I tried to make some MVVM pattern into my app, and i ran into a problem with hte visual representation of data. The data if the binded observablecollecrion is updated, but the visual is not.
some code:
ViewModel:
public class HlavnaViewModel : BaseViewModel
{
public HlavnaViewModel()
{
}
private Doklady _selectedDok;
public Doklady vm_selectedDok
{
get => _selectedDok;
set
{
_selectedDok = value;
OnPropertyChanged(nameof(vm_selectedDok));
update_polozky();
}
}
public async void update_polozky()
{
Polozky dok = new Polozky() { id_doklad = _selectedDok.id };
ObservableCollection<Polozky> pol = new ObservableCollection<Polozky>(await App.Database.GetPolozkyAsync(dok));
vm_polozky = pol;
}
private ObservableCollection<Polozky> _polozky;
public ObservableCollection<Polozky> vm_polozky
{
get => _polozky;
set
{
_polozky =value;
OnPropertyChanged(nameof(vm_polozky));
}
}
}
in the XAML:
<CollectionView x:Name="polozky" SelectionMode="Single" ItemsSource="{Binding vm_polozky}">...
BaseViewModel:
public class BaseViewModel : INotifyPropertyChanged
{
string title = string.Empty;
public string Title
{
get { return title; }
set { SetProperty(ref title, value); }
}
protected bool SetProperty<T>(ref T backingStore, T value,
[CallerMemberName] string propertyName = "",
Action onChanged = null)
{
if (EqualityComparer<T>.Default.Equals(backingStore, value))
return false;
backingStore = value;
onChanged?.Invoke();
OnPropertyChanged(propertyName);
return true;
}
#region INotifyPropertyChanged
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged([CallerMemberName] string propertyName = "")
{
var changed = PropertyChanged;
if (changed == null)
return;
changed.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
finally in View:
public Hlavna()
{
InitializeComponent();
hvm = new HlavnaViewModel();
this.BindingContext = hvm;
}
if i select a row in CollectionView where the vm_selectedDok binding is set, it selects that item, fires the update_polozky(), the vm_polozky gets populated with the right data, but the visual just dont shows the items from vm_polozky.
Ive read couple of similar questions, but i cant figure out where i made a mistake.
EDIT:
so the problem was somewhere else, i had the grid.rowdefinitions set just wrong, therefore the grid was outside of the visible area.
#ToolmakerSteve made good suggestions on calling async/await, please read his answer.
There are two alternative ways to fix this.
One way is Gerald's answer. This is fine for small collections, but might be slower if there are many items being added.
The second way is to do what you've done - replace the collection. But there is a fix needed in your code.
The way you've called update_polozky won't work reliably. You don't start the async/await sequence inside an await.
Replace:
update_polozky();
With:
Device.BeginInvokeOnMainThread(async () => {
await update_polozky();
});
OPTIONAL: Might also make this change. (Though it shouldn't be necessary.) This gives a good place to put a breakpoint, to see whether "result" gets the expected contents.
Replace:
ObservableCollection<Polozky> pol = new ObservableCollection<Polozky>(await App.Database.GetPolozkyAsync(dok));
With:
var result = await App.Database.GetPolozkyAsync(dok);
ObservableCollection<Polozky> pol = new ObservableCollection<Polozky>(result);
IMPORTANT NOTES FOR OTHER CODERS:
The second approach ("replace the collection") relies on OnPropertyChanged(nameof(vm_polozky)); in the setter of the ObservableCollection.
You have that, so not an issue for you. I mention this for anyone else who might adapt this code.
For example, I've seen people attempt to set the private value directly, e.g.:
// Don't do this to replace the collection. XAML won't know you changed it!
_myPrivateField = new ObservableCollection<MyItem>(result);
I've also seen people try to have an ObservableCollection without a setter:
// This is okay UNLESS you replace the collection - in which case you need `OnPropertyChanged(nameof(MyCollection))` somewhere:
public ObservableCollection<MyItem> MyCollection { get; set; }
Basically it comes down to, don't do this: ObservableCollection<Polozky> pol = new ObservableCollection<Polozky>(await App.Database.GetPolozkyAsync(dok));
Whenever you create a new ObservableCollection it will lose the databinding to the UI. Clear your ObservableCollection with .Clear() and add new items to it with a for loop. For example:
public async void update_polozky()
{
Polozky dok = new Polozky() { id_doklad = _selectedDok.id };
var results = await App.Database.GetPolozkyAsync(dok);
vm_polozky.Clear();
foreach(var item in results)
vm_polozky.Add(item);
}
I'm listening to a hardware event message, but I need to debounce it to avoid too many queries.
