将自动完成字符串分配给的访问冲突

Question

我正在从这里修改带有自动完成功能的编辑控件： Auto append/complete from text file to an edit box delphi

我想从数据库加载自动完成字符串。我在自动完成控件后代上声明了新属性：

FACDataSource : TDataSource;
FACFieldName : string;

我称之为加载自动完成字符串：

procedure TAutoCompleteEdit.ReadSuggestions;
begin
  FAutoCompleteSourceList.Clear;

  if (not Assigned(FACDataSource)) or (not Assigned(FACDataSource.DataSet))       or (not ACEnabled) then
      exit;

    with FACDataSource.DataSet do
    begin
       if Active and (RecordCount > 0) and (FACFieldName <> '') then
       begin
         First;
         while not EOF do
         begin
                       FAutoCompleteSourceList.Add(FACDataSource.DataSet.FieldByName(FACFieldName).AsString);
    Next;
  end;
  if FAutoCompleteSourceList.Count > 0 then
    ACStrings := FAutoCompleteSourceList;
end;

结束； 结束；

但是，在将 FAutoCompleteSourceList 分配给 ACStrings 时，我会遇到 AccessViolation。 ACStrings 的设置器是：

procedure TAutoCompleteEdit.SetACStrings(const Value: TStringList);
begin
    if Value <> FACList.FStrings then
      FACList.FStrings.Assign(Value);
end;

我在以下行中收到 AccessViolation：FACList.FStrings.Assign(Value);（读取地址 XXXYYY）。 Value 已定义，此时不是垃圾（例如，我可以在调试器中查看字符串列表）。 'FStrings' 是一个空字符串列表。

将控件放在窗体上时它可以正常工作。但是，如果我将它放在用户输入 DBGridEH 单元格时显示的自定义就地编辑器中，则不会。

就地编辑器是这样的：

    unit UInplaceAutoCompleteEditor;

    interface
    uses UDBAutoComplete, UMyInplaceEditor, classes, windows, Controls, Buttons,     DB;

    type TInplaceAutoCompleteEditor = class(TMyInplaceEditor)
      private
       FEditor : TAutoCompleteEdit;
    FButton : TSpeedButton;
    FShowButton : boolean;
    procedure SetShowButton(value : boolean);
    public
        constructor Create(AOwner : TComponent); override;
        procedure SetFocus; override;
        destructor Destroy; override;
     protected
    procedure EditorKeyDown(Sender : TObject; var Key : Word; Shift : TShiftState);
    function GetACDataSource : TDataSource;
    procedure SetACDataSource(value : TDataSource);
    function GetACFieldName : string;
    procedure SetACFieldName(value : string);
    procedure SetACEnabled(value : boolean);
    function GetACEnabled : boolean;
  published
    property Editor : TAutoCompleteEdit read FEditor;
    property ACDataSource : TDataSource read GetACDataSource write SetACDataSource;
    property ACFieldName : string read GetACFieldName write SetACFieldName;
    property ACEnabled : boolean read GetACEnabled write SetACEnabled;
    property Button : TSpeedButton read FButton;
    property ShowButton : boolean read FShowButton write SetShowButton;
end;

  procedure Register;

implementation

  procedure Register;
  begin
    RegisterComponents('nikolaev', [ TInplaceAutoCompleteEditor ]);
  end;

{ TInplaceAutoCompleteEditor }



constructor TInplaceAutoCompleteEditor.Create(AOwner: TComponent);
begin
  inherited;
  FEditor := TAutoCompleteEdit.Create(self);
  FEditor.Parent := self;
  FEditor.Align := alClient;
  FEditor.Visible := true;
  FEditor.WantTabs := true;
  FEditor.OnKeyDown := EditorKeyDown;

  FButton := TSpeedButton.Create(self);
  FButton.Parent := self;
  FButton.Align := alRight;

  self.FOwnHeight := -1;
  self.FOwnWidth := -1;

