objcode

// Solution #2: Add a Mutex

// Note: This is not optimal for the specific use case of sorting
// or filtering but is a good pattern for network saves.

class ProductsRepository(val productsDao: ProductsDao, val productsApi: ProductsService) {
   val singleRunner = SingleRunner()

   suspend fun loadSortedProducts(ascending: Boolean): List<ProductListing> {
       // wait for the previous sort to complete before starting a new one

objcode

// see the complete implementation at
// https://gist.github.com/objcode/7ab4e7b1df8acd88696cb0ccecad16f7
suspend fun cancelPreviousThenRun(block: suspend () -> T): T {
   // If there is an activeTask, cancel it because it's result is no longer needed
   activeTask?.cancelAndJoin()
   
   // ...

objcode

// Solution #1: Cancel previous work

// This is a great solution for tasks like sorting and filtering that
// can be cancelled if a new request comes in.

class ProductsRepository(val productsDao: ProductsDao, val productsApi: ProductsService) {
   var controlledRunner = ControlledRunner<List<ProductListing>>()

   suspend fun loadSortedProducts(ascending: Boolean): List<ProductListing> {
       // cancel the previous sorts before starting a new one

objcode

// Solution 0: Disable the sort buttons when any sort is running

class ProductsViewModel(val productsRepository: ProductsRepository): ViewModel() {
   private val _sortedProducts = MutableLiveData<List<ProductListing>>()
   val sortedProducts: LiveData<List<ProductListing>> = _sortedProducts
  
   private val _sortButtonsEnabled = MutableLiveData<Boolean>()
   val sortButtonsEnabled: LiveData<Boolean> = _sortButtonsEnabled
  
   init {

objcode

@Dao
interface ProductsDao {
   // Because this is marked suspend, Room will use it's own dispatcher
   //  to run this query in a main-safe way.
   @Query("select * from ProductListing ORDER BY dateStocked ASC")
   suspend fun loadProductsByDateStockedAscending(): List<ProductListing>

   // Because this is marked suspend, Room will use it's own dispatcher
   //  to run this query in a main-safe way.
   @Query("select * from ProductListing ORDER BY dateStocked DESC")

objcode

class ProductsRepository(val productsDao: ProductsDao) {

  /**
    * This is a "regular" suspending function, which means the caller must
    * be in a coroutine. The repository is not responsible for starting or 
    * stopping coroutines since it doesn't have a natural lifecycle to cancel
    * unnecessary work.
    *
    * This *may* be called from Dispatchers.Main and is main-safe because
    * Room will take care of main-safety for us.

objcode

class ProductsViewModel(val productsRepository: ProductsRepository): ViewModel() {
   private val _sortedProducts = MutableLiveData<List<ProductListing>>()
   val sortedProducts: LiveData<List<ProductListing>> = _sortedProducts

   /**
    * Called by the UI when the user clicks the appropriate sort button
    */
   fun onSortAscending() = sortPricesBy(ascending = true)
   fun onSortDescending() = sortPricesBy(ascending = false)

objcode

/* Copyright 2019 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
 *
 *   https://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,

objcode

package com.example.android.gdgfinder.libs

import kotlinx.coroutines.CompletableDeferred
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.channels.Channel
import kotlinx.coroutines.launch

class RequestLimiter<R>(scope: CoroutineScope) {
    val requestChannel = Channel<Pair<suspend () -> R, CompletableDeferred<R>>>()

objcode

private class DownloadActor(val scope: CoroutineScope) {
    private val requested = mutableMapOf<Location, MutableList<Reference>>()
    
    private val apiThread = newFixedThreadPoolContext(1, "DownloadActor")

    private fun downloader(
            references: ReceiveChannel<Reference>,
            locations: SendChannel<Location>
    ) = scope.launch(apiThread) {
        for (ref in references) {