I have a question about using external c++ library (irrKlang.dll) which is an audio playback engine. Now, the problem is that when I get a SoundStopped event out of it, and do an action in the main form, all kinds of stack related errors arise. Let me show the code:
namespace WindowsFormsApplication4
{
public class IsoundFinished : ISoundStopEventReceiver
{
public delegate void OnSoundStoppedEventHandler(object source, EventArgs e);
public event OnSoundStoppedEventHandler IStopped;
public void OnSoundStopped(ISound iSound, StopEventCause reason, object userData)
{
if (reason.ToString() == "SoundFinishedPlaying")
IStopped?.Invoke(this, EventArgs.Empty);
}
}
}
That is an extended class for me to do custom actions (for example - if sound finished, raise the event...) I am creating an instance of it, for the event action to get exposed in my main Form1 class:
IsoundFinished iStopReceiver = new IsoundFinished();
Now in my main form, I have this line in my Form1() method, just under my InitializeComponent():
iStopReceiver.IStopped += new soundFinished.OnSoundStoppedEventHandler(OnStopped);
It's for subscribing to the event handler. And finally - my OnStopped() method which is supposed to do stuff when the song ends it's playback - it's on the same Form1:
private void OnStopped(object sender, EventArgs e)
{
if (InvokeRequired)
{
Invoke(new Action<object, EventArgs>(OnStopped), sender, e);
return;
}
btnStop1.PerformClick();
}
My Stop1 button method is (for those who work with the IrrKlang) ISound.Stop(); and few more lines of code, dealing with the display of playlist and so on. Although I have invoked it from the main UI thread - which should provide me with some degree of thread misalignment protection, all kinds of errors appear, mostly
Cannot evaluate expression because a native frame is on the top of the call stack.
Of course, if I do it without event handler, ISound.Stop(); drops the sound from the engine, like it should. I know something wrong is happening with the threads, but I can't figure out what's going on. If someone would give me few tips, I'd appreciate that a lot.
Well it seems I've solved it myself ! It's all about understanding how the threads are working in Visual C#. The problem was this : I was actually PAUSING the background thread where my audioengine was triggering the event - so 'till I performed an action after INVOKE in the main UI thread, background thread was paused along with the whole irrKlang engine. It was unable to purge itself properly, so it's call stack got clogged!
Using BEGININVOKE solved the problem, as it doesn't PAUSE the background task. It lets it run instead. Diagram on this answer gave me much needed piece of info I was looking for.
Maybe someone will need this answer too, glad I helped myself :P
private void OnStopped(object sender, EventArgs e)
{
if (InvokeRequired)
{
BeginInvoke(new Action<object, EventArgs>(OnStopped), sender, e);
return;
}
btnStop1.PerformClick();
}
Related
tl:dr at the bottom. I am taking a Java course on Udemy and our instructor is demonstrating a situation where background threads can interfere with each other and crash the program. In this case pushing either of the two buttons initiates a method that starts a background Thread, waits, then updates a label. He explains it something along the lines as the threads overwriting each other causing the program to crash with an IllegalStateException. What I don't understand is Threads overwriting each other is not uncommon and just makes the program behave in ways not intended and doesn't usually completely crash the program.
In this case the threads are both trying to change the label to the same String.
Why is this throwing an IllegalStateException instead of just causing the usual interference? After all more than one thread trying to update an object doesn't necessarily crash the program.
tl:dr Why does more than one thread modifying a Label object throw an IllegalStateException in this case but in other multi-threaded programs you just get the usual thread interference? The method of interest is the onButtonClicked() method.
I have tried catching the IllegalStateException to see if I can call Thread.currentThread().getName() but the catch block(both of them actually) seem to get ignored.
