# Executors go/sysui-executors [TOC] ## TLDR In SystemUI, we are encouraging the use of Java's [Executor][Executor] over Android's [Handler][Handler] when shuffling a [Runnable][Runnable] between threads or delaying the execution of a Runnable. We have an implementation of Executor available, as well as our own sub-interface, [DelayableExecutor][DelayableExecutor] available. For test, [FakeExecutor][FakeExecutor] is available. [Executor]: https://developer.android.com/reference/java/util/concurrent/Executor.html [Handler]: https://developer.android.com/reference/android/os/Handler [Runnable]: https://developer.android.com/reference/java/lang/Runnable.html [DelayableExecutor]: /packages/SystemUI/src/com/android/systemui/util/concurrency/DelayableExecutor.java [FakeExecutor]: /packages/SystemUI/tests/src/com/android/systemui/util/concurrency/FakeExecutor.java ## Rationale Executors make testing easier and are generally more flexible than Handlers. They are defined as an interface, making it easy to swap in fake implementations for testing. This also makes it easier to supply alternate implementations generally speaking - shared thread pools; priority queues; etc. For testing, whereas a handler involves trying to directly control its underlying Looper (using things like `Thread.sleep()` as well as overriding internal behaviors), an Executor implementation can be made to be directly controllable and inspectable. See also go/executors-for-the-android-engineer ## Available Executors At present, there are two interfaces of Executor avaiable, each implemented, and each with two instances - `@Background` and `@Main`. ### Executor The simplest Executor available implements the interface directly, making available one method: `Executor.execute()`. You can access an implementation of this Executor through Dependency Injection: ```java public class Foobar { @Inject public Foobar(@Background Executor bgExecutor) { bgExecutor.execute(new Runnable() { // ... }); } } ``` `@Main` will give you an Executor that runs on the ui thread. `@Background` will give you one that runs on a _shared_ non-ui thread. If you ask for an non-annotated Executor, you will get the `@Background` Executor. We do not currently have support for creating an Executor on a new, virgin thread. We do not currently support any sort of shared pooling of threads. If you require either of these, please reach out. ### DelayableExecutor [DelayableExecutor][DelayableExecutor] is the closest analogue we provide to Handler. It adds `executeDelayed(Runnable r, long delayMillis)` and `executeAtTime(Runnable r, long uptimeMillis)` to the interface, just like Handler's [postDelayed][postDelayed] and [postAtTime][postAttime]. It also adds the option to supply a [TimeUnit][TimeUnit] as a third argument. A DelayableExecutor can be accessed via Injection just like a standard Executor. In fact, at this time, it shares the same underlying thread as our basic Executor. ```java public class Foobar { @Inject public Foobar(@Background DelayableExecutor bgExecutor) { bgExecutor.executeDelayed(new Runnable() { // ... }, 1, TimeUnit.MINUTES); } } ``` Unlike Handler, the added methods return a Runnable that, when run, cancels the originally supplied Runnable if it has not yet started execution: ```java public class Foobar { @Inject public Foobar(@Background DelayableExecutor bgExecutor) { Runnable cancel = bgExecutor.executeDelayed(new Runnable() { // ... }, 1, TimeUnit.MINUTES); cancel.run(); // The supplied Runnable will (probably) not run. } } ``` [postDelayed]: https://developer.android.com/reference/android/os/Handler#postDelayed(java.lang.Runnable,%20long) [postAttime]: https://developer.android.com/reference/android/os/Handler#postAtTime(java.lang.Runnable,%20long) [TimeUnit]: https://developer.android.com/reference/java/util/concurrent/TimeUnit ## Moving From Handler Most use cases of Handlers can easily be handled by the above two interfaces above. A minor refactor makes the switch: Handler | Executor | DelayableExecutor ------------- | --------- | ----------------- post() | execute() | execute() postDelayed() | `none` | executeDelayed() postAtTime() | `none` | executeAtTime() There is one notable gap in this implementation: `Handler.postAtFrontOfQueue()`. If you require this method, or similar, please reach out. The idea of a PriorityQueueExecutor has been floated, but will not be implemented until there is a clear need. Note also that "canceling" semantics are different. Instead of passing a `token` object to `Handler.postDelayed()`, you receive a Runnable that, when run, cancels the originally supplied Runnable. ### Message Handling Executors have no concept of message handling. This is an oft used feature of Handlers. There are (as of 2019-12-05) 37 places where we subclass Handler to take advantage of this. However, by-and-large, these subclases take the following form: ```Java mHandler = new Handler(looper) { @Override public void handleMessage(Message msg) { switch (msg.what) { case MSG_A: handleMessageA(); break; case MSG_B: handleMessageB((String) msg.obj); break; case MSG_C: handleMessageC((Foobar) msg.obj); break; // ... } } }; // Elsewhere in the class void doSomething() { mHandler.obtainMessage(MSG_B, "some string"); mHandler.sendMessage(msg); } ``` This could easily be replaced by equivalent, more direct Executor code: ```Java void doSomething() { mExecutor.execute(() -> handleMessageB("some string")); } ``` If you are posting Runnables frequently and you worry that the cost of creating anonymous Runnables is too high, consider creating pre-defined Runnables as fields in your class. If you feel that you have a use case that this does not cover, please reach out. ### Handlers Are Still Necessary Handlers aren't going away. There are Android APIs that still require them (even if future API development discourages them). A simple example is [ContentObserver][ContentObserver]. Use them where necessary. [ContentObserver]: https://developer.android.com/reference/android/database/ContentObserver ## Testing (FakeExecutor) We have a [FakeExecutor][FakeExecutor] available. It implements DelayableExecutor (which in turn is an Executor). It takes a FakeSystemClock in its constructor that allows you to control the flow of time, executing supplied Runnables in a deterministic manner. The implementation is well documented and tested. You are encouraged to read and reference it, but here is a quick overview: