/*
     Add list of states to tutorial.
    */
    fun addAllStates(states: List<TutorialState>)

    /*
     Add one state to tutorial.
    */
    fun addState(state: TutorialState)

    /*
     Add one state by position.
    */
    fun setState(state: TutorialState, index: Int)

    /*
     Remove state from tutorial.
    */
    fun removeState(state: TutorialState)

    /*
     Clear all states of tutorial. 
     Cancel tutorial process.
    */
    fun clearStates()

    /*
     Get current state of tutorial.
    */
    fun getCurrentState(): TutorialState?

    /*
     Start tutorial process.
     
     Should be called in [Activity.onResume] or [Fragment.onResume] or later.
     If you also highlight action bar menu items,
     call it in [Activity.onPrepareOptionsMenu] or [Fragment.onPrepareOptionsMenu] or later.
    */
    fun start()

    /*
     Go to next state of tutorial.
     Finish the current state.
    */
    fun next()

    /*
     Interrupt tutorial process.
     It can be renewed by calling next() or start()
    */
    fun interrupt()
    
    /* 
     Change configuration of
     current [BlurTutorial] instance.
    */
    fun configure(): TutorialBuilder

    /* 
     Save state of tutorial. 
     You must call it in onSaveInstanceState()
     of your Fragment or Activity.
    */
    fun onSaveInstanceState(outState: Bundle)

    /* 
     Restore state of tutorial. 
     You must call it in onRestoreInstanceState() of 
     your Fragment or Activity.
    */
    fun onRestoreInstanceState(savedState: Bundle?)

    /*
     Release resources. 
     You must call it in onDestroy() of 
     your Fragment or Activity.
    */
    fun onDestroy()

interface TutorialListener {

    /*
     Called before state is entered.
     Use it to prepare UI for view highlighting.
     E.g. smoothly scroll to this view.
    */
    override fun onStateEnter(state: TutorialState)

    // Called after state exit.
    override fun onStateExit(state: TutorialState)

    // Called on state error. E.g. highlighted view is not visible, etc.
    override fun onStateError(state: TutorialState, error: StateException)

    /* 
     Called when popup view is inflated.
     Use this method to populate your popup view.
    */
    override fun onPopupViewInflated(state: TutorialState, popupView: View)

    // Called when tutorial process is finished.
    override fun onFinish()
}

companion object {
    private const val VIEW_ID = 0
    private const val MENU_ITEM_ID = 1
    private const val RECYCLER_ITEM_ID = 2
    private const val PATH_ID = 3
    
    private const val RECYCLER_ITEM_INDEX = 10
    ...
}
...

private val path = Path().apply {
        addCircle(500F, 500F, 300F, Path.Direction.CCW) 
}

private val states by lazy {
        listOf(ViewState(VIEW_ID, tvHighlight),
               MenuState(MENU_ITEM_ID, tbMenuOwner, R.id.menu_item),
               RecyclerItemState(RECYCLER_ITEM_ID, rvOwner, RECYCLER_ITEM_INDEX),
               PathState(PATH_ID, path)
}

private var tutorial: BlurTutorial? = null

private val tutorialListener = object : SimpleTutorialListener() {

        override fun onPopupViewInflated(state: TutorialState, popupView: View) {
            popupView.run {
                val tvTitle = findViewById<TextView>(R.id.tvTitle)
                
                tvTitle.textResource = when (state.id) {
                    VIEW_ID -> R.string.view_title
                    MENU_ITEM_ID -> R.string.menu_title
                    RECYCLER_ITEM_ID -> R.string.recycler_title
                    PATH_ID -> R.string.path_state
                }
                setOnClickListener { tutorial?.next() }
            }
        }
    }
    
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        initTutorial()
    }
    
    override fun onResume() {
        super.onResume()
        tutorial?.start()
    }
    
    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        tutorial?.onSaveInstanceState(outState)
    }

    override fun onViewStateRestored(savedInstanceState: Bundle?) {
        super.onViewStateRestored(savedInstanceState)
        tutorial?.onRestoreInstanceState(savedInstanceState)
    }

    override fun onDestroy() {
        tutorial?.onDestroy()
        super.onDestroy()
    }
    
    private fun initTutorial() {
        tutorial = TutorialBuilder()
                .withParent(clParent)
                .withListener(tutorialListener)
                .withPopupLayout(R.layout.popup_window)
                .withPopupCornerRadius(POPUP_RADIUS)
                .withBlurRadius(BLUR_RADIUS)
                .build().apply { addAllStates(states) }
    }