Implement folder dialog, subdirs, files with nfd

src/app/core/platform/file_dialog.cc

@@ -227,17 +227,54 @@ std::string FileDialogWrapper::ShowOpenFileDialog() {
 }
 
 std::string FileDialogWrapper::ShowOpenFolderDialog() {
-  return "Linux: Open folder dialog";
+  NFD_Init();
+  nfdu8char_t *out_path = NULL;
+  nfdresult_t result = NFD_PickFolderU8(&out_path);
+  if (result == NFD_OKAY) {
+    std::string folder_path_linux(out_path);
+    NFD_Free(out_path);
+    NFD_Quit();
+    return folder_path_linux;
+  } else if (result == NFD_CANCEL) {
+    NFD_Quit();
+    return "";
+  }
+  NFD_Quit();
+  return "Error: NFD_PickFolder";
 }
 
 std::vector<std::string> FileDialogWrapper::GetSubdirectoriesInFolder(
     const std::string &folder_path) {
-  return {"Linux: Subdirectories in folder"};
+  std::vector<std::string> subdirectories;
+  DIR *dir;
+  struct dirent *ent;
+  if ((dir = opendir(folder_path.c_str())) != NULL) {
+    while ((ent = readdir(dir)) != NULL) {
+      if (ent->d_type == DT_DIR) {
+        if (strcmp(ent->d_name, ".") != 0 && strcmp(ent->d_name, "..") != 0) {
+          subdirectories.push_back(ent->d_name);
+        }
+      }
+    }
+    closedir(dir);
+  }
+  return subdirectories;
 }
 
 std::vector<std::string> FileDialogWrapper::GetFilesInFolder(
     const std::string &folder_path) {
-  return {"Linux: Files in folder"};
+  std::vector<std::string> files;
+  DIR *dir;
+  struct dirent *ent;
+  if ((dir = opendir(folder_path.c_str())) != NULL) {
+    while ((ent = readdir(dir)) != NULL) {
+      if (ent->d_type == DT_REG) {
+        files.push_back(ent->d_name);
+      }
+    }
+    closedir(dir);
+  }
+  return files;
 }
 
 #endif