# 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.html [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/utils/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 are some notable gaps 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. ### ContentObserver One notable place where Handlers have been a requirement in the past is with [ContentObserver], which takes a Handler as an argument. However, we have created [ExecutorContentObserver], which is a hidden API that accepts an [Executor] in its constructor instead of a [Handler], and is otherwise identical. [ContentObserver]: https://developer.android.com/reference/android/database/ContentObserver.html [ExecutorContentObserver]: /core/java/android/database/ExecutorContentObserver.java ### Handlers Are Still Necessary Handlers aren't going away. There are other Android APIs that still require them. Avoid Handlers when possible, but use them where necessary. ## 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:
Method Description
execute() Queues a Runnable so that it is "ready" to run. (A Runnable is "ready" when its scheduled time is less than or equal to the clock.)
postDelayed() & postAtTime() Queues a runnable to be run at some point in the future.
runNextReady() Run one runnable if it is ready to run according to the supplied clock.
runAllReady() Calls runNextReady() in a loop until there are no more "ready" runnables.
advanceClockToNext() Move the internal clock to the item at the front of the queue, making it "ready".
advanceClockToLast() Makes all currently queued items ready.
numPending() The number of runnables waiting to be run They are not necessarily "ready".
(static method) exhaustExecutors() Given a number of FakeExecutors, it calls runAllReady() repeated on them until none of them have ready work. Useful if you have Executors that post work to one another back and forth.
_If you advance the supplied FakeSystemClock directly, the FakeExecutor will execute pending Runnables accordingly._ If you use the FakeExecutors `advanceClockToNext()` and `advanceClockToLast()`, this behavior will not be seen. You will need to tell the Executor to run its ready items. A quick example shows the difference: Here we advance the clock directly: ```java FakeSystemClock clock = new FakeSystemClock(); FakeExecutor executor = new FakeExecutor(clock); executor.execute(() -> {}); // Nothing run yet. Runs at time-0 executor.executeDelayed(() -> {}, 100); // Nothing run yet. Runs at time-100. executor.executeDelayed(() -> {}, 500); // Nothing run yet. Runs at time-500. clock.synchronizeListeners(); // The clock just told the Executor it's time-0. // One thing run. clock.setUptimeMillis(500); // The clock just told the Executor it's time-500. // Two more items run. ``` Here we have more fine-grained control: ```java FakeSystemClock clock = new FakeSystemClock(); FakeExecutor executor = new FakeExecutor(clock); executor.execute(() -> {}); // Nothing run yet. Runs at time-0 executor.executeDelayed(() -> {}, 100); // Nothing run yet. Runs at time-100. executor.executeDelayed(() -> {}, 500); // Nothing run yet. Runs at time-500. executor.runNextReady(); // One thing run. executor.advanceClockToNext(); // One more thing ready to run. executor.runNextReady(); // One thing run. executor.runNextReady(); // Extra calls do nothing. (Returns false). executor.advanceClockToNext(); // One more thing ready to run. executor.runNextReady(); // Last item run. ``` One gotcha of direct-clock-advancement: If you have interleaved Runnables split between two executors like the following: ```java FakeSystemClock clock = new FakeSystemClock(); FakeExecutor executorA = new FakeExecutor(clock); FakeExecutor executorB = new FakeExecutor(clock); executorA.executeDelayed(() -> {}, 100); executorB.executeDelayed(() -> {}, 200); executorA.executeDelayed(() -> {}, 300); executorB.executeDelayed(() -> {}, 400); clock.setUptimeMillis(500); ``` The Runnables _will not_ interleave. All of one Executor's callbacks will run, then all of the other's. ### Testing Handlers without Loopers If a [Handler] is required because it is used by Android APIs, but is only used in simple ways (i.e. just `Handler.post(Runnable)`), you may still want the benefits of [FakeExecutor] when writing your tests, which you can get by wrapping the [Executor] in a mock for testing. This can be done with `com.android.systemui.util.concurrency.mockExecutorHandler` in `MockExecutorHandler.kt`. ### TestableLooper.RunWithLooper As long as you're using FakeExecutors in all the code under test (and no Handlers or Loopers) you don't need it. Get rid of it. No more TestableLooper; no more Looper at all, for that matter.