[wpilib] Add EventLoop (#4104)

This is a generic expansion of the command-based Trigger framework.

wpilibc/src/main/native/include/frc/event/EventLoop.h

@@ -0,0 +1,48 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+#include <functional>
+#include <list>
+
+#include <wpi/FunctionExtras.h>
+#include <wpi/SmallVector.h>
+
+namespace frc {
+/** The loop polling BooleanEvent objects and executing the actions bound to
+ * them. */
+class EventLoop {
+ public:
+  EventLoop();
+
+  /**
+   * Bind a new action to run whenever the condition is true.
+   *
+   * @param condition the condition to listen to.
+   * @param action the action to run.
+   */
+  void Bind(std::function<bool()> condition,
+            wpi::unique_function<void()> action);
+
+  /**
+   * Poll all bindings.
+   */
+  void Poll();
+
+  /**
+   * Clear all bindings.
+   */
+  void Clear();
+
+ private:
+  struct Binding {
+    std::function<bool()> condition;
+    wpi::unique_function<void()> action;
+
+    void Poll();
+  };
+  std::list<Binding> m_bindings;
+};
+}  // namespace frc