This is an hardware event that sends the machine status and I have to store it in a database for statistical purposes, and it sometimes happens that its status changes very often (flickering?). In this case I would like to store only a "stable" status and I want to implement it by simply waiting for 1-2s before storing the status to the database.
This is my code:
private MachineClass connect()
{
try
{
MachineClass rpc = new MachineClass();
rpc.RxVARxH += eventRxVARxH;
return rpc;
}
catch (Exception e1)
{
log.Error(e1.Message);
return null;
}
}
private void eventRxVARxH(MachineClass Machine)
{
log.Debug("Event fired");
}
I call this behaviour "debounce": wait a few times to really do its job: if the same event is fired again during the debounce time, I have to dismiss the first request and start to wait the debounce time to complete the second event.
What is the best choice to manage it? Simply a one-shot timer?
To explain the "debounce" function please see this javascript implementation for key events:
http://benalman.com/code/projects/jquery-throttle-debounce/examples/debounce/
I've used this to debounce events with some success:
public static Action<T> Debounce<T>(this Action<T> func, int milliseconds = 300)
{
var last = 0;
return arg =>
{
var current = Interlocked.Increment(ref last);
Task.Delay(milliseconds).ContinueWith(task =>
{
if (current == last) func(arg);
task.Dispose();
});
};
}
Usage
Action<int> a = (arg) =>
{
// This was successfully debounced...
Console.WriteLine(arg);
};
var debouncedWrapper = a.Debounce<int>();
while (true)
{
var rndVal = rnd.Next(400);
Thread.Sleep(rndVal);
debouncedWrapper(rndVal);
}
It may not be a robust as what's in RX but it's easy to understand and use.
Followup 2020-02-03
Revised #collie's solution using cancellation tokens as follows
public static Action<T> Debounce<T>(this Action<T> func, int milliseconds = 300)
{
CancellationTokenSource? cancelTokenSource = null;
return arg =>
{
cancelTokenSource?.Cancel();
cancelTokenSource = new CancellationTokenSource();
Task.Delay(milliseconds, cancelTokenSource.Token)
.ContinueWith(t =>
{
if (t.IsCompletedSuccessfully)
{
func(arg);
}
}, TaskScheduler.Default);
};
}
Notes:
Calling Cancel is enough to dispose of the CTS
A successfully completed CTS is not canceled/disposed until the next call
As noted by #collie, tasks get disposed so no need to call Dispose on the task
I've not worked with cancellation tokens before and may not be using them correctly.
This isn't a trivial request to code from scratch as there are several nuances. A similar scenario is monitoring a FileSystemWatcher and waiting for things to quiet down after a big copy, before you try to open the modified files.
Reactive Extensions in .NET 4.5 were created to handle exactly these scenarios. You can use them easily to provide such functionality with methods like Throttle, Buffer, Window or Sample. You post the events to a Subject, apply one of the windowing functions to it, for example to get a notification only if there was no activity for X seconds or Y events, then subscribe to the notification.
Subject<MyEventData> _mySubject=new Subject<MyEventData>();
....
var eventSequenc=mySubject.Throttle(TimeSpan.FromSeconds(1))
.Subscribe(events=>MySubscriptionMethod(events));
Throttle returns the last event in a sliding window, only if there were no other events in the window. Any event resets the window.
You can find a very good overview of the time-shifted functions here
When your code receives the event, you only need to post it to the Subject with OnNext:
_mySubject.OnNext(MyEventData);
If your hardware event surfaces as a typical .NET Event, you can bypass the Subject and manual posting with Observable.FromEventPattern, as shown here:
var mySequence = Observable.FromEventPattern<MyEventData>(
h => _myDevice.MyEvent += h,
h => _myDevice.MyEvent -= h);
_mySequence.Throttle(TimeSpan.FromSeconds(1))
.Subscribe(events=>MySubscriptionMethod(events));
You can also create observables from Tasks, combine event sequences with LINQ operators to request eg: pairs of different hardware events with Zip, use another event source to bound Throttle/Buffer etc, add delays and a lot more.
Reactive Extensions is available as a NuGet package, so it's very easy to add them to your project.
