delphi - 如何检测组件已被释放？

Question

我创建了一个组件，然后将主窗体上的面板传递给它。

这是一个非常简单的示例:

procedure TMy_Socket.StatusPanel_Add(AStatusPanel: TPanel);

此组件将根据需要更新面板标题。

在我的主程序中，如果我在下次组件尝试更新面板时FreeAndNil面板，我会得到一个AV。我明白原因:组件对面板的引用现在指向未定义的位置。

如何在组件内检测面板是否已被释放，以便我知道我无法引用它？

我尝试了if (AStatusPanel = nil)，但它不是nil，它仍然有一个地址。

Answer 1

您必须调用面板的 FreeNotification() 方法，然后让您的 TMy_Socket 组件覆盖虚拟 Notification() 方法，例如(根据您的命名方案，我假设您可以向组件添加多个 TPanel 控件):

type
  TMy_Socket = class(TWhatever)
  ...
  protected
    procedure Notification(AComponent: TComponent; Operation: TOperation); override;
  ...
  public
    procedure StatusPanel_Add(AStatusPanel: TPanel); 
    procedure StatusPanel_Remove(AStatusPanel: TPanel); 
  ...
  end;

procedure TMy_Socket.StatusPanel_Add(AStatusPanel: TPanel); 
begin
  // store AStatusPanel as needed...
  AStatusPanel.FreeNotification(Self);
end;

procedure TMy_Socket.StatusPanel_Remove(AStatusPanel: TPanel); 
begin
  // remove AStatusPanel as needed...
  AStatusPanel.RemoveFreeNotification(Self);
end;

procedure TMy_Socket.Notification(AComponent: TComponent; Operation: TOperation);
begin
  inherited;
  if (AComponent is TPanel) and (Operation = opRemove) then
  begin
    // remove TPanel(AComponent) as needed...
  end;
end; 

如果您一次只跟踪一个 TPanel:

type
  TMy_Socket = class(TWhatever)
  ...
  protected
    FStatusPanel: TPanel;
    procedure Notification(AComponent: TComponent; Operation: TOperation); override;
  ...
  public
    procedure StatusPanel_Add(AStatusPanel: TPanel); 
  ...
  end;

procedure TMy_Socket.StatusPanel_Add(AStatusPanel: TPanel); 
begin
  if (AStatusPanel <> nil) and (FStatusPanel <> AStatusPanel) then
  begin
    if FStatusPanel <> nil then FStatusPanel.RemoveFreeNotification(Self);
    FStatusPanel := AStatusPanel;
    FStatusPanel.FreeNotification(Self);
  end;
end;

procedure TMy_Socket.Notification(AComponent: TComponent; Operation: TOperation);
begin
  inherited;
  if (AComponent = FStatusPanel) and (Operation = opRemove) then
    FStatusPanel := nil;
end;