Kotlin-FRP Metronome sample code

Formatters.kt

class Formatters {
    fun formatBpmInfo(msec: Long)  = "$msec msec\n${(60000/msec).toFloat()} BPM"
}

fragment_metronome.xml

<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="mobi.pooh3.frpstudydummy.metronome.FRPSeekBarMetronomeFragment">


    <SeekBar
        android:id="@+id/seekBar"
        style="@style/Widget.AppCompat.SeekBar.Discrete"
        android:layout_width="200dp"
        android:layout_height="wrap_content"
        android:max="1000"
        android:progress="500"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toEndOf="@+id/textView"
        app:layout_constraintTop_toBottomOf="@+id/pulse_img"/>

    <TextView
        android:id="@+id/out_interval"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toTopOf="@+id/seekBar"
        app:layout_constraintEnd_toStartOf="@+id/pulse_img"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:text="interval"/>

    <ImageView
        android:id="@+id/pulse_img"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:src="@mipmap/ic_launcher_round"
        app:layout_constraintBottom_toBottomOf="@+id/out_interval"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toEndOf="@+id/out_interval"
        app:layout_constraintTop_toTopOf="@+id/out_interval"/>

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Tepmpo"
        app:layout_constraintBottom_toBottomOf="@+id/seekBar"
        app:layout_constraintEnd_toStartOf="@+id/seekBar"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="@+id/seekBar"/>
</android.support.constraint.ConstraintLayout>

FRPSeekBarMetronomeFragment.kt

import android.media.ToneGenerator
import android.os.Bundle
import android.support.v4.app.Fragment
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import com.jakewharton.rxbinding2.widget.text
import com.jakewharton.rxbinding2.widget.userChanges
import io.reactivex.Observable
import io.reactivex.android.schedulers.AndroidSchedulers
import io.reactivex.rxkotlin.withLatestFrom
import io.reactivex.subjects.BehaviorSubject
import kotlinx.android.synthetic.main.fragment_metronome.*
import mobi.pooh3.frpstudydummy.R
import java.util.concurrent.TimeUnit


class FRPSeekBarMetronomeFragment : Fragment() {
    override fun onCreateView(inf: LayoutInflater?, ctr: ViewGroup?, savedInstanceState: Bundle?): View?
            = inf!!.inflate(R.layout.fragment_rx_seekbar_interval, ctr, false)

    override fun onViewCreated(v: View?, bdl: Bundle?) {

        val sProgress = seekBar.userChanges()
                .map { if (it <= 50) 50 else it }.share()

        // inputs to outputs
        val outputs = create(Inputs(
                seekBar.progress.toLong(),
                sProgress,
                sProgress.debounce(500, TimeUnit.MILLISECONDS)
        ))

        outputs.bpmText.subscribe(out_interval.text())
        outputs.duration
                .subscribe { duration ->
                    pulse_img?.also {
                        it.fadeOut(newDuration = duration)
                        ToneGeneratorHolder.instance()
                                .startTone(ToneGenerator.TONE_PROP_PROMPT)
                    }
                }

    }

    fun create(inputs: Inputs): Outputs {
        val mn = Metronome(
                inputs.sProgress,
                inputs.sDebouncedProgress,
                inputs.defaultDuration)

        return Outputs(
                bpmText = mn.bpmText.map { Formatters().formatBpmInfo(it) },
                duration = mn.duration
        )
    }

    companion object {
        fun newInstance(): FRPSeekBarMetronomeFragment {
            return FRPSeekBarMetronomeFragment()
        }
    }
}


data class Inputs(val defaultDuration: Long, val sProgress: Observable<Int>, val sDebouncedProgress: Observable<Int>)
data class Outputs(
        val duration: Observable<Long> = BehaviorSubject.createDefault(0L),
        val bpmText: Observable<String> = BehaviorSubject.createDefault("")
)
class Metronome(sProgress: Observable<Int>, sDebouncedProgress: Observable<Int>, defaultDuration: Long) {
    val bpmText: Observable<Long>
    val duration: Observable<Long>
    init {
        val bpmTextValue = BehaviorSubject.createDefault(defaultDuration)
        bpmText = bpmTextValue
        sProgress.map { it.toLong() }.subscribe(bpmTextValue)

        val durationValue = BehaviorSubject.createDefault(defaultDuration)
        duration = durationValue
                .switchInterval()
        sDebouncedProgress
                .map { it.toLong() }
                .subscribe(durationValue)
    }
}


private fun View.fadeOut(from: Float = 1f, to: Float = 0f, newDuration: Long) {
    this.apply { this.alpha = from }
            .animate()
            .apply { duration = newDuration }
            .alpha(to)
}

fun Observable<Long>.switchInterval(): Observable<Long> =
        this.switchMap {
            Observable.interval(it, TimeUnit.MILLISECONDS)
                    .withLatestFrom(this, {_, duration_ -> duration_})
                    .observeOn(AndroidSchedulers.mainThread())
        }

ToneGeneratorHolder.kt

import android.media.AudioManager
import android.media.ToneGenerator

object ToneGeneratorHolder {
    private val toneGenerator: ToneGenerator by lazy { ToneGenerator(AudioManager.STREAM_SYSTEM, ToneGenerator.MAX_VOLUME) }
    fun instance() = toneGenerator
}