Stephen Cleary's book "Concurrency in C# Cookbook" is a very good resource on Reactive Extensions among other things, and explains how you can use it and how it fits with the rest of the concurrent APIs in .NET like Tasks, Events etc.
Introduction to Rx is an excellent series of articles (that's where I copied the samples from), with several examples.
UPDATE
Using your specific example, you could do something like:
IObservable<MachineClass> _myObservable;
private MachineClass connect()
{
MachineClass rpc = new MachineClass();
_myObservable=Observable
.FromEventPattern<MachineClass>(
h=> rpc.RxVARxH += h,
h=> rpc.RxVARxH -= h)
.Throttle(TimeSpan.FromSeconds(1));
_myObservable.Subscribe(machine=>eventRxVARxH(machine));
return rpc;
}
This can be improved vastly of course - both the observable and the subscription need to be disposed at some point. This code assumes that you only control a single device. If you have many devices, you could create the observable inside the class so that each MachineClass exposes and disposes its own observable.
Recently I was doing some maintenance on an application that was targeting an older version of the .NET framework (v3.5).
I couldn't use Reactive Extensions nor Task Parallel Library, but I needed a nice, clean, consistent way of debouncing events. Here's what I came up with:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
namespace MyApplication
{
public class Debouncer : IDisposable
{
readonly TimeSpan _ts;
readonly Action _action;
readonly HashSet<ManualResetEvent> _resets = new HashSet<ManualResetEvent>();
readonly object _mutex = new object();
public Debouncer(TimeSpan timespan, Action action)
{
_ts = timespan;
_action = action;
}
public void Invoke()
{
var thisReset = new ManualResetEvent(false);
lock (_mutex)
{
while (_resets.Count > 0)
{
var otherReset = _resets.First();
_resets.Remove(otherReset);
otherReset.Set();
}
_resets.Add(thisReset);
}
ThreadPool.QueueUserWorkItem(_ =>
{
try
{
if (!thisReset.WaitOne(_ts))
{
_action();
}
}
finally
{
lock (_mutex)
{
using (thisReset)
_resets.Remove(thisReset);
}
}
});
}
public void Dispose()
{
lock (_mutex)
{
while (_resets.Count > 0)
{
var reset = _resets.First();
_resets.Remove(reset);
reset.Set();
}
}
}
}
}
Here's an example of using it in a windows form that has a search text box:
public partial class Example : Form
{
private readonly Debouncer _searchDebouncer;
public Example()
{
InitializeComponent();
_searchDebouncer = new Debouncer(TimeSpan.FromSeconds(.75), Search);
txtSearchText.TextChanged += txtSearchText_TextChanged;
}
private void txtSearchText_TextChanged(object sender, EventArgs e)
{
_searchDebouncer.Invoke();
}
private void Search()
{
if (InvokeRequired)
{
Invoke((Action)Search);
return;
}
if (!string.IsNullOrEmpty(txtSearchText.Text))
{
// Search here
}
}
}
I ran into issues with this. I tried each of the answers here, and since I'm in a Xamarin universal app, I seem to be missing certain things that are required in each of these answers, and I didn't want to add any more packages or libraries. My solution works exactly how I'd expect it to, and I haven't run into any issues with it. Hope it helps somebody.
using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
namespace OrderScanner.Models
{
class Debouncer
{
private List<CancellationTokenSource> StepperCancelTokens = new List<CancellationTokenSource>();
private int MillisecondsToWait;
private readonly object _lockThis = new object(); // Use a locking object to prevent the debouncer to trigger again while the func is still running
public Debouncer(int millisecondsToWait = 300)
{
this.MillisecondsToWait = millisecondsToWait;
}
public void Debouce(Action func)
{
CancelAllStepperTokens(); // Cancel all api requests;
var newTokenSrc = new CancellationTokenSource();
lock (_lockThis)
{
StepperCancelTokens.Add(newTokenSrc);
}
Task.Delay(MillisecondsToWait, newTokenSrc.Token).ContinueWith(task => // Create new request
{
if (!newTokenSrc.IsCancellationRequested) // if it hasn't been cancelled
{
CancelAllStepperTokens(); // Cancel any that remain (there shouldn't be any)
StepperCancelTokens = new List<CancellationTokenSource>(); // set to new list
lock (_lockThis)
{
func(); // run
}
}
}, TaskScheduler.FromCurrentSynchronizationContext());
}
private void CancelAllStepperTokens()
{
foreach (var token in StepperCancelTokens)
{
if (!token.IsCancellationRequested)
{
token.Cancel();
}
}
}
}
}
It's called like so...
private Debouncer StepperDeboucer = new Debouncer(1000); // one second
StepperDeboucer.Debouce(() => { WhateverMethod(args) });
I wouldn't recommend this for anything where the machine could be sending in hundreds of requests a second, but for user input, it works excellently. I'm using it on a stepper in an android/IOS app that calls to an api on step.
