In WinForms, how do I force an immediate UI update from UI thread?
What I'm doing is roughly:
label.Text = "Please Wait..."
try
{
SomewhatLongRunningOperation();
}
catch(Exception e)
{
label.Text = "Error: " + e.Message;
return;
}
label.Text = "Success!";
Label text does not get set to "Please Wait..." before the operation.
I solved this using another thread for the operation, but it gets hairy and I'd like to simplify the code.
At first I wondered why the OP hadn't already marked one of the responses as the answer, but after trying it myself and still have it not work, I dug a little deeper and found there's much more to this issue then I'd first supposed.
A better understanding can be gained by reading from a similar question: Why won't control update/refresh mid-process
Lastly, for the record, I was able to get my label to update by doing the following:
private void SetStatus(string status)
{
lblStatus.Text = status;
lblStatus.Invalidate();
lblStatus.Update();
lblStatus.Refresh();
Application.DoEvents();
}
Though from what I understand this is far from an elegant and correct approach to doing it. It's a hack that may or may not work depending upon how busy the thread is.
Call Application.DoEvents() after setting the label, but you should do all the work in a separate thread instead, so the user may close the window.
Call label.Invalidate and then label.Update() - usually the update only happens after you exit the current function but calling Update forces it to update at that specific place in code.
From MSDN:
The Invalidate method governs what gets painted or repainted. The Update method governs when the painting or repainting occurs. If you use the Invalidate and Update methods together rather than calling Refresh, what gets repainted depends on which overload of Invalidate you use. The Update method just forces the control to be painted immediately, but the Invalidate method governs what gets painted when you call the Update method.
If you only need to update a couple controls, .update() is sufficient.
btnMyButton.BackColor=Color.Green; // it eventually turned green, after a delay
btnMyButton.Update(); // after I added this, it turned green quickly
I've just stumbled over the same problem and found some interesting information and I wanted to put in my two cents and add it here.
First of all, as others have already mentioned, long-running operations should be done by a thread, which can be a background worker, an explicit thread, a thread from the threadpool or (since .Net 4.0) a task: Stackoverflow 570537: update-label-while-processing-in-windows-forms, so that the UI keeps responsive.
But for short tasks there is no real need for threading although it doesn't hurt of course.
I have created a winform with one button and one label to analyze this problem:
System::Void button1_Click(System::Object^ sender, System::EventArgs^ e)
{
label1->Text = "Start 1";
label1->Update();
System::Threading::Thread::Sleep(5000); // do other work
}
My analysis was stepping over the code (using F10) and seeing what happened. And after reading this article Multithreading in WinForms I have found something interesting. The article says at the bottom of the first page, that the UI thread can not repaint the UI until the currently executed function finishes and the window is marked by Windows as "not responding" instead after a while. I have also noticed that on my test application from above while stepping through it, but only in certain cases.
(For the following test it is important to not have Visual Studio set to fullscreen, you must be able to see your little application window at the same time next to it, You must not have to switch between the Visual Studio window for debugging and your application window to see what happens. Start the application, set a breakpoint at label1->Text ..., put the application window beside the VS window and place the mouse cursor over the VS window.)
When I click once on VS after app start (to put the focues there and enable stepping) and step through it WITHOUT moving the mouse, the new text is set and the label is updated in the update() function. This means, the UI is repainted obviously.
When I step over the first line, then move the mouse around a lot and click somewhere, then step further, the new text is likely set and the update() function is called, but the UI is not updated/repainted and the old text remains there until the button1_click() function finishes. Instead of repainting, the window is marked as "not responsive"! It also doesn't help to add this->Update(); to update the whole form.
Adding Application::DoEvents(); gives the UI a chance to update/repaint. Anyway you have to take care that the user can not press buttons or perform other operations on the UI that are not permitted!! Therefore: Try to avoid DoEvents()!, better use threading (which I think is quite simple in .Net).
But (#Jagd, Apr 2 '10 at 19:25) you can omit .refresh() and .invalidate().
