Sorry if this is a easy question but I am totally new to WP7.
I have a rest service that I am trying to consume however I get an error 'Invalid cross-thread access.'
This is my code
public ObservableCollection<TransactionViewModel> Transactions { get;private set; }
public MainViewModel()
{
this.Transactions = new ObservableCollection<TransactionViewModel>();
}
public void LoadTransactions(string id)
{
var req = (HttpWebRequest)WebRequest.Create(string.Format("http://domain.com/Transactions?Id={0}", id));
req.Method = "POST";
req.ContentType = "application/json; charset=utf-8";
// call async
req.BeginGetResponse(new AsyncCallback(jsonGetRequestStreamCallback), req);
this.IsDataLoaded = true;
}
void jsonGetRequestStreamCallback(IAsyncResult asynchronousResult)
{
WebResponse response = ((HttpWebRequest)asynchronousResult.AsyncState).EndGetResponse(asynchronousResult);
using (StreamReader reader = new StreamReader(response.GetResponseStream()))
{
string responseString = reader.ReadToEnd();
reader.Close();
var s = JsonConvert.DeserializeObject<List<TransactionViewModel>>(responseString);
foreach (var t in s)
{
Transactions.Add(new TransactionViewModel()
{
.........
}
}
Have I done something really stupid here?
When you come back from the request you are no longer on the UI thread. So you need to switch control back to the UI thread before performing any actions that will affect the UI.
You are updating an ObservableCollection, which will be bound on the UI and therefore the update is going to affect the UI.
There are a number of approaches, the simplest for you purposes will be
Deployment.Current.Dispatcher.BeginInvoke(()=> {
foreach (var t in s) {
Transactions.Add(new TransactionViewModel());
}
});
Edit: Also if you want to read a little more about this, I have a blog post about it here http://csainty.blogspot.com/2010/10/windows-phone-7asynchronous-programming.html it starts from code like yours that looks reasonable and should work, explains a few of the gotchas and how to get it working.
Related
I am working on a xamarin mobile application, upon making an async call to the exposed api, i do not get any error, however when i execute the .Result on the task the call never proceeds and it stuck forever.
Click here to see stringResourceResponse details
The same .Result call from a separate project (windows service) in the same solution works.
Any idea if .NET standard is causing limitation in executing async tasks, any advice would be helpful, thanks
Code added below:
//This is code from app.xaml.cs
var stringResourceApi = new StringResourceApiTask();
Task.Run(() =>
{
a = controller.CallStringResourceApi(stringResourceApi);
}).Wait();
public class MobileController
{
public string CallStringResourceApi(StringResourceApiTask stringResourceApiTask)
{
return stringResourceApiTask.Start(StringResourceUrl);
}
}
public override string Start(string URL)
{
var stringResourceResponse = SendRequest(url, "", HttpMethod.Get);
var result = stringResourceResponse.Result;
return result;
}
protected async Task < string > SendRequest(string url, string uri, HttpMethod method, int attempt = 1, int maxAttempts = 5)
{
return await SendRequest(
url, uri, Key, Secret, method, string.Empty, attempt, maxAttempts)
.ConfigureAwait(false);
}
protected async Task<string> SendRequest(string url, string uri, string key, string secret, HttpMethod method,
string requestBody = "", int attempt = 1, int maxAttempts = 5)
{
if (attempt > maxAttempts)
{
return null;
}
var client = InitialiseHttpClient(key, secret);
var request = new HttpRequestMessage
{
RequestUri = string.IsNullOrEmpty(url) ? new Uri(uri) : new Uri(url),
Method = method,
};
if (!string.IsNullOrWhiteSpace(requestBody))
{
request.Content = new StringContent(requestBody, Encoding.UTF8, "application/json");
}
SetOutputText($"Attempting to communicate with {uri}...{Environment.NewLine}");
using (var response = await client.SendAsync(request).ConfigureAwait(false))
{
using (var content = response.Content)
{
try
{
response.EnsureSuccessStatusCode();
}
catch (HttpRequestException ex)
{
if (attempt > maxAttempts)
{
SetOutputText(errorMessage);
}
return await SendRequest(url, uri, key, secret, method, requestBody, attempt + 1).ConfigureAwait(false);
}
var responseBody = await content.ReadAsStringAsync().ConfigureAwait(false);
var isSuccessResponseButEmptyBody = response.IsSuccessStatusCode &&
(string.IsNullOrEmpty(responseBody) ||
string.IsNullOrWhiteSpace(responseBody));
if (!isSuccessResponseButEmptyBody)
{
return responseBody;
}
if (attempt > maxAttempts)
{
SetOutputText(errorMessage);
}
return await SendRequest(url, uri, key, secret, method, requestBody, attempt + 1).ConfigureAwait(false);
}
}
}
when i execute the .Result on the task the call never proceeds and it stuck forever.