RX is probably the easiest choice, especially if you're already using it in your application. But if not, adding it might be a bit of overkill.
For UI based applications (like WPF) I use the following class that use DispatcherTimer:
public class DebounceDispatcher
{
private DispatcherTimer timer;
private DateTime timerStarted { get; set; } = DateTime.UtcNow.AddYears(-1);
public void Debounce(int interval, Action<object> action,
object param = null,
DispatcherPriority priority = DispatcherPriority.ApplicationIdle,
Dispatcher disp = null)
{
// kill pending timer and pending ticks
timer?.Stop();
timer = null;
if (disp == null)
disp = Dispatcher.CurrentDispatcher;
// timer is recreated for each event and effectively
// resets the timeout. Action only fires after timeout has fully
// elapsed without other events firing in between
timer = new DispatcherTimer(TimeSpan.FromMilliseconds(interval), priority, (s, e) =>
{
if (timer == null)
return;
timer?.Stop();
timer = null;
action.Invoke(param);
}, disp);
timer.Start();
}
}
To use it:
private DebounceDispatcher debounceTimer = new DebounceDispatcher();
private void TextSearchText_KeyUp(object sender, KeyEventArgs e)
{
debounceTimer.Debounce(500, parm =>
{
Model.AppModel.Window.ShowStatus("Searching topics...");
Model.TopicsFilter = TextSearchText.Text;
Model.AppModel.Window.ShowStatus();
});
}
Key events are now only processed after keyboard is idle for 200ms - any previous pending events are discarded.
There's also a Throttle method which always fires events after a given interval:
public void Throttle(int interval, Action<object> action,
object param = null,
DispatcherPriority priority = DispatcherPriority.ApplicationIdle,
Dispatcher disp = null)
{
// kill pending timer and pending ticks
timer?.Stop();
timer = null;
if (disp == null)
disp = Dispatcher.CurrentDispatcher;
var curTime = DateTime.UtcNow;
// if timeout is not up yet - adjust timeout to fire
// with potentially new Action parameters
if (curTime.Subtract(timerStarted).TotalMilliseconds < interval)
interval = (int) curTime.Subtract(timerStarted).TotalMilliseconds;
timer = new DispatcherTimer(TimeSpan.FromMilliseconds(interval), priority, (s, e) =>
{
if (timer == null)
return;
timer?.Stop();
timer = null;
action.Invoke(param);
}, disp);
timer.Start();
timerStarted = curTime;
}
Panagiotis's answer is certainly correct, however I wanted to give a simpler example, as it took me a while to sort through how to get it working. My scenario is that a user types in a search box, and as the user types we want to make api calls to return search suggestions, so we want to debounce the api calls so they don't make one every time they type a character.
I'm using Xamarin.Android, however this should apply to any C# scenario...
private Subject<string> typingSubject = new Subject<string> ();
private IDisposable typingEventSequence;
private void Init () {
var searchText = layoutView.FindViewById<EditText> (Resource.Id.search_text);
searchText.TextChanged += SearchTextChanged;
typingEventSequence = typingSubject.Throttle (TimeSpan.FromSeconds (1))
.Subscribe (query => suggestionsAdapter.Get (query));
}
private void SearchTextChanged (object sender, TextChangedEventArgs e) {
var searchText = layoutView.FindViewById<EditText> (Resource.Id.search_text);
typingSubject.OnNext (searchText.Text.Trim ());
}
public override void OnDestroy () {
if (typingEventSequence != null)
typingEventSequence.Dispose ();
base.OnDestroy ();
}
When you first initialize the screen / class, you create your event to listen to the user typing (SearchTextChanged), and then also set up a throttling subscription, which is tied to the "typingSubject".
Next, in your SearchTextChanged event, you can call typingSubject.OnNext and pass in the search box's text. After the debounce period (1 second), it will call the subscribed event (suggestionsAdapter.Get in our case.)
