How to make one shot timer function in Delphi (like setTimeout in JavaScript)?

delphi

Solution

I think you may leave the `TTimer` as it is and try to use the `SetTimer` function and use its callback function. You need to store the timer IDs and their (anonymous) methods in some collection. Since you didn't mentioned your Delphi version I've used a simple classes and `TObjectList` as a collection.

The principle is easy, you just call the `SetTimer` function with the callback function specified and store the new instantiated system timer ID with the anonymous method into the collection. When that callback function is performed, find the timer which caused that callback in the collection by its ID, kill it, execute the anonymous method and delete it from the collection. Here is the sample code:

unit Unit1;

interface

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

type
  TOnTimerProc = reference to procedure;
  TOneShotTimer = class
    ID: UINT_PTR;
    Proc: TOnTimerProc;
  end;
  procedure SetTimeout(AProc: TOnTimerProc; ATimeout: Cardinal);

type
  TForm1 = class(TForm)
    Timer1: TTimer;
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;
  TimerList: TObjectList;

implementation

{$R *.dfm}

procedure TimerProc(hwnd: HWND; uMsg: UINT; idEvent: UINT_PTR;
  dwTime: DWORD); stdcall;
var
  I: Integer;
  Timer: TOneShotTimer;
begin
  for I := 0 to TimerList.Count - 1 do
  begin
    Timer := TOneShotTimer(TimerList[I]);
    if Timer.ID = idEvent then
    begin
      KillTimer(0, idEvent);
      Timer.Proc();
      TimerList.Delete(I);
      Break;
    end;
  end;
end;

procedure SetTimeout(AProc: TOnTimerProc; ATimeout: Cardinal);
var
  Timer: TOneShotTimer;
begin
  Timer := TOneShotTimer.Create;
  Timer.ID := SetTimer(0, 0, ATimeout, @TimerProc);
  Timer.Proc := AProc;
  TimerList.Add(Timer);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  SetTimeout(procedure
    begin
      ShowMessage('OnTimer');
    end,
    1000
  );
end;

initialization
  TimerList := TObjectList.Create;
  TimerList.OwnsObjects := True;

finalization
  TimerList.Free;

end.

Simplified version (Delphi 2009 up):

Like suggested by @David's comment, here is the same code as above, just in a separate unit with the use of generics dictionary. Usage of the `SetTimeout` from this unit is same as in the above code:

unit OneShotTimer;

interface

uses
  Windows, Generics.Collections;

type
  TOnTimerProc = reference to procedure;
  procedure SetTimeout(AProc: TOnTimerProc; ATimeout: Cardinal);

var
  TimerList: TDictionary<UINT_PTR, TOnTimerProc>;

implementation

procedure TimerProc(hwnd: HWND; uMsg: UINT; idEvent: UINT_PTR;
  dwTime: DWORD); stdcall;
var
  Proc: TOnTimerProc;
begin
  if TimerList.TryGetValue(idEvent, Proc) then
  try
    KillTimer(0, idEvent);
    Proc();
  finally
    TimerList.Remove(idEvent);
  end;
end;

procedure SetTimeout(AProc: TOnTimerProc; ATimeout: Cardinal);
begin
  TimerList.Add(SetTimer(0, 0, ATimeout, @TimerProc), AProc);
end;

initialization
  TimerList := TDictionary<UINT_PTR, TOnTimerProc>.Create;
finalization
  TimerList.Free;

end.

Problem

Original source