Android/frameworks_base
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

Documentation on new Executors in SystemUI.

Browse Source 
Also, remove the "removeAll" method. It was going
to cause more harm than good.

Bug: 145135056
Test: n/a
Change-Id: I39ef8c4aa656e6a9d7ef15e8dd528960e07a406a
This commit is contained in:
Dave Mankoff
2019-12-06 10:33:58 -05:00
parent b49e6f44c8
commit 6891682256
5 changed files with 321 additions and 47 deletions
Download Patch File Download Diff File Expand all files Collapse all files

321
packages/SystemUI/docs/executors.md Normal file
View File

@@ -0,0 +1,321 @@
# 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:
<table>
<tr>
<th>Method</th>
<th>Description</th>
</tr>
<tr>
<td>execute()</td>
<td>
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.)
</td>
</tr>
<tr>
<td>postDelayed() & postAtTime()</td>
<td>
Queues a runnable to be run at some
point in the future.
</td>
</tr>
<tr>
<td>runNextReady()</td>
<td>
Run one runnable if it is ready to run
according to the supplied clock.
</td>
</tr>
<tr>
<td>runAllReady()</td>
<td>
Calls runNextReady() in a loop until
there are no more "ready" runnables.
</td>
</tr>
<tr>
<td>advanceClockToNext()</td>
<td>
Move the internal clock to the item at
the front of the queue, making it
"ready".
</td>
</tr>
<tr>
<td>advanceClockToLast()</td>
<td>
Makes all currently queued items ready.
</td>
</tr>
<tr>
<td>numPending()</td>
<td>
The number of runnables waiting to be run
They are not necessarily "ready".
</td>
</tr>
<tr>
<td>(static method) exhaustExecutors()</td>
<td>
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.
</td>
</tr>
</table>
_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.
### 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.