I have implemented a WPF application using background worker in which I am assigning data from database to combobox.
I have used observable collection as ItemSource for combobox .
When I try to fill up the observable collection in DoWork method of background worker, UI is not getting updated but if I move same code to ProgressChanged or RunWorkerCompleted then it is working fine.
I want to know why the DoWork method is not updating the UI.Also the logic for updating UI should be in ProgressChanged or RunWorkerCompleted?
To quote MSDN : "You must be careful not to manipulate any user-interface objects in your DoWork event handler" - so in your DoWork method call ReportProgress event and update your ui in the ProgressChanged event handler
There are many online examples showing how to correctly utilise a BackgroundWorker. As you haven't bothered to show us your code, all we can do is guess, but I'm guessing that you simply haven't implemented your code properly. Not wanting to duplicate this code once again, I'd rather advise you to take a look at my answer to the Progress Bar update from Background worker stalling question here on Stack Overflow.
It clearly demonstrates how to implement a BackgroundWorker correctly.
Related
I have a ListBox bound to an ObservableCollection. The items in the collection each signify a different step with a method to perform that step. What I would like to do is have a BackgroundWorker loop through the collection, run each item's action method, and have the current step be reflected by the ListBox.
My first iteration used no threading and was just to get the step running going. Now I have created a ListCollectionView, set it to represent the data in the ObservableCollection, and have bound the ListBox's ItemsSource to it instead of the ObservableCollection. I noticed running through the steps still blocks the UI thread's updates even though I'm explicitly incrementing the CurrentItem.
I want to use the ListCollectionView inside a BackgroundWorker but most examples I'm finding are written assuming you are modifying the contents or sorting of the list. I don't wish to do this; I simply want to increment the CurrentItem for each iteration. I'm guessing simply referencing it won't get me very far as it is tied to items on the UI thread and the compiler will complain. Any thoughts or pointers would be much appreciated.
You shouldn't be seeing any complaints from the compiler, but you will be getting exceptions at runtime if you try to update anything that fires an INotifyPropertyChanged PropertyChange from the background thread. There are a few ways around this. You can use Dispatcher.Current.Invoke to just do updates from inside your DoWork method. You can try to rig something through a ProgressChanged handler (runs on the calling thread) and calls to ReportProgress for each completed step. Or you could do the updates in the RunWorkerCompleted handler (also runs on calling thread) and maybe use a series of consecutive BackgroundWorker calls that are triggered from the previous one's completed handler (this can get messy if you don't manage the steps through a generic queue or something similar).
You need to check out WPF Cross-Thread Collection Binding – Part 4 – The Grand Solution and the source I think is on QuantumBitDesigns.Core
This allows you to update a list from another thread and have the changes automaticaly reflected on a bindable observable collection.
Figure: The demo app shows multiple updates to a single ObservableCollection
I have used this on multiple projects with fantastic results.
I have a wpf application that takes ~30 seconds to create a map/graphic. I have read there is no easy way to tie into the UI rendering thread to get a progress update. So I was going to use a counter on a value converter which colors my map but that is also on the UI Thread, so my question is has anyone found any slick methods of working with the rendering thread yet?
Thanks.
You could create your map/graphic in a BackgroundWorker which allows you to call ReportProgress in your function, where you can set your percentage of completion and raise the ProgressChanged event to update your UI.
When you say UI rendering thread, you mean that hidden rendering thread from WPF internals or UI thread?
In any case, having a separate thread that builds your map and notifies UI about progress doesn't help you?
im not sure if this is what you are looking for.
I use something similar to the code below to load in around 300 images( about 200 mb ) and have no UI slow down at all. (the user can see each image being loaded in, I just keep an empty placeholder image up till the final image is loaded)
The images are loaded in a background thread, and then the function is called to actually put them into the WPF scene.
here is a simple example using a textbox. You can call this function from any thread and it will work out if it needs to change the to the GUI thread. (for my project of course i am doing it with bitmaps, not a textbox ).
delegate void UpdateUIThreadDelegate(String str);
public void DisplayString(String strMessage)
{
if (this.InvokeRequired)
{
UpdateUIThreadDelegate updateDelegate = DisplayString;
this.BeginInvoke(updateDelegate, strMessage);
return;
}
myTextBox.Text = strMessage;
}
Cheers
Anton
If you use binding to tie your UI with a datasource which can take long time to return, you can set 'IsAsync=True' on your binding so that the binding become asynchronous.
If you want to display some other datas (even an animation I guess) during the time your datasource is loading, you can use a PriorityBinding
HTH
Riana
I tried creating one, but the BackgroundWorker in Window1 couldn't access the ProgressBar in Window2 once the reportProgress was activated, because "The calling thread cannot access this object because a different thread owns it".
Seems there's a lower level thread model I could use, but it also seems a lot more complicated.
You just need to get the ProgressBar disptacher.