Lastly, when the screen is closed, make sure to dispose of the subscription!
This little gem is inspired by Mike Wards diabolically ingenious extension attempt. However, this one cleans up after itself quite nicely.
public static Action Debounce(this Action action, int milliseconds = 300)
{
CancellationTokenSource lastCToken = null;
return () =>
{
//Cancel/dispose previous
lastCToken?.Cancel();
try {
lastCToken?.Dispose();
} catch {}
var tokenSrc = lastCToken = new CancellationTokenSource();
Task.Delay(milliseconds).ContinueWith(task => { action(); }, tokenSrc.Token);
};
}
Note: there's no need to dispose of the task in this case. See here for the evidence.
Usage
Action DebounceToConsole;
int count = 0;
void Main()
{
//Assign
DebounceToConsole = ((Action)ToConsole).Debounce(50);
var random = new Random();
for (int i = 0; i < 50; i++)
{
DebounceToConsole();
Thread.Sleep(random.Next(100));
}
}
public void ToConsole()
{
Console.WriteLine($"I ran for the {++count} time.");
}
I needed something like this but in a web-application, so I can't store the Action in a variable, it will be lost between http requests.
Based on other answers and #Collie idea I created a class that looks at a unique string key for throttling.
public static class Debouncer
{
static ConcurrentDictionary<string, CancellationTokenSource> _tokens = new ConcurrentDictionary<string, CancellationTokenSource>();
public static void Debounce(string uniqueKey, Action action, int seconds)
{
var token = _tokens.AddOrUpdate(uniqueKey,
(key) => //key not found - create new
{
return new CancellationTokenSource();
},
(key, existingToken) => //key found - cancel task and recreate
{
existingToken.Cancel(); //cancel previous
return new CancellationTokenSource();
}
);
Task.Delay(seconds * 1000, token.Token).ContinueWith(task =>
{
if (!task.IsCanceled)
{
action();
_tokens.TryRemove(uniqueKey, out _);
}
}, token.Token);
}
}
Usage:
//throttle for 5 secs if it's already been called with this KEY
Debouncer.Debounce("Some-Unique-ID", () => SendEmails(), 5);
As a side bonus, because it's based on a string key, you can use inline lambda's
Debouncer.Debounce("Some-Unique-ID", () =>
{
//do some work here
}, 5);
Created this class for solving it also for awaitable calls:
public class Debouncer
{
private CancellationTokenSource _cancelTokenSource = null;
public async Task Debounce(Func<Task> method, int milliseconds = 300)
{
_cancelTokenSource?.Cancel();
_cancelTokenSource?.Dispose();
_cancelTokenSource = new CancellationTokenSource();
await Task.Delay(milliseconds, _cancelTokenSource.Token);
await method();
}
}
Sample of use:
private Debouncer _debouncer = new Debouncer();
....
await _debouncer.Debounce(YourAwaitableMethod);
This is inspired by Nieminen's Task.Delay-based Debouncer class. Simplified, some minor corrections, and should clean up after itself better.
class Debouncer: IDisposable
{
private CancellationTokenSource lastCToken;
private int milliseconds;
public Debouncer(int milliseconds = 300)
{
this.milliseconds = milliseconds;
}
public void Debounce(Action action)
{
Cancel(lastCToken);
var tokenSrc = lastCToken = new CancellationTokenSource();
Task.Delay(milliseconds).ContinueWith(task =>
{
action();
},
tokenSrc.Token
);
}
public void Cancel(CancellationTokenSource source)
{
if (source != null)
{
source.Cancel();
source.Dispose();
}
}
public void Dispose()
{
Cancel(lastCToken);
}
~Debouncer()
{
Dispose();
}
}
Usage
private Debouncer debouncer = new Debouncer(500); //1/2 a second
...
debouncer.Debounce(SomeAction);
I needed a Debounce method for Blazor and kept coming back to this page so I wanted to share my solution in case it helps others.
