在Delphi XE中,我可以允许表单接受文件“拖放”,而不必处理裸露的Windows消息吗?
您无需处理消息即可实现此目的。您只需要实现IDropTarget
并调用RegisterDragDrop
/ RevokeDragDrop
。这真的非常非常简单。您实际上可以在表单代码中实现IDropTarget
,但我更喜欢在看起来像这样的帮助器类中实现:
uses
Winapi.Windows,
Winapi.ActiveX,
Winapi.ShellAPI,
System.StrUtils,
Vcl.Forms;
type
IDragDrop = interface
function DropAllowed(const FileNames: array of string): Boolean;
procedure Drop(const FileNames: array of string);
end;
TDropTarget = class(TObject, IInterface, IDropTarget)
private
// IInterface
function QueryInterface(const IID: TGUID; out Obj): HResult; stdcall;
function _AddRef: Integer; stdcall;
function _Release: Integer; stdcall;
private
// IDropTarget
FHandle: HWND;
FDragDrop: IDragDrop;
FDropAllowed: Boolean;
procedure GetFileNames(const dataObj: IDataObject; var FileNames: TArray<string>);
procedure SetEffect(var dwEffect: Integer);
function DragEnter(const dataObj: IDataObject; grfKeyState: Integer; pt: TPoint; var dwEffect: Integer): HResult; stdcall;
function DragOver(grfKeyState: Longint; pt: TPoint; var dwEffect: Longint): HResult; stdcall;
function DragLeave: HResult; stdcall;
function Drop(const dataObj: IDataObject; grfKeyState: Longint; pt: TPoint; var dwEffect: Longint): HResult; stdcall;
public
constructor Create(AHandle: HWND; const ADragDrop: IDragDrop);
destructor Destroy; override;
end;
{ TDropTarget }
constructor TDropTarget.Create(AHandle: HWND; const ADragDrop: IDragDrop);
begin
inherited Create;
FHandle := AHandle;
FDragDrop := ADragDrop;
RegisterDragDrop(FHandle, Self)
end;
destructor TDropTarget.Destroy;
begin
RevokeDragDrop(FHandle);
inherited;
end;
function TDropTarget.QueryInterface(const IID: TGUID; out Obj): HResult;
begin
if GetInterface(IID, Obj) then begin
Result := S_OK;
end else begin
Result := E_NOINTERFACE;
end;
end;
function TDropTarget._AddRef: Integer;
begin
Result := -1;
end;
function TDropTarget._Release: Integer;
begin
Result := -1;
end;
procedure TDropTarget.GetFileNames(const dataObj: IDataObject; var FileNames: TArray<string>);
var
i: Integer;
formatetcIn: TFormatEtc;
medium: TStgMedium;
dropHandle: HDROP;
begin
FileNames := nil;
formatetcIn.cfFormat := CF_HDROP;
formatetcIn.ptd := nil;
formatetcIn.dwAspect := DVASPECT_CONTENT;
formatetcIn.lindex := -1;
formatetcIn.tymed := TYMED_HGLOBAL;
if dataObj.GetData(formatetcIn, medium)=S_OK then begin
(* This cast needed because HDROP is incorrectly declared as Longint in ShellAPI.pas. It should be declared as THandle
which is an unsigned integer. Without this fix the routine fails in top-down memory allocation scenarios. *)
dropHandle := HDROP(medium.hGlobal);
SetLength(FileNames, DragQueryFile(dropHandle, $FFFFFFFF, nil, 0));
for i := 0 to high(FileNames) do begin
SetLength(FileNames[i], DragQueryFile(dropHandle, i, nil, 0));
DragQueryFile(dropHandle, i, @FileNames[i][1], Length(FileNames[i])+1);
end;
end;
end;
procedure TDropTarget.SetEffect(var dwEffect: Integer);
begin
if FDropAllowed then begin
dwEffect := DROPEFFECT_COPY;
end else begin
dwEffect := DROPEFFECT_NONE;
end;
end;
function TDropTarget.DragEnter(const dataObj: IDataObject; grfKeyState: Integer; pt: TPoint; var dwEffect: Integer): HResult;
var
FileNames: TArray<string>;
begin
Result := S_OK;
Try
GetFileNames(dataObj, FileNames);
FDropAllowed := (Length(FileNames)>0) and FDragDrop.DropAllowed(FileNames);
SetEffect(dwEffect);
Except
Result := E_UNEXPECTED;
End;
end;
function TDropTarget.DragLeave: HResult;
begin
Result := S_OK;
end;
function TDropTarget.DragOver(grfKeyState: Integer; pt: TPoint; var dwEffect: Integer): HResult;
begin
Result := S_OK;
Try
SetEffect(dwEffect);
Except
Result := E_UNEXPECTED;
End;
end;
function TDropTarget.Drop(const dataObj: IDataObject; grfKeyState: Integer; pt: TPoint; var dwEffect: Integer): HResult;
var
FileNames: TArray<string>;
begin
Result := S_OK;
Try
GetFileNames(dataObj, FileNames);
if Length(FileNames)>0 then begin
FDragDrop.Drop(FileNames);
end;
Except
Application.HandleException(Self);
End;
end;
这里的想法是在IDropTarget
中包装Windows TDropTarget
的复杂性。您需要做的只是实现更简单的IDragDrop
。无论如何,我认为这应该可以帮助您前进。
从控件的CreateWnd
创建放置目标对象。用DestroyWnd
方法销毁它。这一点很重要,因为VCL窗口重新创建意味着控件可以在其生命周期内销毁并重新创建其窗口句柄。
注意,TDropTarget
上的参考计数被抑制。这是因为调用RegisterDragDrop
会增加参考计数。这将创建一个循环引用,并且此代码可以抑制引用计数,从而打破了这种情况。这意味着您将通过类变量而不是接口变量来使用此类,以避免泄漏。
用法看起来像这样:
type
TMainForm = class(TForm, IDragDrop)
....
private
FDropTarget: TDropTarget;
// implement IDragDrop
function DropAllowed(const FileNames: array of string): Boolean;
procedure Drop(const FileNames: array of string);
protected
procedure CreateWindowHandle; override;
procedure DestroyWindowHandle; override;
end;
....
procedure TMainForm.CreateWindowHandle;
begin
inherited;
FDropTarget := TDropTarget.Create(WindowHandle, Self);
end;
procedure TMainForm.DestroyWindowHandle;
begin
FreeAndNil(FDropTarget);
inherited;
end;
function TMainForm.DropAllowed(const FileNames: array of string): Boolean;
begin
Result := True;
end;
procedure TMainForm.Drop(const FileNames: array of string);
begin
; // do something with the file names
end;
这里我使用表单作为放置目标。但是您可以类似的方式使用任何其他窗口控件。
如果您不喜欢纯WinAPI,则可以使用组件。 Drag and Drop Component Suite是免费的。
否,除非您要仔细阅读一些已内置此功能的自定义TForm后代。
我使用David Heffernan的解决方案作为测试应用程序的基础,并在应用程序关闭时得到了“无效的指针操作”。该问题的解决方案是通过添加'_Release;'
来更改TDropTarget.Create。constructor TDropTarget.Create(AHandle: HWND; const ADragDrop: IDragDrop);
begin
inherited Create;
FHandle := AHandle;
FDragDrop := ADragDrop;
RegisterDragDrop(FHandle, Self);
_Release;
end;
关于此问题的讨论,您可以在Embarcadero forum上看到。
[您必须自己编写代码,或者安装DropMaster之类的第三方产品,也可以拖放旧版本的Delphi。
-jeroen