My explanations is as following: AFAIK winform still uses the WINAPI function. Also MSDN article about System.Windows.Forms Control.Update method refers to WINAPI function WM_PAINT. The MSDN article about WM_PAINT states in its first sentence that the WM_PAINT command is only sent by the system when the message queue is empty. But as the message queue is already filled in the 2nd case, it is not send and thus the label and the application form are not repainted.
<>joke> Conclusion: so you just have to keep the user from using the mouse ;-) <>/joke>
you can try this
using System.Windows.Forms; // u need this to include.
MethodInvoker updateIt = delegate
{
this.label1.Text = "Started...";
};
this.label1.BeginInvoke(updateIt);
See if it works.
After updating the UI, start a task to perform with the long running operation:
label.Text = "Please Wait...";
Task<string> task = Task<string>.Factory.StartNew(() =>
{
try
{
SomewhatLongRunningOperation();
return "Success!";
}
catch (Exception e)
{
return "Error: " + e.Message;
}
});
Task UITask = task.ContinueWith((ret) =>
{
label.Text = ret.Result;
}, TaskScheduler.FromCurrentSynchronizationContext());
This works in .NET 3.5 and later.
It's very tempting to want to "fix" this and force a UI update, but the best fix is to do this on a background thread and not tie up the UI thread, so that it can still respond to events.
Try calling label.Invalidate()
http://msdn.microsoft.com/en-us/library/system.windows.forms.control.invalidate(VS.80).aspx
Think I have the answer, distilled from the above and a little experimentation.
progressBar.Value = progressBar.Maximum - 1;
progressBar.Maximum = progressBar.Value;
I tried decrementing the value and the screen updated even in debug mode, but that would not work for setting progressBar.Value to progressBar.Maximum, because you cannot set the progress bar value above the maximum, so I first set the progressBar.Value to progressBar.Maximum -1, then set progressBar.Maxiumum to equal progressBar.Value. They say there is more than one way of killing a cat. Sometimes I'd like to kill Bill Gates or whoever it is now :o).
With this result, I did not even appear to need to Invalidate(), Refresh(), Update(), or do anything to the progress bar or its Panel container or the parent Form.
myControlName.Refresh() is a simple solution to update a control before moving on to a "SomewhatLongRunningOperation".
From: https://learn.microsoft.com/en-us/dotnet/api/system.windows.forms.control.update?view=windowsdesktop-6.0
There are two ways to repaint a form and its contents:
You can use one of the overloads of the Invalidate method with the Update method.
You can call the Refresh method, which forces the control to redraw itself and all its children. This is equivalent to setting the Invalidate method to true and using it with Update.
The Invalidate method governs what gets painted or repainted. The Update method governs when the painting or repainting occurs. If you use the Invalidate and Update methods together rather than calling Refresh, what gets repainted depends on which overload of Invalidate you use. The Update method just forces the control to be painted immediately, but the Invalidate method governs what gets painted when you call the Update method.
I had the same problem with property Enabled and I discovered a first chance exception raised because of it is not thread-safe.
I found solution about "How to update the GUI from another thread in C#?" here https://stackoverflow.com/a/661706/1529139 And it works !
When I want to update the UI in "real-time" (or based on updates to data or long running operations) I use a helper function to "simplify" the code a bit (here it may seem complex, but it scales upward very nicely). Below is an example of code I use to update my UI:
// a utility class that provides helper functions
// related to Windows Forms and related elements
public static class FormsHelperFunctions {
// This method takes a control and an action
// The action can simply be a method name, some form of delegate, or it could be a lambda function (see: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/lambda-expressions)
public static void InvokeIfNeeded(this Control control, Action action)
{
// control.InvokeRequired checks to see if the current thread is the UI thread,
// if the current thread is not the UI thread it returns True - as in Invoke IS required
if(control.InvokeRequired)
{
// we then ask the control to Invoke the action in the UI thread
control.Invoke(action);
}
// Otherwise, we don't need to Invoke
else
{
// so we can just call the action by adding the parenthesis and semicolon, just like how a method would be called.
action();
}
}
}
// An example user control
public class ExampleUserControl : UserControl {
/*
//
//*****
// declarations of label and other class variables, etc.
