compose: Prototype image/video-from-library upload UI

Fixes: zulip#56

Author: chrisbobbe

ios/Runner/Info.plist

@@ -53,5 +53,7 @@
 	<array>
 		<string>fetch</string>
 	</array>
+	<key>NSPhotoLibraryUsageDescription</key>
+	<string>Choose photos from your library and send them in Zulip messages.</string>
 </dict>
 </plist>

lib/widgets/compose_box.dart

@@ -318,6 +318,44 @@ class _AttachFileButton extends StatelessWidget {
   }
 }
 
+class _AttachMediaButton extends StatelessWidget {
+  const _AttachMediaButton({required this.contentController, required this.contentFocusNode});
+
+  final ContentTextEditingController contentController;
+  final FocusNode contentFocusNode;
+
+  _handlePress(BuildContext context) async {
+    FilePickerResult? result;
+    try {
+      result = await FilePicker.platform.pickFiles(type: FileType.media, allowMultiple: true, withReadStream: true);
+    } catch (e) {
+      // TODO(i18n)
+      showErrorDialog(context: context, title: 'Error', message: e.toString());
+      return;
+    }
+    if (result == null) {
+      return; // User cancelled; do nothing
+    }
+
+    if (context.mounted) {} // https://github.com/dart-lang/linter/issues/4007
+    else {
+      return;
+    }
+
+    final Iterable<_File> files = result.files.map((f) {
+      assert(f.readStream != null);  // We passed `withReadStream: true` to pickFiles.
+      return _File(content: f.readStream!, length: f.size, filename: f.name);
+    });
+    _uploadFiles(context: context, contentController: contentController, contentFocusNode: contentFocusNode,
+        files: files);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return IconButton(icon: const Icon(Icons.image), onPressed: () => _handlePress(context));
+  }
+}
+
 /// The send button for StreamComposeBox.
 class _StreamSendButton extends StatefulWidget {
   const _StreamSendButton({required this.topicController, required this.contentController});
@@ -516,6 +554,7 @@ class _StreamComposeBoxState extends State<StreamComposeBox> {
                   child: Row(
                     children: [
                       _AttachFileButton(contentController: _contentController, contentFocusNode: _contentFocusNode),
+                      _AttachMediaButton(contentController: _contentController, contentFocusNode: _contentFocusNode),
                     ])),
               ]))));
   }