Develop Reference

Issue with MessagingCenter firing multiple times Xamarin

I have looked all over for a solution to an issue. I have noticed that in my android app, every time I fire an event from <button Clicked="GPSToggle_Clicked">, for some reason it increments the number of times my methods get called. So after I compile and load this on my phone, I hit my "GPSToggle_Clicked" button, and then to stop hit that button again. On the first "stop", I'll get a single instance of the below output:
---------------------------------------------------------------Attempting string parsing
---------------------------------------------------------------Sending string to SubmitGeneratedGPX
---------------------------------------------------------------path: /storage/emulated/0/Download/GPX/2022-10-27-02-44-06.gpx
---------------------------------------------------------------GPX File creation success
---------------------------------------------------------------:RawBufferToJsonString: [{"Timestamp":"2022-10-27T18:43:52.849+00:00","Latitude":41.5263818,"Longitude":-81.6507923,"Altitude":153.29998779296875,"Accuracy":20.0,"VerticalAccuracy":1.7990270853042603,"Speed":null,"Course":null,"IsFromMockProvider":false,"AltitudeReferenceSystem":2},{"Timestamp":"2022-10-27T18:43:53.696+00:00","Latitude":41.5263819,"Longitude":-81.6507921,"Altitude":153.29998779296875,"Accuracy":20.0,"VerticalAccuracy":1.7697961330413818,"Speed":null,"Course":null,"IsFromMockProvider":false,"AltitudeReferenceSystem":2},{"Timestamp":"2022-10-27T18:43:54.526+00:00","Latitude":41.5263819,"Longitude":-81.6507921,"Altitude":153.29998779296875,"Accuracy":20.0,"VerticalAccuracy":1.7697961330413818,"Speed":null,"Course":null,"IsFromMockProvider":false,"AltitudeReferenceSystem":2},{"Timestamp":"2022-10-27T18:43:55.374+00:00","Latitude":41.5263819,"Longitude":-81.6507921,"Altitude":153.29998779296875,"Accuracy":20.0,"VerticalAccuracy":1.7697961330413818,"Speed":null,"Course":null,"IsFromMockProvider":false,"AltitudeReferenceSystem":2},{"Timestamp":"2022-10-27T18:43:56.21+00:00","Latitude":41.5263811,"Longitude":-81.650792,"Altitude":153.29998779296875,"Accuracy":20.0,"VerticalAccuracy":1.7160584926605225,"Speed":null,"Course":null,"IsFromMockProvider":false,"AltitudeReferenceSystem":2}]
Every subsequent time I hit start/stop on the app, I get the real-time data in the output multiplied by the number of times I've started/stopped since the last compiling.
the main app page button event thats fired:
private async void GPSToggle_Clicked(object sender, EventArgs e)
{
var LocationPermissionStatus = await Xamarin.Essentials.Permissions.RequestAsync<Xamarin.Essentials.Permissions.LocationAlways>();
var FileReadPermissionStatus = await Xamarin.Essentials.Permissions.RequestAsync<Xamarin.Essentials.Permissions.StorageRead>();
var FileWritePermissionStatus = await Xamarin.Essentials.Permissions.RequestAsync<Xamarin.Essentials.Permissions.StorageWrite>();
if(LocationPermissionStatus == Xamarin.Essentials.PermissionStatus.Denied)
{
// TODO
return;
}
// run if device is android
if(Device.RuntimePlatform == Device.Android)
{
if (!CrossGeolocator.Current.IsGeolocationAvailable || !CrossGeolocator.Current.IsGeolocationEnabled)
{
// gps is not enabled, throw alert
Console.WriteLine("---------------------------------------------------------------GPS is DISABLED");
await DisplayAlert("Error", "GPS is not enabled. You must enable GPS to use this feature", "Ok");
}
else
{
// set our IsTracking = true flag
if (!IsTracking)
{
// start background listening for GPS
await StartListening();
Console.WriteLine("---------------------------------------------------------------Listening: " + CrossGeolocator.Current.IsListening);
StartService();
Console.WriteLine("---------------------------------------------------------------Service initiated");
IsTracking = true;
Console.WriteLine("---------------------------------------------------------------Tracking initiated");