//*****
//
...
*/
// This method updates a label,
// executes a long-running operation,
// and finally updates the label with the resulting message.
public void ExampleUpdateLabel() {
// Update our label with the initial text
UpdateLabelText("Please Wait...");
// result will be what the label gets set to at the end of this method
// we set it to Success here to initialize it, knowing that we will only need to change it if an exception actually occurs.
string result = "Success";
try {
// run the long operation
SomewhatLongRunningOperation();
}
catch(Exception e)
{
// if an exception was caught, we want to update result accordingly
result = "Error: " + e.Message;
}
// Update our label with the result text
UpdateLabelText(result);
}
// This method takes a string and sets our label's text to that value
// (This could also be turned into a method that updates multiple labels based on variables, rather than one input string affecting one label)
private void UpdateLabelText(string value) {
// call our helper funtion on the current control
// here we use a lambda function (an anonymous method) to create an Action to pass into our method
// * The lambda function is like a Method that has no name, here our's just updates the label, but it could do anything else we needed
this.InvokeIfNeeded(() => {
// set the text of our label to the value
// (this is where we could set multiple other UI elements (labels, button text, etc) at the same time if we wanted to)
label.Text = value;
});
}
}
Related
I have a WPF UI with Helix3DToolkit Graphics in it.
I had two buttons there,
1st button - Adds some Visual3D objects into it.
2nd button - Saves a screenshot of the current UI into a jpeg image by using
Code for screenshot:
Viewport3DHelper.Export(vp.Viewport, filename);
All this works fine manually. But now I want to make this automatic. (Several alternating rounds of actions of buttons 1 & 2. i.e. I want to just press one button, which will add some boxes, save a screenshot, add some more, save a screenshot, etc.)
I can control the buttons programmatically by using the following:
ButtonAutomationPeer peer = new ButtonAutomationPeer(MyButton);
IInvokeProvider invokeProv = peer.GetPattern(PatternInterface.Invoke) as IInvokeProvider;
invokeProv.Invoke();
But when I do this, all I get is several copies of the same UI. (This is due to the nature of the UI thread, I believe)
I found a partial solution, which is to use,
TaskScheduler scheduler = TaskScheduler.FromCurrentSynchronizationContext();
CancellationToken cancellationToken = new CancellationToken();
Task.Factory.StartNew(() => GetScreenShotAction()).ContinueWith(w =>
{
AddBoxes();
}, cancellationToken, TaskContinuationOptions.None, scheduler);
But, this is also not working properly (it's working randomly, when I take only 2 screenshots, but never when it's more than 2). This is probably because both the before and after actions are on the UI thread.
Any ideas on how to get this done will be hugely appreciated.
Also I'm using VC# 2010 with .NET 4 framework. So cannot use async and await.
Thanks!
Update
I'm already using, this.Dispatcher.Invoke((Action)(() => { ... })); in the GetScreenShotAction method. And, the other action AddBoxes is on the same thread.
Also, it works randomly, so I'm guessing that because I'm invoking the Dispatcher, both are still happening technically on the same thread.
I suspect that you need to wait for the UI to render (after modifying the visual tree by adding your buttons and what not) before you take your screenshot.
This article shows how to use the dispatcher with DispatcherPriority.Render to get the code to wait for all the rendering to be done before continuing.
To quote the article:
When the DispatcherPriority is set to Render (or lower), the code will
then execute all operations that are of that priority or higher. In
the example, the code already sets label1.Content to something else,
which will result in a render operation. So by calling
Dispatcher.Invoke, the code essentially asks the system to execute all
operations that are Render or higher priority, thus the control will
then render itself (drawing the new content). Afterwards, it will
then execute the provided delegate (which is our empty method).
The code for the Refresh extension method:
public static class ExtensionMethods
{
private static Action EmptyDelegate = delegate() { };
public static void Refresh(this UIElement uiElement)
{
uiElement.Dispatcher.Invoke(DispatcherPriority.Render, EmptyDelegate);
}
}
Use it something like:
AddBoxes();
MyControlContainer.Refresh();
TakeScreenShot();
Where MyControlContainer is a UIElement, maybe the container that you added the boxes to?