Yes. This is a common deadlock situation. When code running on the UI thread blocks on asynchronous code, a deadlock usually occurs.
The same .Result call from a separate project (windows service) in the same solution works.
It works because the Win32 service code does not run on a UI thread.
The proper solution is to remove the blocking code; use await instead. This in turn will cause the calling methods to become async (e.g., StringResourceApiTask.Start), and they should also be awaited, etc. The usage of async and await should "grow" through your code; this is natural.
Alternatively, you can block in a thread pool thread, e.g., Task.Run(() => a = controller.CallStringResourceApi(stringResourceApi)).GetAwaiter().GetResult();. This is a bit of a hack (consuming an unnecessary thread), but it's a quick way to remove the deadlock. Note that this hack is not appropriate for ASP.NET apps; it's acceptable here since this is a UI app.
Im creating a list of components in Blazor, each one of these components need to request some data from a webpage. The list are created as follows on a .razor page:
#foreach(stringcomp in Complist){
<myComponent />
}
around 100 components are created. On all of these components the following URL request is preformed (using this code):
protected override async Task OnAfterRenderAsync(bool firstRender)
{
if (firstRender)
{
await GetUrl("some url here");
}
}
public async Task<string> GetUrl(string url)
{
HttpClient client = new HttpClient();
var request = new HttpRequestMessage(HttpMethod.Get, url);
request.Headers.Add("User-Agent", "get data service");
var response = await client.SendAsync(request).ConfigureAwait(false);
string res = null;
if (response.IsSuccessStatusCode)
{
using var responseStream = await response.Content.ReadAsStreamAsync().ConfigureAwait(false);
var streamReader = new StreamReader(responseStream);
res = await streamReader.ReadToEndAsync().ConfigureAwait(false);
}
return res;
}
Doing this I'm running in to some problems where most of my calls to SendAsync never returns a value. I have come to understand that this is because of a lock-state but for the life of me can't figure out how to solve it. most similar answers suggest setting .ConfigureAwait(false) but this does not yeald a different result in my case.
So my question is: Hos can i request webbpages simultaneously in different components and be sure that they won't hang/lookup. As theres many requests that some times takes a long time (5-10 sec) to complete it's not an alternative to do them synchronously.
It might also be of importance to mention that me code and pages are separated, every .razor page are using #inherits to get its functions/logic
Try to use IHttpClientFactory as follows:
[Inject] public IHttpClientFactory clientFactory { get; set;}
using System.IO;
Your GetUrl method:
public async Task<string> GetUrl(string url)
{
var request = new HttpRequestMessage(HttpMethod.Get, url);
request.Headers.Add("Accept", "application/json");
request.Headers.Add("User-Agent", "get data service");
var client = clientFactory.CreateClient();
var response = await client.SendAsync(request);
string res = null;
if (response.IsSuccessStatusCode)
{
using var responseStream = await response.Content.ReadAsStreamAsync();
var streamReader = new StreamReader(responseStream);
res = await streamReader.ReadToEndAsync().ConfigureAwait(false);
}
return res;
}
Startup.cs
public void ConfigureServices(IServiceCollection services)
{
services.AddHttpClient();
// More code here...
}
Hope this works...
I am facing one error sometimes, when I try get a picture from user gallery.
I get the picture in a Stream object, convert it to a base64 and I send it using a post function.
Sometimes The program says that the image path is big and, because of that, its not possible send the url.
I heard that its cause the image size but I am not sure about it...
Does someone know what really causes a Big base64 string? How can I solve that?
