I have a WebBrowser control and it has InvokeScript method, which you should call only after WebBrowser is loaded.
So I've tried something like this:
private readonly ManualResetEventSlim browserLoaded = new ManualResetEventSlim(false);
private void BrowserLoaded(object sender, NavigationEventArgs navigationEventArgs)
{
browserLoaded.Set();
}
private async Task<object> InvokeScript(string invoke, object[] parameters = null)
{
return await Task.Factory
.StartNew(() =>
{
if (!browserLoaded.Wait(TimeSpan.FromSeconds(10)))
{
throw new Exception("Timeout for waiting browser to load.");
}
})
.ContinueWith(task => parameters == null
? browser.InvokeScript(invoke)
: browser.InvokeScript(invoke, parameters), TaskScheduler.FromCurrentSynchronizationContext());
}
It does not look very nice to me, but works ok when called asynchronously.
Problem appears, when I try to read result value synchronously - app just hangs:
private string GetEnteredText()
{
return (string)InvokeScript("getEnteredText").Result;
}
I know, that I should go all the way async, but I'm wondering what to do with properties:
public override string UserText
{
get
{
return GetEnteredText();
}
set
{
SetEnteredText(value);
}
}
Or async is wrong way to go in this case at all?
Update
Property is a 'glue' between input field value in browser's page and view model in WPF, so I don't see a good way to make it as separate methods, especially because it is a part of the bigger framework (notice override keyword on it).
Once browser control is loaded, execute logic should not take long, I guess less than 10 milliseconds, that is why I would be ok with sync execution in this case. And usually browser control loads fast enough, the only reason here to delay is to make sure InvokeScript is not called before load, not because it taking long time or smth.
app just hangs
We'll, you said it yourself. You know why that happens. You're blocking on the call using Task.Result, that is what causes the deadlock.
Or async is wrong way to go in this case at all?
We don't have async properties. The reason we don't have them are because properties aren't asynchronous by nature. Stephan Cleary describes it nicely:
If your “property” needs to be asynchronously evaluated every time
it’s accessed, then you’re really talking about an asynchronous
operation.The best solution is to change the property to an async
method. Semantically, it shouldn’t be a property.
Instead of a property, make it an asynchronous method which you can await properly.
In regards to using a ManualResetEvent, I would use a TaskCompletionSource<bool> instead. It works "nicer" with TPL:
private TaskCompletionSource<bool> tcs = new TaskCompletionSource<bool>();
private void BrowserLoaded(object sender, NavigationEventArgs navigationEventArgs)
{
tcs.TrySetResult(true);
}
private async Task<object> InvokeScript(string invoke, object[] parameters = null)
{
var timeoutTask = Task.Delay(TimeSpan.FromSeconds(10));
if (timeoutTask == await Task.WhenAny(tcs.Task, timeoutTask))
{
// You've timed out;
}
return Task.Run(() =>
{
parameters == null ? browser.InvokeScript(invoke)
: browser.InvokeScript(invoke, parameters)
});
}
I also see that you used TaskScheduler.FromCurrentSynchronizationContext() in your continuation. If you need this to execute on the UI thread, there is no point in using a threadpool thread at all.
#Noseratio adds that WebBrowser is an STA object. His answer here might help.
Related
I have a method that is "partially" async, meaning that one code path runs async and the other runs synchronously. I can't currently make the synchronous part async, although I may be able to in the future.
public async Task UpdateSomethingAsync(){
if (ConditionIsMet){
await DoSomethingAsync;
}else{
DoSomethingSynchronous;
}
}
Both DoSomethingAsync and DoSomethingSynchronous are I/O bound. Calling this method from the Winforms UI thread with "await" causes it to block the UI thread if the Synchronous path is taken, which is to be expected.
private async void MyDropDownBox_DropDownClosed(object sender, EventArgs e)
{
//This blocks if the DoSomethingSynchronous path is taken, causing UI to
//become unresponsive.
await UpdateSomethingAsync();
}
So off to Stephen Cleary's blog I go. His suggestion (although for CPU bound code instead of I/O bound) is to run the method with Task.Run, as if it were completely synchronous, while documenting that the method is "partially" async. However, events raised by DoSomethingSynchronous now cause an exception, I believe due to the fact that they are now on a different thread from the UI.
private async void MyDropDownBox_DropDownClosed(object sender, EventArgs e)
{
//This no longer blocks, but events will not marshal back to UI Thread
//causing an exception.
await Task.Run(()=> UpdateSomethingAsync());
}
How can this be fixed?