GPSToggle.Text = "Stop Tracking";
}
else
{
//
// verify that the submittal wasn't done in error, before stopping services and submitting data
bool DoneInError = await DisplayAlert("Finish?", "Are you sure you want to stop services and submit?", "No", "Yes");
if (!DoneInError)
{
await StopListening();
Console.WriteLine("---------------------------------------------------------------listening:" + CrossGeolocator.Current.IsListening);
IsTracking = false;
Console.WriteLine("---------------------------------------------------------------Tracking ceased");
// stop the gps service
StopService();
Console.WriteLine("---------------------------------------------------------------Service ceased");
// stop the background listening for gps
Console.WriteLine("---------------------------------------------------------------Attempt GPX parse from buffer obj");
GPSToggle.Text = "Start Tracking";
}
}
}
}
}
Specifically the line:
StartService();
Fires this method off within the same class, specifically the MessagingCenter.Send<>, which initiates my foreground service to handle logging the gps data into a buffer:
private void StartService()
{
var startServiceMessage = new StartServiceMessage();
MessagingCenter.Send(startServiceMessage, "ServiceStarted");
Preferences.Set("LocationServiceRunning", true);
StatusLabel.Text = "Location service has been started";
Console.WriteLine("---------------------------------------------------------------location service has been started. preferences saved");
}
and
StopService();
Fires this method off to stop the services and retrieve the gps buffer data from the foreground to the main thread:
private void StopService()
{
var stopServiceMessage = new StopServiceMessage();
MessagingCenter.Unsubscribe<App, List<Location>>(this, "GPXBufferData");
MessagingCenter.Subscribe<App, List<Location>>(this, "GPXBufferData", (sender, args) =>
{
RawGPXData = args;
Generate_CreateGPX_File(RawGPXData);
RawBufferToJsonString = GPXParse.GenerateJSON_GPXPoints(RawGPXData);
Console.WriteLine("---------------------------------------------------------------:RawBufferToJsonString: " + RawBufferToJsonString);
PromptForSubmission_GPXPoints_API();
});
Console.WriteLine("--------------------------------------------------------------------------");
MessagingCenter.Send(stopServiceMessage, "ServiceStopped");
Preferences.Set("LocationServiceRunning", false);
Console.WriteLine("---------------------------------------------------------------Location service stopped. preferences saved");
}
In the above snippet, this line is subscribed to in the GPSLocationService.cs file:
MessagingCenter.Send(stopServiceMessage, "ServiceStopped");
This is a portion of my GPSLocationService.cs file that is relevant to this:
public async Task Run(CancellationToken token)
{
int ObjCount = 0;
await Task.Run(async () => {
// if the task was stopped
// check the buffer for data, if data, send to GPXGenerator
MessagingCenter.Subscribe<StopServiceMessage>(this, "ServiceStopped",
message =>
{
if (GPSBufferObj != null)
{
Device.BeginInvokeOnMainThread(() =>
{
MessagingCenter.Unsubscribe<App, List<Location>>((App)Xamarin.Forms.Application.Current, "GPXBufferData");
MessagingCenter.Send<App, List<Location>>((App)Xamarin.Forms.Application.Current, "GPXBufferData", GPSBufferObj);
});
}
});
return;
}, token);
}
I believe I have tracked down where the issue is starting. In my StopService() method, I have the following line (just to keep track of where Im at in the buffer) and it is only sent to output once.
Console.WriteLine("--------------------------------------------------------------------------");
BUT if I place that same line within the pasted portion of my GPSLocationService.cs file, I will get the incremented output. I'm leaning towards the nested task being the issue, I wrote this based losely off of this example repro:
https://github.com/jfversluis/XFBackgroundLocationSample