  SetShowButton(false);
end;

destructor TInplaceAutoCompleteEditor.Destroy;
begin
  Feditor.Destroy;
  FButton.Destroy;
  inherited;
end;

procedure TInplaceAutoCompleteEditor.EditorKeyDown(Sender: TObject;
  var Key: Word; Shift: TShiftState);
begin
  if Key in [ VK_Return, VK_Tab ] then
  begin
    self.Value := FEditor.Text;
    Key := 0;
    ConfirmValue;
  end;

  if Key = VK_Escape then
  begin
    Key := 0;
    CancelValue;
  end;

  inherited;
end;



function TInplaceAutoCompleteEditor.GetACDataSource: TDataSource;
begin
  Result := FEditor.ACDataSource;
end;

function TInplaceAutoCompleteEditor.GetACEnabled: boolean;
begin
  Result := FEditor.ACEnabled;
end;

function TInplaceAutoCompleteEditor.GetACFieldName: string;
begin
  Result := FEditor.ACFieldName
end;

procedure TInplaceAutoCompleteEditor.SetACDataSource(value: TDataSource);
begin
  FEditor.ACDataSource := value;
end;

procedure TInplaceAutoCompleteEditor.SetACEnabled(value: boolean);
begin
  FEditor.ACEnabled := value;
end;

procedure TInplaceAutoCompleteEditor.SetACFieldName(value: string);
begin
  FEditor.acfieldname := value;
end;

procedure TInplaceAutoCompleteEditor.SetFocus;
begin
  inherited;
  FEditor.SetFocus;
end;

procedure TInplaceAutoCompleteEditor.SetShowButton(value: boolean);
begin
  if value <> FShowButton then
  begin
    FShowButton := value;
    FButton.Visible := value;
  end;
end;

end.

这个就地编辑器继承自一个抽象类，如下所示：

unit UMyInplaceEditor;

interface
uses Windows, classes, types, dbGridEh, ExtCtrls, Controls;

type TMyInplaceEditor = class (TWinControl)

  private
    FOnValueConfirmed : TNotifyEvent;
    FOnCanceled : TNotifyEvent;
    FWantTabs : boolean;
    procedure AdjustPosition;
  protected
    FOwnHeight, FOwnWidth : integer;
    FValue : Variant;
    function GetIsEditing : boolean;
    procedure SetIsEditing(value : boolean); virtual;
    procedure ConfirmValue;
    procedure CancelValue;
    procedure SetValue(val : Variant); virtual;
  public

    property OnValueConfirmed : TNotifyEvent read FOnValueConfirmed write FOnValueConfirmed;
    property OnCanceled : TNotifyEvent read FOnCanceled write FOnCanceled;
    property Value : Variant read FValue write SetValue;


    property IsEditing : boolean read GetIsEditing write SetIsEditing;
    procedure SetPosition(parentControl : TWinControl; rect : TRect); virtual;

    function ColumnEditable(column : TColumnEH) : boolean; virtual;

    constructor Create(AOwner : TComponent); override;
    property WantTabs : boolean read FWantTabs write FWantTabs;
end;
  procedure Register;

implementation

  procedure Register;
  begin
    RegisterComponents('nikolaev', [TMyInplaceEditor]);
  end;

constructor TMyInplaceEditor.Create(AOwner : TComponent);
begin
  inherited Create(AOwner);
  self.AutoSize := false;
  self.Visible := false;
  self.FOwnHeight := -1;
  self.FOwnWidth := -1;
end;

procedure TMyInplaceEditor.AdjustPosition;
var xOffset, yOffset : integer;
begin
  xoffset := self.Left + self.Width - self.Parent.Width;
  if xOffset > 0 then
    self.Left := self.Left - xOffset;

  yOffset := self.Top + self.Height - self.Parent.height;
  if yOffset > 0 then
    self.Top := self.Top - yOffset;

end;

function TMyInplaceEditor.GetIsEditing : boolean;
begin
  Result := self.Visible;
end;

procedure TMyInplaceEditor.SetIsEditing(value: Boolean);
begin
  self.Visible := value;
  self.BringToFront;
  {if Visible then
    self.SetFocus;}
end;

