How to update GUI with backgroundworker?
C#WpfMultithreadingWinformsBackgroundworkerC# Problem Overview
I have spent the whole day trying to make my application use threads but with no luck. I have read much documentation about it and I still get lots of errors, so I hope you can help me.
I have one big time consuming method which calls the database and updates the GUI. This has to happen all the time(or about every 30 seconds).
public class UpdateController
{
private UserController _userController;
public UpdateController(LoginController loginController, UserController userController)
{
_userController = userController;
loginController.LoginEvent += Update;
}
public void Update()
{
BackgroundWorker backgroundWorker = new BackgroundWorker();
while(true)
{
backgroundWorker.DoWork += new DoWorkEventHandler(backgroundWorker_DoWork);
backgroundWorker.RunWorkerAsync();
}
}
public void backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
{
_userController.UpdateUsersOnMap();
}
}
With this approach I get an exception because the backgroundworker is not and STA thread(but from what I can understand this is what I should use). I have tried with a STA thread and that gave other errors.
I think the problem is because I try to update the GUI while doing the database call(in the background thread). I should only be doing the database call and then somehow it should switch back to the main thread. After the main thread has executed it should go back to the background thread and so on. But I can't see how to do that.
The application should update the GUI right after the database call. Firering events don't seem to work. The backgroundthread just enters them.
EDIT:
Some really great answers :) This is the new code:
public class UpdateController{
private UserController _userController;
private BackgroundWorker _backgroundWorker;
public UpdateController(LoginController loginController, UserController userController)
{
_userController = userController;
loginController.LoginEvent += Update;
_backgroundWorker = new BackgroundWorker();
_backgroundWorker.DoWork += backgroundWorker_DoWork;
_backgroundWorker.RunWorkerCompleted += backgroundWorker_RunWorkerCompleted;
}
public void _backgroundWorker_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
_userController.UpdateUsersOnMap();
}
public void Update()
{
_backgroundWorker.RunWorkerAsync();
}
void backgroundWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
//UI update
System.Threading.Thread.Sleep(10000);
Update();
}
public void backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
{
// Big database task
}
}
But how can I make this run every 10 second? System.Threading.Thread.Sleep(10000) will just make my GUI freeze and while(true) loop in Update() as suggested gives an exception(Thread too busy).
C# Solutions
Solution 1 - C#
You need to declare and configure the BackgroundWorker once - then Invoke the RunWorkerAsync method within your loop...
public class UpdateController
{
private UserController _userController;
private BackgroundWorker _backgroundWorker;
public UpdateController(LoginController loginController, UserController userController)
{
_userController = userController;
loginController.LoginEvent += Update;
_backgroundWorker = new BackgroundWorker();
_backgroundWorker.DoWork += new DoWorkEventHandler(backgroundWorker_DoWork);
_backgroundWorker.ProgressChanged += new ProgressChangedEventHandler(backgroundWorker_ProgressChanged);
_backgroundWorker.WorkerReportsProgress= true;
}
public void Update()
{
_backgroundWorker.RunWorkerAsync();
}
public void backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
{
while (true)
{
// Do the long-duration work here, and optionally
// send the update back to the UI thread...
int p = 0;// set your progress if appropriate
object param = "something"; // use this to pass any additional parameter back to the UI
_backgroundWorker.ReportProgress(p, param);
}
}
// This event handler updates the UI
private void backgroundWorker_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
// Update the UI here
// _userController.UpdateUsersOnMap();
}
}
Solution 2 - C#
You have to use the Control.InvokeRequired property to determine if you are on a background thread. Then you need to invoke your logic that modified your UI via the Control.Invoke method to force your UI operations to occur on the main thread. You do this by creating a delegate and passing it to the Control.Invoke method. The catch here is you need some object derived from Control to call these methods.
