Patrick Leblanc’s Blog

 
namespace bgGUI
{
 
using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Threading;
 
    public partial class Form1 : Form
    {
        delegate void setTextBoxTextCallback(ArrayList a);
        delegate void setLabelTextCallback(string s, Label l); //delegate used to set the text on a label in a Invoke operation
        ArrayList videoSegments; //Imagine this as a collection video/audio segments we need to compute.
        int workingIndex; // This is to track where we're at in the videoSegments during the process.
        object lockObject = new object(); // the lock object used for the thread to safely pick a new segment to compute.
 
        public Form1()
        {
            InitializeComponent();
        }
 
        private void Form1_Load(object sender, EventArgs e)
        {
            //Here we're gonna fake that the stack of videoSegements is full of 10 slice to encode.
            videoSegments = new ArrayList();
 
            for (int i = 0; i < 20; i++)
            {
                videoSegments.Add("this is segement " + i);//for the purpose of the demonstration I'm adding a string and its index.
            }
 
            workingIndex = videoSegments.Count - 1;
            label5.Text = videoSegments.Count.ToString();
 
            //We're starting 4 different thread with parameters (hence the use of the ParameterizedThreadStart Delegate)
            //Becareful though as ParameterizedThreadStart only takes method with an objet argument. Therefore it's not type safe.
            //an alternative, type safe way of doing it is to create an helper class. and start it with ThreadStart
            //more details here: http://msdn.microsoft.com/en-us/library/ts553s52.aspx
            //In my example, I pass as arguments a Label and an int, a time interval in ms.
            Thread t1 = new Thread(new ParameterizedThreadStart(tsChangeLabelText));
            t1.Start(new object[] { label1, 2500 });
 
            Thread t2 = new Thread(new ParameterizedThreadStart(tsChangeLabelText));
            t2.Start(new object[] { label2, 3500 });
 
            Thread t3 = new Thread(new ParameterizedThreadStart(tsChangeLabelText));
            t3.Start(new object[] { label3, 1000 });
 
            Thread t4 = new Thread(new ParameterizedThreadStart(tsChangeLabelText));
            t4.Start(new object[] { label4, 1250 });
 
        }
 
        private void tsChangeLabelText(object o)
        {
 
            //we cast the object as an array.
            //then we exatract the first object at index[x], then we cast it to what we want
            //as i said earlier, it's not type safe. use it with caution!
            Label l = (Label)((object[])o)[0];
            int ms = (int)((object[])o)[1];// the ms is just to simulate heavy load on the thread
 
            int insideIndex = 0; // This will be the index we're going to compute
 
            while (workingIndex >= 0)// the thread will go until all the segments have been computed
            {
                lock (lockObject) // we request a lock on the object.
                {
                    //We have to check again if there are segments remaining!
                    //indeed, another thread could have snatched the last one between our while and this lock!
 
                    if (workingIndex > -1)
                    {
                        insideIndex = workingIndex;//We're going to compute the segment located at this index in our collection;
                        workingIndex--;//The other threads will be able to go to the next one after the end of the lock
                    }
                }// now all other threads (those waiting for permission to use the lock)
                // will be able to check what the working index is and pick the next segment.
 
                //we display infos on the GUI using invoke since it's not the owner thread.
                setLabelTextCallback d = new setLabelTextCallback(setLabelText);
                setTextBoxTextCallback z = new setTextBoxTextCallback(setTextBoxText);
                this.Invoke(d, new object[] { "Working on segment #" + insideIndex, l });
 
                //---------------------------
                Thread.Sleep(ms); // simulate the heavy load of the Encoding
                //---------------------------
 
                this.Invoke(d, new object[] { "DONE WITH segment #" + insideIndex, l });
 
                videoSegments[insideIndex] = (string)videoSegments[insideIndex] + " - COMPUTED"; // let's say that we replaced the old segement with the new one
                this.Invoke(z, new object[] { videoSegments });
                this.Invoke(d, new object[] { (workingIndex + 1).ToString(), label5 });
 
                Thread.Sleep(3000);//this is only for you to see what's going on
 
            }
 
        }
 
        private void setLabelText(string s, Label l)
        {
            l.Text = s.ToString();
        }
        private void setTextBoxText(ArrayList a)
        {
            //to check the order of the ouput
            textBox1.Text = "";
            for (int i = 0; i < a.Count; i++)
            {
                textBox1.Text += a[i].ToString() + Environment.NewLine;
            }
 
        }
 
    }
}