I have a modal dialog with a cancel button only which pops up when the user clicks on a button. Aftre the modal dialog pops up, I would like to start a long process which monitors external event. If the event happens, then the dialog will be closed automatically. The user can cancel the monitoring process by clicking the cancel button.
I assigned the process start to the Shown event
private void ProceedForm_Shown(object sender, System.EventArgs e)
{
controller.StartSwiping();
}
The process itself is a loop
public void StartSwiping()
{
Status status;
do
{
status = CallForFeedback();
} while (status == Status.Pending);
form.DialogResult = DialogResult.OK;
form.Close();
}
The process starts fine, but the dialog does not pop up, so the user can non cancel the process. I also tried to assign the start to the Load event, but nothing changed.
Is there any way to Show the dialog and after that start the process?
Thanks
Your problem is that you are doing everything in the UI thread. You need to put you status monitoring loop in a separate thread so that the UI thread can remain responsive.
There are several ways you can do this, but one easy place to start is with the BackgroundWorker class
Use a Task to do your LongRunning events:
CancellationTokenSource _cancelationTokenSource = new CancellationTokenSource();
new Task(() =>
{
//Do LongRunning task
}, _cancelationTokenSource.Token, TaskCreationOptions.LongRunning).Start();
Use the _cancelationTokenSource to cancel the task when needed.
I would move the long running code onto a background thread as you are blocking the UI thread, which is why the UI never displays.
Use a background worker class for the controller functionality http://msdn.microsoft.com/en-us/library/cc221403(v=vs.95).aspx
When the work is completed on the background worker (i.e. the event is received) then you can use the following mechanism to callback onto the UI thread:
http://msdn.microsoft.com/en-us/library/ms171728(v=vs.80).aspx
Note: the article says you can turn off the crossthreadexception this would be considered bad practice, instead handle it the correct way using the InvokeRequired check and then invoke method on the windows form.
Others have suggested using a BackgroundWorker, or some other sort of background thread. While in many cases this is appropriate here, there is likely an even better solution. You're not just doing some long running task, you're waiting for something to happen. Rather than constantly polling...whatever it is, you should be using events. There should be an event that is triggered when you are done, and you should subscribe to that event to do whatever you need to do (i.e. close the dialog) when the correct conditions are met.
Related
I have a gui. When I press a button my gui interacts with a software. It takes some seconds. During this time I want a dialog box, pop up or some thing like that to appear infront of my gui which tells the user to wait (with a message). When interaction of gui with software finishes the pop up automatically closes and user can again normally interact with gui.
Is there any way to do that ?
The trick is to spin off a thread so as not to tie up the UI thread. This is typically achieved via a BackgroundWorker.
There's a walkthrough for setting all this up on codeplex. The loading form closes when the backgroundworker is complete.
Here is briefly how it can be done using the BackgroundWorker component.
Put a BackgroundWorker onto your Form, then in the button's Click handler show your popup indicator Form above the current form, and start your worker with its RunWorkerAsync method. Handle the workers DoWork event, and it the handler, run the long running task. Also handle the worker's completed event (not sure now how it's called exactly), and in that hide your popup form. You can track the operation result in the DoWork event eventargs (Result property), and also you can catch any exceptions during the long running task with the completed event eventarg's Error peroperty. The operation progress can be reported in the DoWork handler with the worker's ReportProgress method, and it can be catched in the GUI with the worker's corresponding event.
You could also set mouse cursor to wait before long running operation
this.Cursor = Cursors.WaitCursor;
and than back to normal, then it's finished
this.Cursor = Cursors.Default;
I have a main form that contains an edit control that occupies the entire form. There is another worker thread that constantly writes log messages to this edit control. Now I want to show a dialog box with just a cancel button while the main UI's edit control is displaying stuff. The problem is that the cancel dialog is non-responsive while the updates are happening behind it and I cannot click on the cancel button. Any idea on how to resolve it. I was thinking of creating another UI thread and show the cancel button from it. Any other alternatives?
EDIT#1
I should clarify that I already use a worker thread to do the work.
DisplayLogs() is in a seperate thread.
DisplayLogs() is called from other threads.
LogMessage is the delegate that points to the method UpdateMessage in main UI.
The control used is a TextBox. I have tried other controls like listview,
richtextboxsand, etc. still the same result.
Worker Thread
void DisplayLogs()
{
lock (this)
{
while (logQueue.Count > 0)
{
string logMessagemessage = logQueue.Dequeue();
LogMessage(string.Concat(logMessagemessage, Environment.NewLine));
}
}
}
Main UI
public void UpdateMessage( string message)
{
if (!txtLog.IsHandleCreated)
{
return;
}
if (txtLog.InvokeRequired)
txtLog.BeginInvoke( new UpdateLogDelegate( UpdateLog), message);
else
txtLog.AppendText(message);
}
The main solution is to offload the expensive code onto a background worker and leave your UI thread responsive for UI actions. Your form can then simply show a modal dialog or something.