public class Controller {
#FXML
private TextField nameField;
#FXML
private Button helloButton;
#FXML
private Button byeButton;
#FXML
private CheckBox ourCheckBox;
#FXML
private Label ourLabel;
#FXML
public void initialize() {
helloButton.setDisable(true);
byeButton.setDisable(true);
}
#FXML
public void onButtonClicked(ActionEvent event) {
if (event.getSource().equals(helloButton)) {
System.out.println("hello, " + nameField.getText());
} else if (event.getSource().equals(byeButton)) {
System.out.println("Bye, " + nameField.getText());
}
Runnable task = new Runnable() {
#Override
public void run() {
try {
Thread.sleep(5000);
ourLabel.setText("We did something");
} catch (InterruptedException e) {
System.out.println("Interuppted Exception " + e.getMessage());
} catch (IllegalStateException e) {
System.out.println("Illegal State Exception: " + e.getMessage() + " "
+ Thread.currentThread().getName());
}
}
};
new Thread(task).start();
if (ourCheckBox.isSelected()) {
nameField.clear();
byeButton.setDisable(true);
helloButton.setDisable(true);
}
}
#FXML
public void handleKeyReleased() {
String text = nameField.getText();
boolean disableButtons = text.isEmpty() || text.trim().isEmpty();
byeButton.setDisable(disableButtons);
helloButton.setDisable(disableButtons);
}
#FXML
public void handleChange() {
System.out.println("The checkbox is " +
(ourCheckBox.isSelected() ? "checked" : "not checked"));
}
}
Updating active scene graph elements (such as label text) in your own threads has the potential to cause a race condition (please read and understand the Software section of the link) in the internal state of the JavaFX system. The results of which are unpredictable and a corruption or ”crash” of the JavaFX system cannot be ruled out. Perhaps nothing bad will happen, but perhaps something bad will.
Some calls to JavaFX APIs will detect when potential race condition may occur (when they are being invoked by a thread which is not the JavaFX thread), and fail fast by throwing an IllegalStateException. Other JavaFX APIs may not bother to check the calling thread and may not throw an IllegalStateException, thus allowing the potential race condition to occur. Either way, the users calling code is wrong, it should not be trying to modify the active scene graph off of the JavaFX application thread.
JavaFX code which manipulates the active scene graph (elements or properties of elements currently being displayed in a rendered scene) is only ever expected to occur on a single thread (the JavaFX application thread). In that way a race condition cannot occur.
If you want to feedback information from another thread to the UI, then you can either use the Platform.runLater API, which will execute a piece of code at some time in the future on the JavaFX thread, or make use of utilities in the javafx.concurrent package.
I've been reading about AsyncTasks and Hanlders and Loopers but I still can't figure out where I'm going wrong in my code. I'm trying to run code that will look over a Tic Tac Toe grid and determine the best move for the computer. I want this code to run in the background 'cause it can take some time and then I can update the UI level with a textbox that says something like "I'm Thinking". I've tried this a bunch of different ways, none with success.
private class PostTask extends AsyncTask<String, Integer, String> {
private Board _b;
private Welcome.Player _opp;
private int _depth;
#Override
protected void onPreExecute() {
super.onPreExecute();
}
protected void SetVars(Board b, Player p, int depth){
_b = b;
_opp = p;
_depth = depth;
}
#Override
protected String doInBackground(String... params) {
Looper.prepare();
try{
_bestMove = _b.GetBestMove(_opp,_depth);
}
catch(Exception err){
_bestMove = -1;
}
return "All done";
}
#Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
}
#Override
protected void onPostExecute(String result) {
super.onPostExecute(result);
if(_bestMove == -1){
TextView tv = (TextView) findViewById(R.id.tv_Score);
tv.setText("Had and error, couldn't make a move.");
}
FollowUpComputerMove(this);
}
The above code will work for exactly 5 moves and then it crashes. When I watch in the debugger I see new theads being created named Thread<#> AsyncTask #1. Once I get to five of those AsyncTasks it goes back to try and grab the first AsyncTask and crashes. When it crashes I'm shown the ThreadPoolExecutor.class file.
I've also read that I shouldn't be using both the AsyncTask and the Looper objects together so I've tried taking the Loooper.prepare() statement out, but then my AsyncTask fails immediately with the error message:
Can't create handler inside thread that has not called Looper.prepare() - AsyncTask inside a dialog
I've read repeatedly that you shouldn't be trying to update the UI from an AsyncTask and that often the above error is because of that, but GetBestMove isn't updating the UI thread. When I trace through to see where the error comes, it fails when calling a constructor saying it can't find the class.
Can anyone point me in the right direction? My end goal is to use one main thread and only one background thread, and just keep re-using the background thread whenever the computer needs to make a move. I know that the recursive method GetBestMove works when I run this program in a single-thread manner. But the screen freezes for too long on some moves as the method is being run. Thank you so much.
-NifflerX
Apologies for answering my own question, but the issue I was facing had nothing to do with recursion. The class I was calling was extending the class Activity, and while trying to call that from an AsyncTask the program was erroring out. When I removed the extends Activity from the class definition it started working again. Sorry for the post.