public class DebounceHelper
{
private CancellationTokenSource debounceToken = null;
public async Task DebounceAsync(Func<CancellationToken, Task> func, int milliseconds = 1000)
{
try
{
// Cancel previous task
if (debounceToken != null) { debounceToken.Cancel(); }
// Assign new token
debounceToken = new CancellationTokenSource();
// Debounce delay
await Task.Delay(milliseconds, debounceToken.Token);
// Throw if canceled
debounceToken.Token.ThrowIfCancellationRequested();
// Run function
await func(debounceToken.Token);
}
catch (TaskCanceledException) { }
}
}
Example call on a search function
<input type="text" #oninput=#(async (eventArgs) => await OnSearchInput(eventArgs)) />
#code {
private readonly DebounceHelper debouncer = new DebounceHelper();
private async Task OnSearchInput(ChangeEventArgs eventArgs)
{
await debouncer.DebounceAsync(async (cancellationToken) =>
{
// Search Code Here
});
}
}
Simply remember the latest 'hit:
DateTime latestHit = DatetIme.MinValue;
private void eventRxVARxH(MachineClass Machine)
{
log.Debug("Event fired");
if(latestHit - DateTime.Now < TimeSpan.FromXYZ() // too fast
{
// ignore second hit, too fast
return;
}
latestHit = DateTime.Now;
// it was slow enough, do processing
...
}
This will allow a second event if there was enough time after the last event.
Please note: it is not possible (in a simple way) to handle the last event in a series of fast events, because you never know which one is the last...
...unless you are prepared to handle the last event of a burst which is a long time ago. Then you have to remember the last event and log it if the next event is slow enough:
DateTime latestHit = DatetIme.MinValue;
Machine historicEvent;
private void eventRxVARxH(MachineClass Machine)
{
log.Debug("Event fired");
if(latestHit - DateTime.Now < TimeSpan.FromXYZ() // too fast
{
// ignore second hit, too fast
historicEvent = Machine; // or some property
return;
}
latestHit = DateTime.Now;
// it was slow enough, do processing
...
// process historicEvent
...
historicEvent = Machine;
}
I did some more simple solution based on #Mike Ward answer:
public static class CustomTaskExtension
{
#region fields
private static int _last = 0;
#endregion
public static void Debounce(CancellationTokenSource throttleCts, double debounceTimeMs, Action action)
{
var current = Interlocked.Increment(ref _last);
Task.Delay(TimeSpan.FromMilliseconds(debounceTimeMs), throttleCts.Token)
.ContinueWith(task =>
{
if (current == _last) action();
task.Dispose();
});
}
}
Example how to use it:
// security way to cancel the debounce process any time
CancellationTokenSource _throttleCts = new CancellationTokenSource();
public void MethodCalledManyTimes()
{
// will wait 250ms after the last call
CustomTaskExtension.Debounce(_throttleCts, 250, async () =>
{
Console.Write("Execute your code 250ms after the last call.");
});
}
I came up with this in my class definition.
I wanted to run my action immediately if there hasn't been any action for the time period (3 seconds in the example).
If something has happened in the last three seconds, I want to send the last thing that happened within that time.
private Task _debounceTask = Task.CompletedTask;
private volatile Action _debounceAction;
/// <summary>
/// Debounces anything passed through this
/// function to happen at most every three seconds
/// </summary>
/// <param name="act">An action to run</param>
private async void DebounceAction(Action act)
{
_debounceAction = act;
await _debounceTask;
if (_debounceAction == act)
{
_debounceTask = Task.Delay(3000);
act();
}
}
So, if I have subdivide my clock into every quarter of a second
TIME: 1e&a2e&a3e&a4e&a5e&a6e&a7e&a8e&a9e&a0e&a
EVENT: A B C D E F
OBSERVED: A B E F
Note that no attempt is made to cancel the task early, so it's possible for actions to pile up for 3 seconds before eventually being available for garbage collection.
Figured out how to use System.Reactive NuGet package for doing a proper debouncing on a TextBox.
At the class level, we have our field
private IObservable<EventPattern<TextChangedEventArgs>> textChanged;
Then when we want to start listening to the event:
// Debouncing capability
textChanged = Observable.FromEventPattern<TextChangedEventArgs>(txtSearch, "TextChanged");
textChanged.ObserveOnDispatcher().Throttle(TimeSpan.FromSeconds(1)).Subscribe(args => {
Debug.WriteLine("bounce!");
});
Make sure you don't also wire your textbox up to an event handler. The Lambda above is the event handler.