MSDN - How to use a Background Worker
In this situation it's necessary to move the majority of the work to a new thread, and clear up the UI thread for cancel messages etc.
You are going about this backwards. The main thread should, in theory, always be available to accept user input. Anything that may block for extended periods of time (heavy computation, database access, network access) should be done in a background thread. The idea is to have the edit control's data being computed and populated by a background thread (BackgroundWorker objects work nicely here) so that the main thread is always available if the user clicks on the cancel button.
Your problem is that the your UI thread is ALWAYS busy. I am saying this assuming that the number of items in logQueue is quite large. The while loop doesn't quit till the queue is empty. So it keeps hitting the UI thread with request for updates.
Also the if (txtLog.InvokeRequired) is kind of pointless because you are always calling the method from a worker thread.
So, since a .net WinForm application has only a single UI, which in your case is too busy to process other notifications, the new window appears stuck (because the paint messages are stuck in the message queue and cannot be processed as it is already flooded with the text box update messages)
You could stick an Application.DoEvents inside your loop which will give the message loop some time to process the pending notifications. However this is kind of a hack, in my opinion, as the UI behavior is sometimes not predictable. It may lead to things like stuttering while moving a dialog, delayed responses to click events etc.
Another point, MessageBox.Show or a Form.ShowDialog (if this is what you are using for the cancel button) is a blocking call. The thread on which you show it WILL hang till you dismiss the dialog. Try Form.Show and set the parent property to the main form.
Another alternative is to add a timer and process only X notifications per Y seconds. This will give the UI thread some breathing room for performing other activities.
I have the following code, Some times ArchiveFiles() take more time to complete its execution.
When user clicks exit option more than one time from context menu, Application becomes non responding if ArchiveFiles() takes more time. How can I show a wait message when he clicks the exit option again?
private void exitToolStripMenuItem_Click(object sender, EventArgs e)
{
ArchiveFiles();
Application.Exit();
}
Thanks
Bhaskar
You can use BackgroundWorker.
Using BackgroundWorker, you will be able to execute time consuming tasks such as the one you have on another thread so that the UI doesn't freeze. You will be able to report the progress of that task, then report it accomplishement and executing what ever logic you need after its completion.
Handle the BackgroundWorker.DoWork event to start the operation that performs the potentially time-consuming work.
Handle BackgroundWorker.ProgressChanged event to report the progress of an asynchronous operation to the user.
Finaly, handle BackgroundWorker.RunWorkerCompleted event to implement whatever logic you want to be implemented after the task has been completed.
Refer to the following:
BackgroundWorker Component Overview
C# BackgroundWorker Tutorial
Create a new WaitingForm and put an image control on the form and use the below .gif in that image control which which automatically animate. Then use the code below:
private void exitToolStripMenuItem_Click(object sender, EventArgs e)
{
using (var wait = new WaitingForm())
{
wait.Show();
ArchiveFiles();
Application.Exit();
}
}
This may not be the best solution but it is the quickest. If you want the form to be a dialog, use wait.ShowDialog(); instead and carry ArchiveFiles(); Application.Exit();
functions inside it (if that is a probability).
You'd have to run ArchiveFiles asynchronously, by either using a thread or task, or making some delegate and using its BeginInvoke method.
You should use then some synchronization object like ManualResetEvent so the main thread don't continue executing, calling Application.Exit(). Or you could use some event to know when the operation finishes and then call Application.Exit() (or call it inside ArchiveFiles), but I find that to be worse.
Ok, so I found something weird over the weekend. I have a WPF app that spawns off some threads to perform background work. Those background threads then Post work items to my Synchronization Context. This is all working fine except for one case. When my threads finish sometimes they will post an action onto the dispatcher that will open up a Popup window. What ends up happening is that if 2 threads both post an action on the Dispatcher it starts processing one, then if I open up a Popup window with Window.ShowDialog(); the current execution path pauses waiting for feedback from the dialog box as it should. But the problem arises that when the dialog box opens the Dispatcher then goes and starts immediately starts running the second action that was posted. This results in two code paths being executed. The first one with a message box being held open, while the second one is running wild because my application state is unknown because the first action never completed.
