watchfileChanges.cxx

#include <Windows.h>
#include <optional>
#include <Shlwapi.h>
#include <string_view>
#include <functional>
#include <atomic>
namespace timeutil
{
    inline time_t from_filetime(const FILETIME& ft)
    {
        ULARGE_INTEGER ull;
        ull.LowPart = ft.dwLowDateTime;
        ull.HighPart = ft.dwHighDateTime;
        return ull.QuadPart / 10000000ULL - 11644473600ULL;
    }
}

struct FileWatchCancellationToken
{
    std::atomic_bool cancelled = false;
};

std::optional<time_t> GetFileLastWriteTime(std::wstring_view filepath)
{
    auto handle = CreateFileW(filepath.data(), GENERIC_READ, FILE_SHARE_READ | FILE_SHARE_WRITE, nullptr, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, nullptr);
    if (FAILED(GetLastError()))
    {
        return std::nullopt;
    }

    FILETIME lastWrite;
    GetFileTime(handle, nullptr, nullptr, &lastWrite);
    if (FAILED(GetLastError()))
    {
        return std::nullopt;
    }
    CloseHandle(handle);
    return timeutil::from_filetime(lastWrite);
}

void WatchFileChanges(std::wstring_view filepath, std::function<void()> callback, FileWatchCancellationToken& cancelToken)
{
    std::wstring fileToWatch{ filepath };

    std::wstring directoryToWatch{ fileToWatch };
    PathRemoveFileSpecW(directoryToWatch.data());

    std::optional<time_t> currentWriteTime = GetFileLastWriteTime(fileToWatch);

    HANDLE watcherHandle = FindFirstChangeNotificationW(
        directoryToWatch.c_str(),
        false,
        FILE_NOTIFY_CHANGE_LAST_WRITE);

    while (!cancelToken.cancelled)
    {
        constexpr DWORD waitPeriod = 5000;
        if (WaitForSingleObject(watcherHandle, waitPeriod) == WAIT_OBJECT_0)
        {
            std::optional<time_t> refreshedWriteTime = GetFileLastWriteTime(fileToWatch);
            const bool firstWritetime = !currentWriteTime && refreshedWriteTime;
            const bool writeTimeChanged = currentWriteTime && refreshedWriteTime && *currentWriteTime != *refreshedWriteTime;
            if (writeTimeChanged || firstWritetime)
            {
                callback();
                if (refreshedWriteTime)
                {
                    currentWriteTime = refreshedWriteTime;
                }
            }
            FindNextChangeNotification(watcherHandle);
        }
    }
    FindCloseChangeNotification(watcherHandle);
}