I'm trying to implement a wrapper for the Amazon IAP Plugin for Xamarin. It uses an event based system in the following way:
You can initiate method calls and listen for events. Method calls initiate requests, some of which return a response. Events are asynchronous system-generated messages that are sent in response to method calls to return the requested data to you.
See more here
My goal is to wrap this event based system into some API which allows me to use the plugin with tasks, so I can use the async-await syntax. To achieve that I'm using the TaskCompletionSource like in the following example:
public async Task<bool> GetProductInfoAsync(params string[] productIds)
{
var iapService = AmazonIapV2Impl.Instance;
var tcs = new TaskCompletionSource<bool>();
var skus = new SkusInput { Skus = productIds.ToList() };
var requestId = iapService.GetProductData(skus).RequestId;
GetProductDataResponseDelegator delegator = null;
delegator = new GetProductDataResponseDelegator(response =>
{
if(response.Id == requestId) {
var result = GetResultFromResponse(response);
tcs.SetResult(result);
//iapService.RemoveGetProductDataResponseListener(delegator.responseDelegate);
}
});
iapService.AddGetProductDataResponseListener(delegator.responseDelegate);
return await tcs.Task;
}
This code seems to work fine if the method gets called once, but if it gets called two times in a row the app crashes immediately and the only thing printed to the console is the following message..
[mono] Unhandled Exception:
[mono] System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
[mono-rt] [ERROR] FATAL UNHANDLED EXCEPTION: System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
..which kinda makes no sense at all.
So is there something obvious I'm missing here? Or could it be a bug from the plugin?
I have created a repository with the code above so you can reproduce the problem. It's my playground, so please ignore the whole structure of the project and just focus on the classes AmazonIAPService and MainActivity.
Hint 1:
The commented line //iapService.RemoveGetProductDataResponseListener(delegator.responseDelegate); causes also a crash with the same message but already at the first call of the method.
Hint 2:
The AmazonIAPService contains a commented method which uses await Task.Delay(TimeSpan.FromMilliseconds(1)) and solves the problem from above in a very hacky way which I really don't like.
Problem seems to be that those functions have to run asynchronously. Also mentioned here in the doc. So once you run those functions synchronously somehow they throw exception, i dont what is happening in the library but your hacky solution is the actual solution for that. If you write the function as below. it also works.
PurchaseResponseDelegator delegator = null;
delegator = new PurchaseResponseDelegator(async response =>
{
await Task.Run(() =>
{
if (response.RequestId == requestId)
{
var result = GetPurchaseEventHandler(response);
var sucess = taskCompletionSource.TrySetResult(result);
context.RemovePurchaseResponseListener(delegator.responseDelegate);
}
} );
});
// Register for an event
context.AddPurchaseResponseListener(delegator.responseDelegate);
One other exception I had despite the async-await solution, somehow, it always throws exception for the line taskCompletionSource.SetResult(result); for PurchaseUpdates functions only. if i use instead this line var sucess = taskCompletionSource.TrySetResult(result); it works fine
Related
After reading pretty much every question on Stack Overflow and Microsoft's documentation about NetworkStream, I dont understand what is wrong with my code.
The problem I see is that my method GetDataAsync() hangs very often. I call this method from Init Method like so:
public MyView(string id)
{
InitializeComponent();
MyViewModel myViewModel = session.Resolve<MyViewModel>(); //Autofac
myiewModel.Init(id);
BindingContext = myViewModel;
}
Above, my View does its initialization, then resolves MyViewModel from Autofac DiC and then calls MyViewModel Init() method to do some additional setup on the VM.
The Init method then calls my Async method GetDataAsync which return a IList like so:
public void Init()
{
// call this Async method to populate a ListView
foreach (var model in GetDataAsync("111").Result)
{
// The List<MyModel> returned by the GetDataAsync is then
// used to load ListView's ObservableCollection<MyModel>
// This ObservableCollection is data-bound to a ListView in
// this View. So, the ListView shows its data once the View
// displays.