You can access the ProgressBar with:
Window2.prograssbar.Dispatcher.Invoke(
() => /*the code for modifying the progressbar*/ );
In WPF, UI controls and properties may only be activated from the UI thread. In order to change the progress bar's value from a different thread, you can add a command to the GUI thread's dispatcher queue. You can do this by passing a delegate to the Dispatcher.Invoke() method. See the article at http://msdn.microsoft.com/en-us/magazine/cc163328.aspx for more details.
You need to look into Delegates
I have been trying to get a progressbar set to marquee to keep moving while another function is running. After this function runs, I message would display (for this example)
The only way I was able to get this working was with a background worker and then have a
Do
Loop until condition that runs in the main form until the operation is complete followed by my message box.
This seems like a kludge way to accomplish this and a thread.start followed by a thread.join seems like a much nicer way to fix this. However, I was not able to get that working either.
I have included a small demo program if anyone is interested.
http://www.filedropper.com/progressbar
Thanks
Thread.Start and Thread.Join is not the way to do it - that basically blocks your UI thread again. Application.DoEvents isn't the way to go either - you really do want a separate thread.
You could then use Control.Invoke/Control.BeginInvoke to marshal back to the UI thread, but BackgroundWorker makes all this a lot easier. A search for "BackgroundWorker tutorial" yields lots of hits.
EDIT: To show the message when the worker has finished, use the RunWorkerCompleted event. The ReportProgress method and ProgressChanged event are used to handle updating the progress bar. (The UI subscribes to ProgressChanged, and the task calls ReportProgress periodically.)
That is not a kludge. That is the correct way of doing it; what happens with the BackgroundWorker approach? The trick is to use the ReportProgress method to push the change back to the UI (don't update the ProgressBar from the worker).
Use Application.DoEvents() in your function from time to time so that your process has some time to process his events, including redrawing the form.
Alternatively, you can use a worker thread (like a BackgroundWorker) to process your treatement, while the UI thread is displaying your progress bar.
Complementing the answer given by Marc Gravell, the BackbroundWorker has a boolean property WorkerReportsProgress, if it is set to false, when you call ReportProgress, the program will raise an InvalidOperationException
I'm currently designing/reworking the databinding part of an application that makes heavy use of winforms databinding and updates coming from a background thread (once a second on > 100 records).
Let's assume the application is a stock trading application, where a background thread monitors for data changes and putting them onto the data objects. These objects are stored in a BindingList<> and implement INotifyPropertyChanged to propagate the changes via databinding to the winforms controls.
Additionally the data objects are currently marshalling the changes via WinformsSynchronizationContext.Send to the UI thread.
The user is able to enter some of the values in the UI, which means that some values can be changed from both sides. And the user values shouldn't be overritten by updates.
So there are several question coming to my mind:
Is there a general design-guildline how to do that (background updates in databinding)?
When and how to marshal on the UI thread?
What is the best way of the background thread to interact with
binding/data objects?
Which classes/Interfaces should be used? (BindingSource, ...)
...
The UI doesn't really know that there is a background thread, that updates the control, and as of my understanding in databinding scenarios the UI shouldn't know where the data is coming from... You can think of the background thread as something that pushes data to the UI, so I'm not sure if the backgroundworker is the option I'm searching for.
Sometimes you want to get some UI response during an operation in the data-/business object (e.g. setting the background during recalculations). Raising a propertychanged on a status property which is bound to the background isn't enough, as the control get's repainted after the calculation has finished? My idea would be to hook on the propertychanged event and call .update() on the control...
Any other ideas about that?
This is a hard problem since most “solutions” lead to lots of custom code and lots of calls to BeginInvoke() or System.ComponentModel.BackgroundWorker (which itself is just a thin wrapper over BeginInvoke).
In the past, I've also found that you soon wish to delay sending your INotifyPropertyChanged events until the data is stable. The code that handles one propriety-changed event often needs to read other proprieties. You also often have a control that needs to redraw itself whenever the state of one of many properties changes, and you don’t wan the control to redraw itself too often.
Firstly, each custom WinForms control should read all data it needs to paint itself in the PropertyChanged event handler, so it does not need to lock any data objects when it was a WM_PAINT (OnPaint) message. The control should not immediately repaint itself when it gets new data; instead, it should call Control.Invalidate(). Windows will combine the WM_PAINT messages into as few requests as possible and only send them when the UI thread has nothing else to do. This minimizes the number of redraws and the time the data objects are locked. (Standard controls mostly do this with data binding anyway)
The data objects need to record what has changed as the changes are made, then once a set of changes has been completed, “kick” the UI thread into calling the SendChangeEvents method that then calls the PropertyChanged event handler (on the UI thread) for all properties that have changed. While the SendChangeEvents() method is running, the data objects must be locked to stop the background thread(s) from updating them.
The UI thread can be “kicked” with a call to BeginInvoke whenever a set of update have bean read from the database. Often it is better to have the UI thread poll using a timer, as Windows only sends the WM_TIMER message when the UI message queue is empty, hence leading to the UI feeling more responsive.
Also consider not using data binding at all, and having the UI ask each data object “what has changed” each time the timer fires. Databinding always looks nice, but can quickly become part of the problem, rather then part of the solution.
