Prevent users from selecting additional images while processing images in the image picker.

Signal/src/ViewControllers/PhotoLibrary/ImagePickerController.swift

@@ -380,20 +380,33 @@ class ImagePickerGridController: UICollectionViewController, PhotoLibraryDelegat
     }
 
     func complete(withAssets assets: [PHAsset]) {
+
+        ModalActivityIndicatorViewController.present(fromViewController: self,
+                                                     canCancel: false) { (modal) in
                                                         let attachmentPromises: [Promise<SignalAttachment>] = assets.map({
-            return photoCollectionContents.outgoingAttachment(for: $0)
+                                                            return self.photoCollectionContents.outgoingAttachment(for: $0)
                                                         })
 
                                                         firstly {
                                                             when(fulfilled: attachmentPromises)
                                                             }.map { attachments in
                                                                 Logger.debug("built all attachments")
+
+                                                                DispatchQueue.main.async {
+                                                                    modal.dismiss(completion: {
                                                                         self.didComplete(withAttachments: attachments)
+                                                                    })
+                                                                }
                                                             }.catch { error in
                                                                 Logger.error("failed to prepare attachments. error: \(error)")
+                                                                DispatchQueue.main.async {
+                                                                    modal.dismiss(completion: {
                                                                         OWSAlerts.showAlert(title: NSLocalizedString("IMAGE_PICKER_FAILED_TO_PROCESS_ATTACHMENTS", comment: "alert title"))
+                                                                    })
+                                                                }
                                                             }.retainUntilComplete()
         }
+    }
 
     private func didComplete(withAttachments attachments: [SignalAttachment]) {
         AssertIsOnMainThread()