You could follow something similar to this:
http://jamessdixon.wordpress.com/2013/10/01/handling-images-in-webapi/
Maybe you need make some change in your web api like this:
[HttpPost]
public HttpResponseMessage UploadImage(int ID)
{
var result = new HttpResponseMessage(HttpStatusCode.OK);
if (Request.Content.IsMimeMultipartContent())
{
Request.Content.LoadIntoBufferAsync().Wait();
Request.Content.ReadAsMultipartAsync(new MultipartMemoryStreamProvider()).ContinueWith((task) =>
{
MultipartMemoryStreamProvider provider = task.Result;
foreach (HttpContent content in provider.Contents)
{
Stream stream = content.ReadAsStreamAsync().Result;
Image image = Image.FromStream(stream);
var testName = content.Headers.ContentDisposition.Name;
String filePath = HostingEnvironment.MapPath("~/Content/");
String fullPath = Path.Combine(filePath, ID.ToString()+".jpg");
image.Save(fullPath);
}
});
return result;
}
else
{
throw new HttpResponseException(Request.CreateResponse(HttpStatusCode.NotAcceptable, "This request is not properly formatted"));
}
}
After making sure your api web works well you should change from Stream to byte[]
In your Xamarin code try this:
public async Task PostItem(String Controller, String Method, int ID, byte[] item)
{
using (var client = CreateClient ()) {
var da = new ByteArrayContent(item);
try{
var multi = new MultipartContent();
multi.Add(da);
var response = await client.PostAsync (Controller + "/" +Method + "/?ID=" +ID, multi);
}catch(Exception ex)
{
Console.Write(ex.Message);
}
}
}
For more infomation see MultipartContent class documentation
I've got a simple web api which is consumed from a mvc project, I keep on getting the 'Response status code does not indicate success' and was wondering how would I get the response body from the error, I can see the error within a rest viewer but can't navigate through to the error. This is the following code within the MVC app
public ActionResult Index()
{
try
{
var uri = "http://localhost:57089/api/values";
using (var client = new HttpClient())
{
Task<string> response = client.GetStringAsync(uri);
object result = JsonConvert.DeserializeObject(response.Result);
return (ActionResult) result;
}
}
catch (Exception ex)
{
return Content(ex.ToString());
}
return View();
}
Within the API controller I'm sending a bad request, here's the code
public IHttpActionResult Get()
{
return BadRequest("this is a very bad request " + System.DateTime.Now.ToUniversalTime());
}
I've tried to use WebException, HttpRequestException as exceptions to catch the error with no luck.
I can see the response body within the rest viewer
I want to be able to navigate to the Error Message so I can pass that to the client (which later will be changed to a guid).
[EDITED]
I've got a solution without using GetStringAsync, but wanted to use that if possible.
Here's the solution
var httpClient = new HttpClient();
httpClient.BaseAddress = new Uri(url);
HttpResponseMessage responseMessage = httpClient.GetAsync("").Result;
if (responseMessage.IsSuccessStatusCode) return Content(responseMessage.ToString());
var a = responseMessage.Content.ReadAsStringAsync().Result;
var result = JsonConvert.DeserializeObject<HttpError>(a);
object value = "";
return Content(result.TryGetValue("ErrorMessage", out value) ? value.ToString() : responseMessage.ToString());
Is there a better way?
Using WebException you should be able to get to the ResponseStream and the custom error message like this:
catch (WebException e)
{
var message = e.Message;
using (var reader = new StreamReader(e.Response.GetResponseStream()))
{
var content = reader.ReadToEnd();
}
}
Hope that helps.
I'm trying to validate a user in my WP7 app by validating username/pass on a server.
In my "validate user" method I create a HTTPWebRequest with the stuff I need validated.
Then as far as I can see, the only option to execute is to use request.BeginGetResponse, with an async callback.
But I want to return the response from the request in the same method that I created the request in, how can I accomplish this?
I'm not sure why you would like to make it return in the calling method. I would just use the standard WebClient behavior like below:
public void ValidateUser()
{
WebClient webClient = new WebClient();
Uri uri = new Uri(url);
webClient.DownloadStringCompleted += new DownloadStringCompletedEventHandler(webClient_DownloadStringCompleted);
webClient.DownloadStringAsync(new Uri(url));
}
void webClient_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
{
//Validate result from response by using e.Result
}
BeginGetResponse returns an IAsyncResult which has a wait handle that is signalled when the request completes. All you have to do is wait on this handle: [ doesn't work on WP7 ]
UPDATE: use a ManualResetEvent
var mre = new ManualResetEvent( false );
var iar = myHttpWebRequest.BeginGetResponse( state => mre.Set(), null );
mre.WaitOne();
var response = myHttpWebRequest.EndGetResponse( iar );
The only real way to achieve what you need would be to create an event in your worker class, and fire that event when the async call completes, so (pseudocode)...
void StartDownload(object[] parameters)
{
var req = HttpWebRequest.Create("http://google.com");
req.BeginGetRequestStream(Completed_handler, req);
}
void Completed_handler(object sender, DownloadStringCompletedEventArgs e)
{
var request = (HttpWebRequest)result.AsyncState;
var response = request.EndGetResponse(result);
using (var stream = response.GetResponseStream())
using (var reader = new StreamReader(stream))
{
var contents = reader.ReadToEnd();
NotifyCallerOfContent(contents)
}
}
public event EventHandler<MyCustomEventArgsClass> DownloadHasFinished;
public NotifyCallerOfDownload(string content)
{
if(null != DownloadHasFinished)
{
DownloadHasFinished(this, new MyCustomEventArgsClass(content));
}
}
and then subscribe to the DownloadHasFinished event in your calling class.
As for calling out specifically by a HttpWebRequest, take a look at this question, where you will find some working samples.