wpiutil: Add WorkerThread (#1302)

This provides a worker thread that can execute a work function with the result
going into either a future or a uv::Loop functor.

wpiutil/src/test/native/cpp/WorkerThreadTest.cpp

@@ -0,0 +1,73 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2018 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "wpi/WorkerThread.h"  // NOLINT(build/include_order)
+
+#include "gtest/gtest.h"  // NOLINT(build/include_order)
+
+#include <thread>
+
+#include "wpi/uv/Loop.h"
+
+namespace wpi {
+
+TEST(WorkerThread, Future) {
+  WorkerThread<int(bool)> worker;
+  future<int> f =
+      worker.QueueWork([](bool v) -> int { return v ? 1 : 2; }, true);
+  ASSERT_EQ(f.get(), 1);
+}
+
+TEST(WorkerThread, FutureVoid) {
+  int callbacks = 0;
+  WorkerThread<void(int)> worker;
+  future<void> f = worker.QueueWork(
+      [&](int v) {
+        ++callbacks;
+        ASSERT_EQ(v, 3);
+      },
+      3);
+  f.get();
+  ASSERT_EQ(callbacks, 1);
+}
+
+TEST(WorkerThread, Loop) {
+  int callbacks = 0;
+  WorkerThread<int(bool)> worker;
+  auto loop = uv::Loop::Create();
+  worker.SetLoop(*loop);
+  worker.QueueWorkThen([](bool v) -> int { return v ? 1 : 2; },
+                       [&](int v2) {
+                         ++callbacks;
+                         loop->Stop();
+                         ASSERT_EQ(v2, 1);
+                       },
+                       true);
+  auto f = worker.QueueWork([](bool) -> int { return 2; }, true);
+  ASSERT_EQ(f.get(), 2);
+  loop->Run();
+  ASSERT_EQ(callbacks, 1);
+}
+
+TEST(WorkerThread, LoopVoid) {
+  int callbacks = 0;
+  WorkerThread<void(bool)> worker;
+  auto loop = uv::Loop::Create();
+  worker.SetLoop(*loop);
+  worker.QueueWorkThen([](bool) {},
+                       [&]() {
+                         ++callbacks;
+                         loop->Stop();
+                       },
+                       true);
+  auto f = worker.QueueWork([](bool) {}, true);
+  f.get();
+  loop->Run();
+  ASSERT_EQ(callbacks, 1);
+}
+
+}  // namespace wpi