You are on the right track using a Task to multi-thread the operation. The issue is that you need to Invoke your code on the correct Thread. Use the Dispatcher (aka WPF UI Thread) to do this via Application.Current.Dispatcher by calling Invoke() or BeginInvoke() as desired.
I'm having a "invoke" issue that I can't resolve. I'll try to be as thorough in my description as possible, but I'm new to this so bear with me and let me know if you need more information.
I've got a background thread running that when prompted will disable a bunch of check boxes on a form created on the main thread. In order to do this I need to safely cross-thread using an invoke and a delegate but I must be doing it incorrectly. Bottom line, when I check this in the debugger i see that it runs through the ACTION portion of the code twice if InvokeRequired. I can get around this by bracketing ACTION with an else, and although it won't run through the else twice it does still try to go through the method again.
delegate void ManualCurtainShuttoffHandler();
public void ManualCurtainShutoff()
{
if (InvokeRequired)
{
Invoke(new ManualCurtainShuttoffHandler(ManualCurtainShutoff));
}
// ACTION: Disable check boxes
}
I'd just like to know why it runs through the method two times. Let me know if you need any more information and I'd be happy to share it with you.
Just because you call Invoke, it doesn't stop execution of the current method. A quick and simple solution is to simply return after calling Invoke:
delegate void ManualCurtainShuttoffHandler();
public void ManualCurtainShutoff()
{
if (InvokeRequired)
{
Invoke(new ManualCurtainShuttoffHandler(ManualCurtainShutoff));
return;
}
// ACTION: Disable check boxes
}
This will skip the rest of the execution of ManualCurtainShutoff that's running on the background thread while still promoting a new execution of the method on the main thread.
Invoke will cause your function to be called again in a different thread (that's its purpose). You should add a return after the call to Invoke. The idea is that then your function will be called again (that's what you want), and that time InvokeRequired will be false, so your action will take place.
edit: dang, by the time I finish writing I've been beaten to the punch. Oh well!
I've got my main form Form1 running the main bulk of my program.
I have a separate thread started to perform an algorithm.
When I run the method from the new thread, method MyAlgorithm() I get the error
InvalidOperationException with the message, "Control control name accessed from a thread other than the thread it was created on."
How do I get back to the original thread so that I can run the method to update my text boxes with the latest values?
This is the method that I want to run contained in Form1, the main class in my application.
// Reset the results values
public void ShowResults()
{
while (true)
{
loopsNum.Text = Convert.ToString(resultLoopsNum);
nodesVisitedNum.Text = Convert.ToString(resultNodesVisitedNum);
nodesResolvedNum.Text = Convert.ToString(resultNodesResolvedNum);
cpuLoopsNum.Text = Convert.ToString(resultCpuLoopsNum);
shortestPathCostNum.Text = Convert.ToString(resultShortestPathCost);
}
}
I've looked at the Invoke() methods, but I don't know how to get the original instance of my Form1 from the threaded method.
I'm invoking my thread like this...
// Set the algorithm method up in it's own thread
Thread thread = new Thread(new ThreadStart(MyAlgorithm));
// Run the algorithm
thread.Start();
How do I get back to the original thread so that I can run the method to update my text boxes with the latest values?
In Windows Forms, you'd either use Control.Invoke/BeginInvoke or use a BackgroundWorker and perform the update in the progress event handler.
In WPF you'd use Dispatcher.Invoke/BeginInvoke.
In C# 5 and .NET 4.5 you'll be able to use async methods which should make a lot of this much simpler...
I've looked at the Invoke() methods, but I don't know how to get the original instance of my Form1 from the threaded method.
If the "threaded method" is just an instance method of the Form, then you've already got the this reference. If it's not, you'll need to provide that information - ideally as an ISynchronizeInvoke to avoid a direct dependency on Windows Forms if you can express the "update" part separately. (That interface is somewhat deprecated these days, in favour of synchronization contexts, but it still works perfectly well.)
