如何在不处理Windows消息的情况下允许表单接受文件丢弃？

Question

在Delphi XE中,我可以允许我的表单接受文件"拖放"但不必处理裸窗口消息吗？

Answer 1

您不需要处理消息来实现它.你只需要实现IDropTarget并调用RegisterDragDrop/ RevokeDragDrop.这真的非常简单.您实际上可以IDropTarget在表单代码中实现,但我更喜欢在一个看起来像这样的帮助器类中执行它:

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;

这里的想法是用来包在Windows的复杂性IDropTarget在TDropTarget.您需要做的就是实现IDragDrop哪个更简单.无论如何,我认为这应该让你去.

从控件中创建放置目标对象CreateWnd.在DestroyWnd方法中销毁它.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窗口句柄.

请注意,TDropTarget禁止引用计数.那是因为在RegisterDragDrop调用它时会增加引用计数.这会创建一个循环引用,这个代码可以抑制引用计数.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;

在这里,我使用一个表单作为放置目标.但是你可以以类似的方式使用任何其他窗口控件.

Answer 2

如果您不喜欢纯WinAPI,那么您可以使用组件.拖放组件套件是免费的来源.