You don't have MessagingCenter.Unsubscribe<StopServiceMessage> anywhere in your code. StopServiceMessage is what you are accumulating subscriptions to.
You need to make sure Unsubscribe is unsubscribing the instance that was previously subscribed to. It sounds to me like there are multiple instances of GPSLocationService. [In which case, this is no longer referring to the original instance. Unsubscribe won't do anything, unless you have the this that was originally Subscribed.]
If so, instead create an instance of GPSLocationService ONCE, and store it in a static variable. Re-use it. start/stop it, but don't discard it.
Alternatively, if you only want a message ONE TIME from each Subscribe, then Unsubscribe as soon as you receive each message:
MessagingCenter.Subscribe<StopServiceMessage>(this, "ServiceStopped",
message =>
{
MessagingCenter.Unsubscribe<StopServiceMessage>(this, "ServiceStopped");
... your code ...
});
Use this same pattern EVERYWHERE you have a Subscribe (unless you Subscribe ONLY ONE TIME at app start, as Jason suggested.)

How do I get a GPS location update synchronously in Android using Xamarin?

Specifically I am doing C# development using Xamarin.Forms, however working on the native Android side writing a GPS wrapper class which will be usable in the Xamarin.Forms side via dependency injection. The calls should be the same between C# and Java in regards to Android for the most part.
Essentially, I have this method in my Geolocator object (which implements ILocationListener) on the Android side:
public async Task<Tuple<bool, string, GPSData>> GetGPSData() {
gpsData = null;
var success = false;
var error = string.Empty;
if (!manager.IsProviderEnabled(LocationManager.GpsProvider)) {
//request permission or location services enabling
//set error
} else {
manager.RequestSingleUpdate(LocationManager.GpsProvider, this, null);
success = true;
}
return new Tuple<bool, string, GPSData>(success, error, gpsData);
}
and
public void OnLocationChanged(Location location) {
gpsData = new GPSData(location.Latitude, location.Longitude);
}
I want to be able to call GetGPSData and have it return the tuple, which for now the only important thing about the Tuple is gpsData is filled in. I know it can take seconds to find a fix, so I want this method to be async and be awaitable in the Xamarin.Forms side once I actually need the value.
My issue is I can't figure out a way to have manager.RequestSingleUpdate work synchronously, or any work around. You call that method, and then eventually OnLocationChanged fires off. I tried throwing in a disgusting, barbaric
while (gpsData == null);
after the call to force it not to continue until OnLocationChanged was fired, however when I put that line in, OnLocationChanged never gets called. I'm assuming that is because OnLocationChanged is invoked on the same thread instead of being a background thread.
Is there any way for me to take this scenario and have GetGPSData not return until OnLocationChanged has fired off?
Thanks
EDIT: To add, this method will not be regularly called. It's spontaneous and rare, so I don't want to use RequestLocationUpdates, get regular updates and return the most recent one because that would require always having GPS on, while would rain batteries unnecessarily.

You can do what you want with a TaskCompletionSource. I had the same issue and this is how I solved it:
TaskCompletionSource<Tuple<bool, string, GPSData> tcs;
// No need for the method to be async, as nothing is await-ed inside it.
public Task<Tuple<bool, string, GPSData>> GetGPSData() {
tcs = new TaskCompletionSource<Tuple<bool, string, GPSData>>();
gpsData = null;
var success = false;
var error = string.Empty;
if (!manager.IsProviderEnabled(LocationManager.GpsProvider)) {
//request permission or location services enabling
//set error
tcs.TrySetException(new Exception("some error")); // This will throw on the await-ing caller of this method.
} else {
manager.RequestSingleUpdate(LocationManager.GpsProvider, this, null);
success = true;
}
//return new Tuple<bool, string, GPSData>(success, error, gpsData); <-- change this to:
return this.tcs.Task;
}
And:
public void OnLocationChanged(Location location) {
gpsData = new GPSData(location.Latitude, location.Longitude);
// Here you set the result of TaskCompletionSource. Your other method completes the task and returns the result to its caller.
tcs.TrySetResult(new Tuple<bool, string, GPSData>(false, "someString", gpsData));
}