Have a look at Control.Invoke():
public void ShowResults()
{
while (true)
{
Thread.Sleep(1000); // don't spam the UI thread
if (this.InvokeRequired)
{
this.Invoke((Action)UpdateGui);
}
else
{
UpdateGui();
}
}
}
private void UpdateGui()
{
loopsNum.Text = Convert.ToString(resultLoopsNum);
nodesVisitedNum.Text = Convert.ToString(resultNodesVisitedNum);
nodesResolvedNum.Text = Convert.ToString(resultNodesResolvedNum);
cpuLoopsNum.Text = Convert.ToString(resultCpuLoopsNum);
shortestPathCostNum.Text = Convert.ToString(resultShortestPathCost);
}
You can use:
myform.Invoke(ShowResults);
There's other options here too:
Alternately use a System.Forms.Timer to call ShowResults periodically. Or another option would be not to use another thread to do the operation; do it in the GUI thread and call Application.DoEvents() from within the operation when you want to let the GUI update.
The first option is nice because it keeps you from accidentally flooding the GUI with Invoke requests, and the second option is nice because it's all on the GUI thread and allows you to have fine-grain control over when things get displayed on the GUI.
I get the following exception thrown:
Invoke or BeginInvoke cannot be called on a control until the window handle has been created.
This is my code:
if (InvokeRequired)
{
BeginInvoke(new UpdateTextFieldDelegate(WriteToForm), finished, numCount);
}
else
Invoke(new UpdateTextFieldDelegate(WriteToForm), finished, numCount);
I found pages about this topic on this site but I don't know what is wrong.
The difference between Invoke and BeginInvoke is that the former is synchronous (waits for completion) while the later is asynchronous (sort of fire-and-forget). However, both work by posting a message to the UI message loop which will cause the delegate to be executed when it gets to that message.
The InvokeRequired property determines whether you need to Invoke at all or if it is already on the correct thread, not whether you want synchronous or asynchronous calling. If InvokeRequired is false you are (in theory) already running on the UI thread and can simply perform synchronous actions directly (or still BeginInvoke if you need to fire them off asynchronously). This also means you can't use Invoke if InvokeRequired is false, because there's no way for the message loop on the current thread to continue. So that's one big problem with your code above, but not necessarily the error you're reporting. You can actually use BeginInvoke in either case, if you watch out for recursive invocation, and so on.
However, you can't use either one without a window handle. If the Form/Control has been instantiated but not initialized (ie. before it is first shown) it may not have a handle yet. And the handle gets cleared by Dispose(), such as after the Form is closed. In either case InvokeRequired will return false because it is not possible to invoke without a handle. You can check IsDisposed, and there is also a property IsHandleCreated which more specifically tests if the handle exists. Usually, if IsDisposed is true (or if IsHandleCreated is false) you want to punt into a special case such as simply dropping the action as not applicable.
So, the code you want is probably more like:
if (IsHandleCreated)
{
// Always asynchronous, even on the UI thread already. (Don't let it loop back here!)
BeginInvoke(new UpdateTextFieldDelegate(WriteToForm), finished, numCount);
return; // Fired-off asynchronously; let the current thread continue.
// WriteToForm will be called on the UI thread at some point in the near future.
}
else
{
// Handle the error case, or do nothing.
}
Or maybe:
if (IsHandleCreated)
{
// Always synchronous. (But you must watch out for cross-threading deadlocks!)
if (InvokeRequired)
Invoke(new UpdateTextFieldDelegate(WriteToForm), finished, numCount);
else
WriteToForm(finished, numCount); // Call the method (or delegate) directly.
// Execution continues from here only once WriteToForm has completed and returned.
}
else
{
// Handle the error case, or do nothing.
}
This will typically happen in multithreaded scenarios where some external source (maybe a NetworkStream) pushes data to a form before the form has properly initialized.
The message can also appear after a Form is disposed.
You can check IsHandleCreated to see if a form is already created, but you need to put everything in proper error handling as the Invoke statement can throw an exception if you try to update your form while your application is closing.
here is my answer
Let's say you want to write"Hello World" to a text box.
