5 star rating view Android

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:padding="16dp"
        tools:context=".MainActivity">

    <com.example.myapplication.views.RatingView
            android:layout_width="match_parent"
            android:layout_height="wrap_content"/>

</LinearLayout>

MainActivity.kt

package com.example.myapplication

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.view.MotionEvent
import android.view.View
import com.example.myapplication.views.StarView
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
    }
}

rating_view_content.xml

<?xml version="1.0" encoding="utf-8"?>
<merge xmlns:android="http://schemas.android.com/apk/res/android"
       xmlns:tools="http://schemas.android.com/tools"
       android:orientation="horizontal"
       tools:parentTag="android.widget.LinearLayout"
       android:layout_width="match_parent"
       android:layout_height="match_parent">

    <com.example.myapplication.views.StarView
            android:id="@+id/star_one"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"/>

    <com.example.myapplication.views.StarView
            android:id="@+id/star_two"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"/>

    <com.example.myapplication.views.StarView
            android:id="@+id/star_three"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"/>

    <com.example.myapplication.views.StarView
            android:id="@+id/star_four"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"/>

    <com.example.myapplication.views.StarView
            android:id="@+id/star_five"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"/>
</merge>

RatingView.kt

package com.example.myapplication.views

import android.content.Context
import android.util.AttributeSet
import android.view.LayoutInflater
import android.widget.LinearLayout
import com.example.myapplication.R
import kotlinx.android.synthetic.main.rating_view_content.view.*

class RatingView: LinearLayout {

    var selection: Selection = Selection.none()
    val starViews: List<StarView>
        get() = listOf(star_one, star_two, star_three, star_four, star_five)

    constructor(context: Context?) : super(context) {
        init(context)
    }
    constructor(context: Context?, attrs: AttributeSet?) : super(context, attrs) {
        init(context)
    }
    constructor(context: Context?, attrs: AttributeSet?, defStyleAttr: Int) : super(context, attrs, defStyleAttr) {
        init(context)
    }

    private fun init(context: Context?) {
        val inflater = LayoutInflater.from(context)
        inflater.inflate(R.layout.rating_view_content, this)

        for (i in 0 until starViews.count()) {
            starViews[i].onStarClicked = onStarClickedForStar(i)
            starViews[i].state = StarView.State.deselected
        }
    }

    private fun onStarClickedForStar(i: Int): StarView.OnStarClicked {
        return object:StarView.OnStarClicked{
            override fun starClicked(state: StarView.State) {
                when (state) {
                    StarView.State.deselected -> selectUpTo(i)
                    StarView.State.selected -> selectUpTo(i)
                    else -> return
                }
            }
        }
    }

    private fun selectUpTo(starIndex: Int) {
        if (selection.shouldDeselectAll(starIndex)) {
            selection = Selection.none()
            starViews.forEach { it.state = StarView.State.deselected }
        } else {
            selection = Selection.selected(starIndex)

            starViews.forEachIndexed { index, starView ->
                starView.state =  if (index <= starIndex) StarView.State.selected else StarView.State.deselected
            }
        }
    }

    @Suppress("DataClassPrivateConstructor")
    data class Selection private constructor(val value: Int?) {
        val isSelected: Boolean
            get() = value != null

        fun shouldDeselectAll(selectedIndex: Int): Boolean {
            return isSelected && value == 0 && selectedIndex == 0
        }

        companion object Factory {
            fun none(): Selection {
                return Selection(null)
            }

            fun selected(i: Int): Selection {
                return Selection(i)
            }
        }
    }
}

StarView.kt

package com.example.myapplication.views

import android.content.Context
import android.graphics.drawable.Drawable
import android.util.AttributeSet
import android.view.animation.AccelerateDecelerateInterpolator
import android.view.animation.Animation
import android.view.animation.Interpolator
import android.view.animation.ScaleAnimation
import android.widget.ImageSwitcher
import android.widget.ImageView
import com.example.myapplication.R


class StarView : ImageSwitcher {

    enum class State {
        selected,
        deselected,
        disabled
    }

    interface OnStarClicked {
        fun starClicked(state: State)
    }

    var onStarClicked: OnStarClicked = object:OnStarClicked {
        override fun starClicked(state: State) {
        }
    }

    val State.drawable: Drawable?
        get() = when (this) {
            State.selected -> context.getDrawable(R.drawable.star_selected)
            State.deselected -> context.getDrawable(R.drawable.star_deselected)
            State.disabled -> context.getDrawable(R.drawable.star_disabled)
        }

    val State.scale: Float
        get() = when (this) {
            State.selected -> 1.0f
            State.deselected -> 0.9f
            State.disabled -> 0f
        }

    var state: State = State.deselected
        set(value) {
            val previousState = field
            field = value
            if (previousState != value) {
                setImageDrawable(value.drawable)
                // Recover 1.0f scale as the animation scale builds up on it, and it
                // has been set in the init function already
                scaleX = 1.0f
                scaleY = 1.0f

                val anim = ScaleAnimation(
                    previousState.scale, value.scale,
                    previousState.scale, value.scale, // Start and end values for the Y axis scaling
                    Animation.RELATIVE_TO_SELF, 0.5f, // Pivot point of X scaling
                    Animation.RELATIVE_TO_SELF, 0.5f // Pivot point of Y scaling
                )
                anim.fillAfter = true // Needed to keep the result of the animation
                anim.duration = 300
                anim.interpolator = AccelerateDecelerateInterpolator()
                startAnimation(anim)
            }
        }

    constructor(context: Context?) : super(context) { init(context) }
    constructor(context: Context?, attrs: AttributeSet?) : super(context, attrs) { init(context) }

    private fun init(context: Context?) {
        state = State.deselected

        setFactory {
            ImageView(context).apply {
                setImageDrawable(state.drawable)
            }
        }

        setInAnimation(context, android.R.anim.fade_in)
        setOutAnimation(context, android.R.anim.fade_out)

        setOnClickListener {
            onStarClicked.starClicked(state)
        }

        scaleX = state.scale
        scaleY = state.scale
    }
}