[cscore] Add USB camera change event (#3123)

cscore/src/main/native/linux/UsbCameraListener.cpp

@@ -0,0 +1,55 @@
+// 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.
+
+#include "UsbCameraListener.h"
+
+#include <wpi/EventLoopRunner.h>
+#include <wpi/uv/FsEvent.h>
+#include <wpi/uv/Timer.h>
+
+#include "Notifier.h"
+
+using namespace cs;
+
+class UsbCameraListener::Impl {
+ public:
+  explicit Impl(Notifier& notifier) : m_notifier(notifier) {}
+
+  Notifier& m_notifier;
+
+  std::unique_ptr<wpi::EventLoopRunner> m_runner;
+};
+
+UsbCameraListener::UsbCameraListener(wpi::Logger& logger, Notifier& notifier)
+    : m_impl(std::make_unique<Impl>(notifier)) {}
+
+UsbCameraListener::~UsbCameraListener() = default;
+
+void UsbCameraListener::Start() {
+  if (!m_impl->m_runner) {
+    m_impl->m_runner = std::make_unique<wpi::EventLoopRunner>();
+    m_impl->m_runner->ExecAsync([impl = m_impl.get()](wpi::uv::Loop& loop) {
+      auto refreshTimer = wpi::uv::Timer::Create(loop);
+      refreshTimer->timeout.connect([notifier = &impl->m_notifier] {
+        notifier->NotifyUsbCamerasChanged();
+      });
+      refreshTimer->Unreference();
+
+      auto devEvents = wpi::uv::FsEvent::Create(loop);
+      devEvents->fsEvent.connect([refreshTimer](const char* fn, int flags) {
+        if (wpi::StringRef(fn).startswith("video")) {
+          refreshTimer->Start(wpi::uv::Timer::Time(200));
+        }
+      });
+      devEvents->Start("/dev");
+      devEvents->Unreference();
+    });
+  }
+}
+
+void UsbCameraListener::Stop() {
+  if (m_impl->m_runner) {
+    m_impl->m_runner.reset();
+  }
+}