我正在尝试Melander DragDrop套件的TDropFileTarget组件。目标是在拖放文件后执行某些任务。此外，如果在处理过程中出现问题，我希望收到例外情况。
似乎吞下了OnDrop事件处理程序中引发的异常。但是，即使我将raise语句放入组件的源代码中，我仍然无法收到异常。你能帮忙发表评论吗？
object Form4: TForm4
Left = 0
Top = 0
Caption = 'Form4'
ClientHeight = 270
ClientWidth = 392
Color = clBtnFace
Font.Charset = DEFAULT_CHARSET
Font.Color = clWindowText
Font.Height = -11
Font.Name = 'Tahoma'
Font.Style = []
OldCreateOrder = False
PixelsPerInch = 96
TextHeight = 13
object edt1: TEdit
Left = 56
Top = 72
Width = 257
Height = 21
TabOrder = 0
Text = 'edt1'
end
object dropfiletarget2: TDropFileTarget
DragTypes = [dtCopy, dtLink]
OnDrop = dropfiletarget2Drop
Target = edt1
OptimizedMove = True
Left = 56
Top = 120
end
end
unit Unit4;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, DragDrop, DropTarget, DragDropFile, StdCtrls;
type
TForm4 = class(TForm)
edt1: TEdit;
dropfiletarget2: TDropFileTarget;
procedure dropfiletarget2Drop(Sender: TObject; ShiftState: TShiftState; APoint:
TPoint; var Effect: Integer);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form4: TForm4;
implementation
{$R *.dfm}
procedure TForm4.dropfiletarget2Drop(Sender: TObject; ShiftState: TShiftState;
APoint: TPoint; var Effect: Integer);
begin
raise Exception.Create('Error Message');
end;
end.
TCustomDropTarget.Drop。 function TCustomDropTarget.Drop(const dataObj: IDataObject; grfKeyState: Longint;
pt: TPoint; var dwEffect: Longint): HResult;
var
ShiftState: TShiftState;
ClientPt: TPoint;
begin
FScrollTimer.Enabled := False;
// Protect resources against exceptions in OnDrop event handler.
try
// Refuse drop if we have lost the data object somehow.
// This can happen if the drop is rejected in one of the other IDropTarget
// methods (e.g. DragOver).
if (not Enabled) or (FDataObject = nil) then
begin
dwEffect := DROPEFFECT_NONE;
Result := E_UNEXPECTED;
end else
begin
ShiftState := KeysToShiftStatePlus(grfKeyState);
// Create a default drop effect based on the shift state and allowed
// drop effects (or an OnGetDropEffect event if implemented).
if (FTarget <> nil) then
ClientPt := FTarget.ScreenToClient(pt)
else
ClientPt := pt;
dwEffect := GetValidDropEffect(ShiftState, ClientPt, dwEffect);
// Get data from source and generate an OnDrop event unless we failed to
// get data.
try
if (FGetDataOnEnter or GetData(dwEffect)) then
begin
if (not AsyncTransfer) then
DoDrop(ShiftState, ClientPt, dwEffect);
end else
dwEffect := DROPEFFECT_NONE;
Result := S_OK;
except
// We must not allow exceptions to escape from any of the COM methods since
// COM doesn't support exceptions.
dwEffect := DROPEFFECT_NONE;
Result := E_UNEXPECTED;
end;
end;
if (DropTargetHelper <> nil) then
DropTargetHelper.Drop(DataObj, pt, dwEffect)
else
if (FDragImageHandle <> 0) and (FTarget <> nil) then
ImageList_DragLeave(FTarget.Handle);
finally
// clean up!
if (not AsyncTransfer) then
begin
ClearData;
FDataObject := nil;
FTarget := nil;
end;
FDropTargetHelper := nil;
end;
end;
TCustomDropTarget.Drop。 function TCustomDropTarget.Drop(const dataObj: IDataObject; grfKeyState: Longint;
pt: TPoint; var dwEffect: Longint): HResult;
var
ShiftState: TShiftState;
ClientPt: TPoint;
begin
FScrollTimer.Enabled := False;
try
// Protect resources against exceptions in OnDrop event handler.
try
// Refuse drop if we have lost the data object somehow.
// This can happen if the drop is rejected in one of the other IDropTarget
// methods (e.g. DragOver).
if (not Enabled) or (FDataObject = nil) then
begin
dwEffect := DROPEFFECT_NONE;
Result := E_UNEXPECTED;
end else
begin
ShiftState := KeysToShiftStatePlus(grfKeyState);
// Create a default drop effect based on the shift state and allowed
// drop effects (or an OnGetDropEffect event if implemented).
if (FTarget <> nil) then
ClientPt := FTarget.ScreenToClient(pt)
else
ClientPt := pt;
dwEffect := GetValidDropEffect(ShiftState, ClientPt, dwEffect);
// Get data from source and generate an OnDrop event unless we failed to
// get data.
try
if (FGetDataOnEnter or GetData(dwEffect)) then
begin
if (not AsyncTransfer) then
DoDrop(ShiftState, ClientPt, dwEffect);
end else
dwEffect := DROPEFFECT_NONE;
Result := S_OK;
except
// We must not allow exceptions to escape from any of the COM methods since
// COM doesn't support exceptions.
dwEffect := DROPEFFECT_NONE;
Result := E_UNEXPECTED;
raise; // <--- Why can't I get the exception
end;
end;
if (DropTargetHelper <> nil) then
DropTargetHelper.Drop(DataObj, pt, dwEffect)
else
if (FDragImageHandle <> 0) and (FTarget <> nil) then
ImageList_DragLeave(FTarget.Handle);
finally
// clean up!
if (not AsyncTransfer) then
begin
ClearData;
FDataObject := nil;
FTarget := nil;
end;
FDropTargetHelper := nil;
end;
except
raise; // <--- Why can't I get the exception
end;
end;
答案 0 :(得分：6)
您无法在Drop()中引发异常并在应用的代码中捕获它。安德的原始评论很清楚：
// We must not allow exceptions to escape from any of the COM methods since
// COM doesn't support exceptions.
Drop()是TCustomDropTarget接口方法的IDropTarget.Drop()实现。在IDropTarget函数内部调用DoDragDrop()个方法，该函数由启动拖动的应用程序调用。 Drop()会在您应用的流程中运行，但您的应用不会调用它。因此，即使引发异常是安全的（事实并非如此），也无法放置try/except块来捕获异常，因为COM将检测到异常并作为失败传回启动应用程序，而不是您的应用程序。您唯一的选择是处理OnDrop事件处理程序中的错误，而不会引发异常。
答案 1 :(得分：3)
另一个选择是 - 在您的OnDrop事件中 - 只需收集所需的数据（即，丢弃的文件）并将它们存储在应用程序的TStringList中，然后将消息发布到表单的消息队列以通知它下降。
这样，删除文件的实际处理将在程序的正常上下文中处理，因此您可以正常方式处理异常。