我试图用于迭代TObjectList：
program Project1;
{$APPTYPE CONSOLE}
{$R *.res}
uses
System.SysUtils,Contnrs;
var
list: TObjectlist;
o: TObject;
begin
list := TObjectList.Create;
for o in list do
begin
//nothing
end;
end.@H_301_3@
它无法编译：
[dcc32 Error] Project1.dpr(15): E2010 Incompatible types: ‘TObject’ and ‘Pointer’
似乎Delphi的for in构造中没有处理无类型的,未说明的,TObjectList和可枚举的目标.
如何枚举TObjectList中的对象？
我现在应该做什么
我目前的代码是：
procedure TfrmCustomerLocator.OnBatchDataAvailable(BatchList: TObjectList);
var
i: Integer;
o: TObject;
begin
for i := 0 to BatchList.Count-1 do
begin
o := BatchList.Items[i];
//...snip...where we do something with (o as TCustomer)
end;
end;@H_301_3@
没有充分的理由,我希望将其改为：
procedure TfrmCustomerLocator.OnBatchDataAvailable(BatchList: TObjectList);
var
o: TObject;
begin
for o in BatchList do
begin
//...snip...where we do something with (o as TCustomer)
end;
end;@H_301_3@
为什么要使用枚举器？只是因为.
解决方法
How do i enumerate the objects in a TObjectList?
为了回答具体问题,这是引入枚举器的一个例子.
请注意,您必须创建TObjectList的后代才能添加GetEnumerator函数.您可以不使用类助手进行子类化,但我将其留作感兴趣的读者的练习.
type
TObjectListEnumerator = record
private
FIndex: Integer;
FList: TObjectList;
public
constructor Create(AList: TObjectList);
function GetCurrent: TObject;
function MoveNext: Boolean;
property Current: TObject read GetCurrent;
end;
constructor TObjectListEnumerator.Create(AList: TObjectList);
begin
FIndex := -1;
FList := AList;
end;
function TObjectListEnumerator.GetCurrent;
begin
Result := FList[FIndex];
end;
function TObjectListEnumerator.MoveNext: Boolean;
begin
Result := FIndex < FList.Count - 1;
if Result then
Inc(FIndex);
end;
//-- Your new subclassed TObjectList
Type
TMyObjectList = class(TObjectList)
public
function GetEnumerator: TObjectListEnumerator;
end;
function TMyObjectList.GetEnumerator: TObjectListEnumerator;
begin
Result := TObjectListEnumerator.Create(Self);
end;@H_301_3@
枚举器的这种实现使用记录而不是类.这样做的优点是在执行枚举时不会在堆上分配额外的对象.
procedure TfrmCustomerLocator.OnBatchDataAvailable(BatchList: TObjectList);
var
o: TObject;
begin
for o in TMyObjectList(BatchList) do // A simple cast is enough in this example
begin
//...snip...where we do something with (o as TCustomer)
end;
end;@H_301_3@
正如其他人所指出的,有一个泛型类是更好的选择,TObjectList< T>.