FPS as LiveData (with coroutine)

MainViewModel.kt

/*
 * Copyright (C) 2018 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package io.github.yaraki.playground

import android.arch.lifecycle.LiveData
import android.arch.lifecycle.MutableLiveData
import android.arch.lifecycle.ViewModel
import kotlinx.coroutines.experimental.CoroutineScope
import kotlinx.coroutines.experimental.Dispatchers
import kotlinx.coroutines.experimental.Job
import kotlinx.coroutines.experimental.android.Main
import kotlinx.coroutines.experimental.android.awaitFrame
import kotlinx.coroutines.experimental.isActive
import kotlinx.coroutines.experimental.launch
import java.util.concurrent.TimeUnit

class MainViewModel : ViewModel() {

    private val job = Job()
    private val scope = CoroutineScope(Dispatchers.Main + job)

    val fps: LiveData<String> by lazy(LazyThreadSafetyMode.NONE) {
        MutableLiveData<String>().also { liveData ->
            scope.launch {
                var count = 0
                val oneSecond = TimeUnit.SECONDS.toNanos(1)
                var start = System.nanoTime()
                while (isActive) {
                    val now = awaitFrame()
                    count++
                    if (now - start >= oneSecond) {
                        liveData.value = count.toString()
                        count = 0
                        start = now
                    }
                }

            }
        }
    }

    override fun onCleared() {
        super.onCleared()
        job.cancel()
    }

}