}
}
, and here is my GetDataAsync() method including my comments:
public override async Task<IList<MyModel>> GetDataAsync(string id)
{
var timeout = TimeSpan.FromSeconds(20);
try
{
byte[] messageBytes = GetMessageBytes(Id);
using (var cts = new CancellationTokenSource(timeout))
using (TcpClient client = new TcpClient(Ip, Port))
using (NetworkStream stream = client.GetStream())
{
await stream.WriteAsync(messageBytes, 0, messageBytes.Length, cts.Token);
await stream.FlushAsync(cts.Token);
byte[] buffer = new byte[1024];
StringBuilder builder = new StringBuilder();
int bytesRead = 0;
await Task.Delay(500);
while (stream.DataAvailable) // need to Delay to wait for data to be available
{
bytesRead = await stream.ReadAsync(buffer, 0, buffer.Length, cts.Token);
builder.AppendFormat("{0}", Encoding.ASCII.GetString(buffer, 0, bytesRead));
}
string msg = buffer.ToString();
}
return ParseMessageIntoList(msg); // parses message into IList<MyModel>
}
catch (OperationCanceledException oce)
{
return await Task.FromResult<IList<RoomGuestModel>>(new List<RoomGuestModel>());
}
catch (Exception ex)
{
return await Task.FromResult<IList<RoomGuestModel>>(new List<RoomGuestModel>());
}
}
I would expect that a ReadAsync or WriteAsync either complete successfully, throw some exception, or get cancelled after 10 seconds in which case I would catch OperationCanceledException.
However, it just hangs endlessly when I call method above. If I am debugging and have some breakpoints in the code above, I will be able to go through the method entirely but if I call it 2nd time, app just hangs forever.
I am new to Tasks and Async programming, so I am also not sure I do my cancellations and exception handling properly here?
UPDATE AND FIX
I figured out how to fix the deadlock issue. In hope this will help others sho might run into the same issue, I'll first explain it. The articles that helped me a lot are:
https://devblogs.microsoft.com/pfxteam/await-and-ui-and-deadlocks-oh-my/ by Stephen Taub
https://montemagno.com/c-sharp-developers-stop-calling-dot-result/ by James Montemagno
https://msdn.microsoft.com/en-us/magazine/jj991977.aspx by StephenCleary
https://blog.xamarin.com/getting-started-with-async-await/ by Jon Goldberger
#StephenCleary was great help understanding the issue. Calling Result or Wait (above, I call Result when calling GetDataAsync) will lead to dead-lock.
The context thread (UI in this case) is now waiting for GetDataAsync to complete, but GetDataAsync captures the current context-thread (UI thread), so it can resume on it once it gets data from TCP. But since this context-thread is now blocked by call to Result, it cannot resume.
The end result is that it looks like call to GetDataAsync has deadlocked but in reality, it is call to Result that deadlocked.
After reading tons of articles from #StephenTaub, #StephenCleary, #JamesMontemagno, #JoeGoldenberger (thank you all), I started getting understanding of the issue (I am new to TAP/async/await).
Then I discovered continuations in Tasks and how to use them to resolve the issue (thanks to Stephen Taub's article above).
So, instead of calling it like:
IList<MyModel> models = GetDataAsync("111").Result;
foeach(var model in models)
{
MyModelsObservableCollection.Add(model);
}
, I call it with continuation like this:
GetDataAsync(id)
.ContinueWith((antecedant) =>
{
foreach(var model in antecedant.Result)
{
MyModelsObservableCollection.Add(model);
}
}, TaskContinuationOptions.OnlyOnRanToCompletion)
.ContinueWith((antecedant) =>
{
var error = antecedant.Exception.Flatten();
}, TaskContinuationOptions.OnlyOnFaulted);
This seam to have fixed my deadlocking issue and now my list will load fine even though it is loaded from the constructor.