As locking/unlock of the data-objects is a pain and may not allow the updates to be read from the database fast enough, you may wish to pass the UI thread a (virtual) copy of the data objects. Having the data object be persistent/immutable so that any changes to the data object return a new data object rather than changing the current data object can enable this.
Persistent objects sound very slow, but need not be, see this and that for some pointers. Also look at this and that on Stack Overflow.
Also have a look at retlang - Message-based concurrency in .NET. Its message batching may be useful.
(For WPF, I would have a View-Model that sets in the UI thread that was then updated in ‘batches’ from the multi-threaded model by the background thread. However, WPF is a lot better at combining data binding events then WinForms.)
Yes all the books show threaded structures and invokes etc. Which is perfectly correct etc, but it can be a pain to code, and often hard to organise so you can make decent tests for it
A UI only needs to be refreshed so many times a second, so performance is never an issue, and polling will work fine
I like to use a object graph that is being continuously updated by a pool of background threads. They check for actual changes in data values and when they notice an actual change they update a version counter on the root of the object graph (or on each main item whatever makes more sense) and updates the values
Then your foreground process can have a timer (same as UI thread by default) to fire once a second or so and check the version counter, and if it changes, locks it (to stop partial updates) and then refreshes the display
This simple technique totally isolates the UI thread from the background threads
There is an MSDN article specific on that topic. But be prepared to look at VB.NET. ;)
Additionally maybe you could use System.ComponentModel.BackgroundWorker, instead of a generic second thread, since it nicely formalize the kind of interaction with the spawned background thread you are describing. The example given in the MSDN library is pretty decent, so go look at it for a hint on how to use it.
Edit:
Please note: No marshalling is required if you use the ProgressChanged event to communicate back to the UI thread. The background thread calls ReportProgress whenever it has the need to communicate with the UI. Since it is possible to attach any object to that event there is no reason to do manual marshalling. The progress is communicated via another async operation - so there is no need to worry about neither how fast the UI can handle the progress events nor if the background thread gets interruped by waiting for the event to finish.
If you prove that the background thread is raising the progress changed event way too fast then you might want to look at Pull vs. Push models for UI updates an excellent article by Ayende.
I just fought a similar situation - badkground thread updating the UI via BeginInvokes. The background has a delay of 10ms on every loop, but down the road I ran into problems where the UI updates which sometimes get fired every time on that loop, can't keep up with teh freq of updates, and the app effectively stops working (not sure what happens- blew a stack?).
I wound up adding a flag in the object passed over the invoke, which was just a ready flag. I'd set this to false before calling the invoke, and then the bg thread would do no more ui updates until this flag is toggled back to true. The UI thread would do it's screen updates etc, and then set this var to true.
This allowed the bg thread to keep crunching, but allowed the ui to shut off the flow until it was ready for more.
Create a new UserControl, add your control and format it (maybe dock = fill) and add a property.
now configure the property to invoke the usercontrol and update your element, each time you change the property form any thread you want!
thats my solution:
private long value;
public long Value
{
get { return this.value; }
set
{
this.value = value;
UpdateTextBox();
}
}
private delegate void Delegate();
private void UpdateTextBox()
{
if (this.InvokeRequired)
{
this.Invoke(new Delegate(UpdateTextBox), new object[] {});
}
else
{
textBox1.Text = this.value.ToString();
}
}
on my form i bind my view
viewTx.DataBindings.Add(new Binding("Value", ptx.CounterTX, "ReturnValue"));
This is a problem that I solved in Update Controls. I bring this up not to suggest you rewrite your code, but to give you some source to look at for ideas.
The technique that I used in WPF was to use Dispatcher.BeginInvoke to notify the foreground thread of a change. You can do the same thing in Winforms with Control.BeginInvoke. Unfortunately, you have to pass a reference to a Form object into your data object.
Once you do, you can pass an Action into BeginInvoke that fires PropertyChanged. For example:
_form.BeginInvoke(new Action(() => NotifyPropertyChanged(propertyName))) );
You will need to lock the properties in your data object to make them thread-safe.
This post is old but I thought I'd give options to others. It seems once you start doing async programming and Windows Forms databinding you end up with problems updating Bindingsource datasource or updating lists bound to windows forms control. I am going to try using Jeffrey Richters AsyncEnumerator class from his powerthreading tools on wintellect.
Reason:
1. His AsyncEnumerator class automatically marshals background threads to UI threads so you can update controls as you would doing Synchronous code.
2. AsyncEnumerator simplifies Async programming. It does this automatically, so you write your code in a Synchronous fashion, but the code is still running in an asynchronous fashion.
Jeffrey Richter has a video on Channel 9 MSDN, that explains AsyncEnumerator.
Wish me luck.
-R
I am late to the party but I believe this is still a valid question.
I would advise you to avoid using data binding at all and use Observable objects instead.
The reason is, data binding looks cool and when implemented the code looks good, but data binding miserably fails when there is lot os asynchronous UI update or multi-threading as in your case.
I have personally experienced this problem with asynchronous and Databinding in prod, we even didn't detect it in testing, when users started using all different scenarios things started to break down.