CoroutineScope

Defines a scope for new coroutines. Every coroutine builder (like launch, async, etc.) is an extension on CoroutineScope and inherits its coroutineContext to automatically propagate all its elements and cancellation.

The best ways to obtain a standalone instance of the scope are CoroutineScope() and MainScope() factory functions, taking care to cancel these coroutine scopes when they are no longer needed (see section on custom usage below for explanation and example).

Additional context elements can be appended to the scope using the plus operator.

Convention for structured concurrency

Manual implementation of this interface is not recommended, implementation by delegation should be preferred instead. By convention, the context of a scope should contain an instance of a job to enforce the discipline of structured concurrency with propagation of cancellation.

Every coroutine builder (like launch, async, and others) and every scoping function (like coroutineScope and withContext) provides its own scope with its own Job instance into the inner block of code it runs. By convention, they all wait for all the coroutines inside their block to complete before completing themselves, thus enforcing the structured concurrency. See Job documentation for more details.

Android usage

Android has first-party support for coroutine scope in all entities with the lifecycle. See the corresponding documentation.

Custom usage

CoroutineScope should be declared as a property on entities with a well-defined lifecycle that are responsible for launching child coroutines. The corresponding instance of CoroutineScope shall be created with either CoroutineScope() or MainScope():

• CoroutineScope() uses the context provided to it as a parameter for its coroutines and adds a Job if one is not provided as part of the context.

• MainScope() uses Dispatchers.Main for its coroutines and has a SupervisorJob.

The key part of custom usage of CoroutineScope is cancelling it at the end of the lifecycle. The CoroutineScope.cancel extension function shall be used when the entity that was launching coroutines is no longer needed. It cancels all the coroutines that might still be running on behalf of it.

For example:

class MyUIClass {
    val scope = MainScope() // the scope of MyUIClass, uses Dispatchers.Main

    fun destroy() { // destroys an instance of MyUIClass
        scope.cancel() // cancels all coroutines launched in this scope
        // ... do the rest of cleanup here ...
    }

    /*
     * Note: if this instance is destroyed or any of the launched coroutines
     * in this method throws an exception, then all nested coroutines are cancelled.
     */
        fun showSomeData() = scope.launch { // launched in the main thread
           // ... here we can use suspending functions or coroutine builders with other dispatchers
           draw(data) // draw in the main thread
        }
    }

Inheritors