I wrote an async debouncer that doesn't run async-in-sync.
public sealed class Debouncer : IDisposable {
public Debouncer(TimeSpan? delay) => _delay = delay ?? TimeSpan.FromSeconds(2);
private readonly TimeSpan _delay;
private CancellationTokenSource? previousCancellationToken = null;
public async Task Debounce(Action action) {
_ = action ?? throw new ArgumentNullException(nameof(action));
Cancel();
previousCancellationToken = new CancellationTokenSource();
try {
await Task.Delay(_delay, previousCancellationToken.Token);
await Task.Run(action, previousCancellationToken.Token);
}
catch (TaskCanceledException) { } // can swallow exception as nothing more to do if task cancelled
}
public void Cancel() {
if (previousCancellationToken != null) {
previousCancellationToken.Cancel();
previousCancellationToken.Dispose();
}
}
public void Dispose() => Cancel();
}
I use it to debounce changes reported on file changes, see complete example here.
I was inspired by Mike's answer, but needed solution that worked without tasks, which simply swallows subsequent event invocations until debounce time-out runs out. Here's my solution:
public static Action<T> Debounce<T>(this Action<T> action, int milliseconds = 300)
{
DateTime? runningCallTime = null;
var locker = new object();
return arg =>
{
lock (locker)
{
if (!runningCallTime.HasValue ||
runningCallTime.Value.AddMilliseconds(milliseconds) <= DateTime.UtcNow)
{
runningCallTime = DateTime.UtcNow;
action.Invoke(arg);
}
}
};
}
Another implementation
public static class Debounce
{
public static Action Action(Action action, TimeSpan time)
{
var timer = new Timer(_ => action(), null, Timeout.InfiniteTimeSpan, Timeout.InfiniteTimeSpan);
return () => timer.Change(time, Timeout.InfiniteTimeSpan);
}
}
None of the above answers fully worked for me, so I've come up with the following implementation:
public class Debouncer
{
private CancellationTokenSource _cancelTokenSource = null;
public Task Debounce(Func<Task> method, int milliseconds = 250)
{
_cancelTokenSource?.Cancel();
_cancelTokenSource?.Dispose();
_cancelTokenSource = new CancellationTokenSource();
try
{
return Task.Delay(milliseconds, _cancelTokenSource.Token)
.ContinueWith(_ => method(), _cancelTokenSource.Token);
}
catch (TaskCanceledException exception) when (exception.CancellationToken == _cancelTokenSource.Token)
{
}
return Task.CompletedTask;
}
}
Usage:
var debouncer = new Debouncer();
await debouncer.Debounce(async () => await someAction());
I know I'm a couple hundred thousand minutes late to this party but I figured I'd add my 2 cents. I'm surprised no one has suggested this so I'm assuming there's something I don't know that might make it less than ideal so maybe I'll learn something new if this gets shot down.
I often use a solution that uses the System.Threading.Timer's Change() method.
using System.Threading;
Timer delayedActionTimer;
public MyClass()
{
// Setup our timer
delayedActionTimer = new Timer(saveOrWhatever, // The method to call when triggered
null, // State object (Not required)
Timeout.Infinite, // Start disabled
Timeout.Infinite); // Don't repeat the trigger
}
// A change was made that we want to save but not until a
// reasonable amount of time between changes has gone by
// so that we're not saving on every keystroke/trigger event.
public void TextChanged()
{
delayedActionTimer.Change(3000, // Trigger this timers function in 3 seconds,
// overwriting any existing countdown
Timeout.Infinite); // Don't repeat this trigger; Only fire once
}
// Timer requires the method take an Object which we've set to null since we don't
// need it for this example
private void saveOrWhatever(Object obj)
{
/*Do the thing*/
}
In my first view model (renamed to MainViewModel) I have a list of ActionViewModels.
In my xaml i have a listbox which is bound to the list, in the listbox i have a template which binds to properties from the ActionViewModel.
So far so good and everything works.
When selecting one of the listitems i navigate to an ActionViewModel and pass the id with it.
The ActionViewModel retrieves information from a static list in memory from which the MainViewModel also retrieved the information to create the list of actionviewmodels.
So far still so good, i can edit the properties, all the bindings do work fine and i'm all happy.
By clicking the save button the information is gathered and stored in the static list.
When i hit the back button i go back to the list, but unfortunately the values showing there are still the same, is there some way to send a command to reload the items in the list? To pass a complete viewmodel as reference to a new ActionViewModel? Or some property which tells the parent 'this viewmodel in your list has been updated'?
