I do some tests with the new asynchronous pattern of C# 5.0 (async/await) I have a problem with understanding how the asynchronous methods are called.
Considering this code :
private async Task<string> DownloadAsync()
{
progress.ProgressChanged += (s, e) =>
{
progressBar1.Value = e.value;
};
return await DownloadSomething(myurl, progress);
}
private async void CallDownloadAsync()
{
string text = await DownloadAsync();
progressBar1.Value = 0;
label1.Text = "Done!";
}
private void button4_Click(object sender, EventArgs e)
{
CallDownloadAsync();
}
So, this code works very well. When I clic the "button4" a downloading task begins and my ProgressBar is updated correctly.
But, I'd like to compact my code a little bit more by removing CallDownloadAsync() method like this :
private void button4_Click(object sender, EventArgs e)
{
new Action(async () =>
{
string result = await Task.Run<string>(() => DownloadAsync());
}).Invoke();
label1.Text = "Running...";
}
So here, I want to directly initiate an action which calls the DownloadAsync method but when I hit my Button4 I have a Cross-thread operation not valid on the progressBar. So I do not understand what is the main difference between the Action() and the call of my CallDownloadAsync() method.
You may find my async/await intro helpful. In particular, an async method does not run on a background thread; Task.Run is used to run something on a background thread, hence the difference in your code.
In general, you should avoid async void, unless you're writing an async event handler. Like this:
private async void button4_Click(object sender, EventArgs e)
{
label1.Text = "Running...";
string result = await DownloadAsync();
progressBar1.Value = 0;
label1.Text = "Done!";
}
The difference is that in former case you call CallDownloadAsync() from UI thread (context).
In the latter case, DownloadAsync() is called from the initiated Task which is generally executed in a different thread created by TPL (Task Parallel Library) out of UI thread or threads created from it.
In WPF, UI components can be accessed only by a dedicated UI thread or (its children) threads created from under it (i.e with the same UI context).
Related
I have a task that runs in the form_load event of a usercontrol in winforms:
private void ucDeviceInsert_Load(object sender, EventArgs e)
{
System.Threading.Tasks.Task getTBox = System.Threading.Tasks.Task.Run(async () =>
{
await AVeryLongRunningProccess();
});
pbImage.Image = Properties.Resources.Remove;
getTBox.Wait();
pbImage.Image = Properties.Resources.Insert;
btnNext.Visible = true;
tmrDeviceInsert.Enabled = true;
tmrDeviceInsert.Start();
}
private void tmrDeviceInsert_Tick(object sender, EventArgs e)
{
Next();
}
I change the image of the picture box to inform the user the progress of the long running process. That part works fine, however the button doesn't show, and the timer never starts. I've stepped through the code, and I can confirm that it is running without any problems, which makes this even more baffling. Any ideas what would be causing this issue?
Task.Run is for pushing CPU-intensive work off the UI thread. Since you're calling an asynchronous method, I suspect it's not CPU-intensive.
So, you can just use async and await:
private async void ucDeviceInsert_Load(object sender, EventArgs e)
{
pbImage.Image = Properties.Resources.Remove;
await AVeryLongRunningProccess();
pbImage.Image = Properties.Resources.Insert;
btnNext.Visible = true;
tmrDeviceInsert.Enabled = true;
tmrDeviceInsert.Start();
}
Note that at the await, the UI is shown and the user can interact with it (that's the point).
getTBox.Wait() is going to try to complete that task synchronously. Therefore, the rest of the code after it won't happen until after the task completes.
I'd think you don't want your task to run synchronously at all, but rather handle its completion asynchronously, something like this:
getTBox.ContinueWith(() => updateStatusInUI());
I get data from database on a click.
I have an event handler which when triggered should show "data retrieving..." in status bar and should change to "Ready" again just before the event handler ends.
But the text updates only once, the second Ready one. How is it generally done?
private void Next_Click(object sender, RoutedEventArgs e){
this.footerText = "Waiting for dataRetreival";
someRandomTimeTakingMethod(); //Gets Data from DB.
this.footerText = "Ready";
}
Even though code executes line 2, the view updates only when the function is over, ie only the second one actually works.
