Added implementation for simple thread safe async event emitter

chat-android/src/main/java/com/ably/chat/Emitter.kt

@@ -0,0 +1,91 @@
+package com.ably.chat
+
+import io.ably.lib.util.Log.ERROR
+import io.ably.lib.util.Log.LogHandler
+import java.util.LinkedList
+import java.util.concurrent.CopyOnWriteArrayList
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.launch
+
+/**
+ * Emitter interface for supplied value
+ * Ideally, class implementation should work for both kotlin and java
+ */
+interface Emitter<V> {
+    fun emit(value: V)
+    fun on(block: suspend CoroutineScope.(V) -> Unit): Subscription
+    fun offAll()
+}
+
+/**
+ * AsyncEmitter is thread safe, highly performant async emitter implementation for kotlin.
+ * Currently, use-case is limited to handle internal events.
+ * This can be modified in the future to handle external listeners, events etc
+ */
+class AsyncEmitter<V> (private val collectorScope: CoroutineScope = CoroutineScope(Dispatchers.Default)) : Emitter<V> {
+
+    // Read https://www.codejava.net/java-core/concurrency/java-concurrent-collection-copyonwritearraylist-examples
+    // For more information on why it's good to have this list
+    private val subscribers = CopyOnWriteArrayList<AsyncSubscriber<V>>()
+
+    override fun emit(value: V) {
+        for (subscriber in subscribers) {
+            subscriber.notifyAsync(value)
+        }
+    }
+
+    override fun on(block: suspend CoroutineScope.(V) -> Unit): Subscription {
+        val subscriber = AsyncSubscriber(collectorScope, block)
+        subscribers.addIfAbsent(subscriber)
+        return Subscription {
+            subscribers.remove(subscriber)
+        }
+    }
+
+    override fun offAll() {
+        subscribers.clear()
+    }
+}
+
+private class AsyncSubscriber<V>(
+    private val scope: CoroutineScope,
+    private val subscriberBlock: (suspend CoroutineScope.(V) -> Unit),
+    private val logger: LogHandler? = null,
+) {
+    private var isSubscriberRunning = false
+    private val values = LinkedList<V>()
+
+    fun notifyAsync(value: V) {
+        sequentialScope.launch {
+            values.add(value)
+            if (!isSubscriberRunning) {
+                isSubscriberRunning = true
+                while (values.isNotEmpty()) {
+                    val valueTobeEmitted = values.poll()
+                    try {
+                        // Process values sequentially, similar to blocking eventEmitter
+                        scope.launch { subscriberBlock(valueTobeEmitted as V) }.join()
+                    } catch (t: Throwable) {
+                        // TODO - replace with more verbose logging
+                        logger?.println(ERROR, "AsyncSubscriber", "Error processing value $valueTobeEmitted", t)
+                    }
+                }
+                isSubscriberRunning = false
+            }
+        }
+    }
+
+    override fun equals(other: Any?): Boolean {
+        if (other is AsyncSubscriber<*>) {
+            // Avoid registering duplicate anonymous subscriber block with same instance id
+            // Common scenario when Android activity is refreshed or some app components refresh
+            return this.subscriberBlock.hashCode() == other.subscriberBlock.hashCode()
+        }
+        return super.equals(other)
+    }
+
+    companion object {
+        val sequentialScope = CoroutineScope(Dispatchers.Default.limitedParallelism(1))
+    }
+}

chat-android/src/test/java/com/ably/chat/EmitterTest.java

@@ -0,0 +1,38 @@
+package com.ably.chat;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+
+public class EmitterTest {
+
+    @Test
+    public void testEmitter() {
+        AsyncEmitter<String> asyncEmitter = new AsyncEmitter<>();
+        ArrayList<String> receivedValues = new ArrayList<>();
+
+        asyncEmitter.emit("1");
+
+        Subscription subscription = asyncEmitter.on((coroutineScope, s, continuation) -> {
+            receivedValues.add(s);
+            return null;
+        });
+
+        asyncEmitter.emit("2");
+        asyncEmitter.emit("3");
+        asyncEmitter.emit("4");
+
+        subscription.unsubscribe();
+
+        asyncEmitter.emit("5");
+        asyncEmitter.emit("6");
+
+        Exception conditionError = new TestUtils.ConditionalWaiter().
+                wait(() -> receivedValues.size() == 3, 5000);
+        Assert.assertNull(conditionError);
+
+        Assert.assertEquals(Arrays.asList("2", "3", "4"), receivedValues);
+    }
+}

chat-android/src/test/java/com/ably/chat/EmitterTest.kt

@@ -0,0 +1,33 @@
+package com.ably.chat
+
+import kotlinx.coroutines.test.runTest
+import org.junit.Assert
+import org.junit.Test
+
+class AsyncEmitterTest {
+
+    @Test
+    fun `should be able to emit and listen to event`() = runTest {
+        val asyncEmitter = AsyncEmitter<String>()
+        val receivedValues = mutableListOf<String>()
+
+        asyncEmitter.emit("1")
+
+        val subscription = asyncEmitter.on { received: String ->
+            receivedValues.add(received)
+        }
+
+        asyncEmitter.emit("2")
+        asyncEmitter.emit("3")
+        asyncEmitter.emit("4")
+
+        subscription.unsubscribe()
+
+        asyncEmitter.emit("5")
+        asyncEmitter.emit("6")
+
+        assertWaiter { receivedValues.size == 3 }.join()
+
+        Assert.assertEquals(listOf("2", "3", "4"), receivedValues)
+    }
+}

chat-android/src/test/java/com/ably/chat/TestUtils.java

@@ -0,0 +1,38 @@
+package com.ably.chat;
+
+import java.util.Timer;
+import java.util.TimerTask;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+public class TestUtils {
+
+    public interface ConditionFn<O> {
+        O call();
+    }
+
+    public static class ConditionalWaiter {
+        public Exception wait(ConditionFn<Boolean> condition, int timeoutInMs) {
+            AtomicBoolean taskTimedOut = new AtomicBoolean();
+            new Timer().schedule(new TimerTask() {
+                @Override
+                public void run() {
+                    taskTimedOut.set(true);
+                }
+            }, timeoutInMs);
+            while (true) {
+                try {
+                    Boolean result = condition.call();
+                    if (result) {
+                        return null;
+                    }
+                    if (taskTimedOut.get()) {
+                        throw new Exception("Timed out after " + timeoutInMs + "ms waiting for condition");
+                    }
+                    Thread.sleep(200);
+                } catch (Exception e) {
+                    return e;
+                }
+            }
+        }
+    }
+}

chat-android/src/test/java/com/ably/chat/TestUtils.kt

@@ -4,6 +4,12 @@ import com.google.gson.JsonElement
 import io.ably.lib.types.AsyncHttpPaginatedResponse
 import io.mockk.every
 import io.mockk.mockk
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.Job
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.withTimeout
 
 fun buildAsyncHttpPaginatedResponse(items: List<JsonElement>): AsyncHttpPaginatedResponse {
     val response = mockk<AsyncHttpPaginatedResponse>()
@@ -49,3 +55,16 @@ fun mockOccupancyApiResponse(realtimeClientMock: RealtimeClient, response: JsonE
         )
     }
 }
+
+suspend fun assertWaiter(timeoutInMs: Long = 10000, block : () -> Boolean): Job {
+    // Need to create coroutineScope because delay doesn't work in runTest default CoroutineScope
+    val scope = CoroutineScope(Dispatchers.Default)
+    return scope.launch {
+        withTimeout(timeoutInMs) {
+            do {
+                val success = block()
+                delay(100)
+            } while (!success)
+        }
+    }
+}