Don't update the UI, or any model bound to the UI inside of UpdateSomethingAsync or any of the methods that it calls. Create a class that will hold the data required to update your UI, and return an instance of that class from UpdateSomethingAsync.
DoSomethingAsync will return a Task<ThatClassYouCreated> and DoSomethingSynchronous just returns an instance of ThatClassYouCreated. Then, back in MyDropDownBox_DropDownClosed after you await UpdateSomethingAsync, use the instance returned by UpdateSomethingAsync to update your UI or your model.
public class UpdatedInformation
{
public int UpdateId { get; set; }
public string UpdatedName { get; set; }
public DateTimeOffset Stamp { get; set; }
// etc, etc...
}
public class YourForm : Form
{
private async Task<UpdatedInformation> DoSomethingAsync()
{
var result = new UpdatedInformation();
// Something is awaited...
// Populate the properties of result.
// Do not modify your UI controls. Do not modify the model bound to those controls.
return result;
}
private UpdatedInformation DoSomethingSynchronous()
{
var result UpdatedInformation();
// Populate the properties of result.
// Do not modify your UI controls. Do not modify the model bound to those controls.
return result;
}
private async Task<UpdatedInformation> UpdateSomethingAsync()
{
if (ConditionIsMet)
{
return await DoSomethingAsync();
}
else
{
return await Task.Run(DoSomethingSynchronous);
}
}
private async void MyDropDownBox_DropDownClosed(object sender, EventArgs e)
{
var updatedInformation = await UpdateSomethingAsync();
// Now use updatedInformation to update your UI controls, or the model bound to
// your UI controls.
model.Id = updatedInformation.UpdateId;
// etc...
}
}
In your event handler, you can use Invoke() to update the UI like this:
private void someEventHandler() // <- it might have params
{
// ... possibly some other code that does NOT update the UI ...
this.Invoke((MethodInvoker)delegate {
// ... it's safe to update the UI from in here ...
});
// ... possibly some other code that does NOT update the UI ...
}
I don't know who keeps doing it, but my comments below this post keep getting deleted.
This answers the TITLE of the question, which was:
How do I marshal an event from Task.Run back to the UI thread?
When you receive an event from a different thread, this is a perfectly valid way of updating the UI.
Sicne you state that "[..] DoSomethingSynchronous [is] I/O bound" you could also make it async by wrapping the IO bound operation within DoSomethingSynchronous in a Task.Run.
So if DoSomethingSynchronous is something like
public void DoSomethingSynchronous(...)
{
// some UI work
// blocking sysnchornous IO operation
var res = IoOperation();
// some more UI work
}
you could rewrite it to.
public async Task DoSomethingSynchronous(...)
{
// some UI work
// no-UI-Thread blocking IO operation
var res = await Task.Run(() => IoOperation()).ConfigureAwait(true);
// some more UI work
}
the .ConfigureAwait(true) could maybe omited but ensures that the code after the await will be scheduled in the orignal sync-context i.e. the UI-Thread.
You then obviously need to rename the method and such, but this will make the code more maintainable if you someday can use a true asycn IO in DoSomethingSynchronous
Since UpdateSomethingAsync needs to access the UI context, it shouldn't be wrapped in a Task.Run call. (You should very rarely, need to call an async method from Task.Run, usually only if the method is implemented incorrectly and you can't fix it.)
Instead DoSomethingSynchronous should be the thing you call from Task.Run. After all, the purpose of that method is to asynchronously run a synchronous method in a thread pool thread. So only use it for the synchronous method you want run in a thread pool thread, not the (supposedly) asynchronous method that needs to access the UI context.
WinUI 3 respects the below method.
DispatcherQueue.TryEnqueue(() =>
{
//Code to Update the UI
});
Figured I'd answer this myself after some more research. Most of the other answers are correct in some way, but don't necessarily explain the whole deal in one go, so I'll try to sum up here.