You should put your data-intensive work on a background thread so the UI can update properly. This provides the best user experience.
To elaborate on FZysset's answer with some code...
private async void Next_Click(object sender, RoutedEventArgs e)
{
footerText.Text = "Waiting for dataRetreival";
IsEnabled = false;
await SomeRandomTimeTakingMethodAsync();
IsEnabled = true;
footerText.Text = "Ready";
}
private async Task SomeRandomTimeTakingMethodAsync()
{
await Task.Delay(TimeSpan.FromSeconds(new Random().Next(2, 5)));
// or await Task.Run(() => { ... });
}
The above example allows you to leverage await/async that was introduced in .NET 4.5. Notice how nicely it flows? No nonsense!
We're putting stuff onto the background thread so the UI can remain unblocked (thus it will show your updates to your status bar and allow user interaction.) Of course, you have to be careful not to update anything on the UI from your background thread.
If you are using an older version of .NET, you can just use TPL without async/await:
private void Next_Click(object sender, RoutedEventArgs e)
{
footerText.Text = "Waiting for dataRetreival";
IsEnabled = false;
Task.Factory.StartNew(() =>
{
SomeRandomTimeTakingMethod();
}).ContinueWith(t =>
{
IsEnabled = true;
footerText.Text = "Ready";
}, TaskScheduler.FromCurrentSynchronizationContext());
}
private void SomeRandomTimeTakingMethod()
{
Thread.Sleep(TimeSpan.FromSeconds(new Random().Next(2, 5)));
}
Two important things to note about the latter example:
You must provide TaskScheduler.FromCurrentSynchronizationContext() to the ContinueWith call, or you will encounter exceptions because the continuation is not on the UI thread. You must get the context in a method that isn't running on a background thread.
You will want to check for exceptions on the Task object in your ContinueWith.
This example is very rudimentary though. If you were to have a bunch of background operations kicked off with click handlers, you'd want to give yourself some helper classes/services to make life easier. (And investigate MVVM, which I cannot tell if you are using.)
A colleague of mine gave a presentation on using various asynchronous patterns in C# and .NET. You can check it out here: https://github.com/mtusk/TplLunchAndLearn
That's because you're "someRandomTimeTakingMethod" is launched on the UI Thread. Therefore it will not update the view until it is finished.
To go around this you have the following possibilities :
Make your method "someRandom..." asynchronous with a task, and use the await operator : http://msdn.microsoft.com/en-us/library/hh191443.aspx
Launch your randomTimeTaking method into a thread, and launch an event when your execution is finished, to update the footer text
I strongly recommend you the first option, for some sample : http://msdn.microsoft.com/en-us/library/hh873191.aspx
You need to run those lines asynchronously. You can do that using the Task class:
private void Next_Click(object sender, RoutedEventArgs e){
Task.Factory.StartNew(() => footerText = "Waiting for dataRetreival");
someRandomTimeTakingMethod(); //Gets Data from DB.
Task.Factory.StartNew(() => footerText = "Ready");
}
There is one way to do it using Dispatcher. The original post is here.
The code is:-
private void Next_Click(object sender, RoutedEventArgs e){
UpdateUI("Please wait for data retrieval", delegate() { someRandomTimeTakingMethod(); });
this.footerText = "Ready";
}
public delegate void NoArgsDelegate();
public void UpdateUI(string description, NoArgsDelegate operation)
{
this.FooterText= description;
DispatcherFrame frame = new DispatcherFrame();
DispatcherOperation dispatcherOperation = Dispatcher.CurrentDispatcher.BeginInvoke(DispatcherPriority.ContextIdle, operation);
dispatcherOperation.Completed += delegate(object sender, EventArgs e)
{
frame.Continue = false;
};
Dispatcher.PushFrame(frame);
}
If my understanding is right, this uses Asynchronous programming, not different thread. The thread will update UI first and then call the someRandomTimeTakingMethod().
