Fix attachments not being posted in the correct order.

Tusker/Screens/Compose/ComposeView.swift

@@ -229,36 +229,57 @@ struct ComposeView: View {
     private func uploadAttachments(_ completion: @escaping (Result<[Attachment], AttachmentUploadError>) -> Void) {
         let group = DispatchGroup()
         
-        var anyFailed = false
-        var uploadedAttachments = [Result<Attachment, Error>?]()
+        var attachmentDatas = [(Data, String)?]()
         
         for (index, compAttachment) in draft.attachments.enumerated() {
             group.enter()
             
-            uploadedAttachments.append(nil)
+            attachmentDatas.append(nil)
             
             compAttachment.data.getData { (data, mimeType) in
                 postProgress += 1
                 
+                attachmentDatas[index] = (data, mimeType)
+                group.leave()
+            }
+        }
+        
+        group.notify(queue: .global(qos: .userInitiated)) {
+            
+            var anyFailed = false
+            var uploadedAttachments = [Result<Attachment, Error>?]()
+        
+            // Mastodon does not respect the order of the `media_ids` parameter in the create post request,
+            // it determines attachment order by which was uploaded first. Since the upload attachment request
+            // does not include any timestamp data, and requests may arrive at the server out-of-order,
+            // attachments need to be uploaded serially in order to ensure the order of attachments in the
+            // posted status reflects order the user set.
+            // Pleroma does respect the order of the `media_ids` parameter.
+            
+            for (index, (data, mimeType)) in attachmentDatas.map(\.unsafelyUnwrapped).enumerated() {
+                group.enter()
+                
+                let compAttachment = draft.attachments[index]
                 let formAttachment = FormAttachment(mimeType: mimeType, data: data, fileName: "file")
                 let request = Client.upload(attachment: formAttachment, description: compAttachment.attachmentDescription)
                 self.mastodonController.run(request) { (response) in
                     switch response {
                     case let .failure(error):
-                        uploadedAttachments[index] = .failure(error)
+                        uploadedAttachments.append(.failure(error))
                         anyFailed = true
                         
                     case let .success(attachment, _):
-                        postProgress += 1
-                        uploadedAttachments[index] = .success(attachment)
+                        self.postProgress += 1
+                        uploadedAttachments.append(.success(attachment))
                     }
                     
                     group.leave()
                 }
-            }
-        }
                 
-        group.notify(queue: .main) {
+                group.wait()
+            }
+            
+            
             if anyFailed {
                 let errors = uploadedAttachments.map { (result) -> Error? in
                     if case let .failure(error) = result {
@@ -274,6 +295,7 @@ struct ComposeView: View {
                 }
                 completion(.success(uploadedAttachments))
             }
+
         }
     }
 }