add public APIs' dartdoc comments

lib/photo_gallery.dart

@@ -130,6 +130,7 @@ class PhotoGallery {
     });
   }
 
+  /// Clean medium file cache
   static Future<void> cleanCache() async {
     _channel.invokeMethod('cleanCache', {});
   }

lib/src/common/medium_type.dart

@@ -2,10 +2,14 @@ part of photogallery;
 
 /// A medium type.
 enum MediumType {
+  /// MediumType.image
   image,
+
+  /// MediumType.video
   video,
 }
 
+/// Convert MediumType to String
 String? mediumTypeToJson(MediumType? value) {
   switch (value) {
     case MediumType.image:
@@ -17,6 +21,7 @@ String? mediumTypeToJson(MediumType? value) {
   }
 }
 
+/// Parse String to MediumType
 MediumType? jsonToMediumType(String? value) {
   switch (value) {
     case 'image':

lib/src/image_providers/album_thumbnail_provider.dart

@@ -2,6 +2,7 @@ part of photogallery;
 
 /// Fetches the given album thumbnail from the gallery.
 class AlbumThumbnailProvider extends ImageProvider<AlbumThumbnailProvider> {
+  /// ImageProvider of album thumbnail
   const AlbumThumbnailProvider({
     required this.album,
     this.height,
@@ -9,9 +10,16 @@ class AlbumThumbnailProvider extends ImageProvider<AlbumThumbnailProvider> {
     this.highQuality = false,
   });
 
+  /// Album info
   final Album album;
+
+  /// Height of album thumbnail
   final int? height;
+
+  /// Width of album thumbnail
   final int? width;
+
+  /// Whether using high quality of album thumbnail
   final bool? highQuality;
 
   @override

lib/src/image_providers/photo_provider.dart

@@ -2,12 +2,16 @@ part of photogallery;
 
 /// Fetches the given image from the gallery.
 class PhotoProvider extends ImageProvider<PhotoProvider> {
+  /// ImageProvider of photo
   PhotoProvider({
     required this.mediumId,
     this.mimeType,
   });
 
+  /// Medium id
   final String mediumId;
+
+  /// Mime type
   final String? mimeType;
 
   @override

lib/src/image_providers/thumbnail_provider.dart

@@ -2,6 +2,7 @@ part of photogallery;
 
 /// Fetches the given medium thumbnail from the gallery.
 class ThumbnailProvider extends ImageProvider<ThumbnailProvider> {
+  /// ImageProvider of medium thumbnail
   const ThumbnailProvider({
     required this.mediumId,
     this.mediumType,
@@ -10,10 +11,19 @@ class ThumbnailProvider extends ImageProvider<ThumbnailProvider> {
     this.highQuality = false,
   });
 
+  /// Medium id
   final String mediumId;
+
+  /// Medium type
   final MediumType? mediumType;
+
+  /// Height of medium thumbnail
   final int? height;
+
+  /// Width of medium thumbnail
   final int? width;
+
+  /// Whether using high quality of medium thumbnail
   final bool? highQuality;
 
   @override

lib/src/models/media_page.dart

@@ -3,6 +3,7 @@ part of photogallery;
 /// A list of media with pagination support.
 @immutable
 class MediaPage {
+  /// The album that belongs to
   final Album album;
 
   /// The start offset for those media.
@@ -11,6 +12,7 @@ class MediaPage {
   /// The current items.
   final List<Medium> items;
 
+  /// Whether using light weight option
   final bool? lightWeight;
 
   /// The end index in the album.