BeginPaint和GetDC的区别

BeginPaint和GetDC的区别

unit Unit1;

interface

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

type
  TForm1 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  protected
    procedure WndProc(var Message:TMessage); override;
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

{ TForm1 }

procedure TForm1.WndProc(var Message: TMessage);
var
  DC: HDC;
  Str: String;
  PS: tagPaintStruct;
begin
  if Message.Msg = WM_PAINT then
  begin
    Str := 'Hello Ming.';
    DC := BeginPaint(self.Handle,PS);
    TextOut(DC,0,0,PChar(Str),Length(Str));
    EndPaint(Self.Handle,PS);
  end;
  inherited;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  DC: HDC;
  Str: String;
  PS: tagPaintStruct;
begin
  Str := 'Hello Ming.';
  DC := GetWindowDC(Self.Handle);//GetDC(0); 0:Desktop
  TextOut(DC,0,0,PChar(Str),Length(Str));
  ReleaseDC(Self.Handle,DC);//ReleaseDC(0,DC); 0:Desktop
  { TODO :
  下面的代码并不能实现输出,BeginPaint,EndPaint只能在窗体过程中的WM_PAINT中使用.
  BeginPaint()和EndPaint()可以删除消息队列中的WM_PAINT消息,并使无效区域有效.
  GetDC()和ReleaseDC()并不删除也不能使无效区域有效,因此当程序跳出WM_PAINT 时,
  无效区域仍然存在.系统就回不断发送WM_PAINT消息,于是程序不断处理WM_PAINT消息.}
  {
  DC := BeginPaint(self.Handle,PS);
  TextOut(DC,0,0,PChar(Str),Length(Str));
  EndPaint(Self.Handle,PS);
  }
end;

end.