This first snippet from the question works event wise, but blocks if the Synchronous path in UpdateSomethingAsync is taken. Events work because "await" automatically captures the SynchronizationContext (this is key) for the UI thread, such that any events raised from UpdateSomethingAsync are marshalled back to the UI, via the SynchronizationContext. This is just the normal way of using async/await:
private async void MyDropDownBox_DropDownClosed(object sender, EventArgs e)
{
//This blocks if the DoSomethingSynchronous path is taken, causing UI to
//become unresponsive, but events propagate back to the UI correctly.
await UpdateSomethingAsync();
}
Task.Run works in much the same way, if you aren't using it to run an async method. In other words, this works without blocking and will still send events to the UI thread, because UpdateSomethingAsync is replaced with a Synchronous method. This is just the normal usage of Task.Run:
private async void MyDropDownBox_DropDownClosed(object sender, EventArgs e)
{
//UpdateSomethingAsync is replaced with a Synchronous version, and run with
// Task.Run.
await Task.Run(UpdateSomethingSynchronously());
}
However, the original code in question is Async, so the above doesn't apply. The question poses the following snippet as a possible solution, but it errors out with an Illegal Cross Thread call to the UI when an event is raised, because we are using Task.Run to call an Async method, and for some reason this does not set the SynchronizationContext:
private async void MyDropDownBox_DropDownClosed(object sender, EventArgs e)
{
//This no longer blocks, but events raised from UpdateSomethingAsync
//will cause an Illegal Cross Thread Exception to the UI, because the
//SyncrhonizationContext is not correct. Without the SynchronizationContext,
//events are not marshalled back to the UI thread.
await Task.Run(()=> UpdateSomethingAsync());
}
What does seem to work is to use Task.Factory.StartNew to assign the UI SynchronizationContext to the Task using TaskScheduler.FromCurrentSynchronizationContext, like so:
private async void MyDropDownBox_DropDownClosed(object sender, EventArgs e)
{
//This doesn't block and will return events to the UI thread sucessfully,
//because we are explicitly telling it the correct SynchronizationContext to use.
await Task.Factory.StartNew(()=> UpdateSomethingAsync(),
System.Threading.CancellationToken.None,
TaskCreationOptions.None,
TaskScheduler.FromCurrentSynchronizationContext);
}
What also works, and is very simple but "lies" a little to the caller, is to simply wrap DoSomethingSynchronous in Task.Run:
public async Task UpdateSomethingAsync(){
if (ConditionIsMet){
await DoSomethingAsync;
}else{
await Task.Run(DoSomethingSynchronous);
}
}
I consider this a little bit of a lie, because the method is not really fully Async in the sense that it spins off a Thread Pool thread, but may never pose an issue to a caller.
Hopefully this makes sense. If any of this is proven incorrect please let me know, but this is what my testing has uncovered.
I am trying to understand better how can I update a windows forms progress bar from an async operation but I am getting some unexpected behavior from that.
Basically I am having a button which should after is being clicked to update a progress bar and then set it back to 0 once the progress bar gets 100% updated.
This is my code:
private async void button1_Click(object sender, EventArgs e)
{
await CallMethodAsync().ContinueWith((prevTask) =>
{
prevTask.Wait();
progressBar1.Invoke(new Action(() => { progressBar1.Value = 0; }));
});
}
private static async Task ExecuteMethodAsync(IProgress<double> progress = null)
{
double percentComplete = 0;
bool done = false;
while (!done)
{
if (progress != null)
{
progress.Report(percentComplete);
}
percentComplete += 10;
if(percentComplete == 100)
{
done = true;
}
}
}
private async Task CallMethodAsync()
{
var progress = new Progress<double>();
progress.ProgressChanged += (sender, args) => { progressBar1.Increment(10); };
await ExecuteMethodAsync(progress);
}
Having this implementation the progress bar is not being updated at all even if I call "Wait()" on the operation that should update the value of the progress bar.
If i remove this part of code:
progressBar1.Invoke(new Action(() => { progressBar1.Value = 0; }));
the progress bar gets updated but it remains all the time like that, and I want to set it back to 0 once it was entirely filled so that I can update it again when I click again the button.
Could someone please explain me what am I doing wrong ?
One of the reasons async-await syntax was invented because it was difficult to follow the sequence of instructions when tasks were concatenated using functions like ContinueWith.
If you use async-await it is seldom necessary to use statements like ContinueWith. After an await, the thread already continues with the statements after the await.
If the button is clicked, you want to call ExcecuteMethodAsync. This function takes an IProgress, because it wants to report progress regularly. You want to call this function asynchronously, so whenever the function has to wait for something, it doesn't really wait, but returns control to you so you could do other things instead of really waiting, until you encounter an await, in which case your caller continues processing until he encounters an await, etc.
The nice thing with async-await is that the thread that continues after your call to an async function has the same context as the calling thread. This means that you can regard it as your original thread. No InvokeRequired, no need to protect data with mutexes etc.
Your function could be simplified as follows:
async Task CallMethodAsync()
{
var progress = new Progress<double>();
progress.ProgressChanged += OnProgressReported;
await ExecuteMethodAsync(progress);
}
private void OnProgressReported(object sender, ...)