So, this seam to work just fine. But #JoeGoldenberger also suggests another solution in his article https://blog.xamarin.com/getting-started-with-async-await/ which is to use Task.Run(async()=>{...}); and inside that await GetDataAsync and load ObservableCollection. So, I gave that a try as well and that is not blocking either, so working great:
Task.Run(async() =>
{
IList<MyModel> models = await GetDataAsync(id);
foreach (var model in models)
{
MyModelsObservableCollection.Add(model);
}
});
So, it looks like either of these 2 will remove deadlock just fine. And since above my Init method is called from a c-tor; therefore, I cannot make it Async and await on this, using one of the 2 methods described above resolves my problem. I dont know which one is better but in my tests, they do work.
Your problem is most likely due to GetDataAsync("111").Result. You shouldn't block on async code.
This can cause deadocks. E.g., if you're on a UI thread, the UI thread will start GetDataAsync and run it until it hits an await. At this point, GetDataAsync returns an incomplete task, and the .Result call blocks the UI thread until that task is completed.
Eventually, the inner async call completes and GetDataAsync is ready to resume executing after its await. By default, await captures its context and resumes on that context. Which in this example is the UI thread. Which is blocked since it called Result. So, the UI thread is waiting for GetDataAsync to complete, and GetDataAsync is waiting for the UI thread so it can complete: deadlock.
The proper solution is to go async all the way; replace .Result with await, and make the necessary changes to other code for that to happen.
As stated in my update, going async all the way by providing an async lambda like below resolved the issue for me
Task.Run(async() =>
{
IList<MyModel> models = await GetDataAsync(id);
foreach (var model in models)
{
MyModelsObservableCollection.Add(model);
}
});
Loading asynchronously an observable collection in a ctor this way (in my case, ctor calls Init which then uses this Task.Run) solves problem
I am developing an app using Xamarin.Forms to re-use as much code as possible.
Specifically, I have a static class that manages all the POST/GET requests to my Azure Web Server.
Everything single call works fantastically except for one call - this one:
public async static Task<Models.UserParkPosition> GetUserParkPositionForCurrentUserAsync()
{
var body = new JArray { App.User.Id };
var test = await AzureMobileServiceClient.Instance.MobileService.InvokeApiAsync<JArray, Models.UserParkPosition>(ConnectionsAPI, body, HttpMethod.Get, null);
return test;
}
The method above is called when the user presses a button - specifically like this:
private async Task OnGoingToTheParkClicked(object sender, EventArgs e)
{
bool success = false;
var already = await viewModel.AlreadyHavePendingRequestAsync());
Console.WriteLine("TEST");
throw new Exception();
}
When the debugger hits the "var test" line mentioned, above.... nothing happens. The code doesn't deadlock, the UI is still responsive, but the code never returns. I never see the "TEST" word and the exception is not even thrown... what happens??
All my other APIs are called in the same way, and are working correctly.
OTHER INFO:
- Same code works on UWP and Android
- The GET request arrives at the webservice, which responds in a timely fashion (< 1 sec)
Thanks to anyone who might help or even point me in the right direction!
Found the issue - InvokeApiAsync throws an exception which is not catched anywhere from any calling thread. It somehow just disappears without causing any error nor device log, nor crash. Weird, but that's how the world goes.
For some time now, our team has been using the Protractor/Jasmine combo in order to do E2E testing and it has worked out great for us.
Recently, I've been assigned the task of improving the logging and have noticed 2 areas in our code where the logging could be improved.
One of these areas is with using browser.wait(). We're currently using the method in the form of browser.wait(condition, timeOut) and excluding the third parameter which is a message to be written to the console in case of a failure.
I decided to comb the code and insert a message into each of these methods and the result has been OK. Although the code works, I'm wondering if there is a more elegant way of doing this.