Synchronizing asynchronous events

I have a class that receives standard .Net events from an external class.
These events have an address property (in addition to a lot of other properties, of course) that I can use to synchronize my events, so that I should be able to create a method to Get something, wait for the correct event, then return the data from the event in the Get method.
However, I'm fairly new to synchronization in C# and was hoping any of you could help me out. Below is somewhat pseudo code for what I want to accomplish:
Someone calls DoAsynchronousToSynchronousCall
That method waits until an event have been received with the same address (or until it times out)
The event checks against all current requests. If it finds a request with the same address, let DoAsynchronousToSynchronousCall know the reply has arrived
DoAsynchronousCall gets (or retrieves) the reply and returns it to the caller
public class MyMessage
{
public string Address { get; set; }
public string Data { get; set; }
}
public Main
{
externalClass.MessageReceived += MessageReceived;
}
public void MessageReceived(MyMessage message)
{
MyMessage request = _requestQueue.FirstOrDefault(m => m.Address = message.Address);
if (request != null)
{
// Do something to let DoAsynchronousToSynchronousCall() know the reply has arrived
}
}
private List<MyMessage> _requestQueue = new List<MyMessage>();
public MyMessage DoAsynchronousToSynchronousCall(MyMessage message)
{
_requestQueue.Add(message);
externalClass.Send(message);
// Do something to wait for a reply (as checked for above)
MyMessage reply = WaitForCorrectReply(timeout: 10000);
return reply;
}
I feel like I'm missing an opportunity to use async and await (yet I don't know how), and I hope you're able to understand what I'm trying to accomplish based on the information above.

You really can't have multiple calls on the fly and have synchronous responses. If you want synchronous responses for multiple calls then you need to do the calls synchronously too.
I would look at using Microsoft's Reactive Extensions (NuGet "Rx-Main") to make what you're doing as simple as possible. Rx lets you turn events into streams of values that you can query against.
Here's what I would do.
I would first define a stream of the received messages as IObservable<MyMessage> receivedMessages like this:
receivedMessages =
Observable
.FromEvent<MessageReceivedHandler, MyMessage>(
h => externalClass.MessageReceived += h,
h => externalClass.MessageReceived -= h);
(You didn't provide a class def so I've called the event delegate MessageReceivedHandler.)
Now you can redefine DoAsynchronousToSynchronousCall as:
public IObservable<MyMessage> DoAsynchronousCall(MyMessage message)
{
return Observable.Create<MyMessage>(o =>
{
IObservable<MyMessage> result =
receivedMessages
.Where(m => m.Address == message.Address)
.Take(1);
IObservable<MyMessage> timeout =
Observable
.Timer(TimeSpan.FromSeconds(10.0))
.Select(x => (MyMessage)null);
IDisposable subscription =
Observable
.Amb(result, timeout)
.Subscribe(o);
externalClass.Send(message);
return subscription;
});
}
The result observable is the receivedMessages filtered for the current message.Address.
The timeout observable is a default value to return if the call takes longer than TimeSpan.FromSeconds(10.0) to complete.
Finally the subscription uses Observable.Amb(...) to determine which of result or timeout produces a value first and subscribes to that result.
So now to call this you can do this:
DoAsynchronousCall(new MyMessage() { Address = "Foo", Data = "Bar" })
.Subscribe(response => Console.WriteLine(response.Data));
So, if I make a simple definition of ExternalClass like this:
public class ExternalClass
{
public event MessageReceivedHandler MessageReceived;
public void Send(MyMessage message)
{
this.MessageReceived(new MyMessage()
{
Address = message.Address,
Data = message.Data + "!"
});
}
}
...I get the result Bar! printed on the console.
If you have a whole bunch of messages that you want to process you can do this:
var messagesToSend = new List<MyMessage>();
/* populate `messagesToSend` */
var query =
from message in messagesToSend.ToObservable()
from response in DoAsynchronousCall(message)
select new
{
message,
response
};
query
.Subscribe(x =>
{
/* Do something with each correctly paired
`x.message` & `x.response`
*/
});

You're probably looking for ManualResetEvent which functions as a "toggle" of sorts to switch between thread-blocking and non-blocking behavior. The DoAsynchronousToSynchronousCall would Reset and then WaitOne(int timeoutMilliseconds) the event to block the thread, and the thing checking for the correct reply arrived would do the Set call to let the thread continue on its way if the correct thing arrived.