Does it right code, if I update my UI controls in task like this?
Or it's wrong, and I need use smth like Control.Invoke?
private async void button1_Click(object sender, EventArgs e)
{
textBox1.Text = await Task<string>.Factory.StartNew(() =>
{
Foo();
return "Completed";
});
}
private void Foo()
{
for (int i = 0; i < 100; i++)
{
textBox1.Text = i.ToString();
Thread.Sleep(1000);
}
}
The BCL addresses this specific scenario with the IProgress interface, implemented in the Progress class, to provide rich asynchronous progress reporting. This is available in .NET 4.5, or .NET 4 with the BCL Portability Nuget package. A lot of BCL classes accept an IProgress parameter for progress reporting.
Servy's answer addresses the immediate problem of how to update the UI after an asynchronous operation, but that forces you to mix UI code inside the long running operation. IProgress allows you to make an OnReport call with your report data without any concerns about marshalling the call to the proper thread, synchronization contexts, UI specific calls etc.
Your code can be as simple as this:
private async void button1_Click(object sender, EventArgs e)
{
var progress=new Progress<string>(msg=>textBox1.Text = msg);
await Task<string>.Factory.StartNew(() => Foo(progress));
}
private void Foo(IProgress<string> progress)
{
for (int i = 0; i < 100; i++)
{
progress.OnReport(i.ToString());
Thread.Sleep(1000);
}
progress.OnReport("Finished");
}
Or you can use a more complex progress type, eg
class MyProgressData
{
public string Message{get;set;}
public int Iteration {get;set;}
public MyProgressData(string message,int iteration) ...
}
private async void button1_Click(object sender, EventArgs e)
{
var progress=new Progress<MyProgressData>(msg=>{
textBox1.Text = msg.Message;
textBox2.Text=msg.Iteration.ToString();
});
await Task<string>.Factory.StartNew(() => Foo(progress));
}
private void Foo(IProgress<MyProgressData> progress)
{
for (int i = 0; i < 100; i++)
{
progress.OnReport(new MyProgressData("Hi",i));
Thread.Sleep(1000);
}
progress.OnReport("Finished");
}
The beauty of this is that you can completely decouple processing from reporting. You can put the processing code in a completely different class or even project from your UI.
You can just run the code to see that it won't work, and that your program will crash, as you are touching the UI from a non-UI thread.
You should instead update the UI as a continuation to the task that is scheduled to run in the UI thread. await does all of this for you, making the code pretty trivial.
You also shouldn't create a thread pool thread just to have it sit there and do nothing while you sleep for a fixed amount of time. Use Task.Delay instead.
private async void button1_Click(object sender, EventArgs e)
{
for (int i = 0; i < 100; i++)
{
textBox1.Text = i.ToString();
await Task.Delay(1000);
}
textBox1.Text = "Completed";
}
Your Foo method will throw, since you're trying to update a UI control from a background thread.
If Foo did something arbitrary, which doesn't evolve any UI controls, then your textBox1.Text = await Task.Factory.StartNew would be fine, since you use await and the SynchronizationContext would be implicitly captured and used once the await completes, which would make the assignment on the UI thread.
This doesn't make too much sense, but if you wanted to update a control from inside the Task, you'd have to use Control.Invoke.
Never tried asynch calling with Windows Form. I cannot use the new aynch/await because I don't own recent Visual Studio/.NET. What I need is "execute an operation that request long time (populate an IList)" and, when it has finished, write on a TextBox the result of that list.
