In "Lucian Wischik - Async Part 2 -- deep dive into the new language feature of VB/C#" for NDC 2012, the recommended use of Dispatcher.Yield is introduced to me. Does anyone out there have examples (and explanations) of how (and why) this call is used in the wild?
Well for example if you have a long running task but you still need to update your UI you can use Yield.
Yield gives you the ability to leave the current thread context and allow other code to run in the underlying context.
public async void MyButton_Click(object sender, RoutedEventArgs e)
{
for( int i=0; i < 10000; i++)
{
ProcessSomeStuff(i);
// await the Yield to ensure all waiting messages
// are processed before continuing
await Task.Yield();
}
}
In the example above you can process stuff async but calling Yield will allow events on the UI thread to execute also,
I love to use "await Dispatcher.Yield()" whenever I develop WPF applications.
It is easy to use and barely makes any problems in most cases.
The most useful case using Dispatcher.Yield() is when there is a small time of lag in a operation, which can not be bypassed by "Task", "Thread" etc.
For example, let's say that there is a command button which opens a new window or tab.
private void aButton_Click(object sender, RoutedEventArgs e)
{
// Do some ui stuff
// You can not use Task here at large.
}
What happens here is the whole application stopping, and the button UI remains pressed until the new window or tab opens. It is bad user experience and makes the apllicaiton seem so much slow.
and here is a code with a trick.
private async void aButton_Click(object sender, RoutedEventArgs e)
{
await Dispatcher.Yield();
// Do some ui stuff
}
In here, your code works differently. Dispathcer will process other ui operations first before getting into the job. The button pressed motion will be released first and then a new window will come.
Application stopping will be same, but to users the application will be much smooth and fast. So it is usually a good idea to adopt Dispatcher.Yield() in your application.
In addition, Task.Yield() is different from Dispatcher.Yield(). Try two options and see the results.
Related
I have a Winforms application that schedules some work using a service. The service has a callback that updates the database with the work proceedings.
Now let's say I schedule x work items. After the x work items are all completed, I want to generate an HTML report about the work statistics. I think the only way I can check the work completion of all items is to see their completion statuses in the database.
Can someone tell me how and when I can generate the HTML report? I think I can use a thread to poll the db to see if all work completed and inform UI to generate the report, but don't know how to implement that.
You can use BackgroundWorker component for waiting async task completed. It's easy.
First - drag BackgroundWorker from Toolbox to your form.
Second - when you start processing (e.g. on button click event) add following code:
private void button_Click(object sender, EventArgs e)
{
backgroundWorker1.DoWork += backgroundWorker1_DoWork;
backgroundWorker1.RunWorkerCompleted += backgroundWorker1_RunWorkerCompleted;
backgroundWorker1.RunWorkerAsync();
}
Next - add to DoWork event handler code that starts processing and polling database:
private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
// schedule some work
// poll database
}
And last - add RunWorkerCompleted handler, which will run just after DoWork completed (i.e. all tasks updated their state in database):
private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
// generate report
}
Thats it. Btw BackgroundWorker could report progress during polling database. Little searching will help you how to do that :)
You can use Task.ContinueWith and Task.Wait
Example: Using Task
Task.ContinueWith:
This extends the initial task. Run immediatly after it finishes
Task.Wait
You would use this if you are running several task that provide the same result be may do so at different times, i.e. fetching same data from different web services.(Just an example).
This will execute as soon as one of the appropriate task successfully finishes.
When you have a button, and do something like:
Private Function Button_OnClick
Button.Enabled = False
[LONG OPERATION]
End Function
Then the button will not be grayed, because the long operation prevents the UI thread from repainting the control. I know the right design is to start a background thread / dispatcher, but sometimes that's too much hassle for a simple operation.
So how do I force the button to redraw in disabled state? I tried .UpdateLayout() on the Button, but it didn't have any effects. I also tried System.Windows.Forms.DoEvents() which normally works when using WinForms, but it also had no effect.
The following code will do what you're looking for. However I would not use it. Use the BackgroundWorker class for long time operations. It's easy to use and very stable.
Here the code:
public static void ProcessUITasks() {
DispatcherFrame frame = new DispatcherFrame();
Dispatcher.CurrentDispatcher.BeginInvoke(DispatcherPriority.Background, new DispatcherOperationCallback(delegate(object parameter) {
frame.Continue = false;
return null;
}), null);
Dispatcher.PushFrame(frame);
}
Here you will find a sample on how to use the BackgroundWorker.
InvalidateVisual(); #HCL is right... don't do this
Like you say, it is better to start use a background thread / dispatcher and keep the UI thread unblocked. Consider looking at the Reactive Extensions library from Microsoft for high level asynchronous ui programming
In Windows.Forms, you can Button.Refresh().
In Windows.Forms or WPF, you can yield to the message pump to let it redraw. Async/Await were designed to allow you to do this without the nastiness of HCL's answer.
Private Async Function Button_OnClick
Button.Enabled = False
Await Task.Yield
[LONG OPERATION]
End Function
As part of my App's startup procedure, it checks data integrity, and if it finds a problem it pops up a message to the user telling them that it might take a while to repair things.
I'm showing the message using MessageBox.Show. Because the data check is done from a worker thread, I'm switching over to the UI thread to make that call, and then setting a ManualResetEvent to tell the worker thread when the user has acknowledged the message.
I kick off the data check/load very early in the app's lifecycle from the constructor in the main Application class, by spinning off a worker thread (using the ThreadPool).