Shared object among different requests

I'm working with .NET 3.5 with a simple handler for http requests. Right now, on each http request my handler opens a tcp connection with 3 remote servers in order to receive some information from them. Then closes the sockets and writes the server status back to Context.Response.
However, I would prefer to have a separate object that every 5 minutes connects to the remote servers via tcp, gets the information and keeps it. So the HttpRequest, on each request would be much faster just asking this object for the information.
So my questions here are, how to keep a shared global object in memory all the time that can also "wake" an do those tcp connections even when no http requests are coming and have the object accesible to the http request handler.

A service may be overkill for this.
You can create a global object in your application start and have it create a background thread that does the query every 5 minutes. Take the response (or what you process from the response) and put it into a separate class, creating a new instance of that class with each response, and use System.Threading.Interlocked.Exchange to replace a static instance each time the response is retrieved. When you want to look the the response, simply copy a reference the static instance to a stack reference and you will have the most recent data.
Keep in mind, however, that ASP.NET will kill your application whenever there are no requests for a certain amount of time (idle time), so your application will stop and restart, causing your global object to get destroyed and recreated.
You may read elsewhere that you can't or shouldn't do background stuff in ASP.NET, but that's not true--you just have to understand the implications. I have similar code to the following example working on an ASP.NET site that handles over 1000 req/sec peak (across multiple servers).
For example, in global.asax.cs:
public class BackgroundResult
{
public string Response; // for simplicity, just use a public field for this example--for a real implementation, public fields are probably bad
}
class BackgroundQuery
{
private BackgroundResult _result; // interlocked
private readonly Thread _thread;
public BackgroundQuery()
{
_thread = new Thread(new ThreadStart(BackgroundThread));
_thread.IsBackground = true; // allow the application to shut down without errors even while this thread is still running
_thread.Name = "Background Query Thread";
_thread.Start();
// maybe you want to get the first result here immediately?? Otherwise, the first result may not be available for a bit
}
/// <summary>
/// Gets the latest result. Note that the result could change at any time, so do expect to reference this directly and get the same object back every time--for example, if you write code like: if (LatestResult.IsFoo) { LatestResult.Bar }, the object returned to check IsFoo could be different from the one used to get the Bar property.
/// </summary>
public BackgroundResult LatestResult { get { return _result; } }
private void BackgroundThread()
{
try
{
while (true)
{
try
{
HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create("http://example.com/samplepath?query=query");
request.Method = "GET";
using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
{
using (StreamReader reader = new StreamReader(response.GetResponseStream(), System.Text.Encoding.UTF8))
{
// get what I need here (just the entire contents as a string for this example)
string result = reader.ReadToEnd();
// put it into the results
BackgroundResult backgroundResult = new BackgroundResult { Response = result };
System.Threading.Interlocked.Exchange(ref _result, backgroundResult);
}
}
}
catch (Exception ex)
{
// the request failed--cath here and notify us somehow, but keep looping
System.Diagnostics.Trace.WriteLine("Exception doing background web request:" + ex.ToString());
}
// wait for five minutes before we query again. Note that this is five minutes between the END of one request and the start of another--if you want 5 minutes between the START of each request, this will need to change a little.
System.Threading.Thread.Sleep(5 * 60 * 1000);
}
}
catch (Exception ex)
{
// we need to get notified of this error here somehow by logging it or something...
System.Diagnostics.Trace.WriteLine("Error in BackgroundQuery.BackgroundThread:" + ex.ToString());
}
}
}
private static BackgroundQuery _BackgroundQuerier; // set only during application startup
protected void Application_Start(object sender, EventArgs e)
{
// other initialization here...
_BackgroundQuerier = new BackgroundQuery();
// get the value here (it may or may not be set quite yet at this point)
BackgroundResult result = _BackgroundQuerier.LatestResult;
// other initialization here...
}

