delphi – Variant属性可以有默认值吗？

Properties : Storage Specifiers

When saving a component’s state,the storage specifiers of the component’s published properties are checked. If a property’s current value is different from its default value (or if there is no default value) and the stored specifier is True,then the property’s value is saved. Otherwise,the property’s value is not saved.

Note: Property values are not automatically initialized to the default value. That is,the default directive controls only when property values are saved to the form file,but not the initial value of the property on a newly created instance.

这只是组件流系统的一个提示,它不需要在.dfm中显式存储该值 – 您的合同部分是确保您实际将支持字段初始化为该值.进行此类初始化的适当位置是在组件的构造函数中：

constructor TMyComponent.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  FVariantValue := False;
end;

也就是说,False是布尔值,而不是变量,因此它不能用作Variant类型的常量表达式.由于变体是复杂类型,因此不能将其表示为单个常量,因此不能具有默认属性.

Per Remy,如果要在后备变量为False时确保变量未保存在.dfm文件中,则可以使用带有无参数方法的stored指令,当变量求值为布尔值False时,该方法返回False.例如 ：

property VariantValue : Variant read FVariantValue write FVariantValue stored IsVariantValueStored;

哪里

function TMyComponent.IsVariantValueStored : Boolean;
begin
  Result := not VarIsType(FVariantValue,varBoolean);
  if not Result then
    Result := FVariantValue;
end;