Edit: As another user posted, if yo you can wait to the BackgroundWorker.Completed event to update your UI then you can subscribe to that event and call your UI code directly. BackgroundWorker_Completed is called on the main app thread. my code assumes you want to do updates during the operation. One alternative to my method is to subscribe to the BwackgroundWorker.ProgressChanged event, but I believe you'll need to still call Invoke to update your UI in that case.
for example
public class UpdateController
{
private UserController _userController;
BackgroundWorker backgroundWorker = new BackgroundWorker();
public UpdateController(LoginController loginController, UserController userController)
{
_userController = userController;
loginController.LoginEvent += Update;
}
public void Update()
{
// The while loop was unecessary here
backgroundWorker.DoWork += new DoWorkEventHandler(backgroundWorker_DoWork);
backgroundWorker.RunWorkerAsync();
}
public delegate void DoUIWorkHandler();
public void backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
{
// You must check here if your are executing on a background thread.
// UI operations are only allowed on the main application thread
if (someControlOnMyForm.InvokeRequired)
{
// This is how you force your logic to be called on the main
// application thread
someControlOnMyForm.Invoke(new
DoUIWorkHandler(_userController.UpdateUsersOnMap);
}
else
{
_userController.UpdateUsersOnMap()
}
}
}
Solution 3 - C#
You should remove the while(true), you are adding infinite event handlers and invoking them infinite times.
Solution 4 - C#
You can use the RunWorkerCompleted event on the backgroundWorker class to define what should be done when the background task has completed. So you should do the database call in the DoWork handler, and then update the interface in the RunWorkerCompleted handler, something like this:
BackgroundWorker bgw = new BackgroundWorker();
bgw.DoWork += (o, e) => { longRunningTask(); }
bgw.RunWorkerCompleted += (o, e) => {
if(e.Error == null && !e.Cancelled)
{
_userController.UpdateUsersOnMap();
}
}
bgw.RunWorkerAsync();
Solution 5 - C#
In addition to previous comments, take a look at www.albahari.com/threading - best doc on threading you will ever find. It will teach you how to use the BackgroundWorker properly.
You should update the GUI when the BackgroundWorker fires Completed event (which is invoked on UI thread to make it easy for you, so that you don't have to do Control.Invoke yourself).
Solution 6 - C#
Here's a source code pattern you can use based on some WinForms example code, but you can apply it for WPF as well very easily. In this example, I am redirecting output to a Console which I then use to let the background worker write some messages to a textbox while it is processing.
It consists of:
- A helper class
TextBoxStreamWriterused to redirect console output to a textbox - A background worker writing to the redirected console
- A progress bar which needs to be reset after completion of background worker
- Some text boxes (txtPath and txtResult), and a "Start" button
In other words, there is some background task which needs to interact with the UI. Now I am going to show how that is done.
From the context of the background task, you need to use Invoke to access any UI element. I believe the simplest way to do that is to use lambda expression syntax, like
progressBar1.Invoke((Action) (() =>
{ // inside this context, you can safely access the control
progressBar1.Style = ProgressBarStyle.Continuous;
}));
To update the ProgressBar, a local method like
private void UpdateProgress(int value)
{
progressBar1.Invoke((Action)(() => { progressBar1.Value = value; }));
}
helps. It is passing the value parameter to the progress bar as a closure.
This is the helper class TextBoxStreamWriter, which is used to redirect console output:
public class TextBoxStreamWriter : TextWriter
{
TextBox _output = null;
public TextBoxStreamWriter(TextBox output)
{
_output = output;
}
public override void WriteLine(string value)
{
// When character data is written, append it to the text box.
// using Invoke so it works in a different thread as well
_output.Invoke((Action)(() => _output.AppendText(value+"\r\n")));
}
}
You need to use it in the form load event as follows (where txtResult is a textbox, to which the output will be redirected):
private void Form1_Load(object sender, EventArgs e)
{
// Instantiate the writer and redirect the console out
var _writer = new TextBoxStreamWriter(txtResult);
Console.SetOut(_writer);
}
There is also a button on the form which starts the background worker, it passes a path to it:
private void btnStart_Click(object sender, EventArgs e)
{
backgroundWorker1.RunWorkerAsync(txtPath.Text);
}
This is the workload of the background worker, note how it uses the console to output messages to the textbox (because of the redirection that was set up earlier):
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
var selectedPath = e.Argument as string;
Console.Out.WriteLine("Processing Path:"+selectedPath);
// ...
}
The variable selectedPath consists of the path that was passed to the backgroundWorker1 earlier via the parameter txtPath.Text, it is being accessed via e.Argument.