I am sure the above text is a bit confusing, so here is some code to clarify it a bit (hopefully)
MainViewModel.cs
private List<ActionViewModel> _actionViewModels;
public List<ActionViewModel> ActionViewModels
{
get { return _actionViewModels; }
set { _actionViewModels = value; RaisePropertyChanged(() => ActionViewModels); }
}
private Cirrious.MvvmCross.ViewModels.MvxCommand<int> _navigateToAction;
public System.Windows.Input.ICommand NavigateToAction
{
get
{
_navigateToAction = _navigateToAction ?? new Cirrious.MvvmCross.ViewModels.MvxCommand<int>((action) => NavigateToTheDesiredAction(action));
return _navigateToAction;
}
}
private void NavigateToTheDesiredAction(int action)
{
ShowViewModel<ActionViewModel>(new { id = action });
}
// Get DTOs from server or from cache and fill the list of ActionViewModels
public async Task Load()
{
ActionService actionService = new ActionService();
List<ActionViewModel> actionViewModels = new List<ActionViewModel>();
MyActions = await actionService.GetMyActions();
foreach (ActionDTO action in MyActions)
{
ActionViewModel actionViewModel = new ActionViewModel();
await actionViewModel.Load(action.id);
actionViewModels.Add(actionViewModel);
}
ActionViewModels = actionViewModels;
}
ActionViewModel.cs
public int ID
{
get { return TheAction.id; }
set { TheAction.id = value; RaisePropertyChanged(() => ID); }
}
public string Title
{
get { return TheAction.Title; }
set { TheAction.Title = value; RaisePropertyChanged(() => Title); }
}
public async Task Load(int actionId)
{
ActionDTO TheAction = await actionService.GetAction(actionId);
this.ID = TheAction.id;
this.Title = TheAction.Title;
}
private Cirrious.MvvmCross.ViewModels.MvxCommand _save;
public System.Windows.Input.ICommand Save
{
get
{
_save = _save ?? new Cirrious.MvvmCross.ViewModels.MvxCommand(PreSaveModel);
return _save;
}
}
private void PreSaveModel()
{
SaveModel();
}
private async Task SaveModel()
{
ValidationDTO result = await actionService.SaveAction(TheAction);
}
ActionService.cs
public static List<ActionDTO> AllActions = new List<ActionDTO>();
public async Task<ActionDTO> GetAction(int actionId)
{
ActionDTO action = AllActions.FirstOrDefault(a => a.id == actionId);
if (action == null)
{
int tempActionId = await LoadAction(actionId);
if (tempActionId > 0)
return await GetAction(actionId);
else
return new ActionDTO() { Error = new ValidationDTO(false, "Failed to load the action with id " + actionId, ErrorCode.InvalidActionId) };
}
return action;
}
private async Task<int> LoadAction(int actionId)
{
ActionDTO action = await webservice.GetAction(actionId);
AllActions.Add(action);
return action.id;
}
public async Task<ValidationDTO> SaveAction(ActionDTO action)
{
List<ActionDTO> currentList = AllActions;
ActionDTO removeActionFromList = currentList.FirstOrDefault(a => a.id == action.id);
if (removeActionFromList != null)
currentList.Remove(removeActionFromList);
currentList.Add(action);
AllActions = currentList;
return await webservice.SaveAction(action);
}
There are 3 ways I can think of that would allow you to do this.
The ActionService could send out some sort of notification when data changes. One easy way to do this is to use the MvvmCross Messenger plugin. This is the way the CollectABull service works in CollectionService.cs in the N+1 days of mvvmcross videos (for more info watch N=13 in http://mvvmcross.wordpress.com)
This is the approach I generally use. It has low overhead, uses WeakReferences (so doesn't leak memory), it is easily extensible (any object can listen for changes), and it encourages loose coupling of the ViewModel and Model objects
You could implement some kind of Refresh API on the list ViewModel and could call this from appropriate View events (e.g. ViewDidAppear, OnNavigatedTo and OnResume).
I don't generally use this approach for Refreshing known data, but I have used it for enabling/disabling resource intensive objects - e.g. timers
For certain shape of model data (and especially how often it changes), then I can imagine scenarios where this approach might be more efficient than the messenger approach.
You could extend the use of INotifyPropertyChanged and INotifyCollectionChanged back into your model layer.
I've done this a few times and it's worked well for me.
If you do choose this approach, be careful to ensure that all Views do subscribe to change events using WeakReference subscriptions such as those used in MvvmCross binding - see WeakSubscription. If you didn't do this, then it could be possible for the Model to cause Views to persist in memory even after the UI itself has removed them.