How to Implement Progress Bar for Long Running HTTP Request

I have an HTTP server written in C# based off the HttpListenerContext class. The server is for processing binary log files and converting them to text, and can take quite a long time to do the conversion. I would like to indicate progress back to the user, but I am unsure on the best way to do this. On the server side, in handling my HTTP request, I essentially have this function:
public async Task HandleRequest()
{
try
{
await ProcessRequest();
}
catch (HttpListenerException)
{
// Something happened to the http connection, don't try to send anything
}
catch (Exception e)
{
SendFailureResponse(500);
}
}
Currently, ProcessRequest() sends the HTML response when finished, but I would like to essentially add the IProgress interface to the function and somehow indicate that progress back to the Web client. What is the best way to do this?

One way of doing it would be to store progress on server side and periodically pull the information from client.
However, if you want the server to notify the client ( push ), then you will need to implement some kind of bi-directional communication between the server and client (I am currently using ASP.NET Web API and SignalR to achieve this at work).

Here is what I got I'll try to explain and I hope you notice its not FULL FULL complete, you'll have to understand the logic behind this and accept or not as a plausible option.
The Method: Set a custom object to store progress of your ongoing operations, make a global static list containing this metadata. Notice how I track them with Ids: I don't store that on DB, the natural act of instantiating the class will auto_increment their Id.
Then, you can add a new controller to respond the progress of a particular ongoing process.
Now that you have a controller to respond the progress of an ongoing process by Id, you can create a javascript timer to call it and update the DOM.
When creating your process, dont hold the htmlrequest until its over, open a background operation instead and just respond with the newly created ProgressTracker.Id, through that class/list you can keep track of the progress and reply accordingly.
As said in another answer, when an operation finishes you can send a push notification and the clientside javascript will interrupt the timers and proceed to the next view/result/page, or you can increment the looping timer to detect when its done and call the results from another controller. (this way you can avoid using push if needed.)
Here is the partial code:
public class ProgressTracker {
private static GlobalIdProvider = 0;
public int _id = ++GlobalIdProvider;
public int Id { get { return _id; } }
bool IsInProgress = false;
bool IsComplete = false;
float Progress;
public YourProgressObject Data;
}
public class GlobalStatic {
public static List<ProgressTracker> FooOperations = new List<ProgressTracker>();
}
public class SomeWebApiController {
[HttpGet]
[Authorize]
public HttpResponseMessage GetProgress(int Id) {
var query = (from a in GlobalStatic.FooOperations where a.Id==Id select a);
if(!query.Any()) {
return Request.CreateResponse(HttpStatusCode.NotFound, "No operation with this Id found.");
} else {
return Request.CreateResponse(HttpStatusCode.Ok, query.First());
}
}
}
// this is javascript
// ... Your code until it starts the process.
// You'll have to get the ProgressTracker Id from the server somehow.
var InProgress = true;
window.setTimeout(function(e) {
var xmlhttp = new XMLHttpRequest();
var url = "<myHostSomething>/SomeWebApiController/GetProgress?Id="+theId;
xmlhttp.setRequestHeader("Authentication","bearer "+localStorage.getItem("access_token"));
xmlhttp.onreadystatechange = function() {
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
var data = JSON.parse(xmlhttp.responseText);
updateProgressBar(data);
}
}
xmlhttp.open("GET", url, true);
xmlhttp.send();
function updateProgressBar(data) {
document.getElementById("myProgressText").innerHTML = data.Progress;
}
}, 3000);
Disclaimer: If my javascript is shitty, pardon me but I'm too used to using jQuery and all this fancy stuff x_x