If you need to reset some controls afterwards, do it in the following way (as already mentioned above):
private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
progressBar1.Invoke((Action) (() =>
{
progressBar1.MarqueeAnimationSpeed = 0;
progressBar1.Style = ProgressBarStyle.Continuous;
}));
}
In this example, after completion, a progress bar is being reset.
Important: Whenever you access a GUI control, use Invoke as I did in the examples above.
Using Lambda's makes it easy, as you could see in the code.
And here's the complete example, which runs in LinqPad 6 (just copy and paste it into an empty C# Program query) - I decided to use LinqPad this time so you can learn something new, because you all know how to create a new Windows Forms project in Visual Studio (and if you still want to do so, just copy the events below and drag and drop the controls to the form):
// see: https://stackoverflow.com/a/27566468/1016343
using System.ComponentModel;
using System.Windows.Forms;
BackgroundWorker backgroundWorker1 = new System.ComponentModel.BackgroundWorker();
ProgressBar progressBar1 = new ProgressBar() { Text = "Progress", Width = 250, Height=20, Top=10, Left=0 };
TextBox txtPath = new TextBox() { Text =@"C:\temp\", Width = 100, Height=20, Top=30, Left=0 };
TextBox txtResult = new TextBox() { Text = "", Width = 200, Height=250, Top=70, Left=0, Multiline=true, Enabled=false };
Button btnStart = new Button() { Text = "Start", Width = 100, Height=30, Top=320, Left=0 };
void Main()
{
// see: https://www.linqpad.net/CustomVisualizers.aspx
// Instantiate the writer and redirect the console out
var _writer = new TextBoxStreamWriter(txtResult);
Console.SetOut(_writer);
// wire up events
btnStart.Click += (object sender, EventArgs e) => btnStart_Click(sender, e);
backgroundWorker1.DoWork += (object sender, DoWorkEventArgs e) => backgroundWorker1_DoWork(sender, e);
backgroundWorker1.RunWorkerCompleted += (object sender, RunWorkerCompletedEventArgs e)
=> backgroundWorker1_RunWorkerCompleted(sender, e);
using var frm = new Form() {Text="Form", Width = 300, Height=400, Top=0, Left=0};
frm.Controls.Add(progressBar1);
frm.Controls.Add(txtPath);
frm.Controls.Add(txtResult);
frm.Controls.Add(btnStart);
// display controls
frm.ShowDialog();
}
private void btnStart_Click(object sender, EventArgs e)
{
backgroundWorker1.RunWorkerAsync(txtPath.Text);
}
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
InitProgress();
var selectedPath = e.Argument as string;
Console.Out.WriteLine("Processing Path: " + selectedPath);
UpdateProgress(0); Thread.Sleep(300); UpdateProgress(30); Thread.Sleep(300);
UpdateProgress(50); Thread.Sleep(300);
Console.Out.WriteLine("Done.");
// ...
}
private void UpdateProgress(int value)
{
progressBar1.Invoke((Action)(() =>
{
progressBar1.Value = value;
}));
}
private void InitProgress()
{
progressBar1.Invoke((Action)(() =>
{
progressBar1.MarqueeAnimationSpeed = 0;
progressBar1.Style = ProgressBarStyle.Continuous;
}));
}
private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
UpdateProgress(100); // always show 100% when done
}
// You can define other methods, fields, classes and namespaces here
public class TextBoxStreamWriter : TextWriter
{
TextBox _output = null;
public TextBoxStreamWriter(TextBox output)
{
_output = output;
}
public override Encoding Encoding => throw new NotImplementedException();
public override void WriteLine(string value)
{
// When character data is written, append it to the text box.
// using Invoke so it works in a different thread as well
_output.Invoke((Action)(() => _output.AppendText(value + "\r\n")));
}
}
Solution 7 - C#
The if-statement in @Lee's answer should look like:
bgw.RunWorkerCompleted += (o, e) => {
if(e.Error == null && !e.Cancelled)
{
_userController.UpdateUsersOnMap();
}
}
...if you want to invoke UpdateUsersOnMap(); when there are no errors and BgWorker hasn't been cancelled.