I'm currently saving the XPath of whatever element I'm waiting on and then if that fails, displaying a message in the form: getWait() timeout due to <element_name> { XPath = <element_XPath> } being not visible or enabled such that you can click it. The pattern changes if we're using isPresent(), elementToBeClickable(), visibilityOf(), etc.
Is there a way of getting the current context of the code I'm executing when browser.wait() is executed? Can I instead display the element or more information on what caused the timeOut?
Let me know if I can clarify further. Thanks
Even I came faced the same problem when using browser.wait where it won't show any detailed log on time out. So what I have done is, I created a wrapper class for waitHandling and added failure messages based on the wait type. Kindly have a look at below code.
var browserWaitHandler = function () {
var expectedConditions = protractor.ExpectedConditions;
var defaultWaitTime = 5000;
this.waitForElementPresent = function (_element,customWaitTime) {
return browserWait(expectedConditions.presenceOf(_element),customWaitTime,"Wait timeout after waiting for element to be Present with locator "+_element.locator().toString());
};
this.waitForElementVisible = function (_element,customWaitTime) {
return browserWait(expectedConditions.visibilityOf(_element),customWaitTime,"Wait timeout after waiting for element to be Visible with locator "+_element.locator().toString());
};
this.waitForElementClickable = function (_element,customWaitTime) {
return browserWait(expectedConditions.elementToBeClickable(_element),customWaitTime,"Wait timeout after waiting for element to be clickable with locator "+_element.locator().toString())
};
this.waitForElementContainsText = function (_element,expectedText,customWaitTime) {
return browserWait(expectedConditions.textToBePresentInElement(_element,expectedText),customWaitTime,"Wait timeout after waiting for element to Contain text as "+expectedText+" with locator "+_element.locator().toString())
};
var browserWait = function (waitCondition,customWaitTime,timeoutMessage) {
return browser.wait(waitCondition,customWaitTime | defaultWaitTime,timeoutMessage);
};
};
And also in protractor we have a method called locator() which can be used with ElementFinder and ElementArrayFinder objects to get the locator that is used to find the element.Refer below example code,
var loginButton = element(by.buttonText("Login"));
console.log("Locator used for login button is:"+loginButton.locator().toString());
*OUTPUT:*
Locator used for login button is:by.buttonText("Login")
I am trying to fetch Customer data to parse them into customer object to display on TableView. The following code sometimes works, sometimes not. Whenever it does crash, it shows Customer data is empty in the foreach loop even though I run the same code every time. I do not have clue what could be wrong in this circumstances. I am quite new on this platform. If I am missing anything/ extra information, please let me know.
namespace TableViewExample
{
public partial class MyDataServices : ContentPage
{
private ODataClient mODataClient;
private IEnumerable <IDictionary<string,object>> Customers;
public MyDataServices ()
{
InitializeComponent ();
InitializeDataService ();
GetDataFromOdataService ();
TableView tableView = new TableView{ };
var section = new TableSection ("Customer");
foreach (var customers in Customers) {
//System.Diagnostics.Debug.WriteLine ((string)customers ["ContactName"]);
var name = (string)customers ["ContactName"];
var cell = new TextCell{ Text = name };
section.Add (cell);
}
tableView.Root.Add (section);
Padding = new Thickness (10, 20, 10, 10);
Content = new StackLayout () {
Children = { tableView }
};
}
private void InitializeDataService(){
try {
mODataClient = new ODataClient ("myURL is here");
}
catch {
System.Diagnostics.Debug.WriteLine("ERROR!");
}
}
private void GetDataFromOdataService (){
try {
Customers = mODataClient.For ("Customers").FindEntries ();
}
catch {
System.Diagnostics.Debug.WriteLine("ERROR!");
}
}
}
}
Its hard helping out here, however here are some things to consider:-
It sounds like the dataservice could either be not contactable / offline; too busy or it could even be throwing an exception itself and returning a data response that you are not expecting to receive, that then triggers an exception and crash in your application as your always expecting an exact response without catering for any abnormal responses / events.
