Develop Reference

UWP Update UI From Async Worker

I am trying to implement a long-running background process, that periodically reports on its progress, to update the UI in a UWP app. How can I accomplish this? I have seen several helpful topics, but none have all of the pieces, and I have been unable to put them all together.
For example, consider a user who picks a very large file, and the app is reading in and/or operating on the data in the file. The user clicks a button, which populates a list stored on the page with data from the file the user picks.
PART 1
The page and button's click event handler look something like this:
public sealed partial class MyPage : Page
{
public List<DataRecord> DataRecords { get; set; }
private DateTime LastUpdate;
public MyPage()
{
this.InitializeComponent();
this.DataRecords = new List<DataRecord>();
this.LastUpdate = DateTime.Now;
// Subscribe to the event handler for updates.
MyStorageWrapper.MyEvent += this.UpdateUI;
}
private async void LoadButton_Click(object sender, RoutedEventArgs e)
{
StorageFile pickedFile = // … obtained from FileOpenPicker.
if (pickedFile != null)
{
this.DataRecords = await MyStorageWrapper.GetDataAsync(pickedFile);
}
}
private void UpdateUI(long lineCount)
{
// This time check prevents the UI from updating so frequently
// that it becomes unresponsive as a result.
DateTime now = DateTime.Now;
if ((now - this.LastUpdate).Milliseconds > 3000)
{
// This updates a textblock to display the count, but could also
// update a progress bar or progress ring in here.
this.MessageTextBlock.Text = "Count: " + lineCount;
this.LastUpdate = now;
}
}
}
Inside of the MyStorageWrapper class:
public static class MyStorageWrapper
{
public delegate void MyEventHandler(long lineCount);
public static event MyEventHandler MyEvent;
private static void RaiseMyEvent(long lineCount)
{
// Ensure that something is listening to the event.
if (MyStorageWrapper.MyEvent!= null)
{
// Call the listening event handlers.
MyStorageWrapper.MyEvent(lineCount);
}
}
public static async Task<List<DataRecord>> GetDataAsync(StorageFile file)
{
List<DataRecord> recordsList = new List<DataRecord>();
using (Stream stream = await file.OpenStreamForReadAsync())
{
using (StreamReader reader = new StreamReader(stream))
{
while (!reader.EndOfStream)
{
string line = reader.ReadLine();
// Does its parsing here, and constructs a single DataRecord …
recordsList.Add(dataRecord);
// Raises an event.
MyStorageWrapper.RaiseMyEvent(recordsList.Count);
}
}
}
return recordsList;
}
}
The code for the time check I got from following this.
As written, this code makes the app unresponsive with a large file (I tested on a text file on the order of about 8.5 million lines). I thought adding async and await to the GetDataAsync() call would prevent this? Does this not do its work on a thread aside from the UI thread? Through Debug mode in Visual Studio, I have verified the program is progressing as expected... it is just tying up the UI thread, making the app unresponsive (see this page from Microsoft about the UI thread and asynchronous programming).
PART 2
I have successfully implemented before an asynchronous, long-running process that runs on a separate thread AND still updates the UI periodically... but this solution does not allow for the return value - specifically the line from PART 1 that says:
this.DataRecords = await MyStorageWrapper.GetDataAsync(pickedFile);
My previous, successful implementation follows (most of the bodies cut out for brevity). Is there a way to adapt this to allow for return values?
In a Page class:
public sealed partial class MyPage : Page
{
public Generator MyGenerator { get; set; }
public MyPage()
{
this.InitializeComponent();
this.MyGenerator = new Generator();
}
private void StartButton_Click(object sender, RoutedEventArgs e)
{
this.MyGenerator.ProgressUpdate += async (s, f) => await Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, delegate ()
{
// Updates UI elements on the page from here.
}
this.MyGenerator.Start();
}
private void StopButton_Click(object sender, RoutedEventArgs e)
{
this.MyGenerator.Stop();
}
}
And in the Generator class:
public class Generator
{
private CancellationTokenSource cancellationTokenSource;
public event EventHandler<GeneratorStatus> ProgressUpdate;
public Generator()
{
this.cancellationTokenSource = new CancellationTokenSource();
}
public void Start()
{
Task task = Task.Run(() =>
{
while(true)
{
// Throw an Operation Cancelled exception if the task is cancelled.
this.cancellationTokenSource.Token.ThrowIfCancellationRequested();
// Does stuff here.
// Finally raise the event (assume that 'args' is the correct args and datatypes).
this.ProgressUpdate.Raise(this, new GeneratorStatus(args));
}
}, this.cancellationTokenSource.Token);
}
public void Stop()
{
this.cancellationTokenSource.Cancel();
}
}
Finally, there are two supporting classes for the ProgressUpdate event:
public class GeneratorStatus : EventArgs
{
// This class can contain a handful of properties; only one shown.
public int number { get; private set; }
public GeneratorStatus(int n)
{
this.number = n;
}
}
static class EventExtensions
{
public static void Raise(this EventHandler<GeneratorStatus> theEvent, object sender, GeneratorStatus args)
{
theEvent?.Invoke(sender, args);
}
}

