swift4: GLKViewController update method not being called

Hello, I am a new swift developer. I am writing code using glkit. I have trouble in GLKViewController not calling update method. Please help me fix it. Thank You

import GLKit
class gameController: GLKViewController{

    override func viewDidLoad() {
        super.viewDidLoad()
        let glkView: GLKView = view as! GLKView
        glkView.context = EAGLContext(api: .openGLES2)!
        EAGLContext.setCurrent(glkView.context)
    }

    func update(){
        // not being called
    }
    override func glkView(_ view: GLKView, drawIn rect: CGRect) {
     
        glClearColor(1, 1, 1, 1)
        glClear(GLbitfield(GL_COLOR_BUFFER_BIT))
    }


}