procedure TMyInplaceEditor.SetPosition(parentControl : TWinControl; rect: TRect);
begin
  self.Parent := parentControl;
  self.Top := rect.Top;//parentControl.Top;
  self.Left := rect.Left;//parentControl.left;
  if self.FOwnWidth = -1 then
    self.Width := rect.Right - rect.Left
  else
    self.Width := self.FOwnWidth;

    if self.FOwnHeight = -1 then
    self.Height := rect.Bottom - rect.Top
  else
    self.Height := self.FOwnHeight;
  AdjustPosition;
end;

function TMyInplaceEditor.ColumnEditable(column : TColumnEH) : boolean;
begin
  Result := true;
end;

procedure TMyInplaceEditor.ConfirmValue;
begin
  if Assigned(FOnValueConfirmed) then
    FOnValueConfirmed(self);
end;

procedure TMyInplaceEditor.CancelValue;
begin
  if Assigned(FOnCanceled) then
    FOnCanceled(self);
end;

procedure TMyInplaceEditor.SetValue(val : Variant);
begin
  FValue := val;
end;

end.

InplaceEditor 用于DBGridEH 的后代。在某些情况下，我会覆盖 ShowEditor 和 HideEditor 以显示/隐藏我的编辑器。

同样，自动完成控件仅在嵌入到 inplaceeditor 控件时引发异常。

什么导致访问冲突？

Answer 1

问题在于您使用的代码错误地处理了接口引用计数。以下是相关摘录：

type
  TEnumString = class(TInterfacedObject, IEnumString)
  ....

请注意，该类派生自 TInterfacedObject，因此它使用引用计数来管理其生命周期。

然后代码是这样的：

type
  TAutoCompleteEdit = class(TEdit)
  private
    FACList: TEnumString;
  ....

所以我们将持有对对象而不是接口的引用。这看起来已经很可疑了。

然后我们这样做：

constructor TAutoCompleteEdit.Create(AOwner: TComponent);
begin
  inherited;
  FACList := TEnumString.Create;
  ....
end;

destructor TAutoCompleteEdit.Destroy;
begin
  FACList := nil;
  inherited;
end;

这里没有任何东西可以让对象保持活力。在代码的其他地方，我们引用了IEnumString 接口。但是一旦该引用被释放，对象就会认为没有任何引用了。所以它被删除了。然后，稍后，代码引用了FACList，它现在指向一个已被销毁的对象。

---

解决此问题的一种简单方法是确保TAutoCompleteEdit 控件始终持有对接口的引用：

type
  TAutoCompleteEdit = class(TEdit)
  private
    FACList: TEnumString;
    FEnumString: IEnumString;
....
constructor TAutoCompleteEdit.Create(AOwner: TComponent);
begin
  inherited;
  FACList := TEnumString.Create;
  FEnumString := FACList;
  ....
end;

通过此更改，您可以删除 TAutoCompleteEdit 的析构函数，因为 FEnumString 后面的对象将被引用计数机制销毁。

---

解决此问题的另一种方法是更改​​ TEnumString 以禁用自动引用计数。看起来像这样：

type
  TEnumString = class(TObject, IInterface, IEnumString)
  private
    function QueryInterface(const IID: TGUID; out Obj): HResult; stdcall;
    function _AddRef: Integer; stdcall;
    function _Release: Integer; stdcall;
    ....
  end;

function TEnumString.QueryInterface(const IID: TGUID; out Obj): HResult;
begin
  if GetInterface(IID, Obj) then
    Result := 0
  else
    Result := E_NOINTERFACE;
end;

function TEnumString._AddRef: Integer; 
begin
  Result := -1;
end;

function TEnumString._Release: Integer; 
begin
  Result := -1;
end;

然后你需要 TAutoCompleteEdit 析构函数看起来像这样：

destructor TAutoCompleteEdit.Destroy;
begin
  FACList.Free;
  inherited;
end;

---

最后的选择是完全避免持有TEnumString，而只持有IEnumString 引用。让引用计数像第一个解决方案一样管理生命周期。但是你需要实现另一个接口，允许TAutoCompleteEdit 获取TStrings 对象。