Searching on Internet I found this example that seems work, but TOO comply in my opinion (maybe there are somethings fast and simple):
private void button1_Click(object sender, EventArgs e)
{
MyTaskAsync();
}
private void MyTaskWorker()
{
// here I populate the list. I emulate this with a sleep of 3 seconds
Thread.Sleep(3000);
}
private delegate void MyTaskWorkerDelegate();
public void MyTaskAsync()
{
MyTaskWorkerDelegate worker = new MyTaskWorkerDelegate(MyTaskWorker);
AsyncCallback completedCallback = new AsyncCallback(MyTaskCompletedCallback);
AsyncOperation async = AsyncOperationManager.CreateOperation(null);
worker.BeginInvoke(completedCallback, async);
}
private void MyTaskCompletedCallback(IAsyncResult ar)
{
MyTaskWorkerDelegate worker = (MyTaskWorkerDelegate)((AsyncResult)ar).AsyncDelegate;
AsyncOperation async = (AsyncOperation)ar.AsyncState;
worker.EndInvoke(ar);
AsyncCompletedEventArgs completedArgs = new AsyncCompletedEventArgs(null, false, null);
async.PostOperationCompleted(delegate(object e) { OnMyTaskCompleted((AsyncCompletedEventArgs)e); }, completedArgs);
}
public event AsyncCompletedEventHandler MyTaskCompleted;
protected virtual void OnMyTaskCompleted(AsyncCompletedEventArgs e)
{
if (MyTaskCompleted != null)
MyTaskCompleted(this, e);
// here I'll populate the textbox
textBox1.Text = "... content of the Iteration on the List...";
}
really I NEED somethings like 50 lines of code for this easy operation? Or I can remove some stuff? I just need a simple asynch call->callback when finished.
No lock, no concurrency at all...
You can use the TPL with C# 4.0 like so:
private void button1_Click(object sender, EventArgs e)
{
Task.Factory.StartNew(() => DoWork())
.ContinueWith(t => UpdateUIWithResults(t.Result)
, CancellationToken.None
, TaskContinuationOptions.None
, TaskScheduler.FromCurrentSynchronizationContext());
}
This starts DoWork in a thread pool thread, allowing it to do processing out of the UI thread, then runs UpdateUIWithResults in a UI thread, passing it the results of DoWork.
You can use Task.Factory.StartNew to push work onto the thread pool. Task.ContinueWith will give you a "completed callback".
private void button1_Click(object sender, EventArgs e)
{
var ui = TaskScheduler.FromCurrentSynchronizationContext();
Task<List<T>> task = Task.Factory.StartNew(() => MyTaskWorker());
task.ContinueWith(t => OnMyTaskCompleted(t), ui);
}
private List<T> MyTaskWorker()
{
// here I populate the list. I emulate this with a sleep of 3 seconds
Thread.Sleep(3000);
return ...;
}
protected virtual void OnMyTaskCompleted(Task t)
{
// here I'll populate the textbox with t.Result
textBox1.Text = "... content of the Iteration on the List...";
}
i was doing some processor heavy task and every time i start executing that command my winform freezes than i cant even move it around until the task is completed. i used the same procedure from microsoft but nothing seem to be changed.
my working environment is visual studio 2012 with .net 4.5
private async void button2_Click(object sender, EventArgs e)
{
Task<string> task = OCRengine();
rtTextArea.Text = await task;
}
private async Task<string> OCRengine()
{
using (TesseractEngine tess = new TesseractEngine(
"tessdata", "dic", EngineMode.TesseractOnly))
{
Page p = tess.Process(Pix.LoadFromFile(files[0]));
return p.GetText();
}
}
Yes, you're still doing all the work on the UI thread. Using async isn't going to automatically offload the work onto different threads. You could do this though:
private async void button2_Click(object sender, EventArgs e)
{
string file = files[0];
Task<string> task = Task.Run(() => ProcessFile(file));
rtTextArea.Text = await task;
}
private string ProcessFile(string file)
{
using (TesseractEngine tess = new TesseractEngine("tessdata", "dic",
EngineMode.TesseractOnly))
{
Page p = tess.Process(Pix.LoadFromFile(file));
return p.GetText();
}
}
The use of Task.Run will mean that ProcessFile (the heavy piece of work) is executed on a different thread.
You can also do this by starting your task in new thread.
Just use Thread.Start or Thread. ParameterizedThreadStart
See these for your reference:
http://msdn.microsoft.com/en-us/library/system.threading.parameterizedthreadstart.aspx
Start thread with parameters
You could use BackgroundWorker component.