Clear button pressed (and other GUI events) queue
lazarus / 62-General

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richardcgiroux · 2017-11-13 16:31

I have a program that takes some time to complete an action after a
button press.  I run into a problem is the user keeps pressing the
button, queuing additional button presses.  Is there a way that I can
prevent any "queuing" of button presses until the present action is
completed?

This is an issue throughout the entire program, not just with one
button.

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balazsszekely · 2017-11-13 16:39

Try this:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  TForm1 =
class(TForm)    Button1: TButton;    procedure Button1Click(Sender:
TObject);  private    FIsBusy: Boolean;  public  
end; //.... procedure TForm1.Button1Click(Sender: TObject);begin  if
FIsBusy then    Exit;   FIsBusy := True;  try          //lengthy
operation  finally    FIsBusy := False;  end;end;               

You can also disable the button as a visual feedback.

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Thaddy · 2017-11-13 16:57

That is correct:
I also would suggest to give the impatient user some feedback, under
windows normally an hourglass cursor. Linux seems to mimic that...
And check your design...

Anyway: disable the possibility. I would disable the button during
operations AND change the cursor.

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mse · 2017-11-13 17:10

MSEgui has application.beginwait()/application.endwait() for that
purpose. After calling application.beginwait() the mouse pointer shape
becomes cr_wait and mouse- and key-events will be skipped in
application.processmessages(). application.waitescaped() returns true
if the ESC-key has been pressed so one can cancel long running
operations. Maybe Lazarus provides a similar mechanism?

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howardpc · 2017-11-13 17:13

Putting thaddy's suggestion into code gives
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.Button1Click(Sender: TObject);var  b: TButton absolute Sender; 
oldCursor: TCursor;begin  if Sender is TButton then begin   
WriteLn('TForm1.Button1Click');    oldCursor:=Screen.Cursor;   
b.Enabled:=False;    Screen.Cursor:=crHourGlass;    try      // some
lengthy operation    finally      b.Enabled:=True;     
Screen.Cursor:=oldCursor;    end;  end;end;

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balazsszekely · 2017-11-13 17:50

@howardpc
That code(without Application.ProcessMessages) can byte back,
especially under windows. Please try the following: declare a private
variable FCounter then press the button 3-4 times in less then five
seconds. It clearly shows the button wasn't disabled in the first
place.
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.Button1Click(Sender: TObject);var  b: TButton absolute Sender; 
oldCursor: TCursor;begin  if Sender is TButton then begin   
oldCursor:=Screen.Cursor;    b.Enabled:=False;   
Screen.Cursor:=crHourGlass;    try      Sleep(5000);     
Inc(FCounter);      Form1.Caption := IntToStr(FCounter);        //
some lengthy operation    finally      b.Enabled:=True;     
Screen.Cursor:=oldCursor;    end;  end;end;

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Thaddy · 2017-11-13 18:09

Yup, because of messaging (enable / disable are messages). Better to
eat the message queue: Application.processmessages just after
disabled. Not ideal.

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engkin · 2017-11-13 18:22

As Thaddy had pointed out earlier:
Quote from: Thaddy on November 13, 2017, 05:57:06 pmcheck your
design...
The main thread is meant to handle GUI activities. Why would you
assign an activity "that takes some time to complete an action after a
button press" to the GUI thread? This activity should be assigned to a
thread.

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howardpc · 2017-11-13 19:23

@GetMem
You're right of course.
An Application.ProcessMessages is needed after the .Enabled:=False

I enjoyed the idea of code "byting" back ... very Pascalish (as
opposed to biting back)!

engkin's caution is also relevant.

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balazsszekely · 2017-11-13 20:11

@engkin
QuoteThe main thread is meant to handle GUI activities. Why would you
assign an activity "that takes some time to complete an action after a
button press" to the GUI thread? This activity should be assigned to a
thread.
Unfortunately is not that simple. Sometimes opening a big report or
connecting to a remote server can take 3-4 seconds, more then enough
for a "clumsy" user to press the button five times. Even if you move a
lengthy operation to a worker thread the question still remains, how
you prevent the user to create more then one thread. In my opinion the
OP's question is a valid one.

