feat: 新增自動登入功能

利用share perference 套件儲存使用者資料
在開啟時自動尋找是否有儲存及登入後儲存資料進share preference

lib/main.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
+import 'package:shared_preferences/shared_preferences.dart';
 import 'package:topic/HomePage.dart';
 import 'package:mysql_client/mysql_client.dart';
 import 'package:topic/RegisterPage.dart';
@@ -22,6 +23,7 @@ class _LoginPageState extends State<LoginPage> {
   void initState() {//初始化
     super.initState();
     _fetchData();//連資料庫
+    _CheckPreLoginInfo();//確定有先前有無登入
   }
 
   void _fetchData() async {
@@ -40,6 +42,29 @@ class _LoginPageState extends State<LoginPage> {
     await conn.connect();
   }
 
+  void _CheckPreLoginInfo() async{
+    SharedPreferences prefs = await SharedPreferences.getInstance();
+    //get user data from share preference else set empty to check not login before
+    String loginUserEmail = prefs.getString("email") ?? "";
+    //replace screen to HomePage if there are previous data in share preference
+    if(loginUserEmail != ""){
+      Navigator.pushReplacement(
+        context,
+        MaterialPageRoute(
+          builder: (context) => HomePage(
+            email: loginUserEmail,
+          ),
+        ),
+      );
+    }
+  }
+
+  void _setLoginInfo(String email) async{
+    //save user email into share preference to let app can auto login next open
+    SharedPreferences prefs = await SharedPreferences.getInstance();
+    prefs.setString("email", email);
+  }
+
   @override
   Widget build(BuildContext context) {
     debugPaintSizeEnabled=false;
@@ -180,6 +205,7 @@ class _LoginPageState extends State<LoginPage> {
         _emailController.clear();
         _ageController.clear();
         // 如果找到匹配的帳號，登入成功，跳轉到主頁
+        _setLoginInfo(email);
         Navigator.pushReplacement(
           context,
           MaterialPageRoute(