DBEditEh/Edit 文本框内容空白时加上提示文字,输入内容时提示文字不显示

当文本框没有输入内容且输入焦点不在文本框时，显示提示文字：“请输入帐号”
当文本框得到输入焦点时提示文字不显示

unit SkinHintEdit;  

interface  

uses
  Windows,Classes,Controls,SysUtils,Messages,StdCtrls,Forms,
  Graphics;  

const
  //提示文字颜色
  CONST_DEF_HELPTEXT_FONTCOLOR=clGray;
  CONST_DEF_HELPTEXT_FONTNAME='Tahoma';
  CONST_DEF_HELPTEXT_FONTSIZE=9;
  CONST_DEF_HELPTEXT_FONTSTYLE=[];  

type
  TSkinHintEdit=class(TCustomEdit)
  private
    FHelpText: WideString;
    FHelpTextFont: TFont;
    procedure SetHelpText(const Value: WideString);
    procedure SetHelpTextFont(const Value: TFont);
  protected
    procedure WMPaint(var Message:TMessage);message WM_PAINT;
  public
    constructor Create(AOwner:TComponent);override;
    destructor Destroy;override;
  published
    ///////////////自定义扩展属性/////////////////
    //指导性文字字体颜色
    property HelpTextFont:TFont read FHelpTextFont write SetHelpTextFont;
    //指导性文字
    property HelpText:WideString read FHelpText write SetHelpText;
    ///////////////默认文本框自带属性////////////
    property Align;
    property Alignment;
    property Anchors;
    property AutoSelect;
    property AutoSize;
    property BevelEdges;
    property BevelInner;
    property BevelKind default bkNone;
    property BevelOuter;
    property BevelWidth;
    property BiDiMode;
    property BorderStyle;
    property CharCase;
    property Color;
    property Constraints;
    property Ctl3D;
    property DoubleBuffered;
    property DragCursor;
    property DragKind;
    property DragMode;
    property Enabled;
    property Font;
    property HideSelection;
    property ImeMode;
    property ImeName;
    property MaxLength;
    property NumbersOnly;
    property OEMConvert;
    property ParentBiDiMode;
    property ParentColor;
    property ParentCtl3D;
    property ParentDoubleBuffered;
    property ParentFont;
    property ParentShowHint;
    property PasswordChar;
    property PopupMenu;
    property ReadOnly;
    property ShowHint;
    property TabOrder;
    property TabStop;
    property Text;
    property TextHint;
    property Touch;
    property Visible;
    property OnChange;
    property OnClick;
    property OnContextPopup;
    property OnDblClick;
    property OnDragDrop;
    property OnDragOver;
    property OnEndDock;
    property OnEndDrag;
    property OnEnter;
    property OnExit;
    property OnGesture;
    property OnKeyDown;
    property OnKeyPress;
    property OnKeyUp;
    property OnMouseActivate;
    property OnMouseDown;
    property OnMouseEnter;
    property OnMouseLeave;
    property OnMouseMove;
    property OnMouseUp;
    property OnStartDock;
    property OnStartDrag;
  end;  

procedure Register;  

implementation  

procedure Register;
begin
  RegisterComponents('SkinEngine',[TSkinHintEdit]);
end;  

{ TSkinHintEdit }  

constructor TSkinHintEdit.Create(AOwner: TComponent);
begin
  FHelpText:='';
  //提示文字颜色
  FHelpTextFont:=TFont.Create;
  FHelpTextFont.Color:=CONST_DEF_HELPTEXT_FONTCOLOR;
  FHelpTextFont.Name:=CONST_DEF_HELPTEXT_FONTNAME;
  FHelpTextFont.Size:=CONST_DEF_HELPTEXT_FONTSIZE;
  FHelpTextFont.Style:=CONST_DEF_HELPTEXT_FONTSTYLE;
  inherited Create(AOwner);
end;  

destructor TSkinHintEdit.Destroy;
begin
  FreeAndNil(FHelpTextFont);
  inherited;
end;  

procedure TSkinHintEdit.SetHelpText(const Value: WideString);
begin
  FHelpText := Value;
end;  

procedure TSkinHintEdit.SetHelpTextFont(const Value: TFont);
begin
  FHelpTextFont.Assign(Value);
  Invalidate;
end;  

procedure TSkinHintEdit.WMPaint(var Message: TMessage);
var
  ClientDC:HDC;
  ClientCanvas:TCanvas;
  HelpTextDrawLeft,
  HelpTextDrawTop:Integer;
begin
  Inherited;
  if Not Focused and (Text='') and (HelpText<>'') then
  begin
    ClientDC:=GetDC(Handle);
    Try
      ClientCanvas:=TCanvas.Create;
      ClientCanvas.Handle:=ClientDC;
      ClientCanvas.Font.Assign(FHelpTextFont);
      HelpTextDrawLeft:=0;
      HelpTextDrawTop:=(Height-ClientCanvas.TextHeight(FHelpText) ) div 2-2;
      ClientCanvas.TextOut(HelpTextDrawLeft,HelpTextDrawTop,FHelpText);
    Finally
      ReleaseDC(Handle,ClientDC);
      ClientCanvas.Free;
    End;
  end;
end;  

end.