撤销功能

lib/paper/paper_app_bar.dart

@@ -4,10 +4,14 @@ import 'package:flutter/services.dart';
 
 class PaperAppBar extends StatelessWidget implements PreferredSizeWidget {
   final VoidCallback onClear;
+  final VoidCallback? onBack;
+  final VoidCallback? onRevocation;
 
   const PaperAppBar({
     Key? key,
     required this.onClear,
+    this.onBack,
+    this.onRevocation,
   }) : super(key: key);
 
   @override
@@ -19,6 +23,11 @@ class PaperAppBar extends StatelessWidget implements PreferredSizeWidget {
           statusBarIconBrightness: Brightness.dark,
           statusBarColor: Colors.transparent),
       backgroundColor: Colors.white,
+      leading: BackUpButtons(
+        onBack: onBack,
+        onRevocation: onRevocation,
+      ),
+      leadingWidth: 100,
       title: Text(
         '画板绘制',
         style: TextStyle(color: Colors.black, fontSize: 16),
@@ -39,3 +48,42 @@ class PaperAppBar extends StatelessWidget implements PreferredSizeWidget {
   @override
   Size get preferredSize => const Size.fromHeight(kToolbarHeight);
 }
+
+class BackUpButtons extends StatelessWidget {
+  final VoidCallback? onBack;
+  final VoidCallback? onRevocation;
+
+  const BackUpButtons({
+    Key? key,
+    required this.onBack,
+    required this.onRevocation,
+  }) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    const BoxConstraints cts = BoxConstraints(minHeight: 32, minWidth: 32);
+    Color backColor = onBack ==null?Colors.grey:Colors.black;
+    Color revocationColor = onRevocation ==null?Colors.grey:Colors.black;
+    return Center(
+      child: Wrap(
+        children: [
+          Transform.scale(
+            scaleX: -1,
+            child: IconButton(
+              splashRadius: 20,
+              constraints: cts,
+              onPressed: onBack,
+              icon: Icon(Icons.next_plan_outlined,color: backColor),
+            ),
+          ),
+          IconButton(
+            splashRadius: 20,
+            onPressed: onRevocation,
+            constraints: cts,
+            icon: Icon(Icons.next_plan_outlined, color: revocationColor),
+          )
+        ],
+      ),
+    );
+  }
+}