-NifflerX
I am beginner to C# .net. I have simple app in wpf which access a listbox from user thread. in winforms i can use invokerequired, a equivalent for wpf using dispatcher did not help. My system also hangs for the buttons so debugging is though. Please provide solution for the below code. thanks in advance
private void Monitor_mtd()
{
while (AppStatus != 0)
{
if (flag2 == 1)
{
listBox1.Dispatcher.BeginInvoke(DispatcherPriority.Normal,
new list1MtdDelegate(list1Mtd), "Best practice");
}
}
}
private delegate void list1MtdDelegate(string ls1);
private void list1Mtd(string ls1)
{
listBox1.Items.Add(ls1);
}
private void button1_Click_1(object sender, RoutedEventArgs e)
{
Monitor = new Thread(new ThreadStart(Monitor_mtd));
Monitor.Start();
flag1 = 1;
}
private void button2_Click(object sender, RoutedEventArgs e)
{
flag2 = 1;
}
There are a couple of issues that arise in your approach. Firstly, the way that you bind your data to the ListBox and secondly trying to update the ListBox from a user thread.
You can solve the binding of the ListBox by using an ObservableCollection so that the UI is updated with the necessary values (have a look at this post for more information on this). However, this also raises another problem and that is that the ObservableCollection cannot be called from another thread other than the one it is dispatching (see more on this here also). This means that you need another implementation for the ObservableCollection. Thomas Levesque made an AsyncObservableCollection that can be modified from any thread and still notify the UI when its modified.
I made a sample implementation that you can download here showing the full solution.
I'm testing my app with mono in prevision of a Linux port, and I have a threading problem. I initially considered pasting 3000 code lines here, but finally I've devised a small minimal example ;)
You have a form with a button (poetically named Button1, and a label (which bears, without surprise, the name Label1)). The whole lot is living a happy life on a form called Form1. Clicking Button1 launches an infinite loop that increments a local counter and updates Label1 (using Invoke) to reflect its value.
Now in Mono, if you resize the form, the label stops updating, never to restart. This doesn't happen with MS implementation. BeginInvoke doesn't work any better; worse, it makes the UI hang in both cases.
Do you know where this discrepancy comes from? How would you solve it? And finally, why doesn't BeginInvoke work here? I must be making a huge mistake... but which?
EDIT:
Some progress so far:
Calling BeginInvoke does in fact work; only, the UI just doesn't refresh fast enough, so it seems to stop.
On mono, what happens is that the whole thread hangs when you insert a message in the UI queue (eg by resizing the form). In fact, the synchronous Invoke call never returns. I'm trying to understand why.
Of interest: even using BeginInvoke, the asynchronous calls don't get executed before the resizing operation ends. On MS.Net, they keep running while resizing.
The code looks like this (C# version lower):
Public Class Form1
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim T As New Threading.Thread(AddressOf Increment)
T.Start()
End Sub
Sub UpdateLabel(ByVal Text As String)
Label1.Text = Text
End Sub
Delegate Sub UpdateLabelHandler(ByVal Text As String)
Sub Increment()
Dim i As Long = 0
Dim UpdateLabelDelegate As New UpdateLabelHandler(AddressOf UpdateLabel)
Try
While True
i = (i + 1) Mod (Long.MaxValue - 1)
Me.Invoke(UpdateLabelDelegate, New Object() {i.ToString})
End While
Catch Ex As ObjectDisposedException
End Try
End Sub
End Class
Or, in C#,
public class Form1
{
private void Button1_Click(System.Object sender, System.EventArgs e)
{
System.Threading.Thread T = new System.Threading.Thread(Increment);
T.Start();
}
public void UpdateLabel(string Text)
{
Label1.Text = Text;
}
public delegate void UpdateLabelHandler(string Text);
public void Increment()
{
long i = 0;
UpdateLabelHandler UpdateLabelDelegate = new UpdateLabelHandler(UpdateLabel);
try {
while (true) {
i = (i + 1) % (long.MaxValue - 1);
this.Invoke(UpdateLabelDelegate, new object[] { i.ToString() });
}
} catch (ObjectDisposedException Ex) {
}
}
}
This is a bug in the mono runtime, at least I think it is. The code might not be good practice (I'm not a threading expert), but the thing that suggests a bug is the fact that the behaviour differs on windows and Linux.
On Linux, mono has exactly the same behaviour as MS.Net has on windows. No hanging, continuous updates even while resizing.
On Windows, mono displays all the aforementioned problems. I've posted a bug report at https://bugzilla.novell.com/show_bug.cgi?id=690400 .
Do you know where this discrepancy
comes from? How would you solve it?
