🎨 Improve app bar's type definition

Author: AlexV525

example/lib/pages/custom_picker_page.dart

@@ -463,7 +463,7 @@ class FileAssetPickerBuilder
   }
 
   @override
-  Widget appBar(BuildContext context) {
+  PreferredSizeWidget appBar(BuildContext context) {
     return AppBar(
       backgroundColor: theme.appBarTheme.color,
       centerTitle: isAppleOS,

lib/src/delegates/asset_picker_builder_delegate.dart

@@ -382,7 +382,7 @@ abstract class AssetPickerBuilderDelegate<A, P> {
 
   /// Custom app bar for the picker.
   /// 选择器自定义的顶栏
-  Widget appBar(BuildContext context);
+  PreferredSizeWidget appBar(BuildContext context);
 
   /// Layout for Apple OS devices.
   /// 苹果系列设备的选择器布局
@@ -509,7 +509,7 @@ class DefaultAssetPickerBuilderDelegate
   }
 
   @override
-  FixedAppBar appBar(BuildContext context) {
+  PreferredSizeWidget appBar(BuildContext context) {
     return FixedAppBar(
       backgroundColor: theme.appBarTheme.color,
       centerTitle: isAppleOS,

lib/src/widget/fixed_appbar.dart

@@ -173,7 +173,7 @@ class FixedAppBarWrapper extends StatelessWidget {
     required this.body,
   }) : super(key: key);
 
-  final FixedAppBar appBar;
+  final PreferredSizeWidget appBar;
   final Widget body;
 
   @override
@@ -183,15 +183,15 @@ class FixedAppBarWrapper extends StatelessWidget {
       child: Stack(
         children: <Widget>[
           Positioned.fill(
-            top: kToolbarHeight + MediaQuery.of(context).padding.top,
-            child: body,
-          ),
-          PositionedDirectional(
-            top: 0.0,
-            start: 0.0,
-            end: 0.0,
-            child: appBar,
+            top: MediaQuery.of(context).padding.top +
+                appBar.preferredSize.height,
+            child: MediaQuery.removePadding(
+              context: context,
+              removeTop: true,
+              child: body,
+            ),
           ),
+          Positioned.fill(bottom: null, child: appBar),
         ],
       ),
     );