBodyRadius * (child.position.y-bubbleRadius)/bubbleRadius))
        }
        vertices[vertices.count-1] = vertices[1]
        
    }
    func setUpTexCoords()
    {
        let deltaAngle:CGFloat = (2.0 * SwiftBubble.π) / CGFloat(SwiftBubble.numSegments)
        texCoords.append(ccTex2F(u: 0.5, v:0.5))
        
        for i in 0...SwiftBubble.numSegments-1 {
            let coordAngle = SwiftBubble.π + (deltaAngle * CGFloat(i))
            texCoords.append(ccTex2F(u: GLfloat(0.5 + cos(coordAngle)*0.5), v: GLfloat(0.5 + sin(coordAngle)*0.5)))
        }
        
        texCoords.append(texCoords[1])
    }
    //Helper function that creates a CCVertex. Tagged @inline to make it a little faster
    @inline(__always) func makeVertex(v:ccVertex2F, texCoord:ccTex2F, color:ccColor4F, transform: GLKMatrix4) -> CCVertex
    {
        return CCVertex(
            position: GLKMatrix4MultiplyVector4(transform, GLKVector4Make(v.x,v.y,0.0,1.0)),
            texCoord1: GLKVector2Make(texCoord.u, texCoord.v),
            texCoord2: GLKVector2Make(0.0, 0.0),
            color: GLKVector4Make(color.r, color.g, color.b, color.a)
        )
    }
    
    override func draw(renderer: CCRenderer!, transform: UnsafePointer<GLKMatrix4>) {
        
        //Update vertices positions
        updateVertices()
        
        //Buffer
        let buffer = renderer.enqueueTriangles(UInt(SwiftBubble.numSegments), andVertexes: UInt(vertices.count), withState: renderState, globalSortOrder: 0)
        
        //Feed the buffer the vertices
        for i in 0...SwiftBubble.numSegments+1 {
            
              CCRenderBufferSetVertex(buffer,Int32(i),makeVertex(vertices[i], texCoord: texCoords[i], color: texColor, transform: transform.memory))
            
        }
        //Feed the buffer the triangles
        for i in 0...SwiftBubble.numSegments-1 {
            CCRenderBufferSetTriangle(buffer, Int32(i), GLushort(0), GLushort(i+1), GLushort(i+2))
        }
        
        
    }
    
}