在派生自TPanel
的复合组件中,我正在尝试发布一个属性,该属性的唯一优点是设置并获取子组件的链接属性。每次将复合组件添加到窗体时,都会引发访问冲突:
模块"MyRuntimePackage.bpl"中地址 12612D86 的访问冲突。读取地址00000080。
我已经使用TLabel
及其PopupMenu
属性准备了一个简化的示例,但在将复合组件放置在窗体/框架上时仍然遇到同样的问题。
运行时包:
uses
StdCtrls, Menus, ExtCtrls, Classes;
type
TTestCompoundComponent = class(TPanel)
private
FSubCmp : TLabel;
function GetLabelPopupMenu() : TPopupMenu;
procedure SetLabelPopupMenu(AValue : TPopupMenu);
protected
procedure Notification(AComponent: TComponent; Operation: TOperation); override;
public
constructor Create(AOwner : TComponent); override;
destructor Destroy(); override;
published
property LabelPopupMenu : TPopupMenu read GetLabelPopupMenu write SetLabelPopupMenu;
end;
...
function TTestCompoundComponent.GetLabelPopupMenu() : TPopupMenu;
begin
Result := FSubCmp.PopupMenu;
end;
procedure TTestCompoundComponent.SetLabelPopupMenu(AValue : TPopupMenu);
begin
if(GetLabelPopupMenu() <> AValue) then
begin
if(GetLabelPopupMenu() <> nil)
then GetLabelPopupMenu().RemoveFreeNotification(Self);
FSubCmp.PopupMenu := AValue;
if(GetLabelPopupMenu() <> nil)
then GetLabelPopupMenu().FreeNotification(Self);
end;
end;
procedure TTestCompoundComponent.Notification(AComponent: TComponent; Operation: TOperation);
begin
inherited;
if((AComponent = GetLabelPopupMenu()) AND (Operation = opRemove))
then SetLabelPopupMenu(nil);
end;
constructor TTestCompoundComponent.Create(AOwner : TComponent);
begin
inherited;
FSubCmp := TLabel.Create(nil);
FSubCmp.Parent := Self;
end;
destructor TTestCompoundComponent.Destroy();
begin
FSubCmp.Free;
inherited;
end;
设计时包:
procedure Register;
begin
RegisterComponents('MyTestCompoundComponent', [TTestCompoundComponent]);
end;
@kobik的答案解释了AV的根本原因(在创建FSubCmp
之前访问FSubCmp.PopupMenu
属性)。 但是,对于您要实现的目标而言,整个组件代码过于复杂。
你应该将组件设置为TLabel
的Owner
,然后你可以完全删除析构函数。 您还应该在构造函数中调用FSubCmp.SetSubComponent(True)
(特别是如果您打算稍后在对象检查器中公开TLabel
,以便用户可以在设计时自定义其属性):
constructor TTestCompoundComponent.Create(AOwner : TComponent);
begin
inherited;
FSubCmp := TLabel.Create(Self);
FSubCmp.SetSubComponent(True);
FSubCmp.Parent := Self;
end;
您的Notification()
方法应直接设置FSubCmp.PopupMenu := nil
以响应opRemove
,而不是调用SetLabelPopupMenu(nil)
。 您已经知道PopupMenu
已分配并且正在进行销毁,因此检索PopupMenu
(重复),检查它是否nil
并调用RemoveFreeNotification()
的额外代码对于opRemove
操作来说都是矫枉过正的:
procedure TTestCompoundComponent.Notification(AComponent: TComponent; Operation: TOperation);
begin
inherited;
if (Operation = opRemove) and (AComponent = LabelPopupMenu) then
FSubCmp.PopupMenu := nil;
end;
而且您的SetLabelPopupMenu()
方法通常只是碍眼的,所有这些多余的电话都GetLabelPopupMenu()
. 仅调用它一次,并将返回的对象指针存储到局部变量,然后可以根据需要使用该变量:
procedure TTestCompoundComponent.SetLabelPopupMenu(AValue: TPopupMenu);
var
PM: TPopupMenu;
begin
