obsolete files: give error hint on viewer, allow deleting from media store

android/app/src/main/kotlin/deckers/thibault/aves/model/provider/MediaStoreImageProvider.kt

@@ -186,7 +186,7 @@ class MediaStoreImageProvider : ImageProvider() {
     override suspend fun delete(context: Context, uri: Uri, path: String?) {
         path ?: throw Exception("failed to delete file because path is null")
 
-        if (requireAccessPermission(context, path)) {
+        if (File(path).exists() && requireAccessPermission(context, path)) {
             // if the file is on SD card, calling the content resolver `delete()` removes the entry from the Media Store
             // but it doesn't delete the file, even if the app has the permission
             val df = getDocumentFile(context, path, uri)

lib/widgets/viewer/visual/entry_page_view.dart

@@ -129,7 +129,10 @@ class _EntryPageViewState extends State<EntryPageView> {
     } else if (entry.canDecode) {
       child = _buildRasterView();
     }
-    child ??= ErrorView(onTap: () => onTap?.call(null));
+    child ??= ErrorView(
+      entry: entry,
+      onTap: () => onTap?.call(null),
+    );
 
     return widget.heroTag != null
         ? Hero(
@@ -146,7 +149,10 @@ class _EntryPageViewState extends State<EntryPageView> {
       child: RasterImageView(
         entry: entry,
         viewStateNotifier: _viewStateNotifier,
-        errorBuilder: (context, error, stackTrace) => ErrorView(onTap: () => onTap?.call(null)),
+        errorBuilder: (context, error, stackTrace) => ErrorView(
+          entry: entry,
+          onTap: () => onTap?.call(null),
+        ),
       ),
     );
   }

lib/widgets/viewer/visual/error.dart

@@ -1,26 +1,54 @@
+import 'dart:io';
+
+import 'package:aves/model/entry.dart';
 import 'package:aves/theme/icons.dart';
 import 'package:aves/widgets/collection/empty.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 
-class ErrorView extends StatelessWidget {
+class ErrorView extends StatefulWidget {
+  final AvesEntry entry;
   final VoidCallback onTap;
 
-  const ErrorView({@required this.onTap});
+  const ErrorView({
+    @required this.entry,
+    @required this.onTap,
+  });
+
+  @override
+  _ErrorViewState createState() => _ErrorViewState();
+}
+
+class _ErrorViewState extends State<ErrorView> {
+  Future<bool> _exists;
+
+  AvesEntry get entry => widget.entry;
+
+  @override
+  void initState() {
+    super.initState();
+    _exists = entry.path != null ? File(entry.path).exists() : SynchronousFuture(true);
+  }
 
   @override
   Widget build(BuildContext context) {
     return GestureDetector(
-      onTap: () => onTap?.call(),
+      onTap: () => widget.onTap?.call(),
-      // use a `Container` with a dummy color to make it expand
+      // use container to expand constraints, so that the user can tap anywhere
-      // so that we can also detect taps around the title `Text`
       child: Container(
-        color: Colors.transparent,
+        // opaque to cover potential lower quality layer below
-        child: EmptyContent(
+        color: Colors.black,
+        child: FutureBuilder<bool>(
+            future: _exists,
+            builder: (context, snapshot) {
+              if (snapshot.connectionState != ConnectionState.done) return SizedBox();
+              final exists = snapshot.data;
+              return EmptyContent(
                 icon: AIcons.error,
-          text: 'Oops!',
+                text: exists ? 'Oops!' : 'The file no longer exists.',
                 alignment: Alignment.center,
-        ),
+              );
+            }),
       ),
     );
   }