It is key to understand that async/await does not directly say the awaited code will run on a different thread. When you do await GetDataAsync(pickedFile); the execution enters the GetDataAsync method still on the UI thread and continues there until await file.OpenStreamForReadAsync() is reached - and this is the only operation that will actually run asynchronously on a different thread (as file.OpenStreamForReadAsync is actually implemented this way).
However, once OpenStreamForReadAsync is completed (which will be really quick), await makes sure the execution returns to the same thread it started on - which means UI thread. So the actual expensive part of your code (reading the file in while) runs on UI thread.
You could marginally improve this by using reader.ReadLineAsync, but still, you will be returning to UI thread after each await.
ConfigureAwait(false)
The first trick you want to introduce to resolve this problem is ConfigureAwait(false).
Calling this on an asynchronous call tells the runtime that the execution does not have to return to the thread that originally called the asynchronous method - hence this can avoid returning execution to the UI thread. Great place to put it in your case is OpenStreamForReadAsync and ReadLineAsync calls:
public static async Task<List<DataRecord>> GetDataAsync(StorageFile file)
{
List<DataRecord> recordsList = new List<DataRecord>();
using (Stream stream = await file.OpenStreamForReadAsync().ConfigureAwait(false))
{
using (StreamReader reader = new StreamReader(stream))
{
while (!reader.EndOfStream)
{
string line = await reader.ReadLineAsync().ConfigureAwait(false);
// Does its parsing here, and constructs a single DataRecord …
recordsList.Add(dataRecord);
// Raises an event.
MyStorageWrapper.RaiseMyEvent(recordsList.Count);
}
}
}
return recordsList;
}
Dispatcher
Now you freed up your UI thread, but introduced yet another problem with the progress reporting. Because now MyStorageWrapper.RaiseMyEvent(recordsList.Count) runs on a different thread, you cannot update the UI in the UpdateUI method directly, as accessing UI elements from non-UI thread throws synchronization exception. Instead, you must use UI thread Dispatcher to make sure the code runs on the right thread.
In the constructor get reference to the UI thread Dispatcher:
private CoreDispatcher _dispatcher;
public MyPage()
{
this.InitializeComponent();
_dispatcher = Window.Current.Dispatcher;
...
}
Reason to do it ahead is that Window.Current is again accessible only from the UI thread, but the page constructor definitely runs there, so it is the ideal place to use.
Now rewrite UpdateUI as follows
private async void UpdateUI(long lineCount)
{
await _dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
{
// This time check prevents the UI from updating so frequently
// that it becomes unresponsive as a result.
DateTime now = DateTime.Now;
if ((now - this.LastUpdate).Milliseconds > 3000)
{
// This updates a textblock to display the count, but could also
// update a progress bar or progress ring in here.
this.MessageTextBlock.Text = "Count: " + lineCount;
this.LastUpdate = now;
}
});
}

