Add coroutine running controller

Cancel previous and then run new task

Queue new task and run sequentially

Join the previous task

EXTENSIONS.md

@@ -1,4 +1,19 @@
 # Useful Extension Functions
+## Coroutine
+```kotlin
+suspend fun fetchApi(): User { ... }
+
+// Retry with exponential backoff
+retry {
+    fetchApi()
+}
+
+val controller = CoroutineRunningController<User>()
+controller.cancelPreviousThenRun(fetchApi())
+controller.queueTask(fetchApi())
+controller.joinPreviousOrRun(fetchApi())
+```
+
 ## View
 ```kotlin
 // Prevent multiple/duplicate click
@@ -52,3 +67,5 @@ TextView.setTextWithVisibility("5678")
 String?.isValidEmail(): Boolean
 ```
 
+
+

base/src/main/java/com/enginebai/base/extensions/CoroutineExt.kt

@@ -1,7 +1,10 @@
 package com.enginebai.base.extensions
 
-import kotlinx.coroutines.delay
+import kotlinx.coroutines.*
+import kotlinx.coroutines.sync.Mutex
+import kotlinx.coroutines.sync.withLock
 import timber.log.Timber
+import java.util.concurrent.atomic.AtomicReference
 
 /**
  * Retry running block with exponential backoff mechanism.
@@ -26,4 +29,100 @@ suspend fun <T> retry(
         currentDelay = (currentDelay * delayFactor).toLong()
     }
     return block() // last attempt
+}
+
+/**
+ * A helper class that controls the task execution as new task requests to run:
+ *  1. Cancel previous task and run the new task.
+ *  2. Execute all task sequentially.
+ *  3. Continue previous task and don't run the new task.
+ */
+// Source: https://gist.github.com/objcode/7ab4e7b1df8acd88696cb0ccecad16f7
+class CoroutineRunningController<T> {
+
+    // A lock that may only be taken by one coroutine at a time.
+    private val mutex by lazy { Mutex() }
+
+    // The currently running task, this uses atomic reference for thread safety.
+    private val activeTask by lazy { AtomicReference<Deferred<T>?>(null) }
+
+    /**
+     * Cancel all previous tasks before calling block, then run block.
+     */
+    suspend fun cancelPreviousThenRun(block: suspend () -> T): T {
+        // Cancel previous task if there is.
+        activeTask.get()?.cancelAndJoin()
+
+        return coroutineScope {
+            // Create a new coroutine for new task and don't start it until it's decided
+            //  that the new task should execute.
+            val newTask = async(start = CoroutineStart.LAZY) { block() }
+
+            // Reset the currently running task to null as new task completes.
+            newTask.invokeOnCompletion {
+                activeTask.compareAndSet(newTask, null)
+            }
+
+            val result: T
+            // Loop until all previous tasks are canceled and we can run new task.
+            while (true) {
+                // Some other tasks started before the new task got set to running.
+                // If there is still other tasks running, just cancel.
+                if (!activeTask.compareAndSet(null, newTask)) {
+                    activeTask.get()?.cancelAndJoin()
+                    yield()
+                } else {
+                    result = newTask.await()
+                    break
+                }
+            }
+            result
+        }
+    }
+
+    /**
+     * Ensure to execute the tasks one by one, it will always ensure that all previously tasks
+     * completes prior to start the current block task. Any future calls to this method while the
+     * current block task is running will not execute until the current block task completes.
+     */
+    suspend fun queueTask(block: suspend () -> T): T {
+        mutex.withLock {
+            return block()
+        }
+    }
+
+    /**
+     * Don't run the new task while a previous task is running, instead wait for the previous task
+     *  and return its result.
+     */
+    suspend fun joinPreviousOrRun(block: suspend () -> T): T {
+        // If a previous task is running, then wait and return its result.
+        activeTask.get()?.let { return it.await() }
+
+        return coroutineScope {
+            val newTask = async(start = CoroutineStart.LAZY) { block() }
+            newTask.invokeOnCompletion {
+                activeTask.compareAndSet(newTask, null)
+            }
+
+            val result: T
+            while (true) {
+                // Loop to check if there is running tasks, then join.
+                if (!activeTask.compareAndSet(null, newTask)) {
+                    val currentTask = activeTask.get()
+                    if (currentTask != null) {
+                        newTask.cancel()
+                        result = currentTask.await()
+                        break
+                    } else {
+                        yield()
+                    }
+                } else {
+                    result = newTask.await()
+                    break
+                }
+            }
+            result
+        }
+    }
 }