Question

I have a user control that shows a speed in a dial format (An image).

It has a single method: SetSpeed(int speed);

It then makes the dial move to the desired speed, from the last set speed. It does then in incriments. That is, it moves to the new speed.

Problem is, if the car is going 10km/h, and then goes (very quickly) to 100km/h, it takes the dial maybe 2 seconds to reach that speed… (It’s not 100% real time – which is realistic .. it moves to the new speed).

But, if, before the dial gets to 100km/h, the person slows to 50km/h, I need to interupt the movement to 100, and start moving to 50. Regardless of the current position (not speed) of the dial, I need to change the ‘target speed’.

My control is using a BackgroundWorker to handle the refreshing, and not lock the UI.

So, my call is very basic from the UI:

dial1.SetSpeed(int.Parse(value));

And then the user control does this:

BackgroundWorker bw = new BackgroundWorker();

public void SetSpeed(int speed)
        {
            while(bw.IsBusy)
            {
                Thread.Sleep(50);
            }
            bw.RunWorkerAsync(speed);
        }

And the method that does the work:

private void UpdateSpeed(object sender, DoWorkEventArgs e)
        {
            var oldSpeed = airSpeed;
            var newSpeed = (int) e.Argument;


            if(oldSpeed <= newSpeed)
            {
                for (int i = oldSpeed; i < newSpeed; i++)
                {
                    airSpeed++;
                    this.Invoke(new MethodInvoker(Refresh));
                }
            }
            else
            {
                for (int i = oldSpeed; i > newSpeed; i--)
                {
                    airSpeed--;
                    this.Invoke(new MethodInvoker(Refresh));

                }

            }
            airSpeed = newSpeed;
        }

It locks when I send it two values in quick succession…
How do I interrupt the thread, if it’s running, and change the value?

(Also, I think my code to change the speed is bad – can that be made neater?)

Answer 1

You do not. You handle it in a proper way, with locks, and checking whether the value needs changing. You do NOT interrupt it.

Basically, you need a lock area between SetSpeed and the Refresh method, so that one blocks the other. Then, when you set speed and the thread is currently in a critical area, it simply waits until the update is finished.

And your UpdateSpeed makes no sense – the change (airspeed– and airspeed++) should be timer driven… you currently change them in a “arbitrary” speed, depending on processor speed. No timing.