PM := LabelPopupMenu;
if (PM <> AValue) then
begin
if (PM <> nil) then
PM.RemoveFreeNotification(Self);
FSubCmp.PopupMenu := AValue;
if (AValue <> nil) then
AValue.FreeNotification(Self);
end;
end;
但是,您的Notification()
方法实际上是完全多余的,应完全删除。TLabel
已经在自己的PopupMenu
属性上调用FreeNotification()
,并且具有自己的Notification()
实现,该实现将在释放TPopupMenu
对象时将PopupMenu
属性设置为nil
。 您根本不需要手动处理此问题。 因此,SetLabelPopupMenu()
中的所有额外代码都是多余的,应该删除:
procedure TTestCompoundComponent.SetLabelPopupMenu(AValue: TPopupMenu);
begin
FSubCmp.PopupMenu := AValue;
end;
这也意味着@kobik提出的修复程序是多余的,也可以删除1:
function TTestCompoundComponent.GetLabelPopupMenu: TPopupMenu;
begin
Result := FSubCmp.PopupMenu;
end;
1:除非你想处理用户决定直接释放你的TLabel
的情况(这是愚蠢的,在实践中没有人会真正这样做,但这在技术上仍然是可能的),那么你将需要Notification()
来处理这种情况(将你的组件分配为TLabel
的Owner
会为你调用FreeNotificatio()
):
function TTestCompoundComponent.Notification(AComponent: TComponent; Opration: TOperation);
begin
inherited;
if (Operation = opRemove) and (AComponent = FSubCmp) then
FSubCmp := nil;
end;
function TTestCompoundComponent.GetLabelPopupMenu: TPopupMenu;
begin
if FSubCmp <> nil then
Result := FSubCmp.PopupMenu
else
Result := nil;
end;
话虽如此,下面是代码的简化版本:
uses
StdCtrls, Menus, ExtCtrls, Classes;
type
TTestCompoundComponent = class(TPanel)
private
FSubCmp: TLabel;
function GetLabelPopupMenu: TPopupMenu;
procedure SetLabelPopupMenu(AValue: TPopupMenu);
public
constructor Create(AOwner: TComponent); override;
published
property LabelPopupMenu: TPopupMenu read GetLabelPopupMenu write SetLabelPopupMenu;
end;
...
constructor TTestCompoundComponent.Create(AOwner : TComponent);
begin
inherited;
FSubCmp := TLabel.Create(Self);
FSubCmp.SetSubComponent(True);
FSubCmp.Parent := Self;
end;
function TTestCompoundComponent.GetLabelPopupMenu: TPopupMenu;
begin
Result := FSubCmp.PopupMenu;
end;
procedure TTestCompoundComponent.SetLabelPopupMenu(AValue: TPopupMenu);
begin
FSubCmp.PopupMenu := AValue;
end;
甚至只是这个:
uses
StdCtrls, Menus, ExtCtrls, Classes;
type
TTestCompoundComponent = class(TPanel)
private
FSubCmp: TLabel;
public
constructor Create(AOwner: TComponent); override;
published
property SubLabel: TLabel read FSubCmp;
end;
...
constructor TTestCompoundComponent.Create(AOwner : TComponent);
begin
inherited;
FSubCmp := TLabel.Create(Self);
FSubCmp.SetSubComponent(True);
FSubCmp.Parent := Self;
end;
在GetLabelPopupMenu()
中,当Notification()
在创建FSubCmp
之前在构造过程中收到opInsert
通知时,FSubCmp
nil
。 如果FSubCmp
nil
,引用其PopupMenu
属性将导致AV。 因此,您需要在GetLabelPopupMenu()
中检查这一点,例如:
if FSubCmp = nil then
Result := nil
else
Result := FSubCmp.PopupMenu;
否则,请将Notification()
中and
逻辑的顺序更改为以下内容:
if (Operation = opRemove) and (AComponent = GetLabelPopupMenu())
如果条件(Operation = opRemove)
为假,则不会评估右侧条件(短路)。