SceneKit animation with line drawing - ios

Question

I would like to get some help with a scenekit related problem.
I have a dae (collada) file. The scene contains a mesh with bones and one animation. The bones will be moving in the animation and i would like to draw a line from a fix point to a selected moving bone. The expected result should looks like this: img 1, img 2. One end of the line is fixed and the other end of the line should follow the animated bone. Like a rubber band. I tried to implement it in a sample project but i was not able to get the line follow the animation. I don't know how to follow the animation's position per frame and i don't know how could i update the line on the screen. The code:
#import "ViewController.h"
#import SceneKit;
#import <OpenGLES/ES2/gl.h>
#interface ViewController ()<SCNSceneRendererDelegate>
#property (weak, nonatomic) IBOutlet SCNView *scnView;
#property (strong, nonatomic) SCNNode *topBone;
#property (strong, nonatomic) SCNNode *lineNode;
#end
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
self.scnView.allowsCameraControl = YES;
self.scnView.playing = YES;
self.scnView.delegate = self; // Set the delegate to myself. Because during the animation i would like to redraw the line to the correct position
// I have an animation with bones.
SCNScene *mainScene = [SCNScene sceneNamed:#"art.scnassets/test.dae"]; // Loading the test scene
self.topBone = [mainScene.rootNode childNodeWithName:#"TopBone" recursively:YES]; // Find the top bone. I would like to draw a line between the top of the bone and a fix point (5,5,0)
self.lineNode = [self lineFrom:self.topBone.presentationNode.position to:SCNVector3Make(5.0, 5.0, 0.0)]; // Create a node for the line and draw the line
self.scnView.scene = mainScene; // Set the scene
[mainScene.rootNode addChildNode:self.lineNode]; // Add the line node to the scene
}
// I would like to use the render delegate to update the line position. Maybe this is not the right solution...
- (void)renderer:(id<SCNSceneRenderer>)aRenderer willRenderScene:(SCNScene *)scene atTime:(NSTimeInterval)time {
SCNVector3 v = self.topBone.presentationNode.position; // Problem1: The position of the top bone never change. It is always the same value. It is not follow the animation
// Problem 2: if I could get the correct bone position in the animation frame, how could I redraw the line in the Node in every frame?
glLineWidth(20); // Set the line width
}
- (SCNNode *)lineFrom:(SCNVector3)p1 to:(SCNVector3)p2 { // Draw a line between two points and return it as a node
int indices[] = {0, 1};
SCNVector3 positions[] = { p1, p2 };
SCNGeometrySource *vertexSource = [SCNGeometrySource geometrySourceWithVertices:positions count:2];
NSData *indexData = [NSData dataWithBytes:indices length:sizeof(indices)];
SCNGeometryElement *element = [SCNGeometryElement geometryElementWithData:indexData primitiveType:SCNGeometryPrimitiveTypeLine primitiveCount:1 bytesPerIndex:sizeof(int)];
SCNGeometry *line = [SCNGeometry geometryWithSources:#[vertexSource] elements:#[element]];
line.materials.firstObject.diffuse.contents = [UIColor redColor];
SCNNode *lineNode = [SCNNode nodeWithGeometry:line];
return lineNode;
}
#end