@howardpc
QuoteI enjoyed the idea of code "byting" back ... very Pascalish (as
opposed to biting back)!
My limited English knowledge bites back.   Thank you for pointing me
out the mistake.

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Mike.Cornflake · 2017-11-13 21:18

Quote from: GetMem on November 13, 2017, 09:11:36 pm...In my opinion
the OP's question is a valid one
Aye, this very issue caused me no end of pain for many years. 
Intermittent issues reported by end users that I just couldn't track
down (who double clicks a button??).  Once I stumbled onto what was
happening, I slapped code very similar to your FIsBusy all over the
place (though I used .Tag)

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Josh · 2017-11-13 21:43

It maybe beneficial to change the button caption to say 'Busy...' or
'Please wait...'; while the button action in progress..

Code: [Select]procedure TForm1.Button1Click(Sender: TObject);
var
  b: TButton absolute Sender;
  oldCursor: TCursor;
  oldCaption:AnsiString='';
begin
  if Sender is TButton then
  begin
    oldCursor:=Screen.Cursor;
    oldCaption:=b.caption;
    b.Caption:='Working';
    b.Enabled:=False;
    Screen.Cursor:=crHourGlass;
    Application.ProcessMessages;
    try
      Sleep(5000);
      Inc(FCounter);
      Form1.Caption := IntToStr(FCounter);
        // some lengthy operation
    finally
      b.Enabled:=True;
      b.caption:=oldCaption;
      Screen.Cursor:=oldCursor;
      Application.Processmessages;
    end;
  end;
end;

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Josh · 2017-11-13 23:02

if user was wanting to enable/disable  a single control/ panel or form
then I thought this little routine may help, I have not fully tested
but so far so good.
Code: [Select]procedure tForm1.Controls_Enabler(Control:
TControl;aValue:Boolean);
var i: integer;
    WinControl: TWinControl;

begin
  with Control do
  begin
    Control.enabled:=aValue;
    if Control is TWinControl then
    begin
      WinControl := TWinControl(Control);
      If WinControl is TPanel Then
      begin
        for i := 0 to WinControl.ControlCount - 1 do
Controls_Enabler(WinControl.Controls[i],AValue);
      end;
    end;
  end;
  application.ProcessMessages;
end;                  

Also a zip of test application showing its use.

usage Controls_Enabler(control,*required state*);
if control is a panel it goes through controls on panel to do set all
controls to required state; same for a form.

Caution if used on a form all controls are effected ie, timers etc, so
some tweeking for user case would be needed; Can't reaaly see use on
form basis; but tested ok.

Maybe overkill

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engkin · 2017-11-14 01:57

@GetMem
Quote from: GetMem on November 13, 2017, 09:11:36 pmEven if you move a
lengthy operation to a worker thread the question still remains, how
you prevent the user to create more then one thread. In my opinion the
OP's question is a valid one.

I really hate disabled controls, but here you go:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.Button1Click(Sender: TObject);var  Btn: TButton absolute
Sender;begin  Btn.Enabled := False;   Sleep(1000);  inc(FCounter); 
Caption := IntToStr(FCounter);  
Application.QueueAsyncCall(@EnableBtn, PtrInt(Btn));end; procedure
TForm1.EnableBtn(Data: PtrInt);var  Btn: TButton absolute Data;begin 
Btn.Enabled := True;end;
While when using threads where only one is allowed:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.Button1Click(Sender: TObject);begin  if Assigned(LT) then   
exit;   LT := TLengthyThread.Create...

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jamie · 2017-11-19 02:56

I do things like this..
//Global space//
Var
 ExternalStopFlag :Boolean;

Procedure LongCode;
Const Busy :Boolean = false;
begin
 If Busy Then exit else Busy := true;

 While (Not ExternalSTopFlag do
  Begin
    Application.ProcessMessages;
    /// Do the Long code here.
   /// When Done with long Loop: ExternalStopFlag := true;
  End;
ExternalstopFlag := False;
Busy := False;
End;

Just an idea ...