C# object reference gets SOMETIMES lost in Threads that end via EventHandler

I have a strange problem with following code, that is started about every minute.
Normally everything works fine, but sometimes the HandleCalcStatistikMarkersDone function raises an error because of a NullReferenceException.
I try to explain with the code:
I have a class Strategies. This class is started about every minute to calculate and update some information in a MySQL database. This class is instantiated multiple times in separate threads within a ticker on a form.
public partial class mainForm: Form
{
//do something including ticker, that starts RunStatistik about every minute after the previous thread ended
private void RunStatistik()
{
foreach (InternalObject objPar in InternalObjects)
{
Strategies.StrategyParameter giveParms = new Strategies.StrategyParameter();
giveParms.pair= objPar.pair;
Strategies strat = new Strategies();
Thread calcStatistikThread = new Thread(new ParameterizedThreadStart(strat.CalcCoinStatistik));
calcStatistikThread.Start(giveParms);
}
}
}
Internally in the upper initiated Strategies thread, there are stared some additional threads.
Those threads have a "DoneEvent" which is raised at the end of the function.
To notice, that all threads have ended before the main thread ends, I collect all subthreads in a List CalcStatistikMarkersThreads and wait until the list is empty.
The subthreads should remove themselves out of the upper List via the ThreadDone event.
But sometimes the searched thread (CalcStatistikMarkersThreads.Find) is not found anymore and I get a NullReferenceException.
The question is why?!
Could you tell me, why? And possibly how to prevent?
Thanks in advance.
class Strategies
{ public event EventHandler ThreadDone;
private List<Thread> CalcStatistikMarkersThreads;
//do something
public void CalcCoinStatistik(object parameters)
{
StrategyParameter givenParms = (StrategyParameter)parameters;
Pair internalPair = givenParms.pair
//do something
if (CalcStatistikMarkersThreads == null)
{
CalcStatistikMarkersThreads = new List<Thread>();
}
foreach (InternalTodo in InternalToDos)
{
Strategies strat = new Strategies();
CalcStatistikMarkersParameter csp = new CalcStatistikMarkersParameter();
csp.parm1 = param;
strat.ThreadDone += HandleCalcStatistikMarkersDone;
Thread candleCalc = new Thread(new ParameterizedThreadStart(strat.CalcStatistikMarkers));
CalcStatistikMarkersThreads.Add(candleCalc);
candleCalc.Start(csp);
while (CalcStatistikMarkersThreads.Count != 0)
{
Task.Delay(1000).Wait();
}
}
}
public void CalcStatistikMarkers(object parm)
{
//do something
if (ThreadDone != null)
ThreadDone(this, new ThreadInfoEventArgs(Thread.CurrentThread.ManagedThreadId));
}
public void HandleCalcStatistikMarkersDone(object sender, EventArgs e)
{
Guid workGUID = Guid.NewGuid();
ThreadInfoEventArgs tEv = (ThreadInfoEventArgs)e;
Thread currentThread;
try
{
currentThread = CalcStatistikMarkersThreads.Find(xy => xy.ManagedThreadId == tEv.ThreadID);
//HERE THE NullReferenceException is raised sometimes
CalcStatistikMarkersThreads.Remove(currentThread);
}
catch (Exception ex)
{
throw ex;
}
}
public class ThreadInfoEventArgs : EventArgs
{
private int threadID;
public ThreadInfoEventArgs(int trID)
{
this.threadID = trID;
}
public int ThreadID
{
get { return threadID; }
}
}
}
Cheers
Air

How to solve this deadlock?

I have an UWP application in which I am trying to store and retrieve some data from a local text file but no matter how I try to do it the application gets deadlocked. Due to synchronous stuff that needs to happen, I try to use a task and wait for its completion but nevertheless, the application locks.
I have a page in my UWP application called "MainPage" and in it's constructor I have the following code:
var listenkeyViewModel = new ListenkeyViewModel();
listenkeyViewModel.GetKey();
listenkey = listenkeyViewModel.Listenkey;
The get key is the issue here because it calls a method on the ViewModel (which I created to be synchronous because I thought making the call synchronous asap would be preferable.
public void GetKey()
{
try
{
var listenKeyTask = RetrieveListenKey();
_listenkey = listenKeyTask.Result;
}
catch (Exception e)
{
}
}
public static async Task<string> RetrieveListenKey()
{
try
{
var storageFolder = Windows.Storage.ApplicationData.Current.LocalFolder;
var listenkeyFile = await storageFolder.GetFileAsync("listenkey.txt");
return await Windows.Storage.FileIO.ReadTextAsync(listenkeyFile);
}
catch (Exception e)
{
throw new Exception("Could not load file");
}
}
I know the thing is "async all the way down" but this is not possible here. I cannot make the constructor where the original code lies asynchronous. How are you supposed to not get deadlocked? I do not understand.

