add support for custom fields on posts

Author: yringler

lib/flutter_wordpress.dart

@@ -257,6 +257,9 @@ class WordPress {
   ///
   /// [fetchAll] will make as many API requests as is needed to get all posts.
   /// This may take a while.
+  /// 
+  /// Specify any custom fields in [customFieldNames]. They will be loaded into
+  /// [Post.customFields]
   ///
   /// In case of an error, a [WordPressError] object is thrown.
   Future<List<Post>> fetchPosts({
@@ -271,6 +274,7 @@ class WordPress {
     bool fetchAttachments = false,
     String postType = "posts",
     bool fetchAll = false,
+    Set<String>? customFieldNames = null
   }) async {
     if (fetchAll) {
       postParams = postParams.copyWith(perPage: 100);
@@ -288,9 +292,16 @@ class WordPress {
       List<Post> posts = [];
       final list = json.decode(response.body);
 
-      for (final post in list) {
+      for (final Map<String, dynamic> post in list) {
+        Map<String, dynamic>? customFields;
+
+        if (customFieldNames != null && customFieldNames.isNotEmpty) {
+          customFields = Map.fromEntries(
+              customFieldNames.map((key) => MapEntry(key, post[key])));
+        }
+
         posts.add(await _postBuilder(
-          post: Post.fromJson(post),
+          post: Post.fromJson(post)..customFields = customFields ?? {},
           setAuthor: fetchAuthor,
           setComments: fetchComments,
           orderComments: orderComments,

lib/schemas/post.dart

@@ -99,6 +99,9 @@ class Post {
   /// The featured Media of the post.
   Media? featuredMedia;
 
+  /// Custom fields on a post.
+  Map<String, dynamic>? customFields;
+
   Post({
     this.date,
     this.dateGmt,
@@ -118,6 +121,7 @@ class Post {
     this.format = PostFormat.standard,
     this.categoryIDs,
     this.tagIDs,
+    this.customFields,
   })  : this.title = new Title(rendered: title),
         this.featuredMedia = new Media(sourceUrl: featuredMedia),
         this.content = new Content(rendered: content),
@@ -204,6 +208,13 @@ class Post {
     data['categories'] = listToUrlString(this.categoryIDs ?? []);
     data['tags'] = listToUrlString(this.tagIDs ?? []);
 
+    if (customFields != null) {
+      for (final key in customFields!.keys) {
+        data[key] = customFields![key]; 
+      }
+    }
+
+
     return data;
   }