Delphi - reference to (匿名方法)

Delphi 2009 可以使用匿名方法了(使用 reference 定义方法类型, 然后在代码中随用随写方法)

unit Unit1;

interface

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

type
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

Type
  TFun = reference to function(const num: Integer): Integer; {用 reference 定义匿名方法类型}

procedure TForm1.FormCreate(Sender: TObject);
var
  fun: TFun;
  n: Integer;
begin
  {求平方}
  fun := function(const a: Integer): Integer {注意本行最后不能有 ; 号}
  begin
    Result := a * a;
  end;

  n := fun(9);
  ShowMessage(IntToStr(n)); {81}

  {求倍数}
  fun := function(const a: Integer): Integer
  begin
    Result := a + a;
  end;

  n := fun(9);
  ShowMessage(IntToStr(n)); {18}
end;

end.

把匿名方法当作其他方法的参数:

unit Unit1;

interface

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

type
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

Type
  TFun = reference to function(const num: Integer): Integer;

  function FunTest(const n: Integer; fun: TFun): string;
  begin
    Result := Format('%d, %d', [n, fun(n)]);
  end;

procedure TForm1.FormCreate(Sender: TObject);
var
  f: TFun;
  s: string;
begin
  f := function(const a: Integer): Integer {注意本行最后不能有 ; 号}
  begin
    Result := a * a;
  end;

  s := FunTest(9, f);

  ShowMessage(s); {9, 81}
end;

end.

之前我们可以定义方法类型, 然后通过方法类型的变量来使用方法, 譬如:

unit Unit1;

interface

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

type
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

Type
  TFun = function(const num: Integer): Integer; {先定义一个方法类型}

  function MySqr(const num: Integer): Integer;  {再创建一个吻合上面类型的一个方法}
  begin
    Result := num * num;
  end;

{测试}
procedure TForm1.FormCreate(Sender: TObject);
var
  fun: TFun;  {方法变量}
  n: Integer;
begin
  fun := MySqr;             {给变量赋值为相同格式的方法}
  n := fun(9);              {现在这个方法变量可以使用了}
  ShowMessage(IntToStr(n)); {81}
end;

end.

之所以这样做, 是因为有时需要把 "方法" 当作参数, 譬如:

unit Unit1;

interface

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

type
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

Type
  TFun = function(const num: Integer): Integer; {先定义一个方法类型}

  function MySqr(const num: Integer): Integer;  {再创建一个吻合上面类型的一个方法}
  begin
    Result := num * num;
  end;

  {把方法当作参数的方法}
  procedure MyProc(var x: Integer; fun: TFun);
  begin
    x := fun(x);
  end;

{测试}
procedure TForm1.FormCreate(Sender: TObject);
var
  n: Integer;
begin
  n := 9;
  MyProc(n, MySqr);
  ShowMessage(IntToStr(n)); {81}
end;

end.

​