优化更新代码，添加界面功能并整合

WPFluent/Controls/PasswordBox/PasswordHelper.cs

@@ -0,0 +1,112 @@
+
+
+
+using System.Diagnostics;
+
+// ReSharper disable once CheckNamespace
+namespace WPFluent.Controls;
+
+#pragma warning disable SA1601
+
+public partial class PasswordBox
+{
+    private class PasswordHelper
+    {
+        private string _currentPassword;
+        private string _currentText;
+        private string _newPasswordValue;
+        private readonly PasswordBox _passwordBox;
+
+        public PasswordHelper(PasswordBox passwordBox)
+        {
+            _passwordBox = passwordBox;
+            _currentText = string.Empty;
+            _newPasswordValue = string.Empty;
+            _currentPassword = string.Empty;
+        }
+
+        private bool IsDeleteOption()
+        {
+            Debug.Assert(_currentText == _passwordBox.Text, "_currentText == _passwordBox.Text");
+            Debug.Assert(_currentPassword == _passwordBox.Password, "_currentPassword == _passwordBox.Password");
+
+            return _currentText.Length < _currentPassword.Length;
+        }
+
+        private void UpdatePasswordWithInputCharacter(int insertIndex, string insertValue)
+        {
+            Debug.Assert(_currentText == _passwordBox.Text, "_currentText == _passwordBox.Text");
+
+            if(_currentText.Length == _newPasswordValue.Length)
+            {
+                // If it's a direct character replacement, remove the existing one before inserting the new one.
+                _newPasswordValue = _newPasswordValue.Remove(insertIndex, 1).Insert(insertIndex, insertValue);
+            }
+            else
+            {
+                _newPasswordValue = _newPasswordValue.Insert(insertIndex, insertValue);
+            }
+        }
+
+        public string GetNewPassword()
+        {
+            _currentPassword = GetPassword();
+            _newPasswordValue = _currentPassword;
+            _currentText = _passwordBox.Text;
+            var selectionIndex = _passwordBox.SelectionStart;
+            var passwordChar = _passwordBox.PasswordChar;
+            var newCharacters = _currentText.Replace(passwordChar.ToString(), string.Empty);
+            bool isDeleted = false;
+
+            if(IsDeleteOption())
+            {
+                _newPasswordValue = _currentPassword.Remove(
+                    selectionIndex,
+                    _currentPassword.Length - _currentText.Length);
+                isDeleted = true;
+            }
+
+            switch(newCharacters.Length)
+            {
+                case > 1:
+                {
+                    var index = _currentText.IndexOf(newCharacters[0]);
+
+                    _newPasswordValue =
+                            index > _newPasswordValue.Length - 1
+                        ? $"{_newPasswordValue}{newCharacters}"
+                        : _newPasswordValue.Insert(index, newCharacters);
+                    break;
+                }
+
+                case 1:
+                {
+                    for(int i = 0; i < _currentText.Length; i++)
+                    {
+                        if(_currentText[i] == passwordChar)
+                        {
+                            continue;
+                        }
+
+                        UpdatePasswordWithInputCharacter(i, _currentText[i].ToString());
+                        break;
+                    }
+
+                    break;
+                }
+
+                case 0 when !isDeleted:
+                {
+                    // The input is a PasswordChar, which is to be inserted at the designated position.
+                    int insertIndex = selectionIndex - 1;
+                    UpdatePasswordWithInputCharacter(insertIndex, passwordChar.ToString());
+                    break;
+                }
+            }
+
+            return _newPasswordValue;
+        }
+
+        public string GetPassword() => _passwordBox.Password ?? string.Empty;
+    }
+}