Convert GetKey to async/await
public async Task GetKey() {
try {
var listenKeyTask = RetrieveListenKey();
_listenkey = await listenKeyTask;
} catch (Exception e) {
//...should handle/log error
}
}
Move the calling of this out of the constructor and into an event handler. like page load or some other event called early in the lifecycle of the page.
partial class MainPage : Page {
ListenkeyViewModel listenkeyViewModel;
string listenkey;
public MainPage() {
InitializeComponent();
listenkeyViewModel = new ListenkeyViewModel();
// add a handler to be called when the page has been loaded
this.Loaded += OnPageLoaded;
}
async void OnPageLoaded(object sender, RoutedEventArgs e) {
await listenkeyViewModel.GetKey();
listenkey = listenkeyViewModel.Listenkey;
}
// Shown for demonstration purposes only.
// This is typically autogenerated by Visual Studio.
private void InitializeComponent() {
}
}
async void is allowed on event handlers so this should allow the process to flow without deadlock.

C# / WPF Updating the UI from another thread created within another class

I come from an embedded C background and I am working on my first C# application and I have hit a wall on this and my research is not panning out so I thought I would ask here.
Simple app, so far. I have a MainWindow that, among a bunch of other stuff, starts a TCPClient thread on a button click:
public partial class MainWindow : Window
{
....
TCPConnection myCon = new TCPConnection();
....
private void connectButton_Click(object sender, RoutedEventArgs e)
{
networkListBox.Items.Add("Connecting...");
myCon.Connect("localhost", updateNetworkListBox);
}
}
....
public void updateNetworkListBox(string message)
{
networkListBox.Items.Add(message);
}
And in TCPConnection.cs:
public class TCPConnection
{
....
public void Connect(string server, ReportDelegate reportDelegate)
{
this.server = server;
clientThread = new Thread(() => Client(this.server));
clientThread.Start();
reportDelegate("Started client thread...");
}
static void Client(string server)
{
try
{
Int32 port = 25565;
TcpClient client = new TcpClient(server, port);
Byte[] outgoingBytes = new Byte[1024];
string outgoingString = "Hello! I am " + Guid.NewGuid();
outgoingBytes = System.Text.Encoding.ASCII.GetBytes(outgoingString);
NetworkStream stream = client.GetStream();
stream.Write(outgoingBytes, 0, outgoingBytes.Length);
stream.Close();
client.Close();
}
The first thing I would like to do, now that TCP connection works is send a message back to the UI such as "Client thread connecting...", "Client thread connected..." and have it show up in the networkListbox.
Within the Connect() method, I was able to do this by using the delegate but this obviously will not work in the new thread since one is not able to directly access UI controls from another thread.
I have read loads of articles on this and I know that I probably want to use the Dispatcher to to do this. However, almost all of the examples I have seen have created a new thread within the current class and, for example, passed an anonymous method to Dispatcher.Invoke().
One exception to this discussion which advocated using an EventHandler and initializing it in the main window. That seems less than ideal but maybe I am wrong.
Further down, someone else advocated data sharing. Again, that seems less than ideal to me.
Other articles I have read appear to be out of date.
So, I welcome any explanations on how to go about this. It may be that I am just getting hung up syntactically but I suspect that, although I think I am mostly clear on delegates, lambdas, etc., I am probably hung up on what exactly needs to get done.
If you can show how it would be done in this specific example with some explanation, I would greatly appreciate it.
And maybe some specific questions on some points that are a little hazy for me:
1) Can my worker task access on it on its own or must it be provided with the UI's Dispatcher?
2) Should the UI provide a delegate that performs the dispatch or should the dispatch be coded in the worker task, referencing the UI Dispatcher?
Thanks very much.