When I run with the debugger, and the message is displayed, the app just waits for input. When I run without the debugger, the app terminates after displaying the dialog for 10 seconds.
That 10 seconds is a big clue - it tells me that the OS thinks the app took too long to initialize (the OS kills apps that take too long to start up).
I think that my MessageBox.Show is blocking the UI thread before the App.RootFrameNavigating has a chance to be invoked.
My questions:
Does my diagnosis sound right?
I'd prefer to kick off my data load early, because it is almost entirely IO, except for this Message Box, and the sooner I can get my Model loaded, the better, but do you normally delay your data load until later in the app lifecycle?
Any other ideas/suggestions? I can't guarantee which page will be the start page, because the app could be resuming to any page. I'm also thinking of having the MessageBox.Show delay itself until the app has initialized, perhaps polling away for a flag set by App.RootFrameNavigating - does that make sense?
I think your problem is a result of kicking off the worker thread in the Application constructor. You should use the appropriate life-cycle event, in this case: PhoneApplicationService.Activated Event
So, the solution I've come up with is to still kick off the data load in a worker-thread from the Application's constructor, but in my PhoneService's class ShowDialog method that I invoke to invoke MessageBox.Show, I check to see if the initial navigation has occurred:
private readonly ManualResetEvent _appInitialized = new ManualResetEvent(false);
public void AppInitialized()
{
_appInitialized.Set();
}
public void ShowDialog(string caption, string text, Action<MessageBoxResult> callback, MessageBoxButton button = MessageBoxButton.OKCancel)
{
_appInitialized.WaitOne();
DispatcherHelper.CheckBeginInvokeOnUI(() =>
{
var result = MessageBox.Show(text, caption, button);
if (callback != null)
{
callback(result);
}
});
}
Then in my Application class:
private bool _firstNavigate = true;
private void RootFrameNavigating(object sender, NavigatingCancelEventArgs e)
{
if (_firstNavigate)
{
_firstNavigate = false;
var navigationService = (NavigationService) sender;
navigationService.Navigated += NavigationServiceNavigated;
}
....
private void NavigationServiceNavigated(object sender, NavigationEventArgs e)
{
var navigationService = (NavigationService)sender;
navigationService.Navigated -= NavigationServiceNavigated;
PhoneServices.Current.AppInitialized();
}
Anyone see any issues with this approach? Anyone come up with a better way?
I need to check if a c# WinForm Window (FORM Class) has been initialized and waiting for user events. But I could not find out how to manage that.
Therefore I had the idea to set the Control.IsAccessible Flag of the Form to true, within the OnLoad Event of the Windows Form.
My question is now, what is the Control.IsAccessible Flag origin intended for? Or is there an other solution to check if the Winform is initialized.
Thanks for your help
I do not know what IsAccessible is intended for but for the check you are doing you want Created
if(myForm.Created)
{
//Do stuff
}
I had a whole bunch of problems with it, here is one of my old question on SO that helped me out a lot with it.
Control.IsAccessible just means the control is visible to accessibility applications.
You can check myForm.Created to see if the window exists.
You can also register an event handler for the Application.Idle event, which occurs when the application has finished initializing and is ready to begin processing windows messages.
Here is a common usage:
public int Main(string[] args)
{
Application.Idle += WaitUntilInitialized;
}
private void WaitUntilInitialized(object source, EventArgs e)
{
// Avoid processing this method twice
Application.Idle -= WaitUntilInitialized;
// At this point, the UI is visible and waiting for user input.
// Begin work here.
}
I have a windows form on the main thread and another thread that does some calculations. I'd like to update the status bar on my form from the work being done in the other thread. What's the best way to do this?
So far everything I've tried just makes things run really slowly. I'm using Visual Studio 2005.
You can use the marshaling techniques like Control.Invoke to execute a delegate on the UI thread where UI elements can be safely manipulated, but that approach is not very good. Actually, it is a terrible approach if all you want to do is update simple progress information.
By far the best method for doing this is:
Have your worker thread publish progress information to a shared variable.
Have your UI thread poll for it via a System.Windows.Forms.Timers on an interval that works well for you.
Here is what it might look like.
public class Example : Form
{
private volatile int percentComplete = 0;
private void StartThreadButton_Click(object sender, EventArgs args)
{
StatusBarUpdateTimer.Enabled = true;
new Thread(
() =>
{
for (int i = 1; i <= 100; i++)
{
DoSomeWork();
percentComplete = i;
}
}).Start();
}
private void StatusBarUpdateTimer_Tick(object sender, EventArgs args)
{
yourStatusBarPanel.Text = percentComplete.ToString() + "%";
StatusBarUpdateTimer.Enabled = percentComplete < 100;
}
}
This works well because:
The percentComplete field is declared 'volatile' ensuring its value can be reliably read from multiple threads.
The UI thread gets to dictate when and how often the UI gets updated...the way it should be!
The worker thread does not have to wait for a response from the UI thread before it can proceed as would be the case with Invoke.
It breaks the tight coupling between the UI and worker threads that Invoke would impose.
It is more efficient...considerably.
You get more throughput on both the UI and worker threads.
There is no chance of saturating the UI message queue as could be the case with BeginInvoke.
You do not have to litter you code with Invoke calls everytime you need to update the UI from the worker thread.
Make sure that you only update the user interface from the main thread or else you will have problems. You can switch your thread context by calling Invoke. There's a good post here on that.
You can send messages to the main thread and get it to update the progress bar, although you then need to check for the messages. You could also do the same sort of thing as a polling function.