I've posted some example code to demonstrate the behavior I'm talking about. What should happen is that if I post 2 actions and the 1st one opens up a dialog box the second action shouldn't run until after the 1st action has been completed.
public partial class Window1 : Window {
private SynchronizationContext syncContext;
public Window1() {
InitializeComponent();
syncContext = SynchronizationContext.Current;
}
private void Button_ClickWithout(object sender, RoutedEventArgs e) {
// Post an action on the thread pool with the syncContext
ThreadPool.QueueUserWorkItem(BackgroundCallback, syncContext);
}
private void BackgroundCallback(object data) {
var currentContext = data as SynchronizationContext;
System.Console.WriteLine("{1}: Thread {0} started", Thread.CurrentThread.ManagedThreadId, currentContext);
// Simulate work being done
Thread.Sleep(3000);
currentContext.Post(UICallback, currentContext);
System.Console.WriteLine("{1}: Thread {0} finished", Thread.CurrentThread.ManagedThreadId, currentContext);
}
private void UICallback(object data) {
System.Console.WriteLine("{1}: UI Callback started on thread {0}", Thread.CurrentThread.ManagedThreadId, data);
var popup = new Popup();
var result = popup.ShowDialog();
System.Console.WriteLine("{1}: UI Callback finished on thread {0}", Thread.CurrentThread.ManagedThreadId, data);
}
}
The XAML is just a Window with a button that calls Button_ClickWithout OnClick. If you push the button twice and wait 3 seconds, you will see you get 2 dialogs popping up one over the other, where the expected behavior would be the first one pops up, then once it's closed the second one will pop up.
So my question is: Is this a bug ? or how do I mitigate this so I can have only one action be processed at a time when the first action halts execution with a Window.ShowDialog() ?
Thanks,
Raul
As I'm awaiting an answer on my question (Advice on using the Dispatcher Priority and Binding) I thought that would pay-it-forward™.
What you are experiencing is Nested Pumping on the Dispatcher. I recommend reading the MSDN article on the WPF Threading Model, especially the section titled 'Technical Details and Stumbling Points' that is two-thirds down the page. The sub-section describing the Nested Pumping is copied below for convenience.
Nested Pumping
Sometimes it is not feasible to completely lock up the UI thread.
Let’s consider the Show method of the MessageBox class. Show doesn’t
return until the user clicks the OK button. It does, however, create a
window that must have a message loop in order to be interactive. While
we are waiting for the user to click OK, the original application
window does not respond to user input. It does, however, continue to
process paint messages. The original window redraws itself when
covered and revealed.
Some thread must be in charge of the message box window. WPF could
create a new thread just for the message box window, but this thread
would be unable to paint the disabled elements in the original window
(remember the earlier discussion of mutual exclusion). Instead, WPF
uses a nested message processing system. The Dispatcher class includes
a special method called PushFrame, which stores an application’s
current execution point then begins a new message loop. When the
nested message loop finishes, execution resumes after the original
PushFrame call.
In this case, PushFrame maintains the program context at the call to
MessageBox.Show, and it starts a new message loop to repaint the
background window and handle input to the message box window. When the
user clicks OK and clears the pop-up window, the nested loop exits and
control resumes after the call to Show.
A modal dialog box does not prevent the owner window from processing messages, otherwise you'd see it fail to redraw as the modal dialog was moved over its surface (just as an example).
In order to achieve what you want, you have to implement your own queue on the UI thread, possibly with some synchronization to "wake it up" when the first work item arrives.
EDIT:
Also if you examine the call stack of the UI thread while the 2nd modal dialog box is up, you might find out that it has the first ShowDialog call above it in the stack.
EDIT #2:
There might be an easier way of doing this, without implementing your own queue. If instead of the SynchronizationContext, you would use the Dispatcher object, you would be able to call BeginInvoke on it with a priority of DispatcherPriority.Normal, and it will get queued properly (check).
Say I'm showing the user a form, and using a BackgroundWorker to do some work behind the scenes.
When the user clicks OK, I cannot continue until the BackgroundWorker has completed.
If it hasn't finished when the user clicks Ok, I want to show a WaitCursor until it has, and then continue.
What's the best way to implement this?
I know I could use a plain old Thread, and then do Thread.Join, but I like BackgroundWorker.
Can it be done nicely?
You could use this code, instead of BW
var resetEvent = new ManualResetEvent(false);
ThreadPool.QueueUserWorkItem(state =>
{
Thread.Sleep(5000);//LongRunning task, async call
resetEvent.Set();
});
resetEvent.WaitOne();// blocking call, when user clicks OK
Ideally, you should disable all the relevant controls on the form, then re-enable them when the BackgroundWorker completes. That way you won't get a locked UI - and you can have a cancel button etc.
If you wanted to actually block until it had finished, you could just run the task synchronously to start with.
You could check is the BackgroundWorker is running with a check in the btnOK_Click event handler to see if bw.IsBusy is true. If so change the cursor and set a boolean flag that indicates that OK handler is waiting. When BackgroundWorker finishes its job check if ok had been waiting, if so, set the flag to false change the cursor and do the job of the OK button. :-)
I'd just use boolean flags and a lock.
object waitlock = new object();
bool isBackgroundWorkerCompleted = false;
bool isOKButtonPressed = false;
private void onPress(...)
{
lock(waitlock)
{
isOKButtonPressed = true;
if (isBackgroundWorkerCompleted) DoNextStep();
else cursor.Wait(); //psuedo-code - use whatever the actual call is...
}
}
Background thread does the same trick (just remember to BeginInvoke back to the main UI thread)