For your question about providing a sample, if there is a worker class like...
public class Worker
{
public Worker(Action<string>action)
{
Task.Run(() =>
{
int i = 0;
while (true)
{
++i;
Task.Run(() => { action("Current value " + i); });
Task.Run(() =>
{
// doing some work here
});
Thread.Sleep(1000);
}
});
}
}
...which is performing background work on different threads and advising the caller via the delegate. The delegate is a plain vanilla Action that takes a string. Then the View Model should be implemented such that it does not care on which thread the message originated. Here's the corresponding code in the VM...
private readonly SynchronizationContext _context = SynchronizationContext.Current;
private void StartWorker()
{
Worker w = new Worker((s) => _context.Post(delegate { StatusText = s; }, null));
}
This code uses a SynchronizationContext, but could just as easily use a dispatcher. The point being that the responsibility for sync'ing up on the UI thread doesn't belong to a worker. The worker shouldn't care, and similarly the VM is thread-agnostic and posts everything via its SynchronizationContext.
The code for the StatusText property looks like this...
private string _statusText;
public string StatusText
{
[DebuggerStepThrough]
get { return _statusText; }
[DebuggerStepThrough]
set
{
if (value != _statusText)
{
_statusText = value;
OnPropertyChanged("StatusText");
}
}
}
And finally, on the UI, it is presented like this...
<StatusBar DockPanel.Dock="Bottom">
<TextBlock Text="{Binding StatusText}"/>
</StatusBar>
...
So to recap your questions: the worker threads can access it, but they should not have to deal with sync'ing up the UI. That responsibility is the VM's. And VM should be thread-agnostic and sync the UI through the dispatcher or synchronization context or other methods.
Scheduling through the Dispatcher is appropriate if you are manipulating a collection that is the subject of a binding (e.g., an ObservableCollection); otherwise SynchronizationContext is appropriate (it's a bit more light-weight).

just add the delegate and pass a reference to your main form
public partial class MainWindow : Window
{
TCPConnection myCon = new TCPConnection();
private void connectButton_Click(object sender, RoutedEventArgs e)
{
networkListBox.Items.Add("Connecting...");
myCon.Connect("localhost", updateNetworkListBox);
}
public delegate void updateNetworkListBoxDelegate(string message);
public void updateNetworkListBox(string message)
{
if(this.invokeRequired())
{
this.invoke(new updateNetworkListBoxDelegate(updateNetworkListBox), message);
}
else
{
networkListBox.Items.Add(message);
}
}
}
in TCPConnection add a constructor that takes a MainWindow instance
public class TCPConnection
{
//add member to hold instance
private _mainWindow;
//add constructor taking instance
public TCPConnection(MainWindow mw)
{
_mainWindow = mw;
}
public void Connect(string server, ReportDelegate reportDelegate)
{
this.server = server;
clientThread = new Thread(() => Client(this.server));
clientThread.Start();
//reportDelegate("Started client thread...");
//call the method on the UI thread
_mainWindow.updateNetworkListBox("Started client thread...");
}
static void Client(string server)
{
try
{
Int32 port = 25565;
TcpClient client = new TcpClient(server, port);
Byte[] outgoingBytes = new Byte[1024];
string outgoingString = "Hello! I am " + Guid.NewGuid();
outgoingBytes = System.Text.Encoding.ASCII.GetBytes(outgoingString);
NetworkStream stream = client.GetStream();
stream.Write(outgoingBytes, 0, outgoingBytes.Length);
stream.Close();
client.Close();
//call the method ont he ui thread
_mainWindow.updateNetworkListBox("DONE!!")
}
}
}

Run a method and countinue to the WebApp's lifetime

There is an application that collects news from rss of news agencies maybe like Google Reader!.
I want to call a method to update my links in DB in a period of the time and it continues to the Application life time.
something like a clock !!!
without any pause
I know some info about Threading
but the problem is :
Where can I call my Update method?
I have some classes that some of them derive from others and I use to layer in my Project
I call the method in Global.asax:
protected void Application_Start(object sender, EventArgs e)
{
Thread thread = new Thread(new ThreadStart(UpdateRss));
thread.Start();
Thread.Sleep(1000);
}
public void UpdateRss()
{
while (true)
{
using (LinkService linkSrv = new LinkService())
{
linkSrv.UpdateLinksFromRSS();
}
}
}
and the definition of UpdateLinksFromRSS in LinkService is:
public void UpdateLinksFromRSS()
{
List<RssInfo> q;
using (RssService RssSrv = new RssService())
{
q = RssSrv.GetRssInfoes();
}
foreach (var item in q)
{
AddLink(item);
}
}
Honestly the problem is i have a property in BaseService that is defined like this:
public static System.Web.Caching.Cache Cache
{
get { return HttpContext.Current.Cache; }
}
when I run the project!
I got an error from this line: return HttpContext.Current.Cache;