Then IF you use "Ishandlecreated" then your operation will not happen if handlers are not yet created. So You must force itself to CreateHandlers if not yet created.
Here is my code
if (!IsHandleCreated)
this.CreateControl();
this.Invoke((MethodInvoker)delegate
{
cmbEmail.Text = null;
});
Assuming the form is not disposed but not yet fully initialized just put var X = this.Handle; before that if statement... by this the instance of the respective form is meant.
see http://msdn.microsoft.com/en-us/library/system.windows.forms.control.handle.aspx .
If you're going to use a control from another thread before showing the control or doing other things with the control, consider forcing the creation of its handle within the constructor. This is done using the CreateHandle function. In a multi-threaded project, where the "controller" logic isn't in a WinForm, this function is instrumental for avoiding these kinds of errors.
What about this :
public static bool SafeInvoke( this Control control, MethodInvoker method )
{
if( control != null && ! control.IsDisposed && control.IsHandleCreated && control.FindForm().IsHandleCreated )
{
if( control.InvokeRequired )
{
control.Invoke( method );
}
else
{
method();
}
return true;
}
return false;
}
Use it like that :
this.label.SafeInvoke(new MethodInvoker( () => { this.label.Text = yourText; }));
You are probably calling this in the constructor of the form, at that point the underlying system window handle does not exist yet.
Add this before you call your invoke method:
while (!this.IsHandleCreated)
System.Threading.Thread.Sleep(100)
This solution worked for me.
I have a label in a WinForm.in the app, I create a thread for
setting the Text property of the label.
since the code responsible for setting the Text property of the
label is another thread, I wrote it like this :
private void SetLabel8Text(string text)
{
try
{
if (this.label8.InvokeRequired)
{
SetTextCallback d = new SetTextCallback(SetLabel8Text);
this.Invoke(d, new object[] { text });
}
else
{
this.label8.Text = text;
}
}
catch (Exception ex)
{
}
}
now, I also handle the KeyPress event like so :
if (e.KeyChar.ToString() == "\r")
{
SetLabel8Text("Enter key Pressed !");
}
the problem I'm facing is that after pressing the Enter Key (execution of
the KeyPress event), the SetLabel8Text method never gets executed.
everything else seems to flow nicely ,
I tried stepping through the code and it hangs at this place(inside the SetLabe8Text method :
this.Invoke(d, new object[] { text });
it hangs and doesn't move forward a bit.
Well the fact that you're swallowing any exceptions thrown by SetLabel8Text makes it hard to know exactly what's going on. You should never swallow exceptions unconditionally without at least logging what's going on. (You also shouldn't just catch "Exception". Catch a more specific type of exception). Could you post a short but complete program which demonstrates the problem.
Adding logging in the key press event and the SetLabel8Text would also help.
Try calling BeginInvoke instead of Invoke.
Invoke is a blocking call, so it's possible that you have a race condition. (Invoke won't return until the method actually gets executed, but the method can only get executed once the UI thread processes its message loop).
Try the following:
if(e.KeyChar == '\n')
SetLabel8Text("Enter key Pressed !");
Try:
if (e.KeyCode == Keys.Enter) {
}
One possible explanation could be what you're doing next. Assuming the SetLabel8Text function is called in another thread it will require invoke, and thus execute once more. However, the second time it's executed (and thus not requiring invoking) it's executing in the thread that owns the GUI, normally the main thread. So if you have code blocking the main thread for some time somewhere else in your application it would seem like the SetLabel8Text function never got executed the second time. Always thread heavy tasks and keep the main thread idle for simplicity.
You're not following the basic tenet of winforms: Only create UI controls on the UI thread. Your event handler for the KeyPress event is on the UI thread, therefore, if your label was created on the UI thread you wouldn't need to use BeginInvoke/Invoke on it.
If you are creating forms, controls, etc. in threads other than the UI thread you're probably doing something wrong.
See: http://weblogs.asp.net/justin_rogers/pages/126345.aspx for the gory details.