add NFileWatcher

common/nfile_watcher.hh

@@ -0,0 +1,46 @@
+#pragma once
+
+#include <functional>
+#include <filesystem>
+#include <optional>
+
+#include "common/file_base.hh"
+
+
+namespace nf7 {
+
+class NFileWatcher final : public nf7::FileBase::Feature {
+ public:
+  NFileWatcher() = default;
+  NFileWatcher(const NFileWatcher&) = delete;
+  NFileWatcher(NFileWatcher&&) = delete;
+  NFileWatcher& operator=(const NFileWatcher&) = delete;
+  NFileWatcher& operator=(NFileWatcher&&) = delete;
+
+  void Watch(const std::filesystem::path& npath) noexcept {
+    npath_   = npath;
+    lastmod_ = std::nullopt;
+  }
+
+  std::function<void()> onMod;
+
+ protected:
+  void Update() noexcept override
+  try {
+    if (npath_) {
+      const auto lastmod = std::filesystem::last_write_time(*npath_);
+      if (lastmod_ && lastmod > *lastmod_) {
+        onMod();
+      }
+      lastmod_ = lastmod;
+    }
+  } catch (std::filesystem::filesystem_error&) {
+    lastmod_.emplace();
+  }
+
+ private:
+  std::optional<std::filesystem::path> npath_;
+  std::optional<std::filesystem::file_time_type> lastmod_;
+};
+
+}  // namespace nf7