How to pause program execution until button press?

Question

I've got an algorithm. I'd like to pause it at some point and then continue once the user presses a button. How do I do that? I've browsed the documentation, and searched the internet, but no luck yet.

Here's a relevant code snip:

      if A[i]>A[i+1]
        then
          begin
            Zameni(i,i+1);
            done:=true;

            sleep(pauza);

            br:=br+1;
          end;

Right now, I use sleep (pauza is just a constant, means pause in Serbian). Ideally, I'd like to replace that line with a procedure which would sleep for the interval, or wait for a button press based on a configuration setting.

EDIT1: Ah yes, if it wasn't obvious - it's a graphics application, not console, so slapping a "readln" won't work (sadly).

Answer 1

Just use ReadLn;

 ShowConsole("Enter to continue.");
 ReadLn;

Answer 2

That's not how you do event-driven programming. You should do the processing up to the pause point and end your function at that point. In a button press event handler you then do the remainder of the processing - this will only happen when the user presses the button. You can call the same code from an OnTimer event to continue the processing after a given peruiod.

Answer 3

You could launch a dialog which is what we do, a simple message box would be OK, i.e.

  procedure TForm1.Pause1Click(Sender: TObject);
  begin
    // A dialog box will halt thread execution
    Windows.MessageBox(0, 'Paused ...' + sLineBreak + 'Press Enter to Continue',
      '', MB_OK);
  end;

This probably only halts the current thread though, if you have more than one thread, you may want to try something more involved.

Answer 4

Try this

   if A[i]>A[i+1]
        then
          begin
            Zameni(i,i+1);
            done:=true;
            MessageDlg('Pause',mtConfirmation,mbYesNo,0);
            //sleep(pauza);
            br:=br+1;
          end;

Answer 5

I wouldn't recommend it as good application design, but if none of the other suggestions are suitable for you, you may prefer this.

Add a 'Paused: Boolean' field to your class/form, and a 'Continue' button.

When you start the operation, set Paused to False, and Continue.Enabled := False;

When your code reaches the section where you want to pause:

Paused := True;
Continue.Enabled := True;
while Paused do
begin
  sleep(100);
  Application.ProcessMessages;
end;

In your Continue buttons event handler:

procedure Form1.ContinueClick(Sender: TObject);
begin
  Paused := False;
  Continue.Enabled := False;
end;

As I said before, not pretty, but it should get the job done.

Answer 6

Store i in a static variable. Inside the loop add a check for a Pause flag (set by your pause button). If the flag is true, exit the function. When the pause button is clicked again call the function again. Check i's value, if it's already > 0 just enter the loop where you left off.

When the loop completes successfully, reset i to 0;

Answer 7

Use a thread and the built-in suspend/resume functions for the TThread class. Jens Borrisholt wrote a nice little example article on about.com here doing this kind of thing.

If you introduce multiple ProcessMessage locations in the application, you can have problems with code execution from other events being left running when the form closes. For instance, if the form was closed while in the ProcessMessage "sleep" mode, there is no good way to unwind the stack.

MessageDialog may not work if you are using DirectX/OpenGL since you won't have the normal display open. Calling MessageDialog may switch the view from the DirectX view to normal desktop view (ugly but workable) OR it may show the message on the normal desktop view without changing the display mode from DirectX (making it impossible for the user to see the prompt). It's been a long time since I've worked with Dx, but I recall issues about failing to change view modes being a pain.

Edit: As Alexander points out, you shouldn't suspend the thread from anywhere but IN the thread (i.e., Thread A can suspend Thread A but not Thread B). If you want the GUI thread to suspend the sorting thread, send the sorting thread a message and handle the suspend inside the sorting thread. His link contains a good discussion on why this is so.

Answer 8

While I agree with thread approach, I think that using suspend/resume is a very bad idea. Here is why: http://blogs.msdn.com/oldnewthing/archive/2003/12/09/55988.aspx

Create a thread and use event to pause/resume thread. Simple. BTW, you can also use something like AsyncCalls to make thread creation easier (you don't need a TThread class).

Creating a separate thread is good for another reason: you could do heavy-computations in background, so they will not interfere with UI. Your app will have smooth reaction.