在TEdit中显示水印提示

unit MyEdit;

interface

uses Messages, Classes, StdCtrls, Controls, Graphics;

type
  TMyEdit = class(TEdit)
  private
    FCanvas: TControlCanvas;
    FWatermarkHint: string;
  protected
    procedure WMPaint(var Message: TWMPaint); message WM_PAINT;
    procedure CMExit(var Message: TCMExit); message CM_EXIT;
  public
    constructor Create(AOwner: TComponent); override;
    destructor Destroy; override;
  published
    property WatermarkHint: string read FWatermarkHint write FWatermarkHint;
  end;

implementation

{ TMyEdit }

procedure TMyEdit.CMExit(var Message: TCMExit);
begin
  inherited;
  Perform(WM_PAINT, 0, 0);
end;

constructor TMyEdit.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  FCanvas := TControlCanvas.Create;
  FCanvas.Control := Self;
end;

destructor TMyEdit.Destroy;
begin
  FCanvas.Free;
  inherited;
end;

procedure TMyEdit.WMPaint(var Message: TWMPaint);
begin
  inherited;
  if (not Focused) and (Self.Text = '') and (FWatermarkHint <> '') then
  begin
    FCanvas.Font := Self.Font;
    FCanvas.Font.Color := clGrayText;
    FCanvas.TextRect(Self.ClientRect, 1, 1, FWatermarkHint);
  end;
end;

end.