Modal conversation picker, hide loading screen when possible

The first usable screen in the conversation picker, that's the first
thing we want to show the user, and the modal presentation feels like
the right way to introduce this new context.

Long load times should be the exception, not the normal flow, so we
delay it's presentation in hopes that it will generally never be seen.

// FREEBIE

SignalShareExtension/ShareViewController.swift

@@ -20,6 +20,12 @@ public class ShareViewController: UINavigationController, SAELoadViewDelegate, S
 
         Logger.debug("\(self.logTag) \(#function)")
 
+        // We can't show the conversation picker until the DB is set up.
+        // Normally this will only take a moment, so rather than flickering and then hiding the loading screen
+        // We start with as invisible, and only fade it in if it's going to take a while
+        self.view.isHidden = true
+        self.view.alpha = 0
+
         // This should be the first thing we do.
         let appContext = ShareAppExtensionContext(rootViewController:self)
         SetCurrentAppContext(appContext)
@@ -78,7 +84,7 @@ public class ShareViewController: UINavigationController, SAELoadViewDelegate, S
 
         let loadViewController = SAELoadViewController(delegate:self)
         self.pushViewController(loadViewController, animated: false)
-        self.isNavigationBarHidden = false
+        self.isNavigationBarHidden = true
 
         // We don't need to use "screen protection" in the SAE.
 
@@ -225,7 +231,7 @@ public class ShareViewController: UINavigationController, SAELoadViewDelegate, S
         Logger.info("Presenting initial root view controller")
 
         if TSAccountManager.isRegistered() {
-            showConversationPicker()
+            presentConversationPicker()
         } else {
             showNotRegisteredView()
         }
@@ -278,7 +284,6 @@ public class ShareViewController: UINavigationController, SAELoadViewDelegate, S
     private func showErrorView(title: String, message: String) {
         let viewController = SAEFailedViewController(delegate:self, title:title, message:message)
         self.setViewControllers([viewController], animated: false)
-        self.isNavigationBarHidden = false
     }
 
     // MARK: View Lifecycle
@@ -303,6 +308,14 @@ public class ShareViewController: UINavigationController, SAELoadViewDelegate, S
         Logger.debug("\(self.logTag) \(#function)")
 
         super.viewDidAppear(animated)
+
+        // We can't show the conversation picker until the DB is set up.
+        // Normally this will only take a moment, so rather than flickering and then hiding the loading screen
+        // We start with as invisible, and only fade it in if it's going to take a while
+        self.view.isHidden = false
+        UIView.animate(withDuration: 0.1, delay: 0.5, options: [.curveEaseInOut], animations: {
+            self.view.alpha = 1
+        }, completion: nil)
     }
 
     override open func viewWillDisappear(_ animated: Bool) {
@@ -329,11 +342,13 @@ public class ShareViewController: UINavigationController, SAELoadViewDelegate, S
 
     // MARK: Helpers
 
-    private func showConversationPicker() {
-        let conversationPicker = SendExternalFileViewController()
-        buildAttachment().then { attachment -> Void in
+    private func presentConversationPicker() {
+        self.buildAttachment().then { attachment -> Void in
+            let conversationPicker = SendExternalFileViewController()
+            let navigationController = UINavigationController(rootViewController: conversationPicker)
+            navigationController.isNavigationBarHidden = true
             conversationPicker.attachment = attachment
-            self.setViewControllers([conversationPicker], animated: true)
+            self.present(navigationController, animated: true, completion: nil)
             Logger.info("showing picker with attachment: \(attachment)")
         }.catch { error in
             let alertTitle = NSLocalizedString("SHARE_EXTENSION_UNABLE_TO_BUILD_ATTACHMENT_ALERT_TITLE", comment: "Shown when trying to share content to a Signal user for the share extension. Followed by failure details.")