Add onBackpressureDrop

Author: akarnokd

build.gradle

@@ -30,7 +30,7 @@ repositories {
 dependencies {
     compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
     implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-core:1.3.3'
-    testCompile 'junit:junit:4.12'
+    testCompile 'junit:junit:4.13'
     testCompile "org.jetbrains.kotlin:kotlin-test-junit:$kotlin_version"
 }
 

src/main/kotlin/hu/akarnokd/kotlin/flow/FlowExtensions.kt

@@ -158,6 +158,12 @@ fun <T> Flow<T>.toList() : Flow<List<T>> {
     }
 }
 
+/**
+ * Drops items from the upstream when the downstream is not ready to receive them.
+ */
+@FlowPreview
+fun <T> Flow<T>.onBackpressurureDrop() : Flow<T> = FlowOnBackpressureDrop(this)
+
 // -----------------------------------------------------------------------------------------
 // Parallel Extensions
 // -----------------------------------------------------------------------------------------

src/main/kotlin/hu/akarnokd/kotlin/flow/impl/FlowOnBackpressureDrop.kt

@@ -0,0 +1,59 @@
+package hu.akarnokd.kotlin.flow.impl
+
+import hu.akarnokd.kotlin.flow.Resumable
+import kotlinx.coroutines.*
+import kotlinx.coroutines.flow.AbstractFlow
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.FlowCollector
+import kotlinx.coroutines.flow.collect
+import java.util.concurrent.atomic.AtomicBoolean
+import java.util.concurrent.atomic.AtomicReference
+
+@FlowPreview
+internal class FlowOnBackpressureDrop<T>(private val source: Flow<T>) : AbstractFlow<T>() {
+    @ExperimentalCoroutinesApi
+    @InternalCoroutinesApi
+    override suspend fun collectSafely(collector: FlowCollector<T>) {
+        coroutineScope {
+            val consumerReady = AtomicBoolean()
+            val producerReady = Resumable()
+            val value = AtomicReference<T>()
+            val done = AtomicBoolean()
+            val error = AtomicReference<Throwable>();
+
+            launch {
+                try {
+                    source.collect {
+                        if (consumerReady.get()) {
+                            value.set(it);
+                            consumerReady.set(false);
+                            producerReady.resume();
+                        }
+                    }
+                    done.set(true)
+                } catch (ex: Throwable) {
+                    error.set(ex)
+                }
+                producerReady.resume()
+            }
+
+            while (true) {
+                consumerReady.set(true)
+                producerReady.await()
+
+                val d = done.get()
+                val ex = error.get()
+                val v = value.getAndSet(null)
+
+                if (ex != null) {
+                    throw ex;
+                }
+                if (d) {
+                    break;
+                }
+
+                collector.emit(v)
+            }
+        }
+    }
+}

src/test/kotlin/hu/akarnokd/kotlin/flow/impl/FlowOnBackpressureDropTest.kt

@@ -0,0 +1,45 @@
+/*
+ * Copyright 2019 David Karnok
+ *
+ * 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 hu.akarnokd.kotlin.flow.impl
+
+import hu.akarnokd.kotlin.flow.assertResult
+import hu.akarnokd.kotlin.flow.onBackpressurureDrop
+import hu.akarnokd.kotlin.flow.startCollectOn
+import kotlinx.coroutines.*
+import kotlinx.coroutines.flow.*
+import org.junit.Test
+
+@FlowPreview
+class FlowOnBackpressureDropTest {
+    @InternalCoroutinesApi
+    @Test
+    fun basic() = runBlocking {
+
+        flow {
+            for (i in 0 until 10) {
+                emit(i)
+                delay(100)
+            }
+        }
+        .onBackpressurureDrop()
+        .map {
+            delay(130)
+            it
+        }
+        .assertResult(0, 2, 4, 6, 8)
+    }
+}