I am not sure. I do not see anything obvious in your code that would cause the difference between Mono and .NET. If I had to make a wild guess I would say there is a possibility that you have stumbled upon an obscure bug in Mono. Though, I suppose it is possible that Mono uses a sufficiently different mechanism for handling the WM_PAINT messages that cause the form to get refreshed. The constant pounding of the UI thread from repeated calls to Invoke may be disrupting Mono's ability to get the form refreshed.
And finally, why doesn't BeginInvoke
work here?
Calling Invoke in a tight loop is bad enough, but BeginInvoke will be even worse. The worker thread is flooding the UI message pump. BeginInvoke does not wait until the UI thread has finished executing the delegate. It just posts the requests and returns quickly. That is why it appears to hang. The messages that BeginInvoke is posting to the UI message queue keep building up as the worker thread is likely severely out pacing the UI thread's ability to process them.
Other Comments
I should also mention that the worker thread is nearly useless in the code. The reason is because you have a call to Invoke on every iteration. Invoke blocks until the UI has finished executing the delegate. That means your worker thread and UI thread are essentially in lock-step with each other. In other words, the worker is spending most of its time waiting for the UI and vice versa.
Solution
One possible fix is to slow down the rate at which Invoke is called. Instead of calling it on every loop iteration try doing it every 1000 iterations or the like.
Any even better approach is to not use Invoke or BeginInvoke at all. Personally, I think these mechanisms for updating the UI are way overused. It is almost always better to let the UI thread throttle its own update rate especially when the worker thread is doing continuous processing. This means you will need to place a timer on the form and have it tick at the desired refresh rate. From the Tick event you will probe a shared data structure that the worker thread is updating and use that information to update the controls on the form. This has several advantages.
It breaks the tight coupling between the UI and worker threads that Control.Invoke imposes.
It puts the responsibility of updating the UI thread on the UI thread where it should belong anyway.
The UI thread gets to dictate when and how often the update should take place.
There is no risk of the UI message pump being overrun as would be the case with the marshaling techniques initiated by the worker thread.
The worker thread does not have to wait for an acknowledgement that the update was performed before proceeding with its next steps (ie. you get more throughput on both the UI and worker threads).
First and foremost: clicking on Button1 is asynchronous already, so you don't need to create another thread to increment, just call the increment method Sorry, I was reading your question line by line and by the time I got to the while-loop I forgot about the button:
private void Button1_Click(System.Object sender, System.EventArgs e)
{
Thread t = new Thread(Increment);
t.IsBackground = true;
t.Start();
}
Second: if you do need to use a thread then you should always set your thread to background (i.e. foreground prevents your process from terminating), unless you have a good reason for using a foreground thread.
Third: if you're making updates to the UI, then you should check the InvokeRequired property and call BeginInvoke:
public void UpdateLabel(string Text)
{
if (InvokeRequired)
{
BeginInvoke(new UpdateLabelDelegate(UpdateLabel), Text);
}
else
{
Label1.Text = Text;
}
}
public void Increment()
{
int i = 0;
while(true)
{
i++; // just incrementing i??
UpdateLabel(i.ToString());
Thread.Sleep(1000);// slow down a bit so you can see the updates
}
}
You can also "automate" the Invoke Required "pattern": Automating the InvokeRequired code pattern
And now see if you're still having the same problem.
I tried it on my machine and it works like a charm:
public partial class Form1 : Form
{
private delegate void UpdateLabelDelegate(string text);
public Form1()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
Thread t = new Thread(Increment);
t.IsBackground = true;
t.Start();
}
private void UpdateLabel(string text)
{
if (label1.InvokeRequired)
{
BeginInvoke(new UpdateLabelDelegate(UpdateLabel), text);
}
else
{
label1.Text = text;
}
}
private void Increment()
{
int i = 0;
while (true)
{
i++;
UpdateLabel(i.ToString());
Thread.Sleep(1000);
}
}
}
I have a static class with a single method, which I use for delaying an action until the user has stopped clicking/typing/whatever:
public void SomeControl_Click(object sender, EventArgs e)
{
// DoSomething once user has stopped clicking for 300ms
MyClass.MyMethod(() => DoSomething(), 300);
}
Currently I've gone for DelayAction.StartNew, but that doesn't really convey what it does. I wondered about Stabilize, but I'm not sure that's any clearer.
DelayAction.OnIdleUser
Since "being idle" means no activity, I'd go for this naming.
DelayAction.OnTimerExpired()