If you are contacting an external service over the internet it may just be your internet connection is slow / faulty and not returning the information fast enough as other possibilities.
In your code you are assuming that you always get a response from the server - and that this response will always be of an anticipated structure that your expecting to decode - without factoring in any possibility of abnormal responses returned by the dataservice. I have not used ODataClient personally, so not sure how it behaves in the event of maybe no data received / timeout or in your case the dataservice and how it behaves internally in the response to a bad-request etc.
I am assuming an exception would get thrown, and you do get your debug line executed indicating a failure.
You may want to also adjust this statement so that you write out the exception as well, i.e.:-
private void GetDataFromOdataService ()
{
try
{
Customers = mODataClient.For ("Customers").FindEntries ();
}
catch (Exception ex)
{
System.Diagnostics.Debug.WriteLine("ERROR!" + ex.ToString());
}
}
If there was a bad response, then the line at Customers = ..... would throw the exception as there may be no Customers returned or some other information packaged in the response from the dataservice.
The Customers variable would also be null at this point I am assuming due to this failing.
So when you get back to your code at foreach (var customers in Customers) { it will then throw a null reference exception as Customers is infact null.
As all your current code executes in the constructor without any try and catch block around this, it will also crash your application at this point as well.
Also you are doing all of this work in the constructor. Try seperating this out. I haven't investigated exactly where the constructor gets called in an iOS page life-cycle, however, if it is in the viewDidLoad, then you have something like 10 seconds for everything to complete, otherwise it will exit automatically. I imagine in your case, this isn't applicable however.
Going forward also try putting your layout controls in the constructor, and move your data task to maybe the OnAppearing override instead.
Using async would definitely be advisable as well, but remember you need to inspect the response from your dataservice, as the error could be embedded within the response also and you will need to detect when it is OK to process the data.
Below is the code I used to access the asset file for a metro app I am working on.
async void readFileFromDisk (string fileName, string fileType)
{
string fileContent;
StorageFile file = await Windows.ApplicationModel.Package.Current.InstalledLocation.GetFileAsync(fileName);
using (IRandomAccessStream readStream = await file.OpenAsync(FileAccessMode.Read))
{
using (DataReader dataReader = new DataReader(readStream))
{
UInt32 numBytesLoaded = await dataReader.LoadAsync((UInt32)readStream.Size);
fileContent = dataReader.ReadString(numBytesLoaded);
}
}
This code is run in the handler for Loaded event for the page. I am currently getting an exception saying "Value does not fall in range". The error occurs at the first line itself, where I try to get storagefile handle from the installation folder.
On debugging, the fileName string comes out to be Null. I guess, I should be moving the code to some event which is fired at a later stage in page lifecycle, but can't seem to figure out what is the best place to do it. Suggestions??
P.S. I need to read this file before any interaction from user, as it reads the data for the level, that user will be interacting with.
Edit:
Missed a couple things.
The below function is called from the handler for loaded event.
void Game_Loaded(object sender, RoutedEventArgs e)
{
//read all level files to the strings
readFileFromDisk("//Assets/Levels/Start" + selectedLevel + ".txt", "Start");
This handler basically calls above function for different file paths, in similar manner. The string selected level is static variable, while the fileName string is created from the same.
Edit 2:
Found the issue, but solution is still far away. The return type of readFileFromDist method is causing trouble. Changed it to Task, and this part works fine, but I get "Object reference not set to an instance" error. Tried to convert Game_Loaded event handler to async too, to use await operators, but that gives me compiler error for "wrong return type".
SO, I tried removing async completely, but I guess I can't do that. There is no way to open files without using async function. So, I essentially need a way to call the readFileFromDisk function, using await, and continue with rest of the code execution once the task is completed. Something like, "IsCompleted" event for the awaited calls for the function.
Solved! Needed to use "ms:appx///Assets/filename.txt" instead of "//Assets/filename.txt".