{
// because this thread has the context of the main thread no InvokeRequired!
this.progressBar1.Increment(...);
}
private async void button1_Click(object sender, EventArgs e)
{
await CallMethodAsync();
}
So when the button is clicked, CallMethodAsync is called. This function will create A Progress object and subscribes on its Report event. Note that this is still your UI-thread. Then it calls ExecuteMethodAsync, which will regularly raise event Report, which is handled by OnProgressReported.
Because ExecuteMethodAsync is async, you can be sure there is somewhere an await in it. This means that whenever it has to await, control returns to the caller, which is CallMethodAsync, until is encounters an await, which in this case is immediately.
Control goes up the call stack to the caller, which is button1_click, where it immediately encounters an await, so control goes up the call stack, etc.
All these controls have the same context: it is as if they are the same thread.
An article that helped me a lot to understand async-await is this interview with Eric Lippert. Search somewhere in the middle for async await
Another articel that helped me a lot to learn good practices were this article by the ever so helpful Stephen Cleary and Async/Await - Best Practices in Asynchronous Programming also by Stephen Cleary
Your issue is happening because ExecuteMethodAsync(...) is not actually asynchronous.
Add the following before the while loop to make it asynchronous
await Task.Delay(1);
or enclose some synchronous portion of code (e.g. the while loop) into a:
await Task.Run(() => { ... });
or (the best one), add the following at the beginning of the function:
await Task.Yield(); // Make us async right away
I am calling Task.Run(() => DoSomething()).Result which causes the UI to freeze and it happens because am using ".Result". I need Result because i want to return the value.
I don't want the Method StartSomething to be async because I don't want to await the method StartSomething. I want the await to happen at DoSomething().
So basically I need a asynchronous method to be called by a synchronous method, without freezing the UI. Plus I want to return the value from the async method to the top level that is on Button Click.
Can this code be improved or is there any other solution?
private TaskCompletionSource<bool> TaskCompletion = null;
private void Button_Click(object sender, RoutedEventArgs e)
{
bool k = StartSomething();
}
private bool StartSomething()
{
return Task.Run(() => DoSomething()).Result;
}
private async Task<bool> DoSomething()
{
TaskCompletion = new TaskCompletionSource<bool>();
await Task.WhenAny(TaskCompletion.Task, Task.Delay(3000));
MessageBox.Show("DoSomething");
return true;
}
Method StartSomething() doesn't make sense to me. It starts a new Task and then just synchronously waits for the result (.Result) of this task, which is effectively useless - it is nearly [*] the same as calling DoSomething() directly. Also DoSomething() is already asynchronous so you don't need to start a new Task for it.
It looks like you don't need StartSomething() method at all. If you make Button_Click handler async, you can then simply await DoSomething() directly:
private TaskCompletionSource<bool> TaskCompletion = null;
private async void Button_Click(object sender, RoutedEventArgs e)
{
bool k = await DoSomething();
}
private async Task<bool> DoSomething()
{
TaskCompletion = new TaskCompletionSource<bool>();
await Task.WhenAny(TaskCompletion.Task, Task.Delay(3000));
MessageBox.Show("DoSomething");
return true;
}
Edit:
While using async all the way down solution (as shown above) is IMO the preferred way, if you really can't change calling code to async, I can think of two ways to call async method from synchronous method without blocking UI. First is to manually set up a continuation tasks like this:
private void Button_Click(object sender, RoutedEventArgs e)
{
DoSomething().ContinueWith((task) =>
{
bool k = task.Result;
// use the result
},
// TaskScheduler argument is needed only if the continuation task
// must run on the UI thread (eg. because it access UI elements).
// Otherwise this argument can be omitted.
TaskScheduler.FromCurrentSynchronizationContext());
// Method can exit before DoSomething().Result becomes
// available, which keep UI responsive
}
So you basicly split synchronous method (one split instead of each await) into several parts (continuation lambda methods) linked by .ContinueWith. This is similar to what await does under a hood. Problem is that unlike await (which produces nice and clean code), your code will be full of these continuation lambdas. And it will get much worse when you add exception handling blocks, using blocks, etc.
The second approach is using nested loops, eg. Stephen Toub's WaitWithNestedMessageLoop extension method:
static T WaitWithNestedMessageLoop<T>(this Task<T> task)
{
var nested = new DispatcherFrame();
task.ContinueWith(_ => nested.Continue = false, TaskScheduler.Default);
Dispatcher.PushFrame(nested);
return task.Result;
}
Nested loops are quite advanced technique (I actually never used it) and I don't recommend using it unless you have to.
[*] There are differences in exception handling, executing thread, etc., but these are not relevant to this question.
I've got this custom Task code:
public static async Task Run(this CustomForm parent, Action action)
{
parent.Enabled = false;
using (Caricamento form = new Caricamento())
{
form.TopLevel = true;
form.TopMost = true;
form.Show();
await Task.Run(action);
}
parent.Enabled = true;
}
The gif animation and the text inside the form just won't properly load until the async task finished .
ListMessaggi listForm = new ListMessaggi(ListMessaggi.Tipo.Entrata);
listForm.FormClosing += (o, args) =>
{
if (this.Controls.Count == 2)
{
args.Cancel = true;
}
};
listForm.FormBorderStyle = FormBorderStyle.None;
listForm.Dock = DockStyle.Fill;
listForm.TopLevel = false;
panel.Controls.Add(listForm);
listForm.Show();
And then, in the form which shows up upon listForm.Show() method call I've got:
Finally, this the result showing while the async task is running:
How can I improve the code to make things work properly?
Based on the additional information you provided in the comments, I think that you should try to convert your code to entirely use async / await. This will involve converting all your ADO.NET functions to the new async methods added to ADO in .NET 4.5. This will should eliminate all the Task.Run calls, as well as the messy InvokeRequired and BeginInvoke calls that you are doing to marshal control back to the UI thread.
I think you will find that, if properly implemented, you won't even need the special Run extension method; you will be able to have all your code in-line, just as in "traditional" .net development.
For example, you could use code like this to responsively load data, without locking the UI, in a form's Load event.
public async void Form1_Load(object sender, EventArgs e)
{
var data = await _dataProvider.GetSomeDataFromTheDatabase(aTotallyMadeUpVariable);
this.MyDataGrid.DataSource = data;
}
The exact same pattern works for the event handlers on comboboxes and buttons.
As a side note, event handlers in WinForms is practically the only place that async void methods are legitimately valid. Everything else, including async methods called from within event handler, should be Task returning functions.
As a bit more of a "primer" on async / await, this is how it avoids blocking the UI thread in my example above.
The async modifier on a function acts as a marker to the compiler to convert the method in to a state-machine. The compile will segment the code in the method, breaking it up at each await call, in to separate states. When the function is called, the first state is run (up to where the await is), and then the function returns to the caller. When the function that is being awaitted returns, the code after it (in the next state) is invoked as a continuation. Any data that is shared between the states, such as local variables, is moved off to an object that is passed in to each state's continuation.
In CefSharp WinForms, I'm trying to get the html source of the page using JS once the page has loaded, however the application is freezing. I'm using a BackgroundWorker and the concerned functions are as follows:
void bw_DoWork(object sender, DoWorkEventArgs e)
{
browser.Load("http://www.google.com");
browser.FrameLoadEnd += delegate
{
object js = EvaluateScript(browser, "1+1");
MessageBox.Show(js.ToString());
};
}
object EvaluateScript(ChromiumWebBrowser b, string script)
{
var task = b.EvaluateScriptAsync(script);
task.Wait();
return task.Result;
}
As amaitland pointed out, FrameLoadEnd was causing the hang by running in the UI thread. The below code is working:
void bw_DoWork(object sender, DoWorkEventArgs e)
{
first.Load("http://www.google.com");
browser.FrameLoadEnd += delegate
{
Task task = new Task(() => {
object js = EvaluateScript(browser, "document.getElementsByTagName('html')[0].innerHTML;");
MessageBox.Show(js.ToString());
});
task.Start();
};
}
static object EvaluateScript(ChromiumWebBrowser b, string script)
{
var task = b.EvaluateScriptAsync(script);
task.Wait();
JavascriptResponse response = task.Result;
return response.Success ? (response.Result ?? "") : response.Message;
}
Whilst you assign FrameLoadEnd in the BackgroundWorker thread, it's actually executed on the underlying CEF UI thread, for which you cannot block without issues.
I'd typically suggest you spawn a Task from within the event handler to complete your work.
As a general rule, it's a bad idea to use Task.Wait on async code; rather, you should use async "all the way down". See also Don't Block on Async Code by Stephen Cleary. The short answer is that if you do this in an application with a synchronization context, you can cause a circular wait for the synchronization context (and hence a deadlock). The article I linked to has several examples of